Export of internal Abseil changes

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341670bce317dd6af8d3c066970230591a47e80c by Martijn Vels <mvels@google.com>:

Change GetStack() and GetParentStack() to return absl::Span

PiperOrigin-RevId: 368765721

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6aaab9536d6957303c7aba100c3afaa6fb0ea2c8 by Martijn Vels <mvels@google.com>:

Remove locking from parent stack.

This change removes the need to lock all access to `parent_stack' by making the 'copy constructor' logic specify the 'copied from' CordzInfo (where available) to the TrackCord function, after which parent_stack is immutable.

PiperOrigin-RevId: 368760630

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b19e2059cada35a8ede994833018edac94de6ddc by Martijn Vels <mvels@google.com>:

Add cordz instrumentation to Cord

PiperOrigin-RevId: 368746225

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67b8bbf980f0f4e1db79aa32968e9a715a09b51a by Martijn Vels <mvels@google.com>:

Create ABSL_INTERNAL_CORDZ_ENABLED define controlling when Cordz code is enabled

There are specific builds and condtions under which we don't support cordz sampling, which is per this change represented by ABSL_INTERNAL_CORDZ_ENABLED being defined.

PiperOrigin-RevId: 368731603

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8cbfe0e3169637a620f4b66ad2bc2ce340879cb0 by Martijn Vels <mvels@google.com>:

Add a `rep` property to CordzInfo to be managed by Cord logic.

This change adds a `rep` property to CordzInfo, which is intended to be used by collection logic.

Mini design:
Cord invokes TrackCord() providing the active 'root' cordrep of the newly sampled Cord, returning a CordzInfo with a weak (uncounted) reference to this root. Cord invokes `SetCordRep()` each time the root cordrep of the sampled Cord is updated while holding `mutex()`. Cord must also obtain `mutex()` _before_ removing a reference on the old root. i.e.: Cord must guarantee that the (weak) reference held in CordzInfo is at all times valid.

CordzInfo collection code can then safely obtain a (reference counted) rep pointer by adding a reference to `rep_` while holding `mutex()`. This requires only a very brief critical section inside CordzInfo logic, minimizing contention with concurrent Cord updates.

Cord code should typically obtain and hold `mutex()` for the entirety of each mutating Cord operation on a sampled cord. As Cord is thread compatible, it never competes on the lock with any other thread. The only possible concurrent access is from Cordz collection code, which should be a relatively rare event.

PiperOrigin-RevId: 368673758

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1255120dce2bdd6b4205a34a0e555e0b74b6152f by Martijn Vels <mvels@google.com>:

Remove 'depth' from active recorded metrics.

Going forward we do not 'live' record depth (and size), but will observe these at collection time only.

PiperOrigin-RevId: 368636572

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83e5146e35f221736b49e9f0a8805f8c159a51db by Martijn Vels <mvels@google.com>:

Make cordz targets visible in OSS

PiperOrigin-RevId: 368615010

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dcb16a4f1239151f0a8c70a8cfeb29dabbd113b8 by Martijn Vels <mvels@google.com>:

Internal cleanup

PiperOrigin-RevId: 368514666
GitOrigin-RevId: 341670bce317dd6af8d3c066970230591a47e80c
Change-Id: I94cecfbbd441eb386f99fc5186c468a7a5538862

13 files changed, 1765 insertions, 0 deletions

diff --git a/absl/strings/internal/cordz_functions.cc b/absl/strings/internal/cordz_functions.cc
new file mode 100644
index 00000000..6ad864f1
--- /dev/null
+++ b/absl/strings/internal/cordz_functions.cc
@@ -0,0 +1,104 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_functions.h"
+
+#include <atomic>
+#include <cmath>
+#include <limits>
+#include <random>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/exponential_biased.h"
+#include "absl/base/internal/raw_logging.h"
+
+// TODO(b/162942788): weak 'cordz_disabled' value.
+// A strong version is in the 'cordz_disabled_hack_for_odr' library which can
+// be linked in to disable cordz at compile time.
+extern "C" {
+bool absl_internal_cordz_disabled ABSL_ATTRIBUTE_WEAK = false;
+}
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+// The average interval until the next sample. A value of 0 disables profiling
+// while a value of 1 will profile all Cords.
+std::atomic<int> g_cordz_mean_interval(50000);
+
+}  // namespace
+
+#ifdef ABSL_INTERNAL_CORDZ_ENABLED
+
+ABSL_CONST_INIT thread_local int64_t cordz_next_sample = 0;
+
+// kIntervalIfDisabled is the number of profile-eligible events need to occur
+// before the code will confirm that cordz is still disabled.
+constexpr int64_t kIntervalIfDisabled = 1 << 16;
+
+ABSL_ATTRIBUTE_NOINLINE bool cordz_should_profile_slow() {
+  // TODO(b/162942788): check if profiling is disabled at compile time.
+  if (absl_internal_cordz_disabled) {
+    ABSL_RAW_LOG(WARNING, "Cordz info disabled at compile time");
+    // We are permanently disabled: set counter to highest possible value.
+    cordz_next_sample = std::numeric_limits<int64_t>::max();
+    return false;
+  }
+
+  thread_local absl::base_internal::ExponentialBiased
+      exponential_biased_generator;
+  int32_t mean_interval = get_cordz_mean_interval();
+
+  // Check if we disabled profiling. If so, set the next sample to a "large"
+  // number to minimize the overhead of the should_profile codepath.
+  if (mean_interval <= 0) {
+    cordz_next_sample = kIntervalIfDisabled;
+    return false;
+  }
+
+  // Check if we're always sampling.
+  if (mean_interval == 1) {
+    cordz_next_sample = 1;
+    return true;
+  }
+
+  if (cordz_next_sample <= 0) {
+    cordz_next_sample = exponential_biased_generator.GetStride(mean_interval);
+    return true;
+  }
+
+  --cordz_next_sample;
+  return false;
+}
+
+void cordz_set_next_sample_for_testing(int64_t next_sample) {
+  cordz_next_sample = next_sample;
+}
+
+#endif  // ABSL_INTERNAL_CORDZ_ENABLED
+
+int32_t get_cordz_mean_interval() {
+  return g_cordz_mean_interval.load(std::memory_order_acquire);
+}
+
+void set_cordz_mean_interval(int32_t mean_interval) {
+  g_cordz_mean_interval.store(mean_interval, std::memory_order_release);
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_functions.h b/absl/strings/internal/cordz_functions.h
new file mode 100644
index 00000000..c9ba1450
--- /dev/null
+++ b/absl/strings/internal/cordz_functions.h
@@ -0,0 +1,85 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_CORDZ_FUNCTIONS_H_
+#define ABSL_STRINGS_CORDZ_FUNCTIONS_H_
+
+#include <stdint.h>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/optimization.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// Returns the current sample rate. This represents the average interval
+// between samples.
+int32_t get_cordz_mean_interval();
+
+// Sets the sample rate with the average interval between samples.
+void set_cordz_mean_interval(int32_t mean_interval);
+
+// Enable cordz unless any of the following applies:
+// - no thread local support
+// - MSVC build
+// - Android build
+// - Apple build
+// - DLL build
+// Hashtablez is turned off completely in opensource builds.
+// MSVC's static atomics are dynamically initialized in debug mode, which breaks
+// sampling.
+#if defined(ABSL_HAVE_THREAD_LOCAL) && !defined(_MSC_VER)  && \
+    !defined(ABSL_BUILD_DLL) && !defined(ABSL_CONSUME_DLL) && \
+    !defined(__ANDROID__) && !defined(__APPLE__)
+#define ABSL_INTERNAL_CORDZ_ENABLED 1
+#endif
+
+#ifdef ABSL_INTERNAL_CORDZ_ENABLED
+
+// cordz_next_sample is the number of events until the next sample event. If
+// the value is 1 or less, the code will check on the next event if cordz is
+// enabled, and if so, will sample the Cord. cordz is only enabled when we can
+// use thread locals.
+ABSL_CONST_INIT extern thread_local int64_t cordz_next_sample;
+
+// Determines if the next sample should be profiled. If it is, the value pointed
+// at by next_sample will be set with the interval until the next sample.
+bool cordz_should_profile_slow();
+
+// Returns true if the next cord should be sampled.
+inline bool cordz_should_profile() {
+  if (ABSL_PREDICT_TRUE(cordz_next_sample > 1)) {
+    cordz_next_sample--;
+    return false;
+  }
+  return cordz_should_profile_slow();
+}
+
+// Sets the interval until the next sample (for testing only)
+void cordz_set_next_sample_for_testing(int64_t next_sample);
+
+#else  // ABSL_INTERNAL_CORDZ_ENABLED
+
+inline bool cordz_should_profile() { return false; }
+inline void cordz_set_next_sample_for_testing(int64_t) {}
+
+#endif  // ABSL_INTERNAL_CORDZ_ENABLED
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_CORDZ_FUNCTIONS_H_
diff --git a/absl/strings/internal/cordz_functions_test.cc b/absl/strings/internal/cordz_functions_test.cc
new file mode 100644
index 00000000..f2cefae3
--- /dev/null
+++ b/absl/strings/internal/cordz_functions_test.cc
@@ -0,0 +1,131 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_functions.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::testing::Eq;
+using ::testing::Ge;
+using ::testing::Le;
+
+TEST(CordzFunctionsTest, SampleRate) {
+  int32_t orig_sample_rate = get_cordz_mean_interval();
+  int32_t expected_sample_rate = 123;
+  set_cordz_mean_interval(expected_sample_rate);
+  EXPECT_THAT(get_cordz_mean_interval(), Eq(expected_sample_rate));
+  set_cordz_mean_interval(orig_sample_rate);
+}
+
+// Cordz is disabled when we don't have thread_local. All calls to
+// should_profile will return false when cordz is diabled, so we might want to
+// avoid those tests.
+#ifdef ABSL_INTERNAL_CORDZ_ENABLED
+
+TEST(CordzFunctionsTest, ShouldProfileDisable) {
+  int32_t orig_sample_rate = get_cordz_mean_interval();
+
+  set_cordz_mean_interval(0);
+  cordz_set_next_sample_for_testing(0);
+  EXPECT_FALSE(cordz_should_profile());
+  // 1 << 16 is from kIntervalIfDisabled in cordz_functions.cc.
+  EXPECT_THAT(cordz_next_sample, Eq(1 << 16));
+
+  set_cordz_mean_interval(orig_sample_rate);
+}
+
+TEST(CordzFunctionsTest, ShouldProfileAlways) {
+  int32_t orig_sample_rate = get_cordz_mean_interval();
+
+  set_cordz_mean_interval(1);
+  cordz_set_next_sample_for_testing(1);
+  EXPECT_TRUE(cordz_should_profile());
+  EXPECT_THAT(cordz_next_sample, Le(1));
+
+  set_cordz_mean_interval(orig_sample_rate);
+}
+
+TEST(CordzFunctionsTest, ShouldProfileRate) {
+  static constexpr int kDesiredMeanInterval = 1000;
+  static constexpr int kSamples = 10000;
+  int32_t orig_sample_rate = get_cordz_mean_interval();
+
+  set_cordz_mean_interval(kDesiredMeanInterval);
+
+  int64_t sum_of_intervals = 0;
+  for (int i = 0; i < kSamples; i++) {
+    // Setting next_sample to 0 will force cordz_should_profile to generate a
+    // new value for next_sample each iteration.
+    cordz_set_next_sample_for_testing(0);
+    cordz_should_profile();
+    sum_of_intervals += cordz_next_sample;
+  }
+
+  // The sum of independent exponential variables is an Erlang distribution,
+  // which is a gamma distribution where the shape parameter is equal to the
+  // number of summands. The distribution used for cordz_should_profile is
+  // actually floor(Exponential(1/mean)) which introduces bias. However, we can
+  // apply the squint-really-hard correction factor. That is, when mean is
+  // large, then if we squint really hard the shape of the distribution between
+  // N and N+1 looks like a uniform distribution. On average, each value for
+  // next_sample will be about 0.5 lower than we would expect from an
+  // exponential distribution. This squint-really-hard correction approach won't
+  // work when mean is smaller than about 10 but works fine when mean is 1000.
+  //
+  // We can use R to calculate a confidence interval. This
+  // shows how to generate a confidence interval with a false positive rate of
+  // one in a billion.
+  //
+  // $ R -q
+  // > mean = 1000
+  // > kSamples = 10000
+  // > errorRate = 1e-9
+  // > correction = -kSamples / 2
+  // > low = qgamma(errorRate/2, kSamples, 1/mean) + correction
+  // > high = qgamma(1 - errorRate/2, kSamples, 1/mean) + correction
+  // > low
+  // [1] 9396115
+  // > high
+  // [1] 10618100
+  EXPECT_THAT(sum_of_intervals, Ge(9396115));
+  EXPECT_THAT(sum_of_intervals, Le(10618100));
+
+  set_cordz_mean_interval(orig_sample_rate);
+}
+
+#else  // ABSL_INTERNAL_CORDZ_ENABLED
+
+TEST(CordzFunctionsTest, ShouldProfileDisabled) {
+  int32_t orig_sample_rate = get_cordz_mean_interval();
+
+  set_cordz_mean_interval(1);
+  cordz_set_next_sample_for_testing(0);
+  EXPECT_FALSE(cordz_should_profile());
+
+  set_cordz_mean_interval(orig_sample_rate);
+}
+
+#endif  // ABSL_INTERNAL_CORDZ_ENABLED
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_handle.cc b/absl/strings/internal/cordz_handle.cc
new file mode 100644
index 00000000..7e82f85b
--- /dev/null
+++ b/absl/strings/internal/cordz_handle.cc
@@ -0,0 +1,127 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include "absl/strings/internal/cordz_handle.h"
+
+#include <atomic>
+
+#include "absl/base/internal/raw_logging.h"  // For ABSL_RAW_CHECK
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+ABSL_CONST_INIT absl::Mutex CordzHandle::mutex_(absl::kConstInit);
+ABSL_CONST_INIT std::atomic<CordzHandle*> CordzHandle::dq_tail_{nullptr};
+
+CordzHandle::CordzHandle(bool is_snapshot) : is_snapshot_(is_snapshot) {
+  if (is_snapshot) {
+    MutexLock lock(&mutex_);
+    CordzHandle* dq_tail = dq_tail_.load(std::memory_order_acquire);
+    if (dq_tail != nullptr) {
+      dq_prev_ = dq_tail;
+      dq_tail->dq_next_ = this;
+    }
+    dq_tail_.store(this, std::memory_order_release);
+  }
+}
+
+CordzHandle::~CordzHandle() {
+  if (is_snapshot_) {
+    std::vector<CordzHandle*> to_delete;
+    {
+      absl::MutexLock lock(&mutex_);
+      CordzHandle* next = dq_next_;
+      if (dq_prev_ == nullptr) {
+        // We were head of the queue, delete every CordzHandle until we reach
+        // either the end of the list, or a snapshot handle.
+        while (next && !next->is_snapshot_) {
+          to_delete.push_back(next);
+          next = next->dq_next_;
+        }
+      } else {
+        // Another CordzHandle existed before this one, don't delete anything.
+        dq_prev_->dq_next_ = next;
+      }
+      if (next) {
+        next->dq_prev_ = dq_prev_;
+      } else {
+        dq_tail_.store(dq_prev_, std::memory_order_release);
+      }
+    }
+    for (CordzHandle* handle : to_delete) {
+      delete handle;
+    }
+  }
+}
+
+void CordzHandle::Delete(CordzHandle* handle) {
+  if (handle) {
+    if (!handle->is_snapshot_ && !UnsafeDeleteQueueEmpty()) {
+      MutexLock lock(&mutex_);
+      CordzHandle* dq_tail = dq_tail_.load(std::memory_order_acquire);
+      if (dq_tail != nullptr) {
+        handle->dq_prev_ = dq_tail;
+        dq_tail->dq_next_ = handle;
+        dq_tail_.store(handle, std::memory_order_release);
+        return;
+      }
+    }
+    delete handle;
+  }
+}
+
+std::vector<const CordzHandle*> CordzHandle::DiagnosticsGetDeleteQueue() {
+  std::vector<const CordzHandle*> handles;
+  MutexLock lock(&mutex_);
+  CordzHandle* dq_tail = dq_tail_.load(std::memory_order_acquire);
+  for (const CordzHandle* p = dq_tail; p; p = p->dq_prev_) {
+    handles.push_back(p);
+  }
+  return handles;
+}
+
+bool CordzHandle::DiagnosticsHandleIsSafeToInspect(
+    const CordzHandle* handle) const {
+  if (!is_snapshot_) return false;
+  if (handle == nullptr) return true;
+  if (handle->is_snapshot_) return false;
+  bool snapshot_found = false;
+  MutexLock lock(&mutex_);
+  for (const CordzHandle* p = dq_tail_; p; p = p->dq_prev_) {
+    if (p == handle) return !snapshot_found;
+    if (p == this) snapshot_found = true;
+  }
+  ABSL_ASSERT(snapshot_found);  // Assert that 'this' is in delete queue.
+  return true;
+}
+
+std::vector<const CordzHandle*>
+CordzHandle::DiagnosticsGetSafeToInspectDeletedHandles() {
+  std::vector<const CordzHandle*> handles;
+  if (!is_snapshot()) {
+    return handles;
+  }
+
+  MutexLock lock(&mutex_);
+  for (const CordzHandle* p = dq_next_; p != nullptr; p = p->dq_next_) {
+    if (!p->is_snapshot()) {
+      handles.push_back(p);
+    }
+  }
+  return handles;
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_handle.h b/absl/strings/internal/cordz_handle.h
new file mode 100644
index 00000000..0235d4bd
--- /dev/null
+++ b/absl/strings/internal/cordz_handle.h
@@ -0,0 +1,97 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_CORDZ_HANDLE_H_
+#define ABSL_STRINGS_CORDZ_HANDLE_H_
+
+#include <atomic>
+#include <vector>
+
+#include "absl/base/config.h"
+#include "absl/synchronization/mutex.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// This base class allows multiple types of object (CordzInfo and
+// CordzSampleToken) to exist simultaneously on the delete queue (pointed to by
+// global_dq_tail and traversed using dq_prev_ and dq_next_). The
+// delete queue guarantees that once a profiler creates a CordzSampleToken and
+// has gained visibility into a CordzInfo object, that CordzInfo object will not
+// be deleted prematurely. This allows the profiler to inspect all CordzInfo
+// objects that are alive without needing to hold a global lock.
+class CordzHandle {
+ public:
+  CordzHandle() : CordzHandle(false) {}
+
+  bool is_snapshot() const { return is_snapshot_; }
+
+  // Deletes the provided instance, or puts it on the delete queue to be deleted
+  // once there are no more sample tokens (snapshot) instances potentially
+  // referencing the instance. `handle` may be null.
+  static void Delete(CordzHandle* handle);
+
+  // Returns the current entries in the delete queue in LIFO order.
+  static std::vector<const CordzHandle*> DiagnosticsGetDeleteQueue();
+
+  // Returns true if the provided handle is nullptr or guarded by this handle.
+  // Since the CordzSnapshot token is itself a CordzHandle, this method will
+  // allow tests to check if that token is keeping an arbitrary CordzHandle
+  // alive.
+  bool DiagnosticsHandleIsSafeToInspect(const CordzHandle* handle) const;
+
+  // Returns the current entries in the delete queue, in LIFO order, that are
+  // protected by this. CordzHandle objects are only placed on the delete queue
+  // after CordzHandle::Delete is called with them as an argument. Only
+  // CordzHandle objects that are not also CordzSnapshot objects will be
+  // included in the return vector. For each of the handles in the return
+  // vector, the earliest that their memory can be freed is when this
+  // CordzSnapshot object is deleted.
+  std::vector<const CordzHandle*> DiagnosticsGetSafeToInspectDeletedHandles();
+
+ protected:
+  explicit CordzHandle(bool is_snapshot);
+  virtual ~CordzHandle();
+
+ private:
+  // Returns true if the delete queue is empty. This method does not acquire the
+  // lock, but does a 'load acquire' observation on the delete queue tail. It
+  // is used inside Delete() to check for the presence of a delete queue without
+  // holding the lock. The assumption is that the caller is in the state of
+  // 'being deleted', and can not be newly discovered by a concurrent 'being
+  // constructed' snapshot instance. Practically, this means that any such
+  // discovery (`find`, 'first' or 'next', etc) must have proper 'happens before
+  // / after' semantics and atomic fences.
+  static bool UnsafeDeleteQueueEmpty() ABSL_NO_THREAD_SAFETY_ANALYSIS {
+    return dq_tail_.load(std::memory_order_acquire) == nullptr;
+  }
+
+  const bool is_snapshot_;
+  static absl::Mutex mutex_;
+  static std::atomic<CordzHandle*> dq_tail_ ABSL_GUARDED_BY(mutex_);
+  CordzHandle* dq_prev_ ABSL_GUARDED_BY(mutex_) = nullptr;
+  CordzHandle* dq_next_ ABSL_GUARDED_BY(mutex_) = nullptr;
+};
+
+class CordzSnapshot : public CordzHandle {
+ public:
+  CordzSnapshot() : CordzHandle(true) {}
+};
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_CORDZ_HANDLE_H_
diff --git a/absl/strings/internal/cordz_handle_test.cc b/absl/strings/internal/cordz_handle_test.cc
new file mode 100644
index 00000000..c04240e8
--- /dev/null
+++ b/absl/strings/internal/cordz_handle_test.cc
@@ -0,0 +1,253 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include "absl/strings/internal/cordz_handle.h"
+
+#include <random>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/memory/memory.h"
+#include "absl/synchronization/internal/thread_pool.h"
+#include "absl/synchronization/notification.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::testing::ElementsAre;
+using ::testing::Gt;
+using ::testing::IsEmpty;
+using ::testing::SizeIs;
+
+// Local less verbose helper
+std::vector<const CordzHandle*> DeleteQueue() {
+  return CordzHandle::DiagnosticsGetDeleteQueue();
+}
+
+struct CordzHandleDeleteTracker : public CordzHandle {
+  bool* deleted;
+  explicit CordzHandleDeleteTracker(bool* deleted) : deleted(deleted) {}
+  ~CordzHandleDeleteTracker() override { *deleted = true; }
+};
+
+TEST(CordzHandleTest, DeleteQueueIsEmpty) {
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+}
+
+TEST(CordzHandleTest, CordzHandleCreateDelete) {
+  bool deleted = false;
+  auto* handle = new CordzHandleDeleteTracker(&deleted);
+  EXPECT_FALSE(handle->is_snapshot());
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+
+  CordzHandle::Delete(handle);
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+  EXPECT_TRUE(deleted);
+}
+
+TEST(CordzHandleTest, CordzSnapshotCreateDelete) {
+  auto* snapshot = new CordzSnapshot();
+  EXPECT_TRUE(snapshot->is_snapshot());
+  EXPECT_THAT(DeleteQueue(), ElementsAre(snapshot));
+  delete snapshot;
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+}
+
+TEST(CordzHandleTest, CordzHandleCreateDeleteWithSnapshot) {
+  bool deleted = false;
+  auto* snapshot = new CordzSnapshot();
+  auto* handle = new CordzHandleDeleteTracker(&deleted);
+
+  CordzHandle::Delete(handle);
+  EXPECT_THAT(DeleteQueue(), ElementsAre(handle, snapshot));
+  EXPECT_FALSE(deleted);
+
+  delete snapshot;
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+  EXPECT_TRUE(deleted);
+}
+
+TEST(CordzHandleTest, MultiSnapshot) {
+  bool deleted[3] = {false, false, false};
+
+  CordzSnapshot* snapshot[3];
+  CordzHandleDeleteTracker* handle[3];
+  for (int i = 0; i < 3; ++i) {
+    snapshot[i] = new CordzSnapshot();
+    handle[i] = new CordzHandleDeleteTracker(&deleted[i]);
+    CordzHandle::Delete(handle[i]);
+  }
+
+  EXPECT_THAT(DeleteQueue(), ElementsAre(handle[2], snapshot[2], handle[1],
+                                         snapshot[1], handle[0], snapshot[0]));
+  EXPECT_THAT(deleted, ElementsAre(false, false, false));
+
+  delete snapshot[1];
+  EXPECT_THAT(DeleteQueue(), ElementsAre(handle[2], snapshot[2], handle[1],
+                                         handle[0], snapshot[0]));
+  EXPECT_THAT(deleted, ElementsAre(false, false, false));
+
+  delete snapshot[0];
+  EXPECT_THAT(DeleteQueue(), ElementsAre(handle[2], snapshot[2]));
+  EXPECT_THAT(deleted, ElementsAre(true, true, false));
+
+  delete snapshot[2];
+  EXPECT_THAT(DeleteQueue(), SizeIs(0));
+  EXPECT_THAT(deleted, ElementsAre(true, true, deleted));
+}
+
+TEST(CordzHandleTest, DiagnosticsHandleIsSafeToInspect) {
+  CordzSnapshot snapshot1;
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(nullptr));
+
+  auto* handle1 = new CordzHandle();
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(handle1));
+
+  CordzHandle::Delete(handle1);
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(handle1));
+
+  CordzSnapshot snapshot2;
+  auto* handle2 = new CordzHandle();
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(handle1));
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(handle2));
+  EXPECT_FALSE(snapshot2.DiagnosticsHandleIsSafeToInspect(handle1));
+  EXPECT_TRUE(snapshot2.DiagnosticsHandleIsSafeToInspect(handle2));
+
+  CordzHandle::Delete(handle2);
+  EXPECT_TRUE(snapshot1.DiagnosticsHandleIsSafeToInspect(handle1));
+}
+
+TEST(CordzHandleTest, DiagnosticsGetSafeToInspectDeletedHandles) {
+  EXPECT_THAT(DeleteQueue(), IsEmpty());
+
+  auto* handle = new CordzHandle();
+  auto* snapshot1 = new CordzSnapshot();
+
+  // snapshot1 should be able to see handle.
+  EXPECT_THAT(DeleteQueue(), ElementsAre(snapshot1));
+  EXPECT_TRUE(snapshot1->DiagnosticsHandleIsSafeToInspect(handle));
+  EXPECT_THAT(snapshot1->DiagnosticsGetSafeToInspectDeletedHandles(),
+              IsEmpty());
+
+  // This handle will be safe to inspect as long as snapshot1 is alive. However,
+  // since only snapshot1 can prove that it's alive, it will be hidden from
+  // snapshot2.
+  CordzHandle::Delete(handle);
+
+  // This snapshot shouldn't be able to see handle because handle was already
+  // sent to Delete.
+  auto* snapshot2 = new CordzSnapshot();
+
+  // DeleteQueue elements are LIFO order.
+  EXPECT_THAT(DeleteQueue(), ElementsAre(snapshot2, handle, snapshot1));
+
+  EXPECT_TRUE(snapshot1->DiagnosticsHandleIsSafeToInspect(handle));
+  EXPECT_FALSE(snapshot2->DiagnosticsHandleIsSafeToInspect(handle));
+
+  EXPECT_THAT(snapshot1->DiagnosticsGetSafeToInspectDeletedHandles(),
+              ElementsAre(handle));
+  EXPECT_THAT(snapshot2->DiagnosticsGetSafeToInspectDeletedHandles(),
+              IsEmpty());
+
+  CordzHandle::Delete(snapshot1);
+  EXPECT_THAT(DeleteQueue(), ElementsAre(snapshot2));
+
+  CordzHandle::Delete(snapshot2);
+  EXPECT_THAT(DeleteQueue(), IsEmpty());
+}
+
+// Create and delete CordzHandle and CordzSnapshot objects in multiple threads
+// so that tsan has some time to chew on it and look for memory problems.
+TEST(CordzHandleTest, MultiThreaded) {
+  Notification stop;
+  static constexpr int kNumThreads = 4;
+  // Keep the number of handles relatively small so that the test will naturally
+  // transition to an empty delete queue during the test. If there are, say, 100
+  // handles, that will virtually never happen. With 10 handles and around 50k
+  // iterations in each of 4 threads, the delete queue appears to become empty
+  // around 200 times.
+  static constexpr int kNumHandles = 10;
+
+  // Each thread is going to pick a random index and atomically swap its
+  // CordzHandle with one in handles. This way, each thread can avoid
+  // manipulating a CordzHandle that might be operated upon in another thread.
+  std::vector<std::atomic<CordzHandle*>> handles(kNumHandles);
+
+  absl::synchronization_internal::ThreadPool pool(kNumThreads);
+
+  for (int i = 0; i < kNumThreads; ++i) {
+    pool.Schedule([&stop, &handles]() {
+      std::minstd_rand gen;
+      std::uniform_int_distribution<int> dist_type(0, 2);
+      std::uniform_int_distribution<int> dist_handle(0, kNumHandles - 1);
+      size_t max_safe_to_inspect = 0;
+      while (!stop.HasBeenNotified()) {
+        CordzHandle* handle;
+        switch (dist_type(gen)) {
+          case 0:
+            handle = new CordzHandle();
+            break;
+          case 1:
+            handle = new CordzSnapshot();
+            break;
+          default:
+            handle = nullptr;
+            break;
+        }
+        CordzHandle* old_handle = handles[dist_handle(gen)].exchange(handle);
+        if (old_handle != nullptr) {
+          std::vector<const CordzHandle*> safe_to_inspect =
+              old_handle->DiagnosticsGetSafeToInspectDeletedHandles();
+          for (const CordzHandle* handle : safe_to_inspect) {
+            // We're in a tight loop, so don't generate too many error messages.
+            ASSERT_FALSE(handle->is_snapshot());
+          }
+          if (safe_to_inspect.size() > max_safe_to_inspect) {
+            max_safe_to_inspect = safe_to_inspect.size();
+          }
+        }
+        CordzHandle::Delete(old_handle);
+      }
+
+      // Confirm that the test did *something*. This check will be satisfied as
+      // long as this thread has delete a CordzSnapshot object and a
+      // non-snapshot CordzHandle was deleted after the CordzSnapshot was
+      // created. This max_safe_to_inspect count will often reach around 30
+      // (assuming 4 threads and 10 slots for live handles). Don't use a strict
+      // bound to avoid making this test flaky.
+      EXPECT_THAT(max_safe_to_inspect, Gt(0));
+
+      // Have each thread attempt to clean up everything. Some thread will be
+      // the last to reach this cleanup code, and it will be guaranteed to clean
+      // up everything because nothing remains to create new handles.
+      for (size_t i = 0; i < handles.size(); i++) {
+        CordzHandle* handle = handles[i].exchange(nullptr);
+        CordzHandle::Delete(handle);
+      }
+  });
+  }
+
+  // The threads will hammer away.  Give it a little bit of time for tsan to
+  // spot errors.
+  absl::SleepFor(absl::Seconds(3));
+  stop.Notify();
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_info.cc b/absl/strings/internal/cordz_info.cc
new file mode 100644
index 00000000..4dec63d5
--- /dev/null
+++ b/absl/strings/internal/cordz_info.cc
@@ -0,0 +1,138 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_info.h"
+
+#include "absl/base/config.h"
+#include "absl/debugging/stacktrace.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/internal/cordz_statistics.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+constexpr int CordzInfo::kMaxStackDepth;
+
+ABSL_CONST_INIT std::atomic<CordzInfo*> CordzInfo::ci_head_{nullptr};
+ABSL_CONST_INIT absl::Mutex CordzInfo::ci_mutex_(absl::kConstInit);
+
+CordzInfo* CordzInfo::Head(const CordzSnapshot& snapshot) {
+  ABSL_ASSERT(snapshot.is_snapshot());
+  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(ci_head_unsafe()));
+  return ci_head_unsafe();
+}
+
+CordzInfo* CordzInfo::Next(const CordzSnapshot& snapshot) const {
+  ABSL_ASSERT(snapshot.is_snapshot());
+  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(this));
+  ABSL_ASSERT(snapshot.DiagnosticsHandleIsSafeToInspect(ci_next_unsafe()));
+  return ci_next_unsafe();
+}
+
+CordzInfo* CordzInfo::TrackCord(CordRep* rep, const CordzInfo* src) {
+  CordzInfo* ci = new CordzInfo(rep);
+  if (src) {
+    ci->parent_stack_depth_ = src->stack_depth_;
+    memcpy(ci->parent_stack_, src->stack_, sizeof(void*) * src->stack_depth_);
+  }
+  ci->Track();
+  return ci;
+}
+
+CordzInfo* CordzInfo::TrackCord(CordRep* rep) {
+  return TrackCord(rep, nullptr);
+}
+
+void CordzInfo::UntrackCord(CordzInfo* cordz_info) {
+  assert(cordz_info);
+  if (cordz_info) {
+    cordz_info->Untrack();
+    CordzHandle::Delete(cordz_info);
+  }
+}
+
+CordzInfo::CordzInfo(CordRep* rep)
+    : rep_(rep),
+      stack_depth_(absl::GetStackTrace(stack_, /*max_depth=*/kMaxStackDepth,
+                                       /*skip_count=*/1)),
+      parent_stack_depth_(0),
+      create_time_(absl::Now()) {}
+
+CordzInfo::~CordzInfo() {
+  // `rep_` is potentially kept alive if CordzInfo is included
+  // in a collection snapshot (which should be rare).
+  if (ABSL_PREDICT_FALSE(rep_)) {
+    CordRep::Unref(rep_);
+  }
+}
+
+void CordzInfo::Track() {
+  absl::MutexLock l(&ci_mutex_);
+
+  CordzInfo* const head = ci_head_.load(std::memory_order_acquire);
+  if (head != nullptr) {
+    head->ci_prev_.store(this, std::memory_order_release);
+  }
+  ci_next_.store(head, std::memory_order_release);
+  ci_head_.store(this, std::memory_order_release);
+}
+
+void CordzInfo::Untrack() {
+  {
+    // TODO(b/117940323): change this to assuming ownership instead once all
+    // Cord logic is properly keeping `rep_` in sync with the Cord root rep.
+    absl::MutexLock lock(&mutex());
+    rep_ = nullptr;
+  }
+
+  absl::MutexLock l(&ci_mutex_);
+
+  CordzInfo* const head = ci_head_.load(std::memory_order_acquire);
+  CordzInfo* const next = ci_next_.load(std::memory_order_acquire);
+  CordzInfo* const prev = ci_prev_.load(std::memory_order_acquire);
+
+  if (next) {
+    ABSL_ASSERT(next->ci_prev_.load(std::memory_order_acquire) == this);
+    next->ci_prev_.store(prev, std::memory_order_release);
+  }
+  if (prev) {
+    ABSL_ASSERT(head != this);
+    ABSL_ASSERT(prev->ci_next_.load(std::memory_order_acquire) == this);
+    prev->ci_next_.store(next, std::memory_order_release);
+  } else {
+    ABSL_ASSERT(head == this);
+    ci_head_.store(next, std::memory_order_release);
+  }
+}
+
+void CordzInfo::SetCordRep(CordRep* rep) {
+  mutex().AssertHeld();
+  rep_ = rep;
+}
+
+absl::Span<void* const> CordzInfo::GetStack() const {
+  return absl::MakeConstSpan(stack_, stack_depth_);
+}
+
+absl::Span<void* const> CordzInfo::GetParentStack() const {
+  return absl::MakeConstSpan(parent_stack_, parent_stack_depth_);
+}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_info.h b/absl/strings/internal/cordz_info.h
new file mode 100644
index 00000000..c1090a1c
--- /dev/null
+++ b/absl/strings/internal/cordz_info.h
@@ -0,0 +1,168 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_CORDZ_INFO_H_
+#define ABSL_STRINGS_CORDZ_INFO_H_
+
+#include <atomic>
+#include <cstdint>
+#include <functional>
+
+#include "absl/base/config.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/internal/cordz_statistics.h"
+#include "absl/synchronization/mutex.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// CordzInfo tracks a profiled Cord. Each of these objects can be in two places.
+// If a Cord is alive, the CordzInfo will be in the global_cordz_infos map, and
+// can also be retrieved via the linked list starting with
+// global_cordz_infos_head and continued via the cordz_info_next() method. When
+// a Cord has reached the end of its lifespan, the CordzInfo object will be
+// migrated out of the global_cordz_infos list and the global_cordz_infos_map,
+// and will either be deleted or appended to the global_delete_queue. If it is
+// placed on the global_delete_queue, the CordzInfo object will be cleaned in
+// the destructor of a CordzSampleToken object.
+class CordzInfo : public CordzHandle {
+ public:
+  // All profiled Cords should be accompanied by a call to TrackCord.
+  // TrackCord creates a CordzInfo instance which tracks important metrics of
+  // the sampled cord. CordzInfo instances are placed in a global list which is
+  // used to discover and snapshot all actively tracked cords.
+  // Callers are responsible for calling UntrackCord() before the tracked Cord
+  // instance is deleted, or to stop tracking the sampled Cord.
+  static CordzInfo* TrackCord(CordRep* rep);
+
+  // Stops tracking changes for a sampled cord, and deletes the provided info.
+  // This function must be called before the sampled cord instance is deleted,
+  // and before the root cordrep of the sampled cord is unreffed.
+  // This function may extend the lifetime of the cordrep in cases where the
+  // CordInfo instance is being held by a concurrent collection thread.
+  static void UntrackCord(CordzInfo* cordz_info);
+
+  // Identical to TrackCord(), except that this function fills the
+  // 'parent_stack' property of the returned CordzInfo instance from the
+  // provided `src` instance if `src` is not null.
+  // This function should be used for sampling 'copy constructed' cords.
+  static CordzInfo* TrackCord(CordRep* rep, const CordzInfo* src);
+
+  CordzInfo() = delete;
+  CordzInfo(const CordzInfo&) = delete;
+  CordzInfo& operator=(const CordzInfo&) = delete;
+
+  // Retrieves the oldest existing CordzInfo.
+  static CordzInfo* Head(const CordzSnapshot& snapshot);
+
+  // Retrieves the next oldest existing CordzInfo older than 'this' instance.
+  CordzInfo* Next(const CordzSnapshot& snapshot) const;
+
+  // Returns a reference to the mutex guarding the `rep` property of this
+  // instance. CordzInfo instances hold a weak reference to the rep pointer of
+  // sampled cords, and rely on Cord logic to update the rep pointer when the
+  // underlying root tree or ring of the cord changes.
+  absl::Mutex& mutex() const { return mutex_; }
+
+  // Updates the `rep' property of this instance. This methods is invoked by
+  // Cord logic each time the root node of a sampled Cord changes, and before
+  // the old root reference count is deleted. This guarantees that collection
+  // code can always safely take a reference on the tracked cord.
+  // Requires `mutex()` to be held.
+  // TODO(b/117940323): annotate with ABSL_EXCLUSIVE_LOCKS_REQUIRED once all
+  // Cord code is in a state where this can be proven true by the compiler.
+  void SetCordRep(CordRep* rep);
+
+  // Returns the current value of `rep_` for testing purposes only.
+  CordRep* GetCordRepForTesting() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
+    return rep_;
+  }
+
+  // Returns the stack trace for where the cord was first sampled. Cords are
+  // potentially sampled when they promote from an inlined cord to a tree or
+  // ring representation, which is not necessarily the location where the cord
+  // was first created. Some cords are created as inlined cords, and only as
+  // data is added do they become a non-inlined cord. However, typically the
+  // location represents reasonably well where the cord is 'created'.
+  absl::Span<void* const> GetStack() const;
+
+  // Returns the stack trace for a sampled cord's 'parent stack trace'. This
+  // value may be set if the cord is sampled (promoted) after being created
+  // from, or being assigned the value of an existing (sampled) cord.
+  absl::Span<void* const> GetParentStack() const;
+
+  // Retrieve the CordzStatistics associated with this Cord. The statistics are
+  // only updated when a Cord goes through a mutation, such as an Append or
+  // RemovePrefix. The refcounts can change due to external events, so the
+  // reported refcount stats might be incorrect.
+  CordzStatistics GetCordzStatistics() const {
+    CordzStatistics stats;
+    stats.size = size_.load(std::memory_order_relaxed);
+    return stats;
+  }
+
+  // Records size metric for this CordzInfo instance.
+  void RecordMetrics(int64_t size) {
+    size_.store(size, std::memory_order_relaxed);
+  }
+
+ private:
+  static constexpr int kMaxStackDepth = 64;
+
+  explicit CordzInfo(CordRep* tree);
+  ~CordzInfo() override;
+
+  void Track();
+  void Untrack();
+
+  // 'Unsafe' head/next/prev accessors not requiring the lock being held.
+  // These are used exclusively for iterations (Head / Next) where we enforce
+  // a token being held, so reading an 'old' / deleted pointer is fine.
+  static CordzInfo* ci_head_unsafe() ABSL_NO_THREAD_SAFETY_ANALYSIS {
+    return ci_head_.load(std::memory_order_acquire);
+  }
+  CordzInfo* ci_next_unsafe() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
+    return ci_next_.load(std::memory_order_acquire);
+  }
+  CordzInfo* ci_prev_unsafe() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
+    return ci_prev_.load(std::memory_order_acquire);
+  }
+
+  static absl::Mutex ci_mutex_;
+  static std::atomic<CordzInfo*> ci_head_ ABSL_GUARDED_BY(ci_mutex_);
+  std::atomic<CordzInfo*> ci_prev_ ABSL_GUARDED_BY(ci_mutex_){nullptr};
+  std::atomic<CordzInfo*> ci_next_ ABSL_GUARDED_BY(ci_mutex_){nullptr};
+
+  mutable absl::Mutex mutex_;
+  CordRep* rep_ ABSL_GUARDED_BY(mutex());
+
+  void* stack_[kMaxStackDepth];
+  void* parent_stack_[kMaxStackDepth];
+  const int stack_depth_;
+  int parent_stack_depth_;
+  const absl::Time create_time_;
+
+  // Last recorded size for the cord.
+  std::atomic<int64_t> size_{0};
+};
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_CORDZ_INFO_H_
diff --git a/absl/strings/internal/cordz_info_test.cc b/absl/strings/internal/cordz_info_test.cc
new file mode 100644
index 00000000..be20e4a4
--- /dev/null
+++ b/absl/strings/internal/cordz_info_test.cc
@@ -0,0 +1,237 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_info.h"
+
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/debugging/stacktrace.h"
+#include "absl/debugging/symbolize.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/str_cat.h"
+#include "absl/types/span.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::testing::ElementsAre;
+using ::testing::Eq;
+using ::testing::HasSubstr;
+using ::testing::Ne;
+
+struct TestCordRep {
+  CordRepFlat* rep;
+
+  TestCordRep() {
+    rep = CordRepFlat::New(100);
+    rep->length = 100;
+    memset(rep->Data(), 1, 100);
+  }
+  ~TestCordRep() { CordRepFlat::Delete(rep); }
+};
+
+// Local less verbose helper
+std::vector<const CordzHandle*> DeleteQueue() {
+  return CordzHandle::DiagnosticsGetDeleteQueue();
+}
+
+std::string FormatStack(absl::Span<void* const> raw_stack) {
+  static constexpr size_t buf_size = 1 << 14;
+  std::unique_ptr<char[]> buf(new char[buf_size]);
+  std::string output;
+  for (void* stackp : raw_stack) {
+    if (absl::Symbolize(stackp, buf.get(), buf_size)) {
+      absl::StrAppend(&output, "    ", buf.get(), "\n");
+    }
+  }
+  return output;
+}
+
+TEST(CordzInfoTest, TrackCord) {
+  TestCordRep rep;
+  CordzInfo* info = CordzInfo::TrackCord(rep.rep);
+  ASSERT_THAT(info, Ne(nullptr));
+  EXPECT_FALSE(info->is_snapshot());
+  EXPECT_THAT(CordzInfo::Head(CordzSnapshot()), Eq(info));
+  EXPECT_THAT(info->GetCordRepForTesting(), Eq(rep.rep));
+  CordzInfo::UntrackCord(info);
+}
+
+TEST(CordzInfoTest, UntrackCord) {
+  TestCordRep rep;
+  CordzInfo* info = CordzInfo::TrackCord(rep.rep);
+
+  CordzSnapshot snapshot;
+  CordzInfo::UntrackCord(info);
+  EXPECT_THAT(CordzInfo::Head(CordzSnapshot()), Eq(nullptr));
+  EXPECT_THAT(info->GetCordRepForTesting(), Eq(nullptr));
+  EXPECT_THAT(DeleteQueue(), ElementsAre(info, &snapshot));
+}
+
+TEST(CordzInfoTest, SetCordRep) {
+  TestCordRep rep;
+  CordzInfo* info = CordzInfo::TrackCord(rep.rep);
+
+  TestCordRep rep2;
+  {
+    absl::MutexLock lock(&info->mutex());
+    info->SetCordRep(rep2.rep);
+  }
+  EXPECT_THAT(info->GetCordRepForTesting(), Eq(rep2.rep));
+
+  CordzInfo::UntrackCord(info);
+}
+
+#if GTEST_HAS_DEATH_TEST
+
+TEST(CordzInfoTest, SetCordRepRequiresMutex) {
+  TestCordRep rep;
+  CordzInfo* info = CordzInfo::TrackCord(rep.rep);
+  TestCordRep rep2;
+  EXPECT_DEATH(info->SetCordRep(rep2.rep), ".*");
+  CordzInfo::UntrackCord(info);
+}
+
+#endif  // GTEST_HAS_DEATH_TEST
+
+TEST(CordzInfoTest, TrackUntrackHeadFirstV2) {
+  TestCordRep rep;
+  CordzSnapshot snapshot;
+  EXPECT_THAT(CordzInfo::Head(snapshot), Eq(nullptr));
+
+  CordzInfo* info1 = CordzInfo::TrackCord(rep.rep);
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info1));
+  EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
+
+  CordzInfo* info2 = CordzInfo::TrackCord(rep.rep);
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info2));
+  EXPECT_THAT(info2->Next(snapshot), Eq(info1));
+  EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
+
+  CordzInfo::UntrackCord(info2);
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info1));
+  EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
+
+  CordzInfo::UntrackCord(info1);
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(nullptr));
+}
+
+TEST(CordzInfoTest, TrackUntrackTailFirstV2) {
+  TestCordRep rep;
+  CordzSnapshot snapshot;
+  EXPECT_THAT(CordzInfo::Head(snapshot), Eq(nullptr));
+
+  CordzInfo* info1 = CordzInfo::TrackCord(rep.rep);
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info1));
+  EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
+
+  CordzInfo* info2 = CordzInfo::TrackCord(rep.rep);
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info2));
+  EXPECT_THAT(info2->Next(snapshot), Eq(info1));
+  EXPECT_THAT(info1->Next(snapshot), Eq(nullptr));
+
+  CordzInfo::UntrackCord(info1);
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(info2));
+  EXPECT_THAT(info2->Next(snapshot), Eq(nullptr));
+
+  CordzInfo::UntrackCord(info2);
+  ASSERT_THAT(CordzInfo::Head(snapshot), Eq(nullptr));
+}
+
+TEST(CordzInfoTest, StackV2) {
+  TestCordRep rep;
+  // kMaxStackDepth is intentionally less than 64 (which is the max depth that
+  // Cordz will record) because if the actual stack depth is over 64
+  // (which it is on Apple platforms) then the expected_stack will end up
+  // catching a few frames at the end that the actual_stack didn't get and
+  // it will no longer be subset. At the time of this writing 58 is the max
+  // that will allow this test to pass (with a minimum os version of iOS 9), so
+  // rounded down to 50 to hopefully not run into this in the future if Apple
+  // makes small modifications to its testing stack. 50 is sufficient to prove
+  // that we got a decent stack.
+  static constexpr int kMaxStackDepth = 50;
+  CordzInfo* info = CordzInfo::TrackCord(rep.rep);
+  std::vector<void*> local_stack;
+  local_stack.resize(kMaxStackDepth);
+  // In some environments we don't get stack traces. For example in Android
+  // absl::GetStackTrace will return 0 indicating it didn't find any stack. The
+  // resultant formatted stack will be "", but that still equals the stack
+  // recorded in CordzInfo, which is also empty. The skip_count is 1 so that the
+  // line number of the current stack isn't included in the HasSubstr check.
+  local_stack.resize(absl::GetStackTrace(local_stack.data(), kMaxStackDepth,
+                                         /*skip_count=*/1));
+
+  std::string got_stack = FormatStack(info->GetStack());
+  std::string expected_stack = FormatStack(local_stack);
+  // If TrackCord is inlined, got_stack should match expected_stack. If it isn't
+  // inlined, got_stack should include an additional frame not present in
+  // expected_stack. Either way, expected_stack should be a substring of
+  // got_stack.
+  EXPECT_THAT(got_stack, HasSubstr(expected_stack));
+
+  CordzInfo::UntrackCord(info);
+}
+
+// Local helper functions to get different stacks for child and parent.
+CordzInfo* TrackChildCord(CordRep* rep, const CordzInfo* parent) {
+  return CordzInfo::TrackCord(rep, parent);
+}
+CordzInfo* TrackParentCord(CordRep* rep) {
+  return CordzInfo::TrackCord(rep);
+}
+
+TEST(CordzInfoTest, ParentStackV2) {
+  TestCordRep rep;
+  CordzInfo* info_parent = TrackParentCord(rep.rep);
+  CordzInfo* info_child = TrackChildCord(rep.rep, info_parent);
+
+  std::string stack = FormatStack(info_parent->GetStack());
+  std::string parent_stack = FormatStack(info_child->GetParentStack());
+  EXPECT_THAT(stack, Eq(parent_stack));
+
+  CordzInfo::UntrackCord(info_parent);
+  CordzInfo::UntrackCord(info_child);
+}
+
+TEST(CordzInfoTest, ParentStackEmpty) {
+  TestCordRep rep;
+  CordzInfo* info = TrackChildCord(rep.rep, nullptr);
+  EXPECT_TRUE(info->GetParentStack().empty());
+  CordzInfo::UntrackCord(info);
+}
+
+TEST(CordzInfoTest, CordzStatisticsV2) {
+  TestCordRep rep;
+  CordzInfo* info = TrackParentCord(rep.rep);
+
+  CordzStatistics expected;
+  expected.size = 100;
+  info->RecordMetrics(expected.size);
+
+  CordzStatistics actual = info->GetCordzStatistics();
+  EXPECT_EQ(actual.size, expected.size);
+
+  CordzInfo::UntrackCord(info);
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_sample_token.cc b/absl/strings/internal/cordz_sample_token.cc
new file mode 100644
index 00000000..ba1270d8
--- /dev/null
+++ b/absl/strings/internal/cordz_sample_token.cc
@@ -0,0 +1,64 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_sample_token.h"
+
+#include "absl/base/config.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/internal/cordz_info.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+CordzSampleToken::Iterator& CordzSampleToken::Iterator::operator++() {
+  if (current_) {
+    current_ = current_->Next(*token_);
+  }
+  return *this;
+}
+
+CordzSampleToken::Iterator CordzSampleToken::Iterator::operator++(int) {
+  Iterator it(*this);
+  operator++();
+  return it;
+}
+
+bool operator==(const CordzSampleToken::Iterator& lhs,
+                const CordzSampleToken::Iterator& rhs) {
+  return lhs.current_ == rhs.current_ &&
+         (lhs.current_ == nullptr || lhs.token_ == rhs.token_);
+}
+
+bool operator!=(const CordzSampleToken::Iterator& lhs,
+                const CordzSampleToken::Iterator& rhs) {
+  return !(lhs == rhs);
+}
+
+CordzSampleToken::Iterator::reference CordzSampleToken::Iterator::operator*()
+    const {
+  return *current_;
+}
+
+CordzSampleToken::Iterator::pointer CordzSampleToken::Iterator::operator->()
+    const {
+  return current_;
+}
+
+CordzSampleToken::Iterator::Iterator(const CordzSampleToken* token)
+    : token_(token), current_(CordzInfo::Head(*token)) {}
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_sample_token.h b/absl/strings/internal/cordz_sample_token.h
new file mode 100644
index 00000000..28a1d70c
--- /dev/null
+++ b/absl/strings/internal/cordz_sample_token.h
@@ -0,0 +1,97 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/config.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/internal/cordz_info.h"
+
+#ifndef ABSL_STRINGS_CORDZ_SAMPLE_TOKEN_H_
+#define ABSL_STRINGS_CORDZ_SAMPLE_TOKEN_H_
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// The existence of a CordzSampleToken guarantees that a reader can traverse the
+// global_cordz_infos_head linked-list without needing to hold a mutex. When a
+// CordzSampleToken exists, all CordzInfo objects that would be destroyed are
+// instead appended to a deletion queue. When the CordzSampleToken is destroyed,
+// it will also clean up any of these CordzInfo objects.
+//
+// E.g., ST are CordzSampleToken objects and CH are CordzHandle objects.
+//   ST1 <- CH1 <- CH2 <- ST2 <- CH3 <- global_delete_queue_tail
+//
+// This list tracks that CH1 and CH2 were created after ST1, so the thread
+// holding ST1 might have a referece to CH1, CH2, ST2, and CH3. However, ST2 was
+// created later, so the thread holding the ST2 token cannot have a reference to
+// ST1, CH1, or CH2. If ST1 is cleaned up first, that thread will delete ST1,
+// CH1, and CH2. If instead ST2 is cleaned up first, that thread will only
+// delete ST2.
+//
+// If ST1 is cleaned up first, the new list will be:
+//   ST2 <- CH3 <- global_delete_queue_tail
+//
+// If ST2 is cleaned up first, the new list will be:
+//   ST1 <- CH1 <- CH2 <- CH3 <- global_delete_queue_tail
+//
+// All new CordzHandle objects are appended to the list, so if a new thread
+// comes along before either ST1 or ST2 are cleaned up, the new list will be:
+//   ST1 <- CH1 <- CH2 <- ST2 <- CH3 <- ST3 <- global_delete_queue_tail
+//
+// A thread must hold the global_delete_queue_mu mutex whenever it's altering
+// this list.
+//
+// It is safe for thread that holds a CordzSampleToken to read
+// global_cordz_infos at any time since the objects it is able to retrieve will
+// not be deleted while the CordzSampleToken exists.
+class CordzSampleToken : public CordzSnapshot {
+ public:
+  class Iterator {
+   public:
+    using iterator_category = std::input_iterator_tag;
+    using value_type = const CordzInfo&;
+    using difference_type = ptrdiff_t;
+    using pointer = const CordzInfo*;
+    using reference = value_type;
+
+    Iterator() = default;
+
+    Iterator& operator++();
+    Iterator operator++(int);
+    friend bool operator==(const Iterator& lhs, const Iterator& rhs);
+    friend bool operator!=(const Iterator& lhs, const Iterator& rhs);
+    reference operator*() const;
+    pointer operator->() const;
+
+   private:
+    friend class CordzSampleToken;
+    explicit Iterator(const CordzSampleToken* token);
+
+    const CordzSampleToken* token_ = nullptr;
+    pointer current_ = nullptr;
+  };
+
+  CordzSampleToken() = default;
+  CordzSampleToken(const CordzSampleToken&) = delete;
+  CordzSampleToken& operator=(const CordzSampleToken&) = delete;
+
+  Iterator begin() { return Iterator(this); }
+  Iterator end() { return Iterator(); }
+};
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_CORDZ_SAMPLE_TOKEN_H_
diff --git a/absl/strings/internal/cordz_sample_token_test.cc b/absl/strings/internal/cordz_sample_token_test.cc
new file mode 100644
index 00000000..8c052642
--- /dev/null
+++ b/absl/strings/internal/cordz_sample_token_test.cc
@@ -0,0 +1,209 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/cordz_sample_token.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/memory/memory.h"
+#include "absl/random/random.h"
+#include "absl/strings/internal/cord_rep_flat.h"
+#include "absl/strings/internal/cordz_handle.h"
+#include "absl/strings/internal/cordz_info.h"
+#include "absl/synchronization/internal/thread_pool.h"
+#include "absl/synchronization/notification.h"
+#include "absl/time/clock.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+namespace {
+
+using ::testing::ElementsAre;
+using ::testing::Eq;
+using ::testing::Ne;
+
+struct TestCordRep {
+  CordRepFlat* rep;
+
+  TestCordRep() {
+    rep = CordRepFlat::New(100);
+    rep->length = 100;
+    memset(rep->Data(), 1, 100);
+  }
+  ~TestCordRep() { CordRepFlat::Delete(rep); }
+};
+
+TEST(CordzSampleTokenTest, IteratorTraits) {
+  static_assert(std::is_copy_constructible<CordzSampleToken::Iterator>::value,
+                "");
+  static_assert(std::is_copy_assignable<CordzSampleToken::Iterator>::value, "");
+  static_assert(std::is_move_constructible<CordzSampleToken::Iterator>::value,
+                "");
+  static_assert(std::is_move_assignable<CordzSampleToken::Iterator>::value, "");
+  static_assert(
+      std::is_same<
+          std::iterator_traits<CordzSampleToken::Iterator>::iterator_category,
+          std::input_iterator_tag>::value,
+      "");
+  static_assert(
+      std::is_same<std::iterator_traits<CordzSampleToken::Iterator>::value_type,
+                   const CordzInfo&>::value,
+      "");
+  static_assert(
+      std::is_same<
+          std::iterator_traits<CordzSampleToken::Iterator>::difference_type,
+          ptrdiff_t>::value,
+      "");
+  static_assert(
+      std::is_same<std::iterator_traits<CordzSampleToken::Iterator>::pointer,
+                   const CordzInfo*>::value,
+      "");
+  static_assert(
+      std::is_same<std::iterator_traits<CordzSampleToken::Iterator>::reference,
+                   const CordzInfo&>::value,
+      "");
+}
+
+TEST(CordzSampleTokenTest, IteratorEmpty) {
+  CordzSampleToken token;
+  EXPECT_THAT(token.begin(), Eq(token.end()));
+}
+
+TEST(CordzSampleTokenTest, Iterator) {
+  TestCordRep rep1;
+  TestCordRep rep2;
+  TestCordRep rep3;
+  CordzInfo* info1 = CordzInfo::TrackCord(rep1.rep);
+  CordzInfo* info2 = CordzInfo::TrackCord(rep2.rep);
+  CordzInfo* info3 = CordzInfo::TrackCord(rep3.rep);
+
+  CordzSampleToken token;
+  std::vector<const CordzInfo*> found;
+  for (const CordzInfo& cord_info : token) {
+    found.push_back(&cord_info);
+  }
+
+  EXPECT_THAT(found, ElementsAre(info3, info2, info1));
+
+  CordzInfo::UntrackCord(info1);
+  CordzInfo::UntrackCord(info2);
+  CordzInfo::UntrackCord(info3);
+}
+
+TEST(CordzSampleTokenTest, IteratorEquality) {
+  TestCordRep rep1;
+  TestCordRep rep2;
+  TestCordRep rep3;
+  CordzInfo* info1 = CordzInfo::TrackCord(rep1.rep);
+
+  CordzSampleToken token1;
+  // lhs starts with the CordzInfo corresponding to cord1 at the head.
+  CordzSampleToken::Iterator lhs = token1.begin();
+
+  CordzInfo* info2 = CordzInfo::TrackCord(rep2.rep);
+
+  CordzSampleToken token2;
+  // rhs starts with the CordzInfo corresponding to cord2 at the head.
+  CordzSampleToken::Iterator rhs = token2.begin();
+
+  CordzInfo* info3 = CordzInfo::TrackCord(rep3.rep);
+
+  // lhs is on cord1 while rhs is on cord2.
+  EXPECT_THAT(lhs, Ne(rhs));
+
+  rhs++;
+  // lhs and rhs are both on cord1, but they didn't come from the same
+  // CordzSampleToken.
+  EXPECT_THAT(lhs, Ne(rhs));
+
+  lhs++;
+  rhs++;
+  // Both lhs and rhs are done, so they are on nullptr.
+  EXPECT_THAT(lhs, Eq(rhs));
+
+  CordzInfo::UntrackCord(info1);
+  CordzInfo::UntrackCord(info2);
+  CordzInfo::UntrackCord(info3);
+}
+
+TEST(CordzSampleTokenTest, MultiThreaded) {
+  Notification stop;
+  static constexpr int kNumThreads = 4;
+  static constexpr int kNumCords = 3;
+  static constexpr int kNumTokens = 3;
+  absl::synchronization_internal::ThreadPool pool(kNumThreads);
+
+  for (int i = 0; i < kNumThreads; ++i) {
+    pool.Schedule([&stop]() {
+      absl::BitGen gen;
+      TestCordRep reps[kNumCords];
+      CordzInfo* infos[kNumCords] = {nullptr};
+      std::vector<std::unique_ptr<CordzSampleToken>> tokens;
+      tokens.resize(kNumTokens);
+
+      while (!stop.HasBeenNotified()) {
+        // Randomly perform one of five actions:
+        //   1) Untrack
+        //   2) Track
+        //   3) Iterate over Cords visible to a token.
+        //   4) Unsample
+        //   5) Sample
+        int index = absl::Uniform(gen, 0, kNumCords);
+        if (absl::Bernoulli(gen, 0.5)) {
+          // Track/untrack.
+          if (infos[index]) {
+            // 1) Untrack
+            CordzInfo::UntrackCord(infos[index]);
+            infos[index] = nullptr;
+          } else {
+            // 2) Track
+            infos[index] = CordzInfo::TrackCord(reps[index].rep);
+          }
+        } else {
+          if (tokens[index]) {
+            if (absl::Bernoulli(gen, 0.5)) {
+              // 3) Iterate over Cords visible to a token.
+              for (const CordzInfo& info : *tokens[index]) {
+                // This is trivial work to allow us to compile the loop.
+                EXPECT_THAT(info.Next(*tokens[index]), Ne(&info));
+              }
+            } else {
+              // 4) Unsample
+              tokens[index].reset();
+            }
+          } else {
+            // 5) Sample
+            tokens[index] = absl::make_unique<CordzSampleToken>();
+          }
+        }
+      }
+      for (CordzInfo* info : infos) {
+        if (info != nullptr) {
+          CordzInfo::UntrackCord(info);
+        }
+      }
+    });
+  }
+  // The threads will hammer away.  Give it a little bit of time for tsan to
+  // spot errors.
+  absl::SleepFor(absl::Seconds(3));
+  stop.Notify();
+}
+
+}  // namespace
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/cordz_statistics.h b/absl/strings/internal/cordz_statistics.h
new file mode 100644
index 00000000..ce7c39aa
--- /dev/null
+++ b/absl/strings/internal/cordz_statistics.h
@@ -0,0 +1,55 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORDZ_STATISTICS_H_
+#define ABSL_STRINGS_INTERNAL_CORDZ_STATISTICS_H_
+
+#include <cstdint>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// CordzStatistics captures some meta information about a Cord's shape.
+struct CordzStatistics {
+  // The size of the cord in bytes. This matches the result of Cord::size().
+  int64_t size = 0;
+
+  // The estimated memory used by the sampled cord. This value matches the
+  // value as reported by Cord::EstimatedMemoryUsage().
+  // A value of 0 implies the property has not been recorded.
+  int64_t estimated_memory_usage = 0;
+
+  // The effective memory used by the sampled cord, inversely weighted by the
+  // effective indegree of each allocated node. This is a representation of the
+  // fair share of memory usage that should be attributed to the sampled cord.
+  // This value is more useful for cases where one or more nodes are referenced
+  // by multiple Cord instances, and for cases where a Cord includes the same
+  // node multiple times (either directly or indirectly).
+  // A value of 0 implies the property has not been recorded.
+  int64_t estimated_fair_share_memory_usage = 0;
+
+  // The total number of nodes referenced by this cord.
+  // For ring buffer Cords, this includes the 'ring buffer' node.
+  // A value of 0 implies the property has not been recorded.
+  int64_t node_count = 0;
+};
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_CORDZ_STATISTICS_H_