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

1 files changed, 138 insertions, 0 deletions

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