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, 97 insertions, 0 deletions

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_