1 files changed, 97 insertions, 0 deletions
diff --git a/third_party/abseil-cpp/absl/strings/internal/cordz_sample_token.h b/third_party/abseil-cpp/absl/strings/internal/cordz_sample_token.h
new file mode 100644
index 0000000000..28a1d70ccc
--- /dev/null
+++ b/third_party/abseil-cpp/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_