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diff --git a/absl/synchronization/internal/per_thread_sem.h b/absl/synchronization/internal/per_thread_sem.h
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+// Copyright 2017 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
+//
+//      http://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.
+//
+
+// PerThreadSem is a low-level synchronization primitive controlling the
+// runnability of a single thread, used internally by Mutex and CondVar.
+//
+// This is NOT a general-purpose synchronization mechanism, and should not be
+// used directly by applications.  Applications should use Mutex and CondVar.
+//
+// The semantics of PerThreadSem are the same as that of a counting semaphore.
+// Each thread maintains an abstract "count" value associated with its identity.
+
+#ifndef ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
+#define ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_
+
+#include <atomic>
+
+#include "absl/base/internal/thread_identity.h"
+#include "absl/synchronization/internal/create_thread_identity.h"
+#include "absl/synchronization/internal/kernel_timeout.h"
+
+namespace absl {
+
+class Mutex;
+
+namespace synchronization_internal {
+
+class PerThreadSem {
+ public:
+  PerThreadSem() = delete;
+  PerThreadSem(const PerThreadSem&) = delete;
+  PerThreadSem& operator=(const PerThreadSem&) = delete;
+
+  // Routine invoked periodically (once a second) by a background thread.
+  // Has no effect on user-visible state.
+  static void Tick(base_internal::ThreadIdentity* identity);
+
+  // ---------------------------------------------------------------------------
+  // Routines used by autosizing threadpools to detect when threads are
+  // blocked.  Each thread has a counter pointer, initially zero.  If non-zero,
+  // the implementation atomically increments the counter when it blocks on a
+  // semaphore, a decrements it again when it wakes.  This allows a threadpool
+  // to keep track of how many of its threads are blocked.
+  // SetThreadBlockedCounter() should be used only by threadpool
+  // implementations.  GetThreadBlockedCounter() should be used by modules that
+  // block threads; if the pointer returned is non-zero, the location should be
+  // incremented before the thread blocks, and decremented after it wakes.
+  static void SetThreadBlockedCounter(std::atomic<int> *counter);
+  static std::atomic<int> *GetThreadBlockedCounter();
+
+ private:
+  // Create the PerThreadSem associated with "identity".  Initializes count=0.
+  // REQUIRES: May only be called by ThreadIdentity.
+  static void Init(base_internal::ThreadIdentity* identity);
+
+  // Increments "identity"'s count.
+  static inline void Post(base_internal::ThreadIdentity* identity);
+
+  // Waits until either our count > 0 or t has expired.
+  // If count > 0, decrements count and returns true.  Otherwise returns false.
+  // !t.has_timeout() => Wait(t) will return true.
+  static inline bool Wait(KernelTimeout t);
+
+  // White-listed callers.
+  friend class PerThreadSemTest;
+  friend class absl::Mutex;
+  friend absl::base_internal::ThreadIdentity* CreateThreadIdentity();
+};
+
+}  // namespace synchronization_internal
+}  // namespace absl
+
+// In some build configurations we pass --detect-odr-violations to the
+// gold linker.  This causes it to flag weak symbol overrides as ODR
+// violations.  Because ODR only applies to C++ and not C,
+// --detect-odr-violations ignores symbols not mangled with C++ names.
+// By changing our extension points to be extern "C", we dodge this
+// check.
+extern "C" {
+void AbslInternalPerThreadSemPost(
+    absl::base_internal::ThreadIdentity* identity);
+bool AbslInternalPerThreadSemWait(
+    absl::synchronization_internal::KernelTimeout t);
+}  // extern "C"
+
+void absl::synchronization_internal::PerThreadSem::Post(
+    absl::base_internal::ThreadIdentity* identity) {
+  AbslInternalPerThreadSemPost(identity);
+}
+
+bool absl::synchronization_internal::PerThreadSem::Wait(
+    absl::synchronization_internal::KernelTimeout t) {
+  return AbslInternalPerThreadSemWait(t);
+}
+#endif  // ABSL_SYNCHRONIZATION_INTERNAL_PER_THREAD_SEM_H_