Creation of LTS branch "lts_2020_02_25"20200225

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  - 0232c87f21c26718aa3eb2d86678070f3b498a4e Add missing ABSL_HAVE_VDSO_SUPPORT conditional (#622) by Sinan Kaya <41809318+franksinankaya@users.noreply.github.com>
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  - ecc0033b54847f6c9ee37dbb0be8aa17e5b6d37b Always enable proper symbolize implementation on Windows ... by Loo Rong Jie <loorongjie@gmail.com>
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  - e9f9000c7c80993cb589d011616b7a8016e42f4a Fix ABSL_WAITER_MODE detection for mingw (#342) by Joe Sylve <Joe.Sylve@gmail.com>
  - abea769b551f7a100f540967cb95debdb0080df8 Fix ABSL_HAVE_ALARM check on mingw (#341) by Joe Sylve <Joe.Sylve@gmail.com>
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  - 0302d1e5fa4fcdd1763b7d1bb3212943b1ae911d supppress unused variable warning for gcc (#372) by Martin <pizzard@users.noreply.github.com>
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  - 36910d3d7e9fccadd6603f232d0c4f54dcd47c7e [bazel] Add fixes for --incompatible_load_cc_rules_from_b... by Yannic <contact@yannic-bonenberger.com>
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GitOrigin-RevId: 0033c9ea91a52ade7c6b725aa2ef3cbe15463421
Change-Id: I8a2b70063cb3ab40c6943a6db0fe40cae71ed8d7

1 files changed, 118 insertions, 93 deletions

diff --git a/absl/synchronization/internal/waiter.cc b/absl/synchronization/internal/waiter.cc
index 17c6a506..2949f5a8 100644
--- a/absl/synchronization/internal/waiter.cc
+++ b/absl/synchronization/internal/waiter.cc
@@ -49,7 +49,7 @@
 #include "absl/synchronization/internal/kernel_timeout.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 static void MaybeBecomeIdle() {
@@ -123,16 +123,21 @@ class Futex {
   }
 };
 
-void Waiter::Init() {
+Waiter::Waiter() {
   futex_.store(0, std::memory_order_relaxed);
 }
 
+Waiter::~Waiter() = default;
+
 bool Waiter::Wait(KernelTimeout t) {
   // Loop until we can atomically decrement futex from a positive
   // value, waiting on a futex while we believe it is zero.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
   while (true) {
     int32_t x = futex_.load(std::memory_order_relaxed);
-    if (x != 0) {
+    while (x != 0) {
       if (!futex_.compare_exchange_weak(x, x - 1,
                                         std::memory_order_acquire,
                                         std::memory_order_relaxed)) {
@@ -141,6 +146,8 @@ bool Waiter::Wait(KernelTimeout t) {
       return true;  // Consumed a wakeup, we are done.
     }
 
+
+    if (!first_pass) MaybeBecomeIdle();
     const int err = Futex::WaitUntil(&futex_, 0, t);
     if (err != 0) {
       if (err == -EINTR || err == -EWOULDBLOCK) {
@@ -151,14 +158,13 @@ bool Waiter::Wait(KernelTimeout t) {
         ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
       }
     }
-
-    MaybeBecomeIdle();
+    first_pass = false;
   }
 }
 
 void Waiter::Post() {
   if (futex_.fetch_add(1, std::memory_order_release) == 0) {
-    // We incremented from 0, need to wake a potential waker.
+    // We incremented from 0, need to wake a potential waiter.
     Poke();
   }
 }
@@ -196,7 +202,7 @@ class PthreadMutexHolder {
   pthread_mutex_t *mu_;
 };
 
-void Waiter::Init() {
+Waiter::Waiter() {
   const int err = pthread_mutex_init(&mu_, 0);
   if (err != 0) {
     ABSL_RAW_LOG(FATAL, "pthread_mutex_init failed: %d", err);
@@ -207,8 +213,20 @@ void Waiter::Init() {
     ABSL_RAW_LOG(FATAL, "pthread_cond_init failed: %d", err2);
   }
 
-  waiter_count_.store(0, std::memory_order_relaxed);
-  wakeup_count_.store(0, std::memory_order_relaxed);
+  waiter_count_ = 0;
+  wakeup_count_ = 0;
+}
+
+Waiter::~Waiter() {
+  const int err = pthread_mutex_destroy(&mu_);
+  if (err != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_mutex_destroy failed: %d", err);
+  }
+
+  const int err2 = pthread_cond_destroy(&cv_);
+  if (err2 != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_cond_destroy failed: %d", err2);
+  }
 }
 
 bool Waiter::Wait(KernelTimeout t) {
@@ -218,21 +236,13 @@ bool Waiter::Wait(KernelTimeout t) {
   }
 
   PthreadMutexHolder h(&mu_);
-  waiter_count_.fetch_add(1, std::memory_order_relaxed);
+  ++waiter_count_;
   // Loop until we find a wakeup to consume or timeout.
-  while (true) {
-    int x = wakeup_count_.load(std::memory_order_relaxed);
-    if (x != 0) {
-      if (!wakeup_count_.compare_exchange_weak(x, x - 1,
-                                               std::memory_order_acquire,
-                                               std::memory_order_relaxed)) {
-        continue;  // Raced with someone, retry.
-      }
-      // Successfully consumed a wakeup, we're done.
-      waiter_count_.fetch_sub(1, std::memory_order_relaxed);
-      return true;
-    }
-
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (wakeup_count_ == 0) {
+    if (!first_pass) MaybeBecomeIdle();
     // No wakeups available, time to wait.
     if (!t.has_timeout()) {
       const int err = pthread_cond_wait(&cv_, &mu_);
@@ -242,46 +252,56 @@ bool Waiter::Wait(KernelTimeout t) {
     } else {
       const int err = pthread_cond_timedwait(&cv_, &mu_, &abs_timeout);
       if (err == ETIMEDOUT) {
-        waiter_count_.fetch_sub(1, std::memory_order_relaxed);
+        --waiter_count_;
         return false;
       }
       if (err != 0) {
-        ABSL_RAW_LOG(FATAL, "pthread_cond_wait failed: %d", err);
+        ABSL_RAW_LOG(FATAL, "pthread_cond_timedwait failed: %d", err);
       }
     }
-    MaybeBecomeIdle();
+    first_pass = false;
   }
+  // Consume a wakeup and we're done.
+  --wakeup_count_;
+  --waiter_count_;
+  return true;
 }
 
 void Waiter::Post() {
-  wakeup_count_.fetch_add(1, std::memory_order_release);
-  Poke();
+  PthreadMutexHolder h(&mu_);
+  ++wakeup_count_;
+  InternalCondVarPoke();
 }
 
 void Waiter::Poke() {
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
-  }
-  // Potentially a waker. Take the lock and check again.
   PthreadMutexHolder h(&mu_);
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
-  }
-  const int err = pthread_cond_signal(&cv_);
-  if (err != 0) {
-    ABSL_RAW_LOG(FATAL, "pthread_cond_signal failed: %d", err);
+  InternalCondVarPoke();
+}
+
+void Waiter::InternalCondVarPoke() {
+  if (waiter_count_ != 0) {
+    const int err = pthread_cond_signal(&cv_);
+    if (ABSL_PREDICT_FALSE(err != 0)) {
+      ABSL_RAW_LOG(FATAL, "pthread_cond_signal failed: %d", err);
+    }
   }
 }
 
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
 
-void Waiter::Init() {
+Waiter::Waiter() {
   if (sem_init(&sem_, 0, 0) != 0) {
     ABSL_RAW_LOG(FATAL, "sem_init failed with errno %d\n", errno);
   }
   wakeups_.store(0, std::memory_order_relaxed);
 }
 
+Waiter::~Waiter() {
+  if (sem_destroy(&sem_) != 0) {
+    ABSL_RAW_LOG(FATAL, "sem_destroy failed with errno %d\n", errno);
+  }
+}
+
 bool Waiter::Wait(KernelTimeout t) {
   struct timespec abs_timeout;
   if (t.has_timeout()) {
@@ -289,9 +309,12 @@ bool Waiter::Wait(KernelTimeout t) {
   }
 
   // Loop until we timeout or consume a wakeup.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
   while (true) {
     int x = wakeups_.load(std::memory_order_relaxed);
-    if (x != 0) {
+    while (x != 0) {
       if (!wakeups_.compare_exchange_weak(x, x - 1,
                                           std::memory_order_acquire,
                                           std::memory_order_relaxed)) {
@@ -301,6 +324,7 @@ bool Waiter::Wait(KernelTimeout t) {
       return true;
     }
 
+    if (!first_pass) MaybeBecomeIdle();
     // Nothing to consume, wait (looping on EINTR).
     while (true) {
       if (!t.has_timeout()) {
@@ -314,13 +338,16 @@ bool Waiter::Wait(KernelTimeout t) {
         ABSL_RAW_LOG(FATAL, "sem_timedwait failed: %d", errno);
       }
     }
-    MaybeBecomeIdle();
+    first_pass = false;
   }
 }
 
 void Waiter::Post() {
-  wakeups_.fetch_add(1, std::memory_order_release);  // Post a wakeup.
-  Poke();
+  // Post a wakeup.
+  if (wakeups_.fetch_add(1, std::memory_order_release) == 0) {
+    // We incremented from 0, need to wake a potential waiter.
+    Poke();
+  }
 }
 
 void Waiter::Poke() {
@@ -341,31 +368,29 @@ class Waiter::WinHelper {
     return reinterpret_cast<CONDITION_VARIABLE *>(&w->cv_storage_);
   }
 
-  static_assert(sizeof(SRWLOCK) == sizeof(Waiter::SRWLockStorage),
-                "SRWLockStorage does not have the same size as SRWLOCK");
+  static_assert(sizeof(SRWLOCK) == sizeof(void *),
+                "`mu_storage_` does not have the same size as SRWLOCK");
+  static_assert(alignof(SRWLOCK) == alignof(void *),
+                "`mu_storage_` does not have the same alignment as SRWLOCK");
+
+  static_assert(sizeof(CONDITION_VARIABLE) == sizeof(void *),
+                "`ABSL_CONDITION_VARIABLE_STORAGE` does not have the same size "
+                "as `CONDITION_VARIABLE`");
   static_assert(
-      alignof(SRWLOCK) == alignof(Waiter::SRWLockStorage),
-      "SRWLockStorage does not have the same alignment as SRWLOCK");
-
-  static_assert(sizeof(CONDITION_VARIABLE) ==
-                    sizeof(Waiter::ConditionVariableStorage),
-                "ABSL_CONDITION_VARIABLE_STORAGE does not have the same size "
-                "as CONDITION_VARIABLE");
-  static_assert(alignof(CONDITION_VARIABLE) ==
-                    alignof(Waiter::ConditionVariableStorage),
-                "ConditionVariableStorage does not have the same "
-                "alignment as CONDITION_VARIABLE");
-
-  // The SRWLOCK and CONDITION_VARIABLE types must be trivially constuctible
+      alignof(CONDITION_VARIABLE) == alignof(void *),
+      "`cv_storage_` does not have the same alignment as `CONDITION_VARIABLE`");
+
+  // The SRWLOCK and CONDITION_VARIABLE types must be trivially constructible
   // and destructible because we never call their constructors or destructors.
   static_assert(std::is_trivially_constructible<SRWLOCK>::value,
-                "The SRWLOCK type must be trivially constructible");
-  static_assert(std::is_trivially_constructible<CONDITION_VARIABLE>::value,
-                "The CONDITION_VARIABLE type must be trivially constructible");
+                "The `SRWLOCK` type must be trivially constructible");
+  static_assert(
+      std::is_trivially_constructible<CONDITION_VARIABLE>::value,
+      "The `CONDITION_VARIABLE` type must be trivially constructible");
   static_assert(std::is_trivially_destructible<SRWLOCK>::value,
-                "The SRWLOCK type must be trivially destructible");
+                "The `SRWLOCK` type must be trivially destructible");
   static_assert(std::is_trivially_destructible<CONDITION_VARIABLE>::value,
-                "The CONDITION_VARIABLE type must be trivially destructible");
+                "The `CONDITION_VARIABLE` type must be trivially destructible");
 };
 
 class LockHolder {
@@ -385,36 +410,33 @@ class LockHolder {
   SRWLOCK* mu_;
 };
 
-void Waiter::Init() {
+Waiter::Waiter() {
   auto *mu = ::new (static_cast<void *>(&mu_storage_)) SRWLOCK;
   auto *cv = ::new (static_cast<void *>(&cv_storage_)) CONDITION_VARIABLE;
   InitializeSRWLock(mu);
   InitializeConditionVariable(cv);
-  waiter_count_.store(0, std::memory_order_relaxed);
-  wakeup_count_.store(0, std::memory_order_relaxed);
+  waiter_count_ = 0;
+  wakeup_count_ = 0;
 }
 
+// SRW locks and condition variables do not need to be explicitly destroyed.
+// https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
+// https://stackoverflow.com/questions/28975958/why-does-windows-have-no-deleteconditionvariable-function-to-go-together-with
+Waiter::~Waiter() = default;
+
 bool Waiter::Wait(KernelTimeout t) {
   SRWLOCK *mu = WinHelper::GetLock(this);
   CONDITION_VARIABLE *cv = WinHelper::GetCond(this);
 
   LockHolder h(mu);
-  waiter_count_.fetch_add(1, std::memory_order_relaxed);
+  ++waiter_count_;
 
   // Loop until we find a wakeup to consume or timeout.
-  while (true) {
-    int x = wakeup_count_.load(std::memory_order_relaxed);
-    if (x != 0) {
-      if (!wakeup_count_.compare_exchange_weak(x, x - 1,
-                                               std::memory_order_acquire,
-                                               std::memory_order_relaxed)) {
-        continue;  // Raced with someone, retry.
-      }
-      // Successfully consumed a wakeup, we're done.
-      waiter_count_.fetch_sub(1, std::memory_order_relaxed);
-      return true;
-    }
-
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (wakeup_count_ == 0) {
+    if (!first_pass) MaybeBecomeIdle();
     // No wakeups available, time to wait.
     if (!SleepConditionVariableSRW(cv, mu, t.InMillisecondsFromNow(), 0)) {
       // GetLastError() returns a Win32 DWORD, but we assign to
@@ -422,32 +444,35 @@ bool Waiter::Wait(KernelTimeout t) {
       // initialization guarantees this is not a narrowing conversion.
       const unsigned long err{GetLastError()};  // NOLINT(runtime/int)
       if (err == ERROR_TIMEOUT) {
-        waiter_count_.fetch_sub(1, std::memory_order_relaxed);
+        --waiter_count_;
         return false;
       } else {
         ABSL_RAW_LOG(FATAL, "SleepConditionVariableSRW failed: %lu", err);
       }
     }
-
-    MaybeBecomeIdle();
+    first_pass = false;
   }
+  // Consume a wakeup and we're done.
+  --wakeup_count_;
+  --waiter_count_;
+  return true;
 }
 
 void Waiter::Post() {
-  wakeup_count_.fetch_add(1, std::memory_order_release);
-  Poke();
+  LockHolder h(WinHelper::GetLock(this));
+  ++wakeup_count_;
+  InternalCondVarPoke();
 }
 
 void Waiter::Poke() {
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
-  }
-  // Potentially a waker. Take the lock and check again.
   LockHolder h(WinHelper::GetLock(this));
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
+  InternalCondVarPoke();
+}
+
+void Waiter::InternalCondVarPoke() {
+  if (waiter_count_ != 0) {
+    WakeConditionVariable(WinHelper::GetCond(this));
   }
-  WakeConditionVariable(WinHelper::GetCond(this));
 }
 
 #else
@@ -455,5 +480,5 @@ void Waiter::Poke() {
 #endif
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl