absl: fix a priority bug in CondVar wait morphing

Enqueue updates priority of the queued thread.
It was assumed that the queued thread is the current thread.
But it's not the case in CondVar wait morhping,
where we requeue an existing CondVar waiter on the Mutex.
As the result one thread can falsely get priority of another thread.

Fix this by not updating priority in this case.
And make the assumption explicit and checked.

PiperOrigin-RevId: 561249402
Change-Id: I9476c047757090b893a88a2839b795b85fe220ad

1 files changed, 60 insertions, 0 deletions

diff --git a/absl/synchronization/mutex_test.cc b/absl/synchronization/mutex_test.cc
index b585c342..0bca46c5 100644
--- a/absl/synchronization/mutex_test.cc
+++ b/absl/synchronization/mutex_test.cc
@@ -36,10 +36,16 @@
 #include "absl/log/check.h"
 #include "absl/log/log.h"
 #include "absl/memory/memory.h"
+#include "absl/synchronization/internal/create_thread_identity.h"
 #include "absl/synchronization/internal/thread_pool.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
 
+#ifdef ABSL_HAVE_PTHREAD_GETSCHEDPARAM
+#include <pthread.h>
+#include <string.h>
+#endif
+
 namespace {
 
 // TODO(dmauro): Replace with a commandline flag.
@@ -1868,6 +1874,60 @@ TEST(Mutex, WriterPriority) {
   EXPECT_TRUE(saw_wrote.load());
 }
 
+#ifdef ABSL_HAVE_PTHREAD_GETSCHEDPARAM
+TEST(Mutex, CondVarPriority) {
+  // A regression test for a bug in condition variable wait morphing,
+  // which resulted in the waiting thread getting priority of the waking thread.
+  int err = 0;
+  sched_param param;
+  param.sched_priority = 7;
+  std::thread test([&]() {
+    err = pthread_setschedparam(pthread_self(), SCHED_FIFO, &param);
+  });
+  test.join();
+  if (err) {
+    // Setting priority usually requires special privileges.
+    GTEST_SKIP() << "failed to set priority: " << strerror(err);
+  }
+  absl::Mutex mu;
+  absl::CondVar cv;
+  bool locked = false;
+  bool notified = false;
+  bool waiting = false;
+  bool morph = false;
+  std::thread th([&]() {
+    EXPECT_EQ(0, pthread_setschedparam(pthread_self(), SCHED_FIFO, &param));
+    mu.Lock();
+    locked = true;
+    mu.Await(absl::Condition(&notified));
+    mu.Unlock();
+    EXPECT_EQ(absl::synchronization_internal::GetOrCreateCurrentThreadIdentity()
+                  ->per_thread_synch.priority,
+              param.sched_priority);
+    mu.Lock();
+    mu.Await(absl::Condition(&waiting));
+    morph = true;
+    absl::SleepFor(absl::Seconds(1));
+    cv.Signal();
+    mu.Unlock();
+  });
+  mu.Lock();
+  mu.Await(absl::Condition(&locked));
+  notified = true;
+  mu.Unlock();
+  mu.Lock();
+  waiting = true;
+  while (!morph) {
+    cv.Wait(&mu);
+  }
+  mu.Unlock();
+  th.join();
+  EXPECT_NE(absl::synchronization_internal::GetOrCreateCurrentThreadIdentity()
+                ->per_thread_synch.priority,
+            param.sched_priority);
+}
+#endif
+
 TEST(Mutex, LockWhenWithTimeoutResult) {
   // Check various corner cases for Await/LockWhen return value
   // with always true/always false conditions.