Fix out of bounds array access when deadlock detector finds exceptionally large cycles.

PiperOrigin-RevId: 511536497
Change-Id: If70a1c72ef5f7cbb4a80100c4edff459373a5d55

absl/synchronization/mutex.cc

@@ -1403,7 +1403,7 @@ static GraphId DeadlockCheck(Mutex *mu) {
       ABSL_RAW_LOG(ERROR, "Cycle: ");
       ABSL_RAW_LOG(ERROR, "Cycle: ");
       int path_len = deadlock_graph->FindPath(
       int path_len = deadlock_graph->FindPath(
           mu_id, other_node_id, ABSL_ARRAYSIZE(b->path), b->path);
           mu_id, other_node_id, ABSL_ARRAYSIZE(b->path), b->path);
-      for (int j = 0; j != path_len; j++) {
+      for (int j = 0; j != path_len && j != ABSL_ARRAYSIZE(b->path); j++) {
         GraphId id = b->path[j];
         GraphId id = b->path[j];
         Mutex *path_mu = static_cast<Mutex *>(deadlock_graph->Ptr(id));
         Mutex *path_mu = static_cast<Mutex *>(deadlock_graph->Ptr(id));
         if (path_mu == nullptr) continue;
         if (path_mu == nullptr) continue;
@@ -1416,6 +1416,9 @@ static GraphId DeadlockCheck(Mutex *mu) {
                     symbolize);
                     symbolize);
         ABSL_RAW_LOG(ERROR, "%s", b->buf);
         ABSL_RAW_LOG(ERROR, "%s", b->buf);
       }
       }
+      if (path_len > static_cast<int>(ABSL_ARRAYSIZE(b->path))) {
+        ABSL_RAW_LOG(ERROR, "(long cycle; list truncated)");
+      }
       if (synch_deadlock_detection.load(std::memory_order_acquire) ==
       if (synch_deadlock_detection.load(std::memory_order_acquire) ==
           OnDeadlockCycle::kAbort) {
           OnDeadlockCycle::kAbort) {
         deadlock_graph_mu.Unlock();  // avoid deadlock in fatal sighandler
         deadlock_graph_mu.Unlock();  // avoid deadlock in fatal sighandler

absl/synchronization/mutex_test.cc

@@ -1131,6 +1131,25 @@ TEST(Mutex, DeadlockDetectorBazelWarning) {
   absl::SetMutexDeadlockDetectionMode(absl::OnDeadlockCycle::kAbort);
   absl::SetMutexDeadlockDetectionMode(absl::OnDeadlockCycle::kAbort);
 }
 }
 
 
+TEST(Mutex, DeadlockDetectorLongCycle) {
+  absl::SetMutexDeadlockDetectionMode(absl::OnDeadlockCycle::kReport);
+
+  // This test generates a warning if it passes, and crashes otherwise.
+  // Cause bazel to ignore the warning.
+  ScopedDisableBazelTestWarnings disable_bazel_test_warnings;
+
+  // Check that we survive a deadlock with a lock cycle.
+  std::vector<absl::Mutex> mutex(100);
+  for (size_t i = 0; i != mutex.size(); i++) {
+    mutex[i].Lock();
+    mutex[(i + 1) % mutex.size()].Lock();
+    mutex[i].Unlock();
+    mutex[(i + 1) % mutex.size()].Unlock();
+  }
+
+  absl::SetMutexDeadlockDetectionMode(absl::OnDeadlockCycle::kAbort);
+}
+
 // This test is tagged with NO_THREAD_SAFETY_ANALYSIS because the
 // This test is tagged with NO_THREAD_SAFETY_ANALYSIS because the
 // annotation-based static thread-safety analysis is not currently
 // annotation-based static thread-safety analysis is not currently
 // predicate-aware and cannot tell if the two for-loops that acquire and
 // predicate-aware and cannot tell if the two for-loops that acquire and