Ensure consistent return ordering in base::WaitableEvent::WaitMany

base/synchronization/waitable_event_posix.cc

Index: base/synchronization/waitable_event_posix.cc
diff --git a/base/synchronization/waitable_event_posix.cc b/base/synchronization/waitable_event_posix.cc
index 5dfff468ad6acb0f2c81df98bcc31564f7029506..846fa06700e1dcb1c1c5976eb2397643c0698759 100644
--- a/base/synchronization/waitable_event_posix.cc
+++ b/base/synchronization/waitable_event_posix.cc
@@ -5,6 +5,7 @@
 #include <stddef.h>
 
 #include <algorithm>
+#include <limits>
 #include <vector>
 
 #include "base/debug/activity_tracker.h"
@@ -266,12 +267,10 @@ size_t WaitableEvent::WaitMany(WaitableEvent** raw_waitables,
   SyncWaiter sw;
 
   const size_t r = EnqueueMany(&waitables[0], count, &sw);
-  if (r) {
+  if (r < count) {
     // One of the events is already signaled. The SyncWaiter has not been
-    // enqueued anywhere. EnqueueMany returns the count of remaining waitables
-    // when the signaled one was seen, so the index of the signaled event is
-    // @count - @r.
-    return waitables[count - r].second;
+    // enqueued anywhere.
+    return waitables[r].second;
   }
 
   // At this point, we hold the locks on all the WaitableEvents and we have
@@ -319,38 +318,50 @@ size_t WaitableEvent::WaitMany(WaitableEvent** raw_waitables,
 }
 
 // -----------------------------------------------------------------------------
-// If return value == 0:
+// If return value == count:
 //   The locks of the WaitableEvents have been taken in order and the Waiter has
 //   been enqueued in the wait-list of each. None of the WaitableEvents are
 //   currently signaled
 // else:
 //   None of the WaitableEvent locks are held. The Waiter has not been enqueued
-//   in any of them and the return value is the index of the first WaitableEvent
-//   which was signaled, from the end of the array.
+//   in any of them and the return value is the index of the WaitableEvent which
+//   was signaled with the lowest input index from the original WaitMany call.
 // -----------------------------------------------------------------------------
 // static
-size_t WaitableEvent::EnqueueMany
-    (std::pair<WaitableEvent*, size_t>* waitables,
-     size_t count, Waiter* waiter) {
-  if (!count)
-    return 0;
-
-  waitables[0].first->kernel_->lock_.Acquire();
-    if (waitables[0].first->kernel_->signaled_) {
-      if (!waitables[0].first->kernel_->manual_reset_)
-        waitables[0].first->kernel_->signaled_ = false;
-      waitables[0].first->kernel_->lock_.Release();
-      return count;
+size_t WaitableEvent::EnqueueMany(std::pair<WaitableEvent*, size_t>* waitables,
+                                  size_t count,
+                                  Waiter* waiter) {
+  size_t winner = count;
+  size_t winner_index = count;
+  for (size_t i = 0; i < count; ++i) {
+    auto& kernel = waitables[i].first->kernel_;
+    kernel->lock_.Acquire();
+    if (kernel->signaled_ && waitables[i].second < winner) {
+      winner = waitables[i].second;
+      winner_index = i;
     }
+  }
 
-    const size_t r = EnqueueMany(waitables + 1, count - 1, waiter);
-    if (r) {
-      waitables[0].first->kernel_->lock_.Release();
-    } else {
-      waitables[0].first->Enqueue(waiter);
+  // No events signaled. All locks acquired. Enqueue the Waiter on all of them
+  // and return.
+  if (winner == count) {
+    for (size_t i = 0; i < count; ++i)
+      waitables[i].first->Enqueue(waiter);
+    return count;
+  }
+
+  // Unlock in reverse order and possibly clear the chosen winner's signal
+  // before returning its index.
+  for (auto* w = waitables + count - 1; w >= waitables; --w) {
+    auto& kernel = w->first->kernel_;
+    if (w->second == winner) {
+      if (!kernel->manual_reset_)
+        kernel->signaled_ = false;
     }
+    kernel->lock_.Release();
+  }
 
-    return r;
+  return winner_index;
 }
 
 // -----------------------------------------------------------------------------