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

base/synchronization/waitable_event_unittest.cc

Index: base/synchronization/waitable_event_unittest.cc
diff --git a/base/synchronization/waitable_event_unittest.cc b/base/synchronization/waitable_event_unittest.cc
index c0e280aa9740642058d3d3e9ee109b3a1c5e5e8a..3aa1af1619bdaf1a918118f23d6fb7c3493cc439 100644
--- a/base/synchronization/waitable_event_unittest.cc
+++ b/base/synchronization/waitable_event_unittest.cc
@@ -6,6 +6,8 @@
 
 #include <stddef.h>
 
+#include <algorithm>
+
 #include "base/compiler_specific.h"
 #include "base/threading/platform_thread.h"
 #include "base/time/time.h"
@@ -78,6 +80,42 @@ TEST(WaitableEventTest, WaitManyShortcut) {
     delete ev[i];
 }
 
+TEST(WaitableEventTest, WaitManyLeftToRight) {
+  WaitableEvent* ev[5];
+  for (size_t i = 0; i < 5; ++i) {
+    ev[i] = new WaitableEvent(WaitableEvent::ResetPolicy::AUTOMATIC,
+                              WaitableEvent::InitialState::NOT_SIGNALED);
+  }
+
+  // Test for consistent left-to-right return behavior across all permutations
+  // of the input array. This is to verify that only the indices -- and not
+  // the WaitableEvents' addresses -- are relevant in determining who wins when
+  // multiple events are signaled.
+
+  std::sort(ev, ev + 5);
+  do {
+    ev[0]->Signal();
+    ev[1]->Signal();
+    EXPECT_EQ(0u, WaitableEvent::WaitMany(ev, 5));
+
+    ev[2]->Signal();
+    EXPECT_EQ(1u, WaitableEvent::WaitMany(ev, 5));
+    EXPECT_EQ(2u, WaitableEvent::WaitMany(ev, 5));
+
+    ev[3]->Signal();
+    ev[4]->Signal();
+    ev[0]->Signal();
+    EXPECT_EQ(0u, WaitableEvent::WaitMany(ev, 5));
+    EXPECT_EQ(3u, WaitableEvent::WaitMany(ev, 5));
+    ev[2]->Signal();
+    EXPECT_EQ(2u, WaitableEvent::WaitMany(ev, 5));
+    EXPECT_EQ(4u, WaitableEvent::WaitMany(ev, 5));
+  } while (std::next_permutation(ev, ev + 5));
+
+  for (size_t i = 0; i < 5; ++i)
+    delete ev[i];
+}
+
 class WaitableEventSignaler : public PlatformThread::Delegate {
  public:
   WaitableEventSignaler(TimeDelta delay, WaitableEvent* event)