Fix SchedulerLock unittests to actually generate a DCHECK, not an exception

base/task_scheduler/scheduler_lock_impl.cc

Index: base/task_scheduler/scheduler_lock_impl.cc
diff --git a/base/task_scheduler/scheduler_lock_impl.cc b/base/task_scheduler/scheduler_lock_impl.cc
index 7480e18da164edea00b7cafaed5d6e32ac86d835..a23df7d580e4061f435d3f15ff4acd7bbb7e3eed 100644
--- a/base/task_scheduler/scheduler_lock_impl.cc
+++ b/base/task_scheduler/scheduler_lock_impl.cc
@@ -67,19 +67,25 @@ class SafeAcquisitionTracker {
     // Otherwise, make sure that the previous lock acquired is an allowed
     // predecessor.
     AutoLock auto_lock(allowed_predecessor_map_lock_);
+    // Using at() is exception-safe here as |lock| was registered already.
     const SchedulerLockImpl* allowed_predecessor =
         allowed_predecessor_map_.at(lock);
     DCHECK_EQ(acquired_locks->back(), allowed_predecessor);
   }
 
+  // This asserts that |lock|'s registered predecessor is safe (and by

[robliao, 2016/08/04 18:13:29]

Iterating over chat.

[gab, 2016/08/04 18:15:06]

Done.

+  // indicution that the predecessor chain is safe -- cycle free). Since
+  // registration only happens at construction time, the predecessor is safe as
+  // long as it is either null or already registered.
   void AssertSafePredecessor(const SchedulerLockImpl* lock) const {
     allowed_predecessor_map_lock_.AssertAcquired();
-    for (const SchedulerLockImpl* predecessor =
-             allowed_predecessor_map_.at(lock);
-         predecessor != nullptr;
-         predecessor = allowed_predecessor_map_.at(predecessor)) {
-      DCHECK_NE(predecessor, lock) <<
-          "Scheduler lock predecessor cycle detected.";
+    // Using at() is exception-safe here as |lock| was registered already.
+    const SchedulerLockImpl* predecessor = allowed_predecessor_map_.at(lock);
+    if (predecessor) {
+      DCHECK(allowed_predecessor_map_.find(predecessor) !=
+             allowed_predecessor_map_.end())
+          << "SchedulerLock was registered before its predecessor. "
+          << "Potential cycle detected";
     }
   }