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..41de3b0f41120f81ea3ce81e122f27e9efadb3af 100644
--- a/base/task_scheduler/scheduler_lock_impl.cc
+++ b/base/task_scheduler/scheduler_lock_impl.cc
@@ -67,19 +67,29 @@ 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);
   }
 
+  // Asserts that |lock|'s registered predecessor is safe. Because
+  // SchedulerLocks are registered at construction time and any predecessor
+  // specified on a SchedulerLock must already exist, the first registered lock

[robliao, 2016/08/04 18:15:53]

s/registered lock/registered SchedulerLock/

+  // in a potential chain must have a null predecessor and is thus cycle-free.
+  // Any subsequent lock with a predecessor must come from the set of registered

[robliao, 2016/08/04 18:15:53]

s/subsequent lock/subsequent SchedulerLock/, and so forth.

+  // locks. Since the registered locks only contain cycle-free locks, this
+  // subsequent lock is itself cycle-free and may be safely added to the
+  // registered lock set.
   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";
     }
   }