Bug 772643: Don't allocate memory in hang monitor while a thread is suspended. r=ehsan

xpcom/threads/HangMonitor.cpp

@@ -58,6 +58,9 @@ static HANDLE winMainThreadHandle = NULL;
 
 // Default timeout for reporting chrome hangs to Telemetry (5 seconds)
 static const PRInt32 DEFAULT_CHROME_HANG_INTERVAL = 5;
+
+// Maximum number of PCs to gather from the stack
+static const PRInt32 MAX_CALL_STACK_PCS = 400;
 #endif
 
 // PrefChangedFunc
@@ -111,7 +114,9 @@ ChromeStackWalker(void *aPC, void *aSP, void *aClosure)
   MOZ_ASSERT(aClosure);
   Telemetry::HangStack *callStack =
     reinterpret_cast< Telemetry::HangStack* >(aClosure);
-  callStack->AppendElement(reinterpret_cast<uintptr_t>(aPC));
+
+  if (callStack->Length() < MAX_CALL_STACK_PCS)
+    callStack->AppendElement(reinterpret_cast<uintptr_t>(aPC));
 }
 
 static void
@@ -119,6 +124,10 @@ GetChromeHangReport(Telemetry::HangStack &callStack, SharedLibraryInfo &moduleMa
 {
   MOZ_ASSERT(winMainThreadHandle);
 
+  // The thread we're about to suspend might have the alloc lock
+  // so allocate ahead of time
+  callStack.SetCapacity(MAX_CALL_STACK_PCS);
+
   DWORD ret = ::SuspendThread(winMainThreadHandle);
   if (ret == -1) {
     callStack.Clear();