Bug 1214867 - Add HeapSnapshot.prototype.creationTime; r=bholley

devtools/shared/heapsnapshot/HeapSnapshot.h

@@ -10,6 +10,7 @@
 #include "mozilla/ErrorResult.h"
 #include "mozilla/devtools/DeserializedNode.h"
 #include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/Nullable.h"
 #include "mozilla/HashFunctions.h"
 #include "mozilla/Maybe.h"
 #include "mozilla/RefCounted.h"
@@ -139,6 +140,15 @@ public:
 
   void TakeCensus(JSContext* cx, JS::HandleObject options,
                   JS::MutableHandleValue rval, ErrorResult& rv);
+
+  dom::Nullable<uint64_t> GetCreationTime() {
+    static const uint64_t maxTime = uint64_t(1) << 53;
+    if (timestamp.isSome() && timestamp.ref() <= maxTime) {
+      return dom::Nullable<uint64_t>(timestamp.ref());
+    }
+
+    return dom::Nullable<uint64_t>();
+  }
 };
 
 // A `CoreDumpWriter` is given the data we wish to save in a core dump and

devtools/shared/heapsnapshot/tests/unit/test_HeapSnapshot_creationTime_01.js

@@ -0,0 +1,29 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// HeapSnapshot.prototype.creationTime returns the expected time.
+
+function waitForTenMilliseconds() {
+  const start = Date.now();
+  while (Date.now() - start < 10) ;
+}
+
+function run_test() {
+  const start = Date.now() * 1000;
+  do_print("start                 = " + start);
+
+  // Because Date.now() is less precise than the snapshot's time stamp, give it
+  // a little bit of head room.
+  waitForTenMilliseconds();
+  const path = ChromeUtils.saveHeapSnapshot({ runtime: true });
+  waitForTenMilliseconds();
+
+  const end = Date.now() * 1000;
+  do_print("end                   = " + end);
+
+  const snapshot = ChromeUtils.readHeapSnapshot(path);
+  do_print("snapshot.creationTime = " + snapshot.creationTime);
+
+  ok(snapshot.creationTime >= start);
+  ok(snapshot.creationTime <= end);
+}

devtools/shared/heapsnapshot/tests/unit/xpcshell.ini

@@ -25,6 +25,7 @@ support-files =
 [test_HeapAnalyses_takeCensus_03.js]
 [test_HeapAnalyses_takeCensus_04.js]
 [test_HeapAnalyses_takeCensus_05.js]
+[test_HeapSnapshot_creationTime_01.js]
 [test_HeapSnapshot_takeCensus_01.js]
 [test_HeapSnapshot_takeCensus_02.js]
 [test_HeapSnapshot_takeCensus_03.js]

dom/webidl/HeapSnapshot.webidl

@@ -9,6 +9,12 @@
  */
 [ChromeOnly, Exposed=(Window,System,Worker)]
 interface HeapSnapshot {
+  /**
+   * A time stamp of when the heap snapshot was taken, if available. Units are
+   * microseconds since midnight (00:00:00) 1 January 1970 UTC.
+   */
+  readonly attribute unsigned long long? creationTime;
+
   /**
    * Take a census of the heap snapshot.
    *