Bug 1167292 - Part 1: Add a telemetry probe to time how long it takes to save a heap snapshot; r=ejpbruel

toolkit/components/telemetry/Histograms.json

@@ -7078,6 +7078,13 @@
     "n_buckets": "100",
     "description": "The mean number of pinned tabs (app tabs) in all windows for a session for devtools users."
   },
+  "DEVTOOLS_SAVE_HEAP_SNAPSHOT_MS": {
+    "expires_in_version": "never",
+    "kind": "exponential",
+    "high": "100000",
+    "n_buckets": "1000",
+    "description": "The time (in milliseconds) that it took to save a heap snapshot in mozilla::devtools::ChromeUtils::SaveHeapSnapshot."
+  },
   "BROWSER_IS_USER_DEFAULT": {
     "expires_in_version": "never",
     "kind": "boolean",

toolkit/devtools/server/HeapSnapshot.cpp

@@ -19,6 +19,8 @@
 #include "mozilla/devtools/ZeroCopyNSIOutputStream.h"
 #include "mozilla/dom/ChromeUtils.h"
 #include "mozilla/dom/HeapSnapshotBinding.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/TimeStamp.h"
 #include "mozilla/UniquePtr.h"
 
 #include "jsapi.h"
@@ -550,6 +552,8 @@ ThreadSafeChromeUtils::SaveHeapSnapshot(GlobalObject& global,
                                         const HeapSnapshotBoundaries& boundaries,
                                         ErrorResult& rv)
 {
+  auto start = TimeStamp::Now();
+
   bool wantNames = true;
   ZoneSet zones;
   Maybe<AutoCheckCannotGC> maybeNoGC;
@@ -587,12 +591,15 @@ ThreadSafeChromeUtils::SaveHeapSnapshot(GlobalObject& global,
                       wantNames,
                       zones.initialized() ? &zones : nullptr,
                       maybeNoGC.ref()))
-    {
-      rv.Throw(zeroCopyStream.failed()
-               ? zeroCopyStream.result()
-               : NS_ERROR_UNEXPECTED);
-      return;
-    }
+  {
+    rv.Throw(zeroCopyStream.failed()
+             ? zeroCopyStream.result()
+             : NS_ERROR_UNEXPECTED);
+    return;
+  }
+
+  Telemetry::AccumulateTimeDelta(Telemetry::DEVTOOLS_SAVE_HEAP_SNAPSHOT_MS,
+                                 start);
 }
 
 /* static */ already_AddRefed<HeapSnapshot>