Bug 1848459 - add telemetry to count closed tabs lost in session restore r=sfoster,sessionstore-reviewers

* Add histogram to sessionrestore code to count how often and the number of
closed tabs that aren't saved on window close due to no open saveable tabs
* Add test coverage

Differential Revision: https://phabricator.services.mozilla.com/D193055

browser/components/sessionstore/SessionStore.sys.mjs

@@ -2360,7 +2360,7 @@ var SessionStoreInternal = {
           // It's possible that a tab switched its privacy state at some point
           // before our flush, so we need to filter again.
           lazy.PrivacyFilter.filterPrivateTabs(winData);
-          this.maybeSaveClosedWindow(winData, isLastWindow);
+          this.maybeSaveClosedWindow(winData, isLastWindow, true);
 
           if (!isLastWindow && winData.closedId > -1) {
             this._addClosedAction(
@@ -2439,7 +2439,7 @@ var SessionStoreInternal = {
    *        to call this method again asynchronously (for example, after
    *        a window flush).
    */
-  maybeSaveClosedWindow(winData, isLastWindow) {
+  maybeSaveClosedWindow(winData, isLastWindow, recordTelemetry = false) {
     // Make sure SessionStore is still running, and make sure that we
     // haven't chosen to forget this window.
     if (
@@ -2498,6 +2498,14 @@ var SessionStoreInternal = {
         }
         if (alreadyStored) {
           this._removeClosedWindow(winIndex);
+          return;
+        }
+        // we only do this after the TabStateFlusher promise resolves in ssi_onClose
+        if (recordTelemetry) {
+          let closedTabsHistogram = Services.telemetry.getHistogramById(
+            "FX_SESSION_RESTORE_CLOSED_TABS_NOT_SAVED"
+          );
+          closedTabsHistogram.add(winData._closedTabs.length);
         }
       }
     }

browser/components/sessionstore/test/browser_closed_tabs_closed_windows.js

@@ -1,7 +1,9 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
+const { TelemetryTestUtils } = ChromeUtils.import(
+  "resource://testing-common/TelemetryTestUtils.jsm"
+);
 const TEST_URLS = [
   "http://mochi.test:8888/browser/",
   "https://www.example.com/",
@@ -74,17 +76,37 @@ async function prepareClosedData() {
   closedIds.tab5 =
     SessionStore.getClosedTabDataForWindow(privateWin)[0].closedId;
 
+  const testWindow6 = await BrowserTestUtils.openNewBrowserWindow();
+
+  const testWindow7 = await BrowserTestUtils.openNewBrowserWindow();
+  await openAndCloseTab(testWindow7, TEST_URLS[4]);
+
+  let closedTabsHistogram = TelemetryTestUtils.getAndClearHistogram(
+    "FX_SESSION_RESTORE_CLOSED_TABS_NOT_SAVED"
+  );
+
   await BrowserTestUtils.closeWindow(testWindow1);
   closedIds.testWindow1 = SessionStore.getClosedWindowData()[0].closedId;
   await BrowserTestUtils.closeWindow(testWindow2);
+
   closedIds.testWindow2 = SessionStore.getClosedWindowData()[0].closedId;
   await BrowserTestUtils.closeWindow(testWindow3);
+
   closedIds.testWindow3 = SessionStore.getClosedWindowData()[0].closedId;
   await BrowserTestUtils.closeWindow(privateWin);
   Assert.ok(
     closedIds.testWindow2 > closedIds.testWindow1,
     "We got the closedIds in the expected order"
   );
+
+  await BrowserTestUtils.closeWindow(testWindow6);
+  TelemetryTestUtils.assertHistogram(closedTabsHistogram, 0, 1);
+  closedTabsHistogram.clear();
+
+  await BrowserTestUtils.closeWindow(testWindow7);
+  TelemetryTestUtils.assertHistogram(closedTabsHistogram, 1, 1);
+  closedTabsHistogram.clear();
+
   return closedIds;
 }
 

toolkit/components/telemetry/Histograms.json

@@ -9307,6 +9307,17 @@
     "n_values": 50,
     "description": "Session restore: Number of tabs restored eagerly in the session that has just been restored."
   },
+  "FX_SESSION_RESTORE_CLOSED_TABS_NOT_SAVED": {
+    "record_in_processes": ["main", "content"],
+    "products": ["firefox"],
+    "expires_in_version": "127",
+    "alert_emails": ["firefox-view-engineers@mozilla.com"],
+    "releaseChannelCollection": "opt-out",
+    "bug_numbers": [1848459],
+    "kind": "enumerated",
+    "n_values": 25,
+    "description": "Session restore: Number of closed tabs that are NOT saved due to lack of open tabs worth saving on window close."
+  },
   "FX_TABLETMODE_PAGE_LOAD": {
     "record_in_processes": ["main", "content"],
     "products": ["firefox", "fennec"],