Bug 1715858 - Part1. Add a new telemetry histogram TAB_UNLOAD_TO_RELOAD. r=NeilDeakin

The new histogram `TAB_UNLOAD_TO_RELOAD` records how long a tab had been
unloaded until it was reload by a user.  With this data, we can evaluate
the selection logic to choose a tab to unload.  For example, if many of
unloaded tabs are reloaded within 30 seconds or so, we unload a wrong tab.

Differential Revision: https://phabricator.services.mozilla.com/D120019

browser/base/content/tabbrowser-tab.js

@@ -265,6 +265,22 @@
       this._lastAccessed = this.selected ? Infinity : aDate || Date.now();
     }
 
+    updateLastUnloadedByTabUnloader() {
+      this._lastUnloaded = Date.now();
+    }
+
+    recordTimeFromUnloadToReload() {
+      if (!this._lastUnloaded) {
+        return;
+      }
+
+      const diff_in_msec = Date.now() - this._lastUnloaded;
+      Services.telemetry
+        .getHistogramById("TAB_UNLOAD_TO_RELOAD")
+        .add(diff_in_msec / 1000);
+      delete this._lastUnloaded;
+    }
+
     on_mouseover(event) {
       if (event.target.classList.contains("tab-close-button")) {
         this.mOverCloseButton = true;

browser/base/content/tabbrowser.js

@@ -1156,6 +1156,7 @@
       }
 
       if (!this._previewMode) {
+        newTab.recordTimeFromUnloadToReload();
         newTab.updateLastAccessed();
         oldTab.updateLastAccessed();
 

browser/modules/TabUnloader.jsm

@@ -279,6 +279,7 @@ var TabUnloader = {
         Services.console.logStringMessage(
           `TabUnloader discarded <${remoteType}>`
         );
+        tabInfo.tab.updateLastUnloadedByTabUnloader();
         return true;
       }
     }

browser/modules/test/browser/browser_TabUnloader.js

@@ -209,6 +209,10 @@ add_task(async function test() {
   ok(!pinnedSoundTab.linkedPanel, "unloaded a pinned tab playing sound");
   await compareTabOrder([]); // note that no tabs are returned when there are no discardable tabs.
 
+  const histogram = TelemetryTestUtils.getAndClearHistogram(
+    "TAB_UNLOAD_TO_RELOAD"
+  );
+
   // It's possible that we're already in the memory-pressure state
   // and we may receive the "ongoing" message.
   const message = await pressureAndObserve("memory-pressure");
@@ -224,6 +228,12 @@ add_task(async function test() {
   await BrowserTestUtils.switchTab(gBrowser, tab1);
   await BrowserTestUtils.switchTab(gBrowser, soundTab);
   await BrowserTestUtils.switchTab(gBrowser, pinnedTab);
+
+  const hist = histogram.snapshot();
+  const numEvents = Object.values(hist.values).reduce((a, b) => a + b);
+  Assert.equal(numEvents, 3, "three tabs have been reloaded.");
+
+  // tab0 has never been unloaded.  No data is added to the histogram.
   await BrowserTestUtils.switchTab(gBrowser, tab0);
 
   // Audio from the first sound tab was stopped when the tab was discarded earlier,

toolkit/components/telemetry/Histograms.json

@@ -4957,6 +4957,18 @@
     "releaseChannelCollection": "opt-out",
     "description": "Number of tabs opened across all windows, collected at most every 5 minutes whenever the user interacts with the browser in the following ways: open tab/window, page load."
   },
+  "TAB_UNLOAD_TO_RELOAD": {
+    "record_in_processes": ["main"],
+    "products": ["firefox"],
+    "alert_emails": ["tkikuchi@mozilla.com"],
+    "bug_numbers": [1715858],
+    "expires_in_version": "never",
+    "kind": "exponential",
+    "high": 100000,
+    "n_buckets": 50,
+    "releaseChannelCollection": "opt-out",
+    "description": "How long (sec) a tab had been unloaded until it was reloaded."
+  },
   "LAZYLOAD_IMAGE_TOTAL": {
     "record_in_processes": ["main", "content"],
     "products": ["firefox"],