Bug 1031298 - Add a histogram for counting unrecoverable sessions. r=yoric

browser/components/sessionstore/SessionFile.jsm

@@ -234,6 +234,12 @@ let SessionFileInternal = {
         }
       }
     }
+
+    // All files are corrupted if files found but none could deliver a result.
+    let allCorrupt = !noFilesFound && !result;
+    Telemetry.getHistogramById("FX_SESSION_RESTORE_ALL_FILES_CORRUPT").
+      add(allCorrupt);
+
     if (!result) {
       // If everything fails, start with an empty session.
       result = {

browser/components/sessionstore/test/unit/test_histogram_corrupt_files.js

@@ -0,0 +1,114 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/*
+ * The primary purpose of this test is to ensure that
+ * the sessionstore component records information about
+ * corrupted backup files into a histogram.
+ */
+
+"use strict";
+Cu.import("resource://gre/modules/osfile.jsm", this);
+
+const Telemetry = Services.telemetry;
+const Path = OS.Path;
+const HistogramId = "FX_SESSION_RESTORE_ALL_FILES_CORRUPT";
+
+// Prepare the session file.
+let profd = do_get_profile();
+Cu.import("resource:///modules/sessionstore/SessionFile.jsm", this);
+
+/**
+ * A utility function for resetting the histogram and the contents
+ * of the backup directory.
+ */
+function reset_session(backups = {}) {
+
+  // Reset the histogram.
+  Telemetry.getHistogramById(HistogramId).clear();
+
+  // Reset the contents of the backups directory
+  OS.File.makeDir(SessionFile.Paths.backups);
+  for (let key of SessionFile.Paths.loadOrder) {
+    if (backups.hasOwnProperty(key)) {
+      OS.File.copy(backups[key], SessionFile.Paths[key]);
+    } else {
+      OS.File.remove(SessionFile.Paths[key]);
+    }
+  }
+}
+
+/**
+ * In order to use FX_SESSION_RESTORE_ALL_FILES_CORRUPT histogram
+ * it has to be registered in "toolkit/components/telemetry/Histograms.json".
+ * This test ensures that the histogram is registered and empty.
+ */
+add_task(function* test_ensure_histogram_exists_and_empty() {
+  let s = Telemetry.getHistogramById(HistogramId).snapshot();
+  Assert.equal(s.sum, 0, "Initially, the sum of probes is 0");
+});
+
+/**
+ * Makes sure that the histogram is negatively updated when no
+ * backup files are present.
+ */
+add_task(function* test_no_files_exist() {
+  // No session files are available to SessionFile.
+  reset_session();
+
+  yield SessionFile.read();
+  // Checking if the histogram is updated negatively
+  let h = Telemetry.getHistogramById(HistogramId);
+  let s = h.snapshot();
+  Assert.equal(s.counts[0], 1, "One probe for the 'false' bucket.");
+  Assert.equal(s.counts[1], 0, "No probes in the 'true' bucket.");
+});
+
+/**
+ * Makes sure that the histogram is negatively updated when at least one
+ * backup file is not corrupted.
+ */
+add_task(function* test_one_file_valid() {
+  // Corrupting some backup files.
+  let invalidSession = "data/sessionstore_invalid.js";
+  let validSession = "data/sessionstore_valid.js";
+  reset_session({
+    clean : invalidSession,
+    cleanBackup: validSession,
+    recovery: invalidSession,
+    recoveryBackup: invalidSession
+  });
+
+  yield SessionFile.read();
+  // Checking if the histogram is updated negatively.
+  let h = Telemetry.getHistogramById(HistogramId);
+  let s = h.snapshot();
+  Assert.equal(s.counts[0], 1, "One probe for the 'false' bucket.");
+  Assert.equal(s.counts[1], 0, "No probes in the 'true' bucket.");
+});
+
+/**
+ * Makes sure that the histogram is positively updated when all
+ * backup files are corrupted.
+ */
+add_task(function* test_all_files_corrupt() {
+  // Corrupting all backup files.
+  let invalidSession = "data/sessionstore_invalid.js";
+  reset_session({
+    clean : invalidSession,
+    cleanBackup: invalidSession,
+    recovery: invalidSession,
+    recoveryBackup: invalidSession
+  });
+
+  yield SessionFile.read();
+  // Checking if the histogram is positively updated.
+  let h = Telemetry.getHistogramById(HistogramId);
+  let s = h.snapshot();
+  Assert.equal(s.counts[1], 1, "One probe for the 'true' bucket.");
+  Assert.equal(s.counts[0], 0, "No probes in the 'false' bucket.");
+});
+
+function run_test() {
+  run_next_test();
+}

browser/components/sessionstore/test/unit/xpcshell.ini

@@ -11,3 +11,4 @@ support-files =
 [test_startup_nosession_async.js]
 [test_startup_session_async.js]
 [test_startup_invalid_session.js]
+[test_histogram_corrupt_files.js]

toolkit/components/telemetry/Histograms.json

@@ -3823,6 +3823,11 @@
     "kind": "boolean",
     "description": "Session restore: Whether the file read on startup contained parse-able JSON"
   },
+  "FX_SESSION_RESTORE_ALL_FILES_CORRUPT": {
+    "expires_in_version": "default",
+    "kind": "boolean",
+    "description": "Session restore: Whether none of the backup files contained parse-able JSON"
+  },
   "FX_SESSION_RESTORE_RESTORE_WINDOW_MS": {
     "expires_in_version": "default",
     "kind": "exponential",