Bug 1910953 - Use a Web Lock to sequence deletions and creations of backups. r=backup-reviewers,fchasen

This allows us to avoid creating a backup while we're in the middle of deleting
one, and deleting a backup when we're in the middle of creating one.

An AbortController is used to clear the lock's request queue on shutdown
in the (unlikely) event that a whole slew of backup creation and deletion
requests have queued up.

Differential Revision: https://phabricator.services.mozilla.com/D218240

browser/components/backup/BackupService.sys.mjs

@@ -648,6 +648,14 @@ export class BackupService extends EventTarget {
    */
   #encState = undefined;
 
+  /**
+   * The AbortController used to abort any queued requests to create or delete
+   * backups that might be waiting on the WRITE_BACKUP_LOCK_NAME lock.
+   *
+   * @type {AbortController}
+   */
+  #backupWriteAbortController = null;
+
   /**
    * The path of the default parent directory for saving backups.
    * The current default is the Documents directory.
@@ -840,6 +848,16 @@ export class BackupService extends EventTarget {
     return AppConstants.MOZ_APP_BASENAME + " Backup Recovery Storage";
   }
 
+  /**
+   * The name of the exclusive Web Lock that will be requested and held when
+   * creating or deleting a backup.
+   *
+   * @type {string}
+   */
+  static get WRITE_BACKUP_LOCK_NAME() {
+    return "write-backup";
+  }
+
   /**
    * Returns a reference to a BackupService singleton. If this is the first time
    * that this getter is accessed, this causes the BackupService singleton to be
@@ -897,6 +915,7 @@ export class BackupService extends EventTarget {
     let { promise, resolve } = Promise.withResolvers();
     this.#postRecoveryPromise = promise;
     this.#postRecoveryResolver = resolve;
+    this.#backupWriteAbortController = new AbortController();
   }
 
   /**
@@ -1069,11 +1088,17 @@ export class BackupService extends EventTarget {
       return null;
     }
 
+    return locks.request(
+      BackupService.WRITE_BACKUP_LOCK_NAME,
+      { signal: this.#backupWriteAbortController.signal },
+      async () => {
         this.#backupInProgress = true;
         const backupTimer = Glean.browserBackup.totalBackupTime.start();
 
         try {
-      lazy.logConsole.debug(`Creating backup for profile at ${profilePath}`);
+          lazy.logConsole.debug(
+            `Creating backup for profile at ${profilePath}`
+          );
 
           let archiveDestFolderPath = await this.resolveArchiveDestFolderPath(
             lazy.backupDirPref
@@ -1189,7 +1214,9 @@ export class BackupService extends EventTarget {
           );
           await IOUtils.writeJSON(manifestPath, manifest);
 
-      let renamedStagingPath = await this.#finalizeStagingFolder(stagingPath);
+          let renamedStagingPath = await this.#finalizeStagingFolder(
+            stagingPath
+          );
           lazy.logConsole.log(
             "Wrote backup to staging directory at ",
             renamedStagingPath
@@ -1223,7 +1250,10 @@ export class BackupService extends EventTarget {
           // backups folder while it gets written. Once that's done, we'll attempt
           // to move it to the user's configured backup path.
           let archiveTmpPath = PathUtils.join(backupDirPath, "archive.html");
-      lazy.logConsole.log("Exporting single-file archive to ", archiveTmpPath);
+          lazy.logConsole.log(
+            "Exporting single-file archive to ",
+            archiveTmpPath
+          );
           await this.createArchive(
             archiveTmpPath,
             BackupService.ARCHIVE_TEMPLATE,
@@ -1257,7 +1287,10 @@ export class BackupService extends EventTarget {
           );
 
           let nowSeconds = Math.floor(Date.now() / 1000);
-      Services.prefs.setIntPref(LAST_BACKUP_TIMESTAMP_PREF_NAME, nowSeconds);
+          Services.prefs.setIntPref(
+            LAST_BACKUP_TIMESTAMP_PREF_NAME,
+            nowSeconds
+          );
           this.#_state.lastBackupDate = nowSeconds;
           Glean.browserBackup.totalBackupTime.stopAndAccumulate(backupTimer);
 
@@ -1269,6 +1302,8 @@ export class BackupService extends EventTarget {
           this.#backupInProgress = false;
         }
       }
+    );
+  }
 
   /**
    * Generates a string from a Date in the form of:
@@ -3121,6 +3156,7 @@ export class BackupService extends EventTarget {
       }
       case "quit-application-granted": {
         this.uninitBackupScheduler();
+        this.#backupWriteAbortController.abort();
         break;
       }
     }
@@ -3279,6 +3315,10 @@ export class BackupService extends EventTarget {
    * @returns {Promise<undefined>}
    */
   async deleteLastBackup() {
+    return locks.request(
+      BackupService.WRITE_BACKUP_LOCK_NAME,
+      { signal: this.#backupWriteAbortController.signal },
+      async () => {
         if (this.#_state.lastBackupFileName) {
           let backupFilePath = PathUtils.join(
             lazy.backupDirPref,
@@ -3313,4 +3353,6 @@ export class BackupService extends EventTarget {
           }
         }
       }
+    );
+  }
 }