Sanity-check backup agent name prior to instantiation

Fixes a crash that would occur if an app with a pending backup
pass in the pipeline was updated to remove its agent declaration
from the manifest (or other more esoteric ways that a backup
pass was expected to run for an app without their own agent).

Bug 5776591

Change-Id: I5a8bc8c12de6a2bfa82f5093fe3a15b754109ab1

Author: Christopher Tate

services/java/com/android/server/BackupManagerService.java

@@ -1986,6 +1986,16 @@ void invokeNextAgent() {
             try {
                 mCurrentPackage = mPackageManager.getPackageInfo(request.packageName,
                         PackageManager.GET_SIGNATURES);
+                if (mCurrentPackage.applicationInfo.backupAgentName == null) {
+                    // The manifest has changed but we had a stale backup request pending.
+                    // This won't happen again because the app won't be requesting further
+                    // backups.
+                    Slog.i(TAG, "Package " + request.packageName
+                            + " no longer supports backup; skipping");
+                    addBackupTrace("skipping - no agent, completion is noop");
+                    executeNextState(BackupState.RUNNING_QUEUE);
+                    return;
+                }
 
                 IBackupAgent agent = null;
                 try {