Support downgrades

Author: schmidt-sebastian

firebase-firestore/src/main/java/com/google/firebase/firestore/local/SQLitePersistence.java

@@ -320,18 +320,7 @@ public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
     @Override
     public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
       ensureConfigured(db);
-
-      // For now, we can safely do nothing.
-      //
-      // The only case that's possible at this point would be to downgrade from version 1 (present
-      // in our first released version) to 0 (uninstalled). Nobody would want us to just wipe the
-      // data so instead we just keep it around in the hope that they'll upgrade again :-).
-      //
-      // Note that if you uninstall a Firestore-based app, the database goes away completely. The
-      // downgrade-then-upgrade case can only happen in very limited circumstances.
-      //
-      // We'll have to revisit this once we ship a migration past version 1, but this will
-      // definitely be good enough for our initial launch.
+      new SQLiteSchema(db, serializer).runMigrations(oldVersion);
     }
 
     @Override

firebase-firestore/src/main/java/com/google/firebase/firestore/local/SQLiteSchema.java

@@ -153,8 +153,11 @@ void runMigrations(int fromVersion, int toVersion) {
       dropLastLimboFreeSnapshotVersion();
     }
 
+    // Schema version 11 changed the format of canonical IDs in the target cache.
     if (fromVersion < 11 && toVersion >= 11) {
-      // Schema version 11 changed the format of canonical IDs in the target cache.
+      rewriteCanonicalIds();
+    } else if (fromVersion >= 11 && toVersion < 11) {
+      // Rewrite the canonical IDs in the old format.
       rewriteCanonicalIds();
     }
 

firebase-firestore/src/test/java/com/google/firebase/firestore/local/SQLiteSchemaTest.java

@@ -552,6 +552,19 @@ public void rewritesCanonicalIds() {
         });
 
     schema.runMigrations(10, 11);
+    assertTargetIds();
+
+    // Simulate a downgrade that requires a canonical ID migration.
+    db.execSQL("UPDATE targets SET canonical_id = ?", new Object[] {"invalid_canonical_id"});
+    schema.runMigrations(11, 10);
+    assertTargetIds();
+  }
+
+  /**
+   * Evaluates the data in the `targets` table to make sure that the canonical ID matches the
+   * current format.
+   */
+  private void assertTargetIds() {
     new SQLitePersistence.Query(db, "SELECT canonical_id, target_proto, canonical_id FROM targets")
         .forEach(
             cursor -> {