firestore: fix undefined document snapshot data after "clear site data"

.changeset/slow-students-fry.md

@@ -0,0 +1,6 @@
+---
+'@firebase/firestore': patch
+'firebase': patch
+---
+
+Fix issue where Firestore would produce `undefined` for document snapshot data if using IndexedDB persistence and "clear site data" (or equivalent) button was pressed in the web browser.

packages/firestore/src/local/simple_db.ts

@@ -158,6 +158,7 @@ export class SimpleDbTransaction {
  */
 export class SimpleDb {
   private db?: IDBDatabase;
+  private lastDbVersion: number | null = null;
   private versionchangelistener?: (event: IDBVersionChangeEvent) => void;
 
   /** Deletes the specified database. */
@@ -344,6 +345,24 @@ export class SimpleDb {
             event.oldVersion
           );
           const db = (event.target as IDBOpenDBRequest).result;
+          if (
+            this.lastDbVersion !== null &&
+            this.lastDbVersion !== event.oldVersion
+          ) {
+            // This thrown error will get passed to the `onerror` callback
+            // registered above, and will then be propagated correctly.
+            throw new Error(
+              `refusing to open IndexedDB database due to potential ` +
+                `corruption of the IndexedDB database data; this corruption ` +
+                `could be caused by clicking the "clear site data" button in ` +
+                `a web browser; try reloading the web page to re-initialize ` +
+                `the IndexedDB database: ` +
+                `lastDbVersion=${this.lastDbVersion}, ` +
+                `event.oldVersion=${event.oldVersion}, ` +
+                `event.newVersion=${event.newVersion}, ` +
+                `db.version=${db.version}`
+            );
+          }
           this.schemaConverter
             .createOrUpgrade(
               db,
@@ -359,11 +378,21 @@ export class SimpleDb {
             });
         };
       });
+
+      this.db.addEventListener(
+        'close',
+        event => {
+          const db = event.target as IDBDatabase;
+          this.lastDbVersion = db.version;
+        },
+        { passive: true }
+      );
     }
 
     if (this.versionchangelistener) {
       this.db.onversionchange = event => this.versionchangelistener!(event);
     }
+
     return this.db;
   }