IndexMigrator: fix non blocking migration wrapper promise rejection

src/core/server/saved_objects/migrations/core/index_migrator.test.ts

@@ -369,6 +369,30 @@ describe('IndexMigrator', () => {
       ],
     });
   });
+
+  test('rejects when the migration function throws an error', async () => {
+    const { client } = testOpts;
+    const migrateDoc = jest.fn((doc: SavedObjectUnsanitizedDoc) => {
+      throw new Error('error migrating document');
+    });
+
+    testOpts.documentMigrator = {
+      migrationVersion: { foo: '1.2.3' },
+      migrate: migrateDoc,
+    };
+
+    withIndex(client, {
+      numOutOfDate: 1,
+      docs: [
+        [{ _id: 'foo:1', _source: { type: 'foo', foo: { name: 'Bar' } } }],
+        [{ _id: 'foo:2', _source: { type: 'foo', foo: { name: 'Baz' } } }],
+      ],
+    });
+
+    await expect(new IndexMigrator(testOpts).migrate()).rejects.toThrowErrorMatchingInlineSnapshot(
+      `"error migrating document"`
+    );
+  });
 });
 
 function withIndex(

src/core/server/saved_objects/migrations/core/migrate_raw_docs.test.ts

@@ -90,4 +90,18 @@ describe('migrateRawDocs', () => {
 
     expect(logger.error).toBeCalledTimes(1);
   });
+
+  test('rejects when the transform function throws an error', async () => {
+    const transform = jest.fn<any, any>((doc: any) => {
+      throw new Error('error during transform');
+    });
+    await expect(
+      migrateRawDocs(
+        new SavedObjectsSerializer(new SavedObjectTypeRegistry()),
+        transform,
+        [{ _id: 'a:b', _source: { type: 'a', a: { name: 'AAA' } } }],
+        createSavedObjectsMigrationLoggerMock()
+      )
+    ).rejects.toThrowErrorMatchingInlineSnapshot(`"error during transform"`);
+  });
 });

src/core/server/saved_objects/migrations/core/migrate_raw_docs.ts

@@ -78,10 +78,14 @@ function transformNonBlocking(
 ): (doc: SavedObjectUnsanitizedDoc) => Promise<SavedObjectUnsanitizedDoc> {
   // promises aren't enough to unblock the event loop
   return (doc: SavedObjectUnsanitizedDoc) =>
-    new Promise((resolve) => {
+    new Promise((resolve, reject) => {
       // set immediate is though
       setImmediate(() => {
-        resolve(transform(doc));
+        try {
+          resolve(transform(doc));
+        } catch (e) {
+          reject(e);
+        }
       });
     });
 }