Use an error class instead of string matching

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

@@ -18,6 +18,23 @@ import {
 import { MigrateAndConvertFn } from './document_migrator';
 import { SavedObjectsMigrationLogger } from '.';
 
+/**
+ * Error thrown when saved object migrations encounter a corrupt saved object.
+ * Corrupt saved objects cannot be serialized because:
+ *  - there's no `[type]` property which contains the type attributes
+ *  - the type or namespace in the _id doesn't match the `type` or `namespace`
+ *    properties
+ */
+export class CorruptSavedObjectError extends Error {
+  constructor(public readonly rawId: string) {
+    super(`Unable to migrate the corrupt saved object document with _id: '${rawId}'.`);
+
+    // Set the prototype explicitly, see:
+    // https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#extending-built-ins-like-error-array-and-map-may-no-longer-work
+    Object.setPrototypeOf(this, CorruptSavedObjectError.prototype);
+  }
+}
+
 /**
  * Applies the specified migration function to every saved object document in the list
  * of raw docs. Any raw docs that are not valid saved objects will simply be passed through.
@@ -48,9 +65,7 @@ export async function migrateRawDocs(
         )
       );
     } else {
-      throw new Error(
-        `Unable to migrate the corrupt saved object document with _id: '${raw._id}'.`
-      );
+      throw new CorruptSavedObjectError(raw._id);
     }
   }
   return processedDocs;

src/core/server/saved_objects/migrationsv2/migrations_state_action_machine.ts

@@ -10,6 +10,7 @@ import { errors as EsErrors } from '@elastic/elasticsearch';
 import * as Option from 'fp-ts/lib/Option';
 import { performance } from 'perf_hooks';
 import { Logger, LogMeta } from '../../logging';
+import { CorruptSavedObjectError } from '../migrations/core/migrate_raw_docs';
 import { Model, Next, stateActionMachine } from './state_action_machine';
 import { State } from './types';
 
@@ -165,7 +166,7 @@ export async function migrationStateActionMachine({
       logger.error(e);
 
       dumpExecutionLog(logger, logMessagePrefix, executionLog);
-      if (e.message.startsWith('Unable to migrate the corrupt saved object document')) {
+      if (e instanceof CorruptSavedObjectError) {
         throw new Error(
           `${e.message} To allow migrations to proceed, please delete this document from the [${initialState.indexPrefix}_${initialState.kibanaVersion}_001] index.`
         );