Tweak migration loop

1. The migration loop no longer resets if the object does not have
a [type] field. Objects never have this field after being
deserialized from their raw document format, this functionality
appears to be unnecessary and can cause unexpected behavior
(though migrations still worked because the loop would get
restarted).
2. Changed one unit test case that depended upon the aforementioned
condition. This appeared to be an invalid test case which would not
actually be encountered in a real world scenario, since it relied
on an object containing a [type] field.
3. The migration loop now resets if an object's migrationVersion
has explicitly been increased by a transform function. This is
expected behavior and a unit test exercises it, but it was
a side effect of the aforementioned condition.

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

@@ -382,36 +382,26 @@ describe('DocumentMigrator', () => {
     it('allows props to be renamed', () => {
       const migrator = new DocumentMigrator({
         ...testOpts(),
-        typeRegistry: createRegistry(
-          {
-            name: 'animal',
-            migrations: {
-              '1.0.0': setAttr('animal', (name: string) => `Animal: ${name}`),
-              '3.2.1': renameAttr('animal', 'dawg'),
-            },
+        typeRegistry: createRegistry({
+          name: 'dog',
+          migrations: {
+            '1.0.0': setAttr('attributes.name', (name: string) => `Name: ${name}`),
+            '1.0.1': renameAttr('attributes.name', 'attributes.title'),
+            '1.0.2': setAttr('attributes.title', (name: string) => `Title: ${name}`),
           },
-          {
-            name: 'dawg',
-            migrations: {
-              '2.2.4': renameAttr('dawg', 'animal'),
-              '3.2.0': setAttr('dawg', (name: string) => `Dawg3.x: ${name}`),
-            },
-          }
-        ),
+        }),
       });
       const actual = migrator.migrate({
         id: 'smelly',
-        type: 'foo',
+        type: 'dog',
         attributes: { name: 'Callie' },
-        dawg: 'Yo',
         migrationVersion: {},
-      } as SavedObjectUnsanitizedDoc);
+      });
       expect(actual).toEqual({
         id: 'smelly',
-        type: 'foo',
-        attributes: { name: 'Callie' },
-        dawg: 'Dawg3.x: Animal: Yo',
-        migrationVersion: { animal: '3.2.1', dawg: '3.2.0' },
+        type: 'dog',
+        attributes: { title: 'Title: Name: Callie' },
+        migrationVersion: { dog: '1.0.2' },
       });
     });
 

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

@@ -361,14 +361,20 @@ function migrateProp(
 ): SavedObjectUnsanitizedDoc {
   const originalType = doc.type;
   let migrationVersion = _.clone(doc.migrationVersion) || {};
-  const typeChanged = () => !doc.hasOwnProperty(prop) || doc.type !== originalType;
 
   for (const { version, transform } of applicableTransforms(migrations, doc, prop)) {
+    const currentVersion = propVersion(doc, prop);
+    if (currentVersion && Semver.gt(currentVersion, version)) {
+      // the previous transform function increased the object's migrationVersion; break out of the loop
+      break;
+    }
+
     doc = transform(doc);
     migrationVersion = updateMigrationVersion(doc, migrationVersion, prop, version);
     doc.migrationVersion = _.clone(migrationVersion);
 
-    if (typeChanged()) {
+    if (doc.type !== originalType) {
+      // the transform function changed the object's type; break out of the loop
       break;
     }
   }