Fix TagManifests with shared digests under the same repository.

TagManifests can (apparently, in very rare scenarios) share manifests with the exact same digests, so we need to support that case in the backfill worker. We also need to remove a unique constraint on the manifest column in the mapping table to support this case.

workers/test/test_manifestbackfillworker.py

@@ -138,3 +138,33 @@ def test_manifestbackfillworker_mislinked_invalid_manifest(clear_rows, initializ
 
   manifest_blobs = list(ManifestBlob.select().where(ManifestBlob.manifest == manifest_row))
   assert len(manifest_blobs) == 0
+
+
+def test_manifestbackfillworker_repeat_digest(clear_rows, initialized_db):
+  """ Tests that a manifest with a shared digest will be properly linked. """
+  # Delete existing tag manifest so we can reuse the tag.
+  TagManifestLabel.delete().execute()
+  TagManifest.delete().execute()
+
+  repo = model.repository.get_repository('devtable', 'gargantuan')
+  tag_v30 = model.tag.get_active_tag('devtable', 'gargantuan', 'v3.0')
+  tag_v50 = model.tag.get_active_tag('devtable', 'gargantuan', 'v5.0')
+
+  # Build a manifest and assign it to both tags (this is allowed in the old model).
+  builder = DockerSchema1ManifestBuilder('devtable', 'gargantuan', 'sometag')
+  builder.add_layer('sha256:deadbeef', '{"id": "foo"}')
+  manifest = builder.build(docker_v2_signing_key)
+
+  manifest_1 = TagManifest.create(json_data=manifest.bytes, digest=manifest.digest,
+                                  tag=tag_v30)
+  manifest_2 = TagManifest.create(json_data=manifest.bytes, digest=manifest.digest,
+                                  tag=tag_v50)
+
+  # Backfill "both" manifests and ensure both are pointed to by a single resulting row.
+  assert backfill_manifest(manifest_1)
+  assert backfill_manifest(manifest_2)
+
+  map_row1 = TagManifestToManifest.get(tag_manifest=manifest_1)
+  map_row2 = TagManifestToManifest.get(tag_manifest=manifest_2)
+
+  assert map_row1.manifest == map_row2.manifest