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.

data/database.py

@@ -1435,7 +1435,7 @@ class TagManifest(BaseModel):
 class TagManifestToManifest(BaseModel):
   """ NOTE: Only used for the duration of the migrations. """
   tag_manifest = ForeignKeyField(TagManifest, index=True, unique=True)
-  manifest = ForeignKeyField(Manifest, index=True, unique=True)
+  manifest = ForeignKeyField(Manifest, index=True)
   broken = BooleanField(index=True, default=False)
 
 

data/migrations/versions/13411de1c0ff_remove_unique_from_tagmanifesttomanifest.py

@@ -0,0 +1,43 @@
+"""Remove unique from TagManifestToManifest
+
+Revision ID: 13411de1c0ff
+Revises: 654e6df88b71
+Create Date: 2018-08-19 23:30:24.969549
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '13411de1c0ff'
+down_revision = '654e6df88b71'
+
+from alembic import op
+import sqlalchemy as sa
+
+def upgrade(tables, tester):
+    # Note: Because of a restriction in MySQL, we cannot simply remove the index and re-add
+    # it without the unique=False, nor can we simply alter the index. To make it work, we'd have to
+    # remove the primary key on the field, so instead we simply drop the table entirely and
+    # recreate it with the modified index. The backfill will re-fill this in.
+    op.drop_table('tagmanifesttomanifest')
+
+    op.create_table('tagmanifesttomanifest',
+    sa.Column('id', sa.Integer(), nullable=False),
+    sa.Column('tag_manifest_id', sa.Integer(), nullable=False),
+    sa.Column('manifest_id', sa.Integer(), nullable=False),
+    sa.Column('broken', sa.Boolean(), nullable=False, server_default=sa.sql.expression.false()),
+    sa.ForeignKeyConstraint(['manifest_id'], ['manifest.id'], name=op.f('fk_tagmanifesttomanifest_manifest_id_manifest')),
+    sa.ForeignKeyConstraint(['tag_manifest_id'], ['tagmanifest.id'], name=op.f('fk_tagmanifesttomanifest_tag_manifest_id_tagmanifest')),
+    sa.PrimaryKeyConstraint('id', name=op.f('pk_tagmanifesttomanifest'))
+    )
+    op.create_index('tagmanifesttomanifest_broken', 'tagmanifesttomanifest', ['broken'], unique=False)
+    op.create_index('tagmanifesttomanifest_manifest_id', 'tagmanifesttomanifest', ['manifest_id'], unique=False)
+    op.create_index('tagmanifesttomanifest_tag_manifest_id', 'tagmanifesttomanifest', ['tag_manifest_id'], unique=True)
+
+    tester.populate_table('tagmanifesttomanifest', [
+        ('manifest_id', tester.TestDataType.Foreign('manifest')),
+        ('tag_manifest_id', tester.TestDataType.Foreign('tagmanifest')),
+    ])
+
+
+def downgrade(tables, tester):
+    pass