Prevent inconsistency between release and release_files tables.

The work in PR pypi#1448 was meant to replicate the information of the
`requires_python` of the `release` table to the `release_files` table
for efficiency when generating the list of available packages for pip.

While the work on pypi#1448 seem sufficient for warehouse itself, it need to
consider that legacy-pypi also access the same database, and legacy-pypi
violate some constraint.

In particular when using `setup.py register` followed by `twine upload`,
the file upload insert files into `release_files` after inserting into
`releases`. Thus the value in releases is not propagated, leading to an
inconsistency and a listing in pip missing information about
python-version compatibility.

While I doubt there are any packages released between the merge of pypi#1448
and a fix, this update the tables, and bind an already existing trigger
to update the information during insertion in `release_files`

warehouse/migrations/versions/99291f0fe9c2_update_release_file_on_insert.py

@@ -0,0 +1,60 @@
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""
+
+The release.requires_python and release_files.requires_python can be out of
+sync if a file is uploaded after the release has been created. It's
+requires_python field will stay empty.
+
+We need to :
+ - Fix potentially existing mistakes
+ - Create a trigger that ensure consistency
+
+Revision ID: 99291f0fe9c2
+Revises: be4cf6b58557
+Create Date: 2016-12-02 00:58:53.109880
+"""
+
+from alembic import op
+
+
+revision = '99291f0fe9c2'
+down_revision = 'be4cf6b58557'
+
+
+def upgrade():
+
+    op.execute(
+         """ UPDATE release_files
+             SET requires_python = releases.requires_python
+             FROM releases
+             WHERE
+                 release_files.name=releases.name
+                 AND release_files.version=releases.version;
+          """
+      )
+
+    # Establish a trigger such that on INSERT on release_files.
+    # The requires_python value is no supposed to be set directly here
+    # and UPDATES of the `releases` table already propagate.
+    # Also triggering on UPDATE might create a recursion.
+    op.execute(
+        """ CREATE TRIGGER release_files_requires_python
+              AFTER INSERT ON release_files
+              FOR EACH ROW
+                  EXECUTE PROCEDURE update_release_files_requires_python();
+        """)
+
+
+def downgrade():
+    op.execute("DROP TRIGGER release_files_requires_python ON release_files")