Fix DB migrations after alembic integration

With a change to swap sqlalchemy-migrate with alembic [1]
a `db_sync --check` was broken.

This is due to both `upgrades.get_db_version` and
`upgrades.get_current_heads` are actually checking "current" state of
the dabase by calling _get_current_heads[2][3], while obvious intention
was to compare intended state with current state.

With that we're introducing upgrade.get_head_revisions which will fetch
revisions not from the database, but from the environment [4]

As a result `db_sync --check` does compare desired state of the DB with
actual state and exists with corresponsive status again.

[1] f174b4fa7c
[2] https://opendev.org/openstack/keystone/src/branch/master/keystone/common/sql/upgrades.py#L147
[3] https://opendev.org/openstack/keystone/src/branch/master/keystone/common/sql/upgrades.py#L191
[4] https://alembic.sqlalchemy.org/en/latest/api/runtime.html#alembic.runtime.environment.EnvironmentContext.get_head_revisions

Closes-Bug: #2080542
Change-Id: I854d37e3b4a34a7880f157564466bde61a3f886a
(cherry picked from commit 5125d9feed)

keystone/cmd/cli.py

@@ -366,7 +366,7 @@ class DbSync(BaseApp):
         except db_exception.DBMigrationError:
             contract_version = None
 
-        heads = upgrades.get_current_heads()
+        heads = upgrades.get_head_revisions()
 
         if (
             upgrades.EXPAND_BRANCH not in heads or

keystone/common/sql/upgrades.py

@@ -19,6 +19,7 @@ import os
 from alembic import command as alembic_api
 from alembic import config as alembic_config
 from alembic import migration as alembic_migration
+from alembic.runtime import environment as alembic_environment
 from alembic import script as alembic_script
 from oslo_db import exception as db_exception
 from oslo_log import log as logging
@@ -111,6 +112,11 @@ def _get_current_heads(engine, config):
         context = alembic_migration.MigrationContext.configure(conn)
         heads = context.get_current_heads()
 
+    heads_map = _get_head_maps(heads, script)
+    return heads_map
+
+
+def _get_head_maps(heads, script):
     heads_map = {}
 
     for head in heads:
@@ -146,6 +152,17 @@ def get_current_heads():
     return heads
 
 
+def get_head_revisions():
+    """Get the available head for each the expand and contract branches."""
+    config = _find_alembic_conf()
+    script = alembic_script.ScriptDirectory.from_config(config)
+    context = alembic_environment.EnvironmentContext(config, script)
+    heads = context.get_head_revisions()
+    heads_map = _get_head_maps(heads, script)
+
+    return heads_map
+
+
 def _is_database_under_alembic_control(engine):
     with engine.connect() as conn:
         context = alembic_migration.MigrationContext.configure(conn)