printedparts 0003 was an unguarded ALTER, and no test could see it

Unlike its twenty siblings it ran a bare add_column plus create_index, so
`flask plugin upgrade-all` failed with "duplicate column name: gagelabtag" on
any database whose tables were built by create_all and stamped nowhere - and a
chain stops at its first error, so 0004 never applied either.

The index is guarded by COLUMN SET rather than by name: create_all names indexes
by SQLAlchemy's convention and the migration by this repo's, so a name-keyed
guard would have added a second index over the same column instead of skipping.

The test that should have caught it was restricted to plugins whose whole chain
is a single baseline, on the reasoning that a longer chain meets a second
problem - an ALTER whose column is already present - that guarding a create_table
does not fix. True, but the conclusion was wrong: the answer is to guard the
ALTERs too, not to stop testing the chains that have them. It now covers every
post-cutover chain, and was confirmed to fail against the unguarded migration
before the guard went in.

plugins/printedparts/migrations/versions/0003_gagelabtag.py

@@ -3,6 +3,15 @@
 The gage lab issues WJRP asset numbers for printed parts; the internal
 itemcode stays auto-generated, and this optional unique tag carries the
 lab's number. The kiosk resolves scans/typed digits against both.
+
+Guarded/idempotent: skips when the table is absent (plugin disabled) or the
+column already exists (e.g. a test DB built by db.create_all() from the model).
+Unguarded, this failed `flask plugin upgrade-all` on any create-all-built
+database and took 0004 down with it, since a chain stops at its first error.
+
+The index is matched by COLUMN SET, not by name: create_all names indexes by
+SQLAlchemy's convention and this migration by the repo's, so a name-keyed guard
+would add a second index over the same column rather than skip.
 """
 from alembic import op
 import sqlalchemy as sa
@@ -15,13 +24,31 @@ branch_labels = None
 depends_on = None
 
 
+def _indexed(inspector):
+    return any(tuple(ix.get('column_names') or ()) == ('gagelabtag',)
+               for ix in inspector.get_indexes('printeditems'))
+
+
 def upgrade():
+    inspector = sa.inspect(op.get_bind())
+    if 'printeditems' not in inspector.get_table_names():
+        return
+    columns = {column['name'] for column in inspector.get_columns('printeditems')}
+    if 'gagelabtag' not in columns:
         op.add_column('printeditems',
                       sa.Column('gagelabtag', sa.String(length=50), nullable=True))
+    inspector = sa.inspect(op.get_bind())
+    if not _indexed(inspector):
         op.create_index('ix_printeditems_gagelabtag', 'printeditems',
                         ['gagelabtag'], unique=True)
 
 
 def downgrade():
+    inspector = sa.inspect(op.get_bind())
+    if 'printeditems' not in inspector.get_table_names():
+        return
+    if _indexed(inspector):
         op.drop_index('ix_printeditems_gagelabtag', table_name='printeditems')
+    columns = {column['name'] for column in inspector.get_columns('printeditems')}
+    if 'gagelabtag' in columns:
         op.drop_column('printeditems', 'gagelabtag')

tests/test_plugin_migrations.py

@@ -282,18 +282,21 @@ def test_upgrade_all_on_fresh_db_is_clean_and_idempotent(tmp_path, monkeypatch):
 
 
 
-# Plugins whose whole chain is a single baseline. Their tables existing already
+# EVERY post-cutover chain, not just the single-baseline ones. This was once
-# is the ONLY divergence possible between schema and stamp, so upgrade-all must
+# restricted to plugins whose whole chain is one baseline, on the reasoning that
-# handle it end to end. A longer chain can also meet an ALTER whose column is
+# a longer chain meets a second problem - an ALTER whose column is already
-# already present, which no amount of guarding a create_table fixes - see the
+# present - that guarding a create_table does not fix. True, but the conclusion
-# test below for why that is a different problem.
+# was wrong: the answer is to guard the ALTERs too, not to stop testing the
-SINGLE_BASELINE_PLUGINS = tuple(
+# chains that have them. While this was narrowed, printedparts 0003 sat
-    plugin for plugin in POST_CUTOVER_PLUGINS
+# unguarded for a month and would fail upgrade-all on any create-all-built
-    if len(list((PLUGINS_DIR / plugin / 'migrations' / 'versions').glob('0*.py'))) == 1
+# database, taking 0004 down with it, with no test able to see it.
-)
+#
+# The invariant is now the whole one: a post-cutover chain must run to its head
+# against a database whose tables were built from the models and stamped
+# nowhere, and must do it twice.
 
 
-@pytest.mark.parametrize('plugin', SINGLE_BASELINE_PLUGINS)
+@pytest.mark.parametrize('plugin', POST_CUTOVER_PLUGINS)
 def test_baseline_skips_tables_that_already_exist(plugin, tmp_path, monkeypatch):
     """A baseline must stamp, not fail, when its table is already there.