turn pg provision error into a warning

We haven't had any real cases of the PG "cant drop tables"
condition since this error was first introduced; instead we
seem to get it for a non-critical query during pool reconnect
tests, and I have not been able to isolate what is causing it.
Therefore turn the error into a new class of warning that can
emit within the test suite without failing the test, so that
if we do get a real PG drop timeout, the warning will be there
to show us what the query was in which it was stuck.

Change-Id: I1a9b3c4f7a25b7b9c1af722a721fc44ad5575b0f

3 files changed, 18 insertions, 5 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/provision.py b/lib/sqlalchemy/dialects/postgresql/provision.py
index 9196337ba..68a01e483 100644
--- a/lib/sqlalchemy/dialects/postgresql/provision.py
+++ b/lib/sqlalchemy/dialects/postgresql/provision.py
@@ -3,6 +3,7 @@ import time
 from ... import exc
 from ... import inspect
 from ... import text
+from ...testing import warn_test_suite
 from ...testing.provision import create_db
 from ...testing.provision import drop_all_schema_objects_post_tables
 from ...testing.provision import drop_all_schema_objects_pre_tables
@@ -118,8 +119,9 @@ def prepare_for_drop_tables(config, connection):
         "and pid != pg_backend_pid()"
     )
     rows = result.all()  # noqa
-    assert not rows, (
-        "PostgreSQL may not be able to DROP tables due to "
-        "idle in transaction: %s"
-        % ("; ".join(row._mapping["query"] for row in rows))
-    )
+    if rows:
+        warn_test_suite(
+            "PostgreSQL may not be able to DROP tables due to "
+            "idle in transaction: %s"
+            % ("; ".join(row._mapping["query"] for row in rows))
+        )
diff --git a/lib/sqlalchemy/testing/__init__.py b/lib/sqlalchemy/testing/__init__.py
index a311efa74..1bb2943e1 100644
--- a/lib/sqlalchemy/testing/__init__.py
+++ b/lib/sqlalchemy/testing/__init__.py
@@ -74,6 +74,7 @@ from .util import rowset
 from .util import run_as_contextmanager
 from .util import teardown_events
 from .warnings import assert_warnings
+from .warnings import warn_test_suite
 
 
 def against(*queries):
diff --git a/lib/sqlalchemy/testing/warnings.py b/lib/sqlalchemy/testing/warnings.py
index 8810dff5d..fd7748050 100644
--- a/lib/sqlalchemy/testing/warnings.py
+++ b/lib/sqlalchemy/testing/warnings.py
@@ -11,6 +11,15 @@ import warnings
 
 from . import assertions
 from .. import exc as sa_exc
+from ..util.langhelpers import _warnings_warn
+
+
+class SATestSuiteWarning(sa_exc.SAWarning):
+    """warning for a condition detected during tests that is non-fatal"""
+
+
+def warn_test_suite(message):
+    _warnings_warn(message, category=SATestSuiteWarning)
 
 
 def setup_filters():
@@ -21,6 +30,7 @@ def setup_filters():
     )
     warnings.filterwarnings("error", category=sa_exc.SADeprecationWarning)
     warnings.filterwarnings("error", category=sa_exc.SAWarning)
+    warnings.filterwarnings("always", category=SATestSuiteWarning)
 
     # some selected deprecations...
     warnings.filterwarnings("error", category=DeprecationWarning)