Propagate query-arg-only URL to psycopg2; don't send blank host

Fixed regression from release 1.3.2 caused by :ticket:`4562` where a URL
that contained only a query string and no hostname, such as for the
purposes of specifying a service file with connection information, would no
longer be propagated to psycopg2 properly.   The change in :ticket:`4562`
has been adjusted to further suit psycopg2's exact requirements, which is
that if there are any connection parameters whatsoever, the "dsn" parameter
is no longer required, so in this case the query string parameters are
passed alone.

Fixes: #4601
Change-Id: Ic29a8b77bcf50ee996968bab25aaac7ae4bfc26f

3 files changed, 39 insertions, 1 deletions

diff --git a/doc/build/changelog/unreleased_13/4601.rst b/doc/build/changelog/unreleased_13/4601.rst
new file mode 100644
index 000000000..950d74650
--- /dev/null
+++ b/doc/build/changelog/unreleased_13/4601.rst
@@ -0,0 +1,12 @@
+.. change::
+    :tags: bug, postgresql
+    :tickets: 4601
+
+    Fixed regression from release 1.3.2 caused by :ticket:`4562` where a URL
+    that contained only a query string and no hostname, such as for the
+    purposes of specifying a service file with connection information, would no
+    longer be propagated to psycopg2 properly.   The change in :ticket:`4562`
+    has been adjusted to further suit psycopg2's exact requirements, which is
+    that if there are any connection parameters whatsoever, the "dsn" parameter
+    is no longer required, so in this case the query string parameters are
+    passed alone.
\ No newline at end of file
diff --git a/lib/sqlalchemy/dialects/postgresql/psycopg2.py b/lib/sqlalchemy/dialects/postgresql/psycopg2.py
index 199c1c776..fb127f1c7 100644
--- a/lib/sqlalchemy/dialects/postgresql/psycopg2.py
+++ b/lib/sqlalchemy/dialects/postgresql/psycopg2.py
@@ -769,8 +769,13 @@ class PGDialect_psycopg2(PGDialect):
             # send individual dbname, user, password, host, port
             # parameters to psycopg2.connect()
             return ([], opts)
+        elif url.query:
+            # any other connection arguments, pass directly
+            opts.update(url.query)
+            return ([], opts)
         else:
-            # send a blank string for "dsn" to psycopg2.connect()
+            # no connection arguments whatsoever; psycopg2.connect()
+            # requires that "dsn" be present as a blank string.
             return ([''], opts)
 
     def is_disconnect(self, e, connection, cursor):
diff --git a/test/dialect/postgresql/test_dialect.py b/test/dialect/postgresql/test_dialect.py
index 8c19e31bb..1a735c41d 100644
--- a/test/dialect/postgresql/test_dialect.py
+++ b/test/dialect/postgresql/test_dialect.py
@@ -129,6 +129,27 @@ class DialectTest(fixtures.TestBase):
         eq_(cargs, [])
         eq_(cparams, {"host": "host"})
 
+    def test_psycopg2_empty_connection_string_w_query_one(self):
+        dialect = psycopg2_dialect.dialect()
+        u = url.make_url("postgresql:///?service=swh-log")
+        cargs, cparams = dialect.create_connect_args(u)
+        eq_(cargs, [])
+        eq_(cparams, {"service": "swh-log"})
+
+    def test_psycopg2_empty_connection_string_w_query_two(self):
+        dialect = psycopg2_dialect.dialect()
+        u = url.make_url("postgresql:///?any_random_thing=yes")
+        cargs, cparams = dialect.create_connect_args(u)
+        eq_(cargs, [])
+        eq_(cparams, {"any_random_thing": "yes"})
+
+    def test_psycopg2_nonempty_connection_string_w_query(self):
+        dialect = psycopg2_dialect.dialect()
+        u = url.make_url("postgresql://somehost/?any_random_thing=yes")
+        cargs, cparams = dialect.create_connect_args(u)
+        eq_(cargs, [])
+        eq_(cparams, {"host": "somehost", "any_random_thing": "yes"})
+
 
 class BatchInsertsTest(fixtures.TablesTest):
     __only_on__ = "postgresql+psycopg2"