Do not fail on empty values for mutli-valued headers

Before this change user would receive a ValueError: not enough values to
unpack if the session property, role or prepared statements headers
existed but had empty values. While Trino itself doesn't send empty
headers it's possible for them to be present because of some proxy or
gateway sitting between client and the server.

tests/unit/test_http.py

@@ -11,7 +11,12 @@
 # limitations under the License.
 
 from trino import constants
-from trino.client import get_header_values, get_session_property_values
+from trino.client import (
+    get_header_values,
+    get_prepared_statement_values,
+    get_roles_values,
+    get_session_property_values,
+)
 
 
 def test_get_header_values():
@@ -24,3 +29,21 @@ def test_get_session_property_values():
     headers = {constants.HEADER_SET_SESSION: "a=1, b=2, c=more%3Dv1%2Cv2"}
     values = get_session_property_values(headers, constants.HEADER_SET_SESSION)
     assert values == [("a", "1"), ("b", "2"), ("c", "more=v1,v2")]
+
+
+def test_get_session_property_values_ignores_empty_values():
+    headers = {constants.HEADER_SET_SESSION: ""}
+    values = get_session_property_values(headers, constants.HEADER_SET_SESSION)
+    assert len(values) == 0
+
+
+def test_get_prepared_statement_values_ignores_empty_values():
+    headers = {constants.HEADER_SET_SESSION: ""}
+    values = get_prepared_statement_values(headers, constants.HEADER_SET_SESSION)
+    assert len(values) == 0
+
+
+def test_get_roles_values_ignores_empty_values():
+    headers = {constants.HEADER_SET_SESSION: ""}
+    values = get_roles_values(headers, constants.HEADER_SET_SESSION)
+    assert len(values) == 0

trino/client.py

@@ -225,23 +225,23 @@ def get_session_property_values(headers, header):
     kvs = get_header_values(headers, header)
     return [
         (k.strip(), urllib.parse.unquote_plus(v.strip()))
-        for k, v in (kv.split("=", 1) for kv in kvs)
+        for k, v in (kv.split("=", 1) for kv in kvs if kv)
     ]
 
 
 def get_prepared_statement_values(headers, header):
     kvs = get_header_values(headers, header)
     return [
         (k.strip(), urllib.parse.unquote_plus(v.strip()))
-        for k, v in (kv.split("=", 1) for kv in kvs)
+        for k, v in (kv.split("=", 1) for kv in kvs if kv)
     ]
 
 
 def get_roles_values(headers, header):
     kvs = get_header_values(headers, header)
     return [
         (k.strip(), urllib.parse.unquote_plus(v.strip()))
-        for k, v in (kv.split("=", 1) for kv in kvs)
+        for k, v in (kv.split("=", 1) for kv in kvs if kv)
     ]