Add hdb_catalog.current_setting abstraction for reading Hasura settings

As the comment in the function’s definition explains, this is needed to
work around an awkward Postgres behavior.

server/src-lib/Hasura/RQL/DML/Internal.hs

@@ -226,7 +226,7 @@ sessVarFromCurrentSetting' ty sessVar =
                  [currentSession, S.SELit $ sessionVariableToText sessVar]
 
 currentSession :: S.SQLExp
-currentSession = S.SEUnsafe "current_setting('hasura.user')::json"
+currentSession = S.SEUnsafe "hdb_catalog.current_setting('user')"
 
 checkSelPerm
   :: (UserInfoM m, QErrM m, CacheRM m)

server/src-rsr/catalog_version.txt

@@ -1 +1 @@
-37
+38

server/src-rsr/initialise.sql

@@ -596,46 +596,55 @@ CREATE VIEW hdb_catalog.hdb_function_info_agg AS (
         hdb_catalog.hdb_function_agg
 );
 
-CREATE OR REPLACE FUNCTION
-  hdb_catalog.insert_event_log(schema_name text, table_name text, trigger_name text, op text, row_data json)
-  RETURNS text AS $$
+CREATE FUNCTION hdb_catalog.current_setting(setting_name text) RETURNS json
+  LANGUAGE plpgsql AS $$
   DECLARE
-    id text;
-    payload json;
-    session_variables json;
     server_version_num int;
-    trace_context json;
+    setting_value text;
   BEGIN
-    id := gen_random_uuid();
     server_version_num := current_setting('server_version_num');
     IF server_version_num >= 90600 THEN
-      session_variables := current_setting('hasura.user', 't');
-      trace_context := current_setting('hasura.tracecontext', 't');
+      setting_value := current_setting('hasura.' || setting_name, true);
     ELSE
+      -- We still support Postgres 9.5, which doesn’t support the second
+      -- argument to current_setting, so emulate it by just catching the error.
       BEGIN
-        session_variables := current_setting('hasura.user');
-      EXCEPTION WHEN OTHERS THEN
-                  session_variables := NULL;
-      END;
-      BEGIN
-        trace_context := current_setting('hasura.tracecontext');
+        setting_value := current_setting('hasura.' || setting_name);
       EXCEPTION WHEN OTHERS THEN
-        trace_context := NULL;
+        setting_value := NULL;
       END;
     END IF;
+    -- Note: we test for the empty string here to work around a Postgres bug.
+    -- When SET LOCAL is used to set the value of a setting that does not
+    -- currently exist, Postgres implicitly creates a /session-wide/ definition
+    -- of the setting with the empty string as its value. This is annoying, but
+    -- fortunately an empty string is not a legal JSON value, so it always means
+    -- no value was set in the current transaction.
+    IF setting_value IS NULL OR setting_value = '' THEN
+      RETURN NULL;
+    ELSE
+      RETURN setting_value::json;
+    END IF;
+  END $$;
+
+CREATE OR REPLACE FUNCTION hdb_catalog.insert_event_log(schema_name text, table_name text, trigger_name text, op text, row_data json)
+  RETURNS text LANGUAGE plpgsql AS $$
+  DECLARE
+    id text;
+    payload json;
+  BEGIN
+    id := gen_random_uuid();
     payload := json_build_object(
       'op', op,
       'data', row_data,
-      'session_variables', session_variables,
-      'trace_context', trace_context
+      'session_variables', hdb_catalog.current_setting('user'),
+      'trace_context', hdb_catalog.current_setting('tracecontext')
     );
     INSERT INTO hdb_catalog.event_log
                 (id, schema_name, table_name, trigger_name, payload)
-    VALUES
-    (id, schema_name, table_name, trigger_name, payload);
+      VALUES (id, schema_name, table_name, trigger_name, payload);
     RETURN id;
-  END;
-$$ LANGUAGE plpgsql;
+  END $$;
 
 CREATE TABLE hdb_catalog.hdb_query_collection
 (

server/src-rsr/migrations/37_to_38.sql

@@ -0,0 +1,49 @@
+CREATE FUNCTION hdb_catalog.current_setting(setting_name text) RETURNS json
+  LANGUAGE plpgsql AS $$
+  DECLARE
+    server_version_num int;
+    setting_value text;
+  BEGIN
+    server_version_num := current_setting('server_version_num');
+    IF server_version_num >= 90600 THEN
+      setting_value := current_setting('hasura.' || setting_name, true);
+    ELSE
+      -- We still support Postgres 9.5, which doesn’t support the second
+      -- argument to current_setting, so emulate it by just catching the error.
+      BEGIN
+        setting_value := current_setting('hasura.' || setting_name);
+      EXCEPTION WHEN OTHERS THEN
+        setting_value := NULL;
+      END;
+    END IF;
+    -- Note: we test for the empty string here to work around a Postgres bug.
+    -- When SET LOCAL is used to set the value of a setting that does not
+    -- currently exist, Postgres implicitly creates a /session-wide/ definition
+    -- of the setting with the empty string as its value. This is annoying, but
+    -- fortunately an empty string is not a legal JSON value, so it always means
+    -- no value was set in the current transaction.
+    IF setting_value IS NULL OR setting_value = '' THEN
+      RETURN NULL;
+    ELSE
+      RETURN setting_value::json;
+    END IF;
+  END $$;
+
+CREATE OR REPLACE FUNCTION hdb_catalog.insert_event_log(schema_name text, table_name text, trigger_name text, op text, row_data json)
+  RETURNS text LANGUAGE plpgsql AS $$
+  DECLARE
+    id text;
+    payload json;
+  BEGIN
+    id := gen_random_uuid();
+    payload := json_build_object(
+      'op', op,
+      'data', row_data,
+      'session_variables', hdb_catalog.current_setting('user'),
+      'trace_context', hdb_catalog.current_setting('tracecontext')
+    );
+    INSERT INTO hdb_catalog.event_log
+                (id, schema_name, table_name, trigger_name, payload)
+      VALUES (id, schema_name, table_name, trigger_name, payload);
+    RETURN id;
+  END $$;

server/src-rsr/migrations/38_to_37.sql

@@ -0,0 +1,41 @@
+CREATE OR REPLACE FUNCTION hdb_catalog.insert_event_log(schema_name text, table_name text, trigger_name text, op text, row_data json)
+  RETURNS text AS $$
+  DECLARE
+    id text;
+    payload json;
+    session_variables json;
+    server_version_num int;
+    trace_context json;
+  BEGIN
+    id := gen_random_uuid();
+    server_version_num := current_setting('server_version_num');
+    IF server_version_num >= 90600 THEN
+      session_variables := current_setting('hasura.user', 't');
+      trace_context := current_setting('hasura.tracecontext', 't');
+    ELSE
+      BEGIN
+        session_variables := current_setting('hasura.user');
+      EXCEPTION WHEN OTHERS THEN
+                  session_variables := NULL;
+      END;
+      BEGIN
+        trace_context := current_setting('hasura.tracecontext');
+      EXCEPTION WHEN OTHERS THEN
+        trace_context := NULL;
+      END;
+    END IF;
+    payload := json_build_object(
+      'op', op,
+      'data', row_data,
+      'session_variables', session_variables,
+      'trace_context', trace_context
+    );
+    INSERT INTO hdb_catalog.event_log
+                (id, schema_name, table_name, trigger_name, payload)
+    VALUES
+    (id, schema_name, table_name, trigger_name, payload);
+    RETURN id;
+  END;
+$$ LANGUAGE plpgsql;
+
+DROP FUNCTION hdb_catalog.current_setting(text);