use real time for events

NOW() is time of transaction start. For long transaction, there could be
race and some other transactions which ended earlier can have newer
events. clock_timestamp returns real time (changes during transaction),
so it should be better for this case.

Fixes: https://pagure.io/koji/issue/1747

docs/schema-upgrade-1.20-1.21.sql

@@ -0,0 +1,16 @@
+-- upgrade script to migrate the Koji database schema
+-- from version 1.20 to 1.21
+
+
+BEGIN;
+
+-- make better events
+ALTER TABLE events ALTER COLUMN time SET NOT NULL;
+ALTER TABLE events ALTER COLUMN time SET DEFAULT clock_timestamp();
+
+CREATE OR REPLACE FUNCTION get_event() RETURNS INTEGER AS '
+    INSERT INTO events (time) VALUES (clock_timestamp());
+    SELECT currval(''events_id_seq'')::INTEGER;
+' LANGUAGE SQL;
+
+COMMIT;

docs/schema.sql

@@ -7,12 +7,12 @@ BEGIN WORK;
 -- in the event that the system clock rolls back, event_ids will retain proper sequencing
 CREATE TABLE events (
 	id SERIAL NOT NULL PRIMARY KEY,
-	time TIMESTAMP NOT NULL DEFAULT NOW()
+	time TIMESTAMP NOT NULL DEFAULT clock_timestamp()
 ) WITHOUT OIDS;
 
 -- A function that creates an event and returns the id, used as DEFAULT value for versioned tables
 CREATE FUNCTION get_event() RETURNS INTEGER AS '
-	INSERT INTO events (time) VALUES (''now'');
+	INSERT INTO events (time) VALUES (clock_timestamp());
 	SELECT currval(''events_id_seq'')::INTEGER;
 ' LANGUAGE SQL;