Increase the number of different values used when seeding random().

When a backend process is forked, we initialize the system's random number
generator with srandom(). The seed used is derived from the backend's pid
and the timestamp. However, we only used the microseconds part of the
timestamp, and it was XORed with the pid, so the total range of different
seed values chosen was 0-999999. That's quite limited.

Change the code to also use the seconds part of the timestamp in the seed,
and shift the microseconds so that all 32 bits of the seed are used.

Honza Horak

src/backend/postmaster/postmaster.c

@@ -4033,9 +4033,9 @@ BackendRun(Port *port)
 	 */
 	random_seed = 0;
 	random_start_time.tv_usec = 0;
-	/* slightly hacky way to get integer microseconds part of timestamptz */
+	/* slightly hacky way to convert timestamptz into integers */
 	TimestampDifference(0, port->SessionStartTime, &secs, &usecs);
-	srandom((unsigned int) (MyProcPid ^ usecs));
+	srandom((unsigned int) (MyProcPid ^ (usecs << 12) ^ secs));
 
 	/*
 	 * Now, build the argv vector that will be given to PostgresMain.