Initialize random() in bootstrap/stand-alone postgres and in initdb.

nmisch · nmisch · commit 402da7054f34 · 2018-09-23T22:57:43.000-07:00
This removes a difference between the standard IsUnderPostmaster execution environment and that of --boot and --single. In a stand-alone backend, "SELECT random()" always started at the same seed. On a system capable of using posix shared memory, initdb could still conclude "selecting dynamic shared memory implementation ... sysv". Crashed --boot or --single postgres processes orphaned shared memory objects having names that collided with the not-actually-random names that initdb probed. The sysv fallback appeared after ten crashes of --boot or --single postgres. Since --boot and --single are rare in production use, systems used for PostgreSQL development are the principal candidate to notice this symptom. Back-patch to 9.3 (all supported versions). PostgreSQL 9.4 introduced dynamic shared memory, but 9.3 does share the "SELECT random()" problem. Reviewed by Tom Lane and Kyotaro HORIGUCHI. Discussion: https://postgr.es/m/20180915221546.GA3159382@rfd.leadboat.com
diff --git a/src/backend/bootstrap/bootstrap.c b/src/backend/bootstrap/bootstrap.c
@@ -202,6 +202,14 @@ AuxiliaryProcessMain(int argc, char *argv[])
 
 	MyStartTime = time(NULL);
 
+	/*
+	 * Initialize random() for the first time, like PostmasterMain() would.
+	 * In a regular IsUnderPostmaster backend, BackendRun() computes a
+	 * high-entropy seed before any user query.  Fewer distinct initial seeds
+	 * can occur here.
+	 */
+	srandom((unsigned int) (MyProcPid ^ MyStartTime));
+
 	/* Compute paths, if we didn't inherit them from postmaster */
 	if (my_exec_path[0] == '\0')
 	{
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
@@ -3620,6 +3620,14 @@ PostgresMain(int argc, char *argv[],
 		MyProcPid = getpid();
 
 		MyStartTime = time(NULL);
+
+		/*
+		 * Initialize random() for the first time, like PostmasterMain()
+		 * would.  In a regular IsUnderPostmaster backend, BackendRun()
+		 * computes a high-entropy seed before any user query.  Fewer distinct
+		 * initial seeds can occur here.
+		 */
+		srandom((unsigned int) (MyProcPid ^ MyStartTime));
 	}
 
 	SetProcessingMode(InitProcessing);
