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

nmisch · nmisch · commit 401228183a63 · 2018-09-23T22:56:57.000-07:00
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
@@ -197,6 +197,14 @@ 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
@@ -3611,6 +3611,14 @@ 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);
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
@@ -1055,6 +1055,9 @@ choose_dsm_implementation(void)
 #ifdef HAVE_SHM_OPEN
 	int			ntries = 10;
 
+	/* Initialize random(); this function is its only user in this program. */
+	srandom((unsigned int) (getpid() ^ time(NULL)));
+
 	while (ntries > 0)
 	{
 		uint32		handle;

Original file line number	Diff line number	Diff line change
`@@ -1055,6 +1055,9 @@ choose_dsm_implementation(void)`
`1055`	`1055`	`#ifdef HAVE_SHM_OPEN`
`1056`	`1056`	`int ntries = 10;`
`1057`	`1057`
	`1058`	`+ /* Initialize random(); this function is its only user in this program. */`
	`1059`	`+ srandom((unsigned int) (getpid() ^ time(NULL)));`
	`1060`	`+`
`1058`	`1061`	`while (ntries > 0)`
`1059`	`1062`	`{`
`1060`	`1063`	`uint32 handle;`