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

nmisch · nmisch · commit d18f6674bd60 · 2018-09-23T22:56:39.000-07:00
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/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
@@ -325,6 +325,14 @@ InitStandaloneProcess(const char *argv0)
 
 	MyStartTime = time(NULL);	/* set our start time in case we call elog */
 
+	/*
+	 * 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));
+
 	/* Initialize process-local latch support */
 	InitializeLatchSupport();
 	MyLatch = &LocalLatchData;
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
@@ -923,6 +923,9 @@ 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
`@@ -923,6 +923,9 @@ choose_dsm_implementation(void)`
`923`	`923`	`#ifdef HAVE_SHM_OPEN`
`924`	`924`	`int ntries = 10;`
`925`	`925`
	`926`	`+ /* Initia 5295 lize random(); this function is its only user in this program. */`
	`927`	`+ srandom((unsigned int) (getpid() ^ time(NULL)));`
	`928`	`+`
`926`	`929`	`while (ntries > 0)`
`927`	`930`	`{`
`928`	`931`	`uint32 handle;`