From 401228183a63254a8edbded3693124ad466b185b Mon Sep 17 00:00:00 2001
From: Noah Misch <noah@leadboat.com>
Date: Sun, 23 Sep 2018 22:56:39 -0700
Subject: [PATCH] Initialize random() in bootstrap/stand-alone postgres and in
 initdb.

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
---
 src/backend/bootstrap/bootstrap.c | 8 ++++++++
 src/backend/tcop/postgres.c       | 8 ++++++++
 src/bin/initdb/initdb.c           | 3 +++
 3 files changed, 19 insertions(+)

diff --git a/src/backend/bootstrap/bootstrap.c b/src/backend/bootstrap/bootstrap.c
index 0e14b9272b5..0943a59d660 100644
--- 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
index 5903b228905..98180ed52ab 100644
--- 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
index 6de22ba65e1..fb16897adf0 100644
--- 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;