Refactor pid, random seed and start time initialization.

Background workers, including parallel workers, were generating
the same sequence of numbers in random().  This showed up as DSM
handle collisions when Parallel Hash created multiple segments,
but any code that calls random() in background workers could be
affected if it cares about different backends generating different
numbers.

Repair by making sure that all new processes initialize the seed
at the same time as they set MyProcPid and MyStartTime in a new
function InitProcessGlobals(), called by the postmaster, its
children and also standalone processes.  Also add a new high
resolution MyStartTimestamp as a potentially useful by-product,
and remove SessionStartTime from struct Port as it is now
redundant.

No back-patch for now, as the known consequences so far are just
a bunch of harmless shm_open(O_EXCL) collisions.

Author: Thomas Munro
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CAEepm%3D2eJj_6%3DB%2B2tEpGu2nf1BjthCf9nXXUouYvJJ4C5WSwhg%40mail.gmail.com

src/backend/postmaster/pgstat.c

@@ -2784,21 +2784,13 @@ pgstat_initialize(void)
 void
 pgstat_bestart(void)
 {
-	TimestampTz proc_start_timestamp;
 	SockAddr	clientaddr;
 	volatile PgBackendStatus *beentry;
 
 	/*
 	 * To minimize the time spent modifying the PgBackendStatus entry, fetch
 	 * all the needed data first.
-	 *
-	 * If we have a MyProcPort, use its session start time (for consistency,
-	 * and to save a kernel call).
 	 */
-	if (MyProcPort)
-		proc_start_timestamp = MyProcPort->SessionStartTime;
-	else
-		proc_start_timestamp = GetCurrentTimestamp();
 
 	/*
 	 * We may not have a MyProcPort (eg, if this is the autovacuum process).
@@ -2883,7 +2875,7 @@ pgstat_bestart(void)
 	} while ((beentry->st_changecount & 1) == 0);
 
 	beentry->st_procpid = MyProcPid;
-	beentry->st_proc_start_timestamp = proc_start_timestamp;
+	beentry->st_proc_start_timestamp = MyStartTimestamp;
 	beentry->st_activity_start_timestamp = 0;
 	beentry->st_state_start_timestamp = 0;
 	beentry->st_xact_start_timestamp = 0;

src/backend/postmaster/postmaster.c

@@ -129,6 +129,7 @@
 #include "utils/pidfile.h"
 #include "utils/ps_status.h"
 #include "utils/timeout.h"
+#include "utils/timestamp.h"
 #include "utils/varlena.h"
 
 #ifdef EXEC_BACKEND
@@ -581,9 +582,9 @@ PostmasterMain(int argc, char *argv[])
 	int			i;
 	char	   *output_config_variable = NULL;
 
-	MyProcPid = PostmasterPid = getpid();
+	InitProcessGlobals();
 
-	MyStartTime = time(NULL);
+	PostmasterPid = MyProcPid;
 
 	IsPostmasterEnvironment = true;
 
@@ -597,16 +598,6 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	umask(PG_MODE_MASK_OWNER);
 
-	/*
-	 * Initialize random(3) so we don't get the same values in every run.
-	 *
-	 * Note: the seed is pretty predictable from externally-visible facts such
-	 * as postmaster start time, so avoid using random() for security-critical
-	 * random values during postmaster startup.  At the time of first
-	 * connection, PostmasterRandom will select a hopefully-more-random seed.
-	 */
-	srandom((unsigned int) (MyProcPid ^ MyStartTime));
-
 	/*
 	 * By default, palloc() requests in the postmaster will be allocated in
 	 * the PostmasterContext, which is space that can be recycled by backends.
@@ -2513,6 +2504,32 @@ ClosePostmasterPorts(bool am_syslogger)
 }
 
 
+/*
+ * InitProcessGlobals -- set MyProcPid, MyStartTime[stamp], random seeds
+ *
+ * Called early in every backend.
+ */
+void
+InitProcessGlobals(void)
+{
+	MyProcPid = getpid();
+	MyStartTimestamp = GetCurrentTimestamp();
+	MyStartTime = timestamptz_to_time_t(MyStartTimestamp);
+
+	/*
+	 * Don't want backend to be able to see the postmaster random number
+	 * generator state.  We have to clobber the static random_seed.
+	 */
+#ifndef HAVE_STRONG_RANDOM
+	random_seed = 0;
+	random_start_time.tv_usec = 0;
+#endif
+
+	/* Set a different seed for random() in every backend. */
+	srandom((unsigned int) MyProcPid ^ (unsigned int) MyStartTimestamp);
+}
+
+
 /*
  * reset_shared -- reset shared memory and semaphores
  */
@@ -4154,10 +4171,6 @@ BackendInitialize(Port *port)
 	/* This flag will remain set until InitPostgres finishes authentication */
 	ClientAuthInProgress = true;	/* limit visibility of log messages */
 
-	/* save process start time */
-	port->SessionStartTime = GetCurrentTimestamp();
-	MyStartTime = timestamptz_to_time_t(port->SessionStartTime);
-
 	/* set these to empty in case they are needed before we set them up */
 	port->remote_host = "";
 	port->remote_port = "";
@@ -4315,23 +4328,8 @@ BackendRun(Port *port)
 	char	  **av;
 	int			maxac;
 	int			ac;
-	long		secs;
-	int			usecs;
 	int			i;
 
-	/*
-	 * Don't want backend to be able to see the postmaster random number
-	 * generator state.  We have to clobber the static random_seed *and* start
-	 * a new random sequence in the random() library function.
-	 */
-#ifndef HAVE_STRONG_RANDOM
-	random_seed = 0;
-	random_start_time.tv_usec = 0;
-#endif
-	/* slightly hacky way to convert timestamptz into integers */
-	TimestampDifference(0, port->SessionStartTime, &secs, &usecs);
-	srandom((unsigned int) (MyProcPid ^ (usecs << 12) ^ secs));
-
 	/*
 	 * Now, build the argv vector that will be given to PostgresMain.
 	 *

src/backend/tcop/postgres.c

@@ -4632,7 +4632,7 @@ log_disconnections(int code, Datum arg)
 				minutes,
 				seconds;
 
-	TimestampDifference(port->SessionStartTime,
+	TimestampDifference(MyStartTimestamp,
 						GetCurrentTimestamp(),
 						&secs, &usecs);
 	msecs = usecs / 1000;

src/backend/utils/init/globals.c

@@ -39,6 +39,7 @@ volatile uint32 CritSectionCount = 0;
 
 int			MyProcPid;
 pg_time_t	MyStartTime;
+TimestampTz	MyStartTimestamp;
 struct Port *MyProcPort;
 int32		MyCancelKey;
 int			MyPMChildSlot;

src/backend/utils/init/miscinit.c

@@ -274,9 +274,7 @@ InitPostmasterChild(void)
 {
 	IsUnderPostmaster = true;	/* we are a postmaster subprocess now */
 
-	MyProcPid = getpid();		/* reset MyProcPid */
-
-	MyStartTime = time(NULL);	/* set our start time in case we call elog */
+	InitProcessGlobals();
 
 	/*
 	 * make sure stderr is in binary mode before anything can possibly be
@@ -321,17 +319,7 @@ InitStandaloneProcess(const char *argv0)
 {
 	Assert(!IsPostmasterEnvironment);
 
-	MyProcPid = getpid();		/* reset MyProcPid */
-
-	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));
+	InitProcessGlobals();
 
 	/* Initialize process-local latch support */
 	InitializeLatchSupport();