Fix coding rules violations in walreceiver.c

1. Since commit b1a9bad9e7 we had pstrdup() inside a
spinlock-protected critical section; reported by Andreas Seltenreich.
Turn those into strlcpy() to stack-allocated variables instead.
Backpatch to 9.6.

2. Since commit 9ed551e0a4 we had a pfree() uselessly inside a
spinlock-protected critical section.  Tom Lane noticed in code review.
Move down.  Backpatch to 9.6.

3. Since commit 64233902d2 we had GetCurrentTimestamp() (a kernel
call) inside a spinlock-protected critical section.  Tom Lane noticed in
code review.  Move it up.  Backpatch to 9.2.

4. Since commit 1bb2558046 we did elog(PANIC) while holding spinlock.
Tom Lane noticed in code review.  Release spinlock before dying.
Backpatch to 9.2.

Discussion: https://postgr.es/m/87h8vhtgj2.fsf@ansel.ydns.eu

src/backend/replication/walreceiver.c

@@ -196,6 +196,7 @@ WalReceiverMain(void)
 	bool		first_stream;
 	WalRcvData *walrcv = WalRcv;
 	TimestampTz last_recv_timestamp;
+	TimestampTz now;
 	bool		ping_sent;
 	char	   *err;
 
@@ -205,6 +206,8 @@ WalReceiverMain(void)
 	 */
 	Assert(walrcv != NULL);
 
+	now = GetCurrentTimestamp();
+
 	/*
 	 * Mark walreceiver as running in shared memory.
 	 *
@@ -235,6 +238,7 @@ WalReceiverMain(void)
 		case WALRCV_RESTARTING:
 		default:
 			/* Shouldn't happen */
+			SpinLockRelease(&walrcv->mutex);
 			elog(PANIC, "walreceiver still running according to shared memory state");
 	}
 	/* Advertise our PID so that the startup process can kill us */
@@ -249,7 +253,8 @@ WalReceiverMain(void)
 	startpointTLI = walrcv->receiveStartTLI;
 
 	/* Initialise to a sanish value */
-	walrcv->lastMsgSendTime = walrcv->lastMsgReceiptTime = walrcv->latestWalEndTime = GetCurrentTimestamp();
+	walrcv->lastMsgSendTime =
+		walrcv->lastMsgReceiptTime = walrcv->latestWalEndTime = now;
 
 	SpinLockRelease(&walrcv->mutex);
 
@@ -308,13 +313,13 @@ WalReceiverMain(void)
 	SpinLockAcquire(&walrcv->mutex);
 	memset(walrcv->conninfo, 0, MAXCONNINFO);
 	if (tmp_conninfo)
-	{
 		strlcpy((char *) walrcv->conninfo, tmp_conninfo, MAXCONNINFO);
-		pfree(tmp_conninfo);
-	}
 	walrcv->ready_to_display = true;
 	SpinLockRelease(&walrcv->mutex);
 
+	if (tmp_conninfo)
+		pfree(tmp_conninfo);
+
 	first_stream = true;
 	for (;;)
 	{
@@ -1390,8 +1395,8 @@ pg_stat_get_wal_receiver(PG_FUNCTION_ARGS)
 	TimestampTz last_receipt_time;
 	XLogRecPtr	latest_end_lsn;
 	TimestampTz latest_end_time;
-	char	   *slotname;
-	char	   *conninfo;
+	char		slotname[NAMEDATALEN];
+	char		conninfo[MAXCONNINFO];
 
 	/* Take a lock to ensure value consistency */
 	SpinLockAcquire(&WalRcv->mutex);
@@ -1406,8 +1411,8 @@ pg_stat_get_wal_receiver(PG_FUNCTION_ARGS)
 	last_receipt_time = WalRcv->lastMsgReceiptTime;
 	latest_end_lsn = WalRcv->latestWalEnd;
 	latest_end_time = WalRcv->latestWalEndTime;
-	slotname = pstrdup(WalRcv->slotname);
-	conninfo = pstrdup(WalRcv->conninfo);
+	strlcpy(slotname, (char *) WalRcv->slotname, sizeof(slotname));
+	strlcpy(conninfo, (char *) WalRcv->conninfo, sizeof(conninfo));
 	SpinLockRelease(&WalRcv->mutex);
 
 	/*