Prevent starting a standalone backend with standby_mode on.

This can't really work because standby_mode expects there to be more
WAL arriving, which there will not ever be because there's no WAL
receiver process to fetch it.  Moreover, if standby_mode is on then
hot standby might also be turned on, causing even more strangeness
because that expects read-only sessions to be executing in parallel.
Bernd Helmle reported a case where btree_xlog_delete_get_latestRemovedXid
got confused, but rather than band-aiding individual problems it seems
best to prevent getting anywhere near this state in the first place.
Back-patch to all supported branches.

In passing, also fix some omissions of errcodes in other ereport's in
readRecoveryCommandFile().

Michael Paquier (errcode hacking by me)

Discussion: <00F0B2CEF6D0CEF8A90119D4@eje.credativ.lan>

src/backend/access/transam/xlog.c

@@ -4220,7 +4220,8 @@ readRecoveryCommandFile(void)
 				rtli = (TimeLineID) strtoul(item->value, NULL, 0);
 				if (errno == EINVAL || errno == ERANGE)
 					ereport(FATAL,
-							(errmsg("recovery_target_timeline is not a valid number: \"%s\"",
+							(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+							 errmsg("recovery_target_timeline is not a valid number: \"%s\"",
 									item->value)));
 			}
 			if (rtli)
@@ -4236,7 +4237,8 @@ readRecoveryCommandFile(void)
 			recoveryTargetXid = (TransactionId) strtoul(item->value, NULL, 0);
 			if (errno == EINVAL || errno == ERANGE)
 				ereport(FATAL,
-				 (errmsg("recovery_target_xid is not a valid number: \"%s\"",
+						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				  errmsg("recovery_target_xid is not a valid number: \"%s\"",
 						 item->value)));
 			ereport(DEBUG2,
 					(errmsg_internal("recovery_target_xid = %u",
@@ -4327,7 +4329,8 @@ readRecoveryCommandFile(void)
 		}
 		else
 			ereport(FATAL,
-					(errmsg("unrecognized recovery parameter \"%s\"",
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("unrecognized recovery parameter \"%s\"",
 							item->name)));
 	}
 
@@ -4346,10 +4349,20 @@ readRecoveryCommandFile(void)
 	{
 		if (recoveryRestoreCommand == NULL)
 			ereport(FATAL,
-					(errmsg("recovery command file \"%s\" must specify restore_command when standby mode is not enabled",
+					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+					 errmsg("recovery command file \"%s\" must specify restore_command when standby mode is not enabled",
 							RECOVERY_COMMAND_FILE)));
 	}
 
+	/*
+	 * We don't support standby_mode in standalone backends; that requires
+	 * other processes such as the WAL receiver to be alive.
+	 */
+	if (StandbyModeRequested && !IsUnderPostmaster)
+		ereport(FATAL,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			errmsg("standby mode is not supported by single-user servers")));
+
 	/* Enable fetching from archive recovery area */
 	ArchiveRecoveryRequested = true;
 
@@ -4366,7 +4379,8 @@ readRecoveryCommandFile(void)
 			/* Timeline 1 does not have a history file, all else should */
 			if (rtli != 1 && !existsTimeLineHistory(rtli))
 				ereport(FATAL,
-						(errmsg("recovery target timeline %u does not exist",
+						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+						 errmsg("recovery target timeline %u does not exist",
 								rtli)));
 			recoveryTargetTLI = rtli;
 			recoveryTargetIsLatest = false;