Coordinate log_line_prefix options 'm' and 'n' to share a timeval.

Commit f828654e introduced the 'n' option, but it invoked
gettimeofday() independently of the 'm' option. If both options were
in use (or multiple 'n' options), or if 'n' was in use along with
csvlog, then the reported times could be different for the same log
message.

To fix, initialize a global variable with gettimeofday() once per log
message, and use that for both formats.

Don't bother coordinating the time for the 't' option, which has much
lower resolution.

Per complaint by Alvaro Herrera.

src/backend/utils/error/elog.c

@@ -143,10 +143,14 @@ static int	errordata_stack_depth = -1; /* index of topmost active frame */
 
 static int	recursion_depth = 0;	/* to detect actual recursion */
 
-/* buffers for formatted timestamps that might be used by both
- * log_line_prefix and csv logs.
+/*
+ * Saved timeval and buffers for formatted timestamps that might be used by
+ * both log_line_prefix and csv logs.
  */
 
+static struct timeval	saved_timeval;
+static bool				saved_timeval_set = false;
+
 #define FORMATTED_TS_LEN 128
 static char formatted_start_time[FORMATTED_TS_LEN];
 static char formatted_log_time[FORMATTED_TS_LEN];
@@ -2195,12 +2199,16 @@ write_console(const char *line, int len)
 static void
 setup_formatted_log_time(void)
 {
-	struct timeval tv;
 	pg_time_t	stamp_time;
 	char		msbuf[8];
 
-	gettimeofday(&tv, NULL);
-	stamp_time = (pg_time_t) tv.tv_sec;
+	if (!saved_timeval_set)
+	{
+		gettimeofday(&saved_timeval, NULL);
+		saved_timeval_set = true;
+	}
+
+	stamp_time = (pg_time_t) saved_timeval.tv_sec;
 
 	/*
 	 * Note: we expect that guc.c will ensure that log_timezone is set up (at
@@ -2213,7 +2221,7 @@ setup_formatted_log_time(void)
 				pg_localtime(&stamp_time, log_timezone));
 
 	/* 'paste' milliseconds into place... */
-	sprintf(msbuf, ".%03d", (int) (tv.tv_usec / 1000));
+	sprintf(msbuf, ".%03d", (int) (saved_timeval.tv_usec / 1000));
 	memcpy(formatted_log_time + 19, msbuf, 4);
 }
 
@@ -2440,11 +2448,16 @@ log_line_prefix(StringInfo buf, ErrorData *edata)
 				break;
 			case 'n':
 				{
-					struct	timeval tv;
 					char	strfbuf[128];
 
-					gettimeofday(&tv, NULL);
-					sprintf(strfbuf, "%ld.%03d", tv.tv_sec, (int)(tv.tv_usec / 1000));
+					if (!saved_timeval_set)
+					{
+						gettimeofday(&saved_timeval, NULL);
+						saved_timeval_set = true;
+					}
+
+					sprintf(strfbuf, "%ld.%03d", saved_timeval.tv_sec,
+							(int)(saved_timeval.tv_usec / 1000));
 
 					if (padding != 0)
 						appendStringInfo(buf, "%*s", padding, strfbuf);
@@ -2825,6 +2838,7 @@ send_message_to_server_log(ErrorData *edata)
 
 	initStringInfo(&buf);
 
+	saved_timeval_set = false;
 	formatted_log_time[0] = '\0';
 
 	log_line_prefix(&buf, edata);