Provide log_status_format(), useful for an emit_log_hook.

Refactor so that log_line_prefix() is a thin wrapper over a new
function log_status_format(), and move the implementation to the
latter. Export log_status_format() so that it can be used by an
emit_log_hook.

Discussion: https://postgr.es/m/39c8197652f4d3050aedafae79fa5af31096505f.camel%40j-davis.com
Reviewed-by: Michael Paquier, Alvaro Herrera

Author: jeff-davis

src/backend/utils/error/elog.c

@@ -2438,10 +2438,19 @@ process_log_prefix_padding(const char *p, int *ppadding)
 }
 
 /*
- * Format tag info for log lines; append to the provided buffer.
+ * Format log status information using Log_line_prefix.
  */
 static void
 log_line_prefix(StringInfo buf, ErrorData *edata)
+{
+	log_status_format(buf, Log_line_prefix, edata);
+}
+
+/*
+ * Format log status info; append to the provided buffer.
+ */
+void
+log_status_format(StringInfo buf, const char *format, ErrorData *edata)
 {
 	/* static counter for line numbers */
 	static long log_line_number = 0;
@@ -2465,10 +2474,10 @@ log_line_prefix(StringInfo buf, ErrorData *edata)
 	}
 	log_line_number++;
 
-	if (Log_line_prefix == NULL)
+	if (format == NULL)
 		return;					/* in case guc hasn't run yet */
 
-	for (p = Log_line_prefix; *p != '\0'; p++)
+	for (p = format; *p != '\0'; p++)
 	{
 		if (*p != '%')
 		{

src/include/utils/elog.h

@@ -16,6 +16,8 @@
 
 #include <setjmp.h>
 
+#include "lib/stringinfo.h"
+
 /* Error level codes */
 #define DEBUG5		10			/* Debugging messages, in categories of
 								 * decreasing detail. */
@@ -439,6 +441,8 @@ extern PGDLLIMPORT bool syslog_split_messages;
 #define LOG_DESTINATION_JSONLOG	16
 
 /* Other exported functions */
+extern void log_status_format(StringInfo buf, const char *format,
+							  ErrorData *edata);
 extern void DebugFileOpen(void);
 extern char *unpack_sql_state(int sql_state);
 extern bool in_error_recursion_trouble(void);