Implement %m in src/port/snprintf.c, and teach elog.c to rely on that.

I started out with the idea that we needed to detect use of %m format specs
in contexts other than elog/ereport calls, because we couldn't rely on that
working in *printf calls.  But a better answer is to fix things so that it
does work.  Now that we're using snprintf.c all the time, we can implement
%m in that and we've fixed the problem.

This requires also adjusting our various printf-wrapping functions so that
they ensure "errno" is preserved when they call snprintf.c.

Remove elog.c's handmade implementation of %m, and let it rely on
snprintf to support the feature.  That should provide some performance
gain, though I've not attempted to measure it.

There are a lot of places where we could now simplify 'printf("%s",
strerror(errno))' into 'printf("%m")', but I'm not in any big hurry
to make that happen.

Patch by me, reviewed by Michael Paquier

Discussion: https://postgr.es/m/2975.1526862605@sss.pgh.pa.us

src/backend/lib/stringinfo.c

@@ -77,12 +77,15 @@ resetStringInfo(StringInfo str)
 void
 appendStringInfo(StringInfo str, const char *fmt,...)
 {
+	int			save_errno = errno;
+
 	for (;;)
 	{
 		va_list		args;
 		int			needed;
 
 		/* Try to format the data. */
+		errno = save_errno;
 		va_start(args, fmt);
 		needed = appendStringInfoVA(str, fmt, args);
 		va_end(args);
@@ -105,6 +108,9 @@ appendStringInfo(StringInfo str, const char *fmt,...)
  * pass the return value to enlargeStringInfo() before trying again; see
  * appendStringInfo for standard usage pattern.
  *
+ * Caution: callers must be sure to preserve their entry-time errno
+ * when looping, in case the fmt contains "%m".
+ *
  * XXX This API is ugly, but there seems no alternative given the C spec's
  * restrictions on what can portably be done with va_list arguments: you have
  * to redo va_start before you can rescan the argument list, and we can't do

src/backend/utils/error/elog.c

@@ -177,7 +177,6 @@ static void write_csvlog(ErrorData *edata);
 static void send_message_to_server_log(ErrorData *edata);
 static void write_pipe_chunks(char *data, int len, int dest);
 static void send_message_to_frontend(ErrorData *edata);
-static char *expand_fmt_string(const char *fmt, ErrorData *edata);
 static const char *error_severity(int elevel);
 static void append_with_tabs(StringInfo buf, const char *str);
 static bool is_log_level_output(int elevel, int log_min_level);
@@ -705,13 +704,10 @@ errcode_for_socket_access(void)
  */
 #define EVALUATE_MESSAGE(domain, targetfield, appendval, translateit)	\
 	{ \
-		char		   *fmtbuf; \
 		StringInfoData	buf; \
 		/* Internationalize the error format string */ \
 		if ((translateit) && !in_error_recursion_trouble()) \
 			fmt = dgettext((domain), fmt);				  \
-		/* Expand %m in format string */ \
-		fmtbuf = expand_fmt_string(fmt, edata); \
 		initStringInfo(&buf); \
 		if ((appendval) && edata->targetfield) { \
 			appendStringInfoString(&buf, edata->targetfield); \
@@ -722,15 +718,14 @@ errcode_for_socket_access(void)
 		{ \
 			va_list		args; \
 			int			needed; \
+			errno = edata->saved_errno; \
 			va_start(args, fmt); \
-			needed = appendStringInfoVA(&buf, fmtbuf, args); \
+			needed = appendStringInfoVA(&buf, fmt, args); \
 			va_end(args); \
 			if (needed == 0) \
 				break; \
 			enlargeStringInfo(&buf, needed); \
 		} \
-		/* Done with expanded fmt */ \
-		pfree(fmtbuf); \
 		/* Save the completed message into the stack item */ \
 		if (edata->targetfield) \
 			pfree(edata->targetfield); \
@@ -746,15 +741,12 @@ errcode_for_socket_access(void)
 #define EVALUATE_MESSAGE_PLURAL(domain, targetfield, appendval)  \
 	{ \
 		const char	   *fmt; \
-		char		   *fmtbuf; \
 		StringInfoData	buf; \
 		/* Internationalize the error format string */ \
 		if (!in_error_recursion_trouble()) \
 			fmt = dngettext((domain), fmt_singular, fmt_plural, n); \
 		else \
 			fmt = (n == 1 ? fmt_singular : fmt_plural); \
-		/* Expand %m in format string */ \
-		fmtbuf = expand_fmt_string(fmt, edata); \
 		initStringInfo(&buf); \
 		if ((appendval) && edata->targetfield) { \
 			appendStringInfoString(&buf, edata->targetfield); \
@@ -765,15 +757,14 @@ errcode_for_socket_access(void)
 		{ \
 			va_list		args; \
 			int			needed; \
+			errno = edata->saved_errno; \
 			va_start(args, n); \
-			needed = appendStringInfoVA(&buf, fmtbuf, args); \
+			needed = appendStringInfoVA(&buf, fmt, args); \
 			va_end(args); \
 			if (needed == 0) \
 				break; \
 			enlargeStringInfo(&buf, needed); \
 		} \
-		/* Done with expanded fmt */ \
-		pfree(fmtbuf); \
 		/* Save the completed message into the stack item */ \
 		if (edata->targetfield) \
 			pfree(edata->targetfield); \
@@ -3328,59 +3319,6 @@ send_message_to_frontend(ErrorData *edata)
  */
 
 
-/*
- * expand_fmt_string --- process special format codes in a format string
- *
- * We must replace %m with the appropriate strerror string, since vsnprintf
- * won't know what to do with it.
- *
- * The result is a palloc'd string.
- */
-static char *
-expand_fmt_string(const char *fmt, ErrorData *edata)
-{
-	StringInfoData buf;
-	const char *cp;
-
-	initStringInfo(&buf);
-
-	for (cp = fmt; *cp; cp++)
-	{
-		if (cp[0] == '%' && cp[1] != '\0')
-		{
-			cp++;
-			if (*cp == 'm')
-			{
-				/*
-				 * Replace %m by system error string.  If there are any %'s in
-				 * the string, we'd better double them so that vsnprintf won't
-				 * misinterpret.
-				 */
-				const char *cp2;
-
-				cp2 = strerror(edata->saved_errno);
-				for (; *cp2; cp2++)
-				{
-					if (*cp2 == '%')
-						appendStringInfoCharMacro(&buf, '%');
-					appendStringInfoCharMacro(&buf, *cp2);
-				}
-			}
-			else
-			{
-				/* copy % and next char --- this avoids trouble with %%m */
-				appendStringInfoCharMacro(&buf, '%');
-				appendStringInfoCharMacro(&buf, *cp);
-			}
-		}
-		else
-			appendStringInfoCharMacro(&buf, *cp);
-	}
-
-	return buf.data;
-}
-
-
 /*
  * error_severity --- get string representing elevel
  *

src/bin/pg_dump/pg_backup_archiver.c

@@ -1507,6 +1507,7 @@ archputs(const char *s, Archive *AH)
 int
 archprintf(Archive *AH, const char *fmt,...)
 {
+	int			save_errno = errno;
 	char	   *p;
 	size_t		len = 128;		/* initial assumption about buffer size */
 	size_t		cnt;
@@ -1519,6 +1520,7 @@ archprintf(Archive *AH, const char *fmt,...)
 		p = (char *) pg_malloc(len);
 
 		/* Try to format the data. */
+		errno = save_errno;
 		va_start(args, fmt);
 		cnt = pvsnprintf(p, len, fmt, args);
 		va_end(args);
@@ -1640,6 +1642,7 @@ RestoreOutput(ArchiveHandle *AH, OutputContext savedContext)
 int
 ahprintf(ArchiveHandle *AH, const char *fmt,...)
 {
+	int			save_errno = errno;
 	char	   *p;
 	size_t		len = 128;		/* initial assumption about buffer size */
 	size_t		cnt;
@@ -1652,6 +1655,7 @@ ahprintf(ArchiveHandle *AH, const char *fmt,...)
 		p = (char *) pg_malloc(len);
 
 		/* Try to format the data. */
+		errno = save_errno;
 		va_start(args, fmt);
 		cnt = pvsnprintf(p, len, fmt, args);
 		va_end(args);

src/bin/pg_dump/pg_backup_tar.c

@@ -1026,6 +1026,7 @@ _EndBlobs(ArchiveHandle *AH, TocEntry *te)
 static int
 tarPrintf(ArchiveHandle *AH, TAR_MEMBER *th, const char *fmt,...)
 {
+	int			save_errno = errno;
 	char	   *p;
 	size_t		len = 128;		/* initial assumption about buffer size */
 	size_t		cnt;
@@ -1038,6 +1039,7 @@ tarPrintf(ArchiveHandle *AH, TAR_MEMBER *th, const char *fmt,...)
 		p = (char *) pg_malloc(len);
 
 		/* Try to format the data. */
+		errno = save_errno;
 		va_start(args, fmt);
 		cnt = pvsnprintf(p, len, fmt, args);
 		va_end(args);

src/common/psprintf.c

@@ -45,6 +45,7 @@
 char *
 psprintf(const char *fmt,...)
 {
+	int			save_errno = errno;
 	size_t		len = 128;		/* initial assumption about buffer size */
 
 	for (;;)
@@ -60,6 +61,7 @@ psprintf(const char *fmt,...)
 		result = (char *) palloc(len);
 
 		/* Try to format the data. */
+		errno = save_errno;
 		va_start(args, fmt);
 		newlen = pvsnprintf(result, len, fmt, args);
 		va_end(args);
@@ -89,6 +91,9 @@ psprintf(const char *fmt,...)
  * Other error cases do not return, but exit via elog(ERROR) or exit().
  * Hence, this shouldn't be used inside libpq.
  *
+ * Caution: callers must be sure to preserve their entry-time errno
+ * when looping, in case the fmt contains "%m".
+ *
  * Note that the semantics of the return value are not exactly C99's.
  * First, we don't promise that the estimated buffer size is exactly right;
  * callers must be prepared to loop multiple times to get the right size.

src/interfaces/libpq/pqexpbuffer.c

@@ -233,6 +233,7 @@ enlargePQExpBuffer(PQExpBuffer str, size_t needed)
 void
 printfPQExpBuffer(PQExpBuffer str, const char *fmt,...)
 {
+	int			save_errno = errno;
 	va_list		args;
 	bool		done;
 
@@ -244,6 +245,7 @@ printfPQExpBuffer(PQExpBuffer str, const char *fmt,...)
 	/* Loop in case we have to retry after enlarging the buffer. */
 	do
 	{
+		errno = save_errno;
 		va_start(args, fmt);
 		done = appendPQExpBufferVA(str, fmt, args);
 		va_end(args);
@@ -261,6 +263,7 @@ printfPQExpBuffer(PQExpBuffer str, const char *fmt,...)
 void
 appendPQExpBuffer(PQExpBuffer str, const char *fmt,...)
 {
+	int			save_errno = errno;
 	va_list		args;
 	bool		done;
 
@@ -270,6 +273,7 @@ appendPQExpBuffer(PQExpBuffer str, const char *fmt,...)
 	/* Loop in case we have to retry after enlarging the buffer. */
 	do
 	{
+		errno = save_errno;
 		va_start(args, fmt);
 		done = appendPQExpBufferVA(str, fmt, args);
 		va_end(args);
@@ -281,6 +285,9 @@ appendPQExpBuffer(PQExpBuffer str, const char *fmt,...)
  * Shared guts of printfPQExpBuffer/appendPQExpBuffer.
  * Attempt to format data and append it to str.  Returns true if done
  * (either successful or hard failure), false if need to retry.
+ *
+ * Caution: callers must be sure to preserve their entry-time errno
+ * when looping, in case the fmt contains "%m".
  */
 static bool
 appendPQExpBufferVA(PQExpBuffer str, const char *fmt, va_list args)

src/pl/plpython/plpy_elog.c

@@ -46,6 +46,7 @@ static bool set_string_attr(PyObject *obj, char *attrname, char *str);
 void
 PLy_elog_impl(int elevel, const char *fmt,...)
 {
+	int			save_errno = errno;
 	char	   *xmsg;
 	char	   *tbmsg;
 	int			tb_depth;
@@ -96,6 +97,7 @@ PLy_elog_impl(int elevel, const char *fmt,...)
 			va_list		ap;
 			int			needed;
 
+			errno = save_errno;
 			va_start(ap, fmt);
 			needed = appendStringInfoVA(&emsg, dgettext(TEXTDOMAIN, fmt), ap);
 			va_end(ap);

src/port/snprintf.c

@@ -64,6 +64,14 @@
  *
  * 5. Space and '#' flags are not implemented.
  *
+ * In addition, we support some extensions over C99:
+ *
+ * 1. Argument order control through "%n$" and "*n$", as required by POSIX.
+ *
+ * 2. "%m" expands to the value of strerror(errno), where errno is the
+ * value that variable had at the start of the call.  This is a glibc
+ * extension, but a very useful one.
+ *
  *
  * Historically the result values of sprintf/snprintf varied across platforms.
  * This implementation now follows the C99 standard:
@@ -155,6 +163,13 @@ static void flushbuffer(PrintfTarget *target);
 static void dopr(PrintfTarget *target, const char *format, va_list args);
 
 
+/*
+ * Externally visible entry points.
+ *
+ * All of these are just wrappers around dopr().  Note it's essential that
+ * they not change the value of "errno" before reaching dopr().
+ */
+
 int
 pg_vsnprintf(char *str, size_t count, const char *fmt, va_list args)
 {
@@ -315,11 +330,12 @@ static void trailing_pad(int *padlen, PrintfTarget *target);
 
 
 /*
- * dopr(): poor man's version of doprintf
+ * dopr(): the guts of *printf for all cases.
  */
 static void
 dopr(PrintfTarget *target, const char *format, va_list args)
 {
+	int			save_errno = errno;
 	const char *format_start = format;
 	int			ch;
 	bool		have_dollar;
@@ -497,6 +513,7 @@ nextch1:
 				else
 					have_non_dollar = true;
 				break;
+			case 'm':
 			case '%':
 				break;
 		}
@@ -802,6 +819,15 @@ nextch2:
 						 precision, pointflag,
 						 target);
 				break;
+			case 'm':
+				{
+					char		errbuf[PG_STRERROR_R_BUFLEN];
+					const char *errm = strerror_r(save_errno,
+												  errbuf, sizeof(errbuf));
+
+					dostr(errm, strlen(errm), target);
+				}
+				break;
 			case '%':
 				dopr_outch('%', target);
 				break;