Fix unsafe usage of strerror(errno) within ereport().

tglsfdc · tglsfdc · commit 1545ca9a7463 · 2018-05-21T00:32:44.000-04:00
This is the converse of the unsafe-usage-of-%m problem: the reason
ereport/elog provide that format code is mainly to dodge the hazard
of errno getting changed before control reaches functions within the
arguments of the macro.  I only found one instance of this hazard,
but it's been there since 9.4 :-(.
diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
@@ -1752,10 +1752,12 @@ auth_peer(hbaPort *port)
 	pw = getpwuid(uid);
 	if (!pw)
 	{
+		int			save_errno = errno;
+
 		ereport(LOG,
 				(errmsg("could not look up local user ID %ld: %s",
 						(long) uid,
-						errno ? strerror(errno) : _("user does not exist"))));
+						save_errno ? strerror(save_errno) : _("user does not exist"))));
 		return STATUS_ERROR;
 	}
 

Original file line number	Diff line number	Diff line change
`@@ -1752,10 +1752,12 @@ auth_peer(hbaPort *port)`
`1752`	`1752`	`pw = getpwuid(uid);`
`1753`	`1753`	`if (!pw)`
`1754`	`1754`	`{`
	`1755`	`+ int save_errno = errno;`
	`1756`	`+`
`1755`	`1757`	`ereport(LOG,`
`1756`	`1758`	`(errmsg("could not look up local user ID %ld: %s",`
`1757`	`1759`	`(long) uid,`
`1758`		`- errno ? strerror(errno) : _("user does not exist"))));`
	`1760`	`+ save_errno ? strerror(save_errno) : _("user does not exist"))));`
`1759`	`1761`	`return STATUS_ERROR;`
`1760`	`1762`	`}`
`1761`	`1763`