Clean up assorted misuses of snprintf()'s result value.

Fix a small number of places that were testing the result of snprintf()
but doing so incorrectly.  The right test for buffer overrun, per C99,
is "result >= bufsize" not "result > bufsize".  Some places were also
checking for failure with "result == -1", but the standard only says
that a negative value is delivered on failure.

(Note that this only makes these places correct if snprintf() delivers
C99-compliant results.  But at least now these places are consistent
with all the other places where we assume that.)

Also, make psql_start_test() and isolation_start_test() check for
buffer overrun while constructing their shell commands.  There seems
like a higher risk of overrun, with more severe consequences, here
than there is for the individual file paths that are made elsewhere
in the same functions, so this seemed like a worthwhile change.

Also fix guc.c's do_serialize() to initialize errno = 0 before
calling vsnprintf.  In principle, this should be unnecessary because
vsnprintf should have set errno if it returns a failure indication ...
but the other two places this coding pattern is cribbed from don't
assume that, so let's be consistent.

These errors are all very old, so back-patch as appropriate.  I think
that only the shell command overrun cases are even theoretically
reachable in practice, but there's not much point in erroneous error
checks.

Discussion: https://postgr.es/m/17245.1534289329@sss.pgh.pa.us

2 files changed, 4 insertions, 4 deletions

diff --git a/src/backend/libpq/ip.c b/src/backend/libpq/ip.c
index 0196c02223b..9b23ede7f62 100644
--- a/src/backend/libpq/ip.c
+++ b/src/backend/libpq/ip.c
@@ -240,7 +240,7 @@ getnameinfo_unix(const struct sockaddr_un * sa, int salen,
 				 char *service, int servicelen,
 				 int flags)
 {
-	int			ret = -1;
+	int			ret;
 
 	/* Invalid arguments. */
 	if (sa == NULL || sa->sun_family != AF_UNIX ||
@@ -250,14 +250,14 @@ getnameinfo_unix(const struct sockaddr_un * sa, int salen,
 	if (node)
 	{
 		ret = snprintf(node, nodelen, "%s", "[local]");
-		if (ret == -1 || ret > nodelen)
+		if (ret < 0 || ret >= nodelen)
 			return EAI_MEMORY;
 	}
 
 	if (service)
 	{
 		ret = snprintf(service, servicelen, "%s", sa->sun_path);
-		if (ret == -1 || ret > servicelen)
+		if (ret < 0 || ret >= servicelen)
 			return EAI_MEMORY;
 	}
 
diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index 236ebadddda..9a9c51648bb 100644
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -3768,7 +3768,7 @@ get_dbstat_filename(bool permanent, bool tempname, Oid databaseid,
 					   pgstat_stat_directory,
 					   databaseid,
 					   tempname ? "tmp" : "stat");
-	if (printed > len)
+	if (printed >= len)
 		elog(ERROR, "overlength pgstat path");
 }