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

1 files changed, 1 insertions, 1 deletions

diff --git a/src/test/regress/pg_regress.c b/src/test/regress/pg_regress.c
index b1c73c6d23b..eb889f388ce 100644
--- a/src/test/regress/pg_regress.c
+++ b/src/test/regress/pg_regress.c
@@ -1097,7 +1097,7 @@ config_sspi_auth(const char *pgdata)
 	} while (0)
 
 	res = snprintf(fname, sizeof(fname), "%s/pg_hba.conf", pgdata);
-	if (res < 0 || res >= sizeof(fname) - 1)
+	if (res < 0 || res >= sizeof(fname))
 	{
 		/*
 		 * Truncating this name is a fatal error, because we must not fail to