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

3 files changed, 39 insertions, 15 deletions

diff --git a/src/test/isolation/isolation_main.c b/src/test/isolation/isolation_main.c
index 58402b74d80..d3ada6d2bcf 100644
--- a/src/test/isolation/isolation_main.c
+++ b/src/test/isolation/isolation_main.c
@@ -75,15 +75,27 @@ isolation_start_test(const char *testname,
 	add_stringlist_item(expectfiles, expectfile);
 
 	if (launcher)
+	{
 		offset += snprintf(psql_cmd + offset, sizeof(psql_cmd) - offset,
 						   "%s ", launcher);
+		if (offset >= sizeof(psql_cmd))
+		{
+			fprintf(stderr, _("command too long\n"));
+			exit(2);
+		}
+	}
 
-	snprintf(psql_cmd + offset, sizeof(psql_cmd) - offset,
-			 "\"%s\" \"dbname=%s\" < \"%s\" > \"%s\" 2>&1",
-			 isolation_exec,
-			 dblist->str,
-			 infile,
-			 outfile);
+	offset += snprintf(psql_cmd + offset, sizeof(psql_cmd) - offset,
+					   "\"%s\" \"dbname=%s\" < \"%s\" > \"%s\" 2>&1",
+					   isolation_exec,
+					   dblist->str,
+					   infile,
+					   outfile);
+	if (offset >= sizeof(psql_cmd))
+	{
+		fprintf(stderr, _("command too long\n"));
+		exit(2);
+	}
 
 	pid = spawn_process(psql_cmd);
 
diff --git a/src/test/regress/pg_regress.c b/src/test/regress/pg_regress.c
index 2ff2acc641b..6890678fa81 100644
--- a/src/test/regress/pg_regress.c
+++ b/src/test/regress/pg_regress.c
@@ -1024,7 +1024,7 @@ 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
diff --git a/src/test/regress/pg_regress_main.c b/src/test/regress/pg_regress_main.c
index a2bd6a2cd57..bd613e4fda1 100644
--- a/src/test/regress/pg_regress_main.c
+++ b/src/test/regress/pg_regress_main.c
@@ -63,20 +63,32 @@ psql_start_test(const char *testname,
 	add_stringlist_item(expectfiles, expectfile);
 
 	if (launcher)
+	{
 		offset += snprintf(psql_cmd + offset, sizeof(psql_cmd) - offset,
 						   "%s ", launcher);
+		if (offset >= sizeof(psql_cmd))
+		{
+			fprintf(stderr, _("command too long\n"));
+			exit(2);
+		}
+	}
+
+	offset += snprintf(psql_cmd + offset, sizeof(psql_cmd) - offset,
+					   "\"%s%spsql\" -X -a -q -d \"%s\" < \"%s\" > \"%s\" 2>&1",
+					   bindir ? bindir : "",
+					   bindir ? "/" : "",
+					   dblist->str,
+					   infile,
+					   outfile);
+	if (offset >= sizeof(psql_cmd))
+	{
+		fprintf(stderr, _("command too long\n"));
+		exit(2);
+	}
 
 	appnameenv = psprintf("PGAPPNAME=pg_regress/%s", testname);
 	putenv(appnameenv);
 
-	snprintf(psql_cmd + offset, sizeof(psql_cmd) - offset,
-			 "\"%s%spsql\" -X -a -q -d \"%s\" < \"%s\" > \"%s\" 2>&1",
-			 bindir ? bindir : "",
-			 bindir ? "/" : "",
-			 dblist->str,
-			 infile,
-			 outfile);
-
 	pid = spawn_process(psql_cmd);
 
 	if (pid == INVALID_PID)