libpq: Fix minor TOCTOU violation

libpq checks the permissions of the password file before opening it.
The way this is done in two separate operations, a static analyzer
would flag as a time-of-check-time-of-use violation.  In practice, you
can't do anything with that, but it still seems better style to fix
it.

To fix it, open the file first and then check the permissions on the
opened file handle.

Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Reviewed-by: Andreas Karlsson <andreas@proxel.se>
Discussion: https://www.postgresql.org/message-id/flat/a3356054-14ae-4e7a-acc6-249d19dac20b%40eisentraut.org

src/interfaces/libpq/fe-connect.c

@@ -7452,7 +7452,9 @@ passwordFromFile(const char *hostname, const char *port, const char *dbname,
 				 const char *username, const char *pgpassfile)
 {
 	FILE	   *fp;
+#ifndef WIN32
 	struct stat stat_buf;
+#endif
 	PQExpBufferData buf;
 
 	if (dbname == NULL || dbname[0] == '\0')
@@ -7477,10 +7479,14 @@ passwordFromFile(const char *hostname, const char *port, const char *dbname,
 		port = DEF_PGPORT_STR;
 
 	/* If password file cannot be opened, ignore it. */
-	if (stat(pgpassfile, &stat_buf) != 0)
+	fp = fopen(pgpassfile, "r");
+	if (fp == NULL)
 		return NULL;
 
 #ifndef WIN32
+	if (fstat(fileno(fp), &stat_buf) != 0)
+		return NULL;
+
 	if (!S_ISREG(stat_buf.st_mode))
 	{
 		fprintf(stderr,
@@ -7505,10 +7511,6 @@ passwordFromFile(const char *hostname, const char *port, const char *dbname,
 	 */
 #endif
 
-	fp = fopen(pgpassfile, "r");
-	if (fp == NULL)
-		return NULL;
-
 	/* Use an expansible buffer to accommodate any reasonable line length */
 	initPQExpBuffer(&buf);