[autoboot] Use next-server from filename's settings block

Locate the settings block containing the filename, and search only
that settings block for the next-server address.  This avoids problems
caused by misconfigured DHCP servers which provide a next-server
address (often defaulting to the DHCP server's own IP address) even
when not providing a filename.

Originally-implemented-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/usr/autoboot.c

  * @ret uri		URI, or NULL on failure
  */
 struct uri * fetch_next_server_and_filename ( struct settings *settings ) {
-	struct in_addr next_server;
-	char buf[256];
+	struct in_addr next_server = { 0 };
+	char *raw_filename = NULL;
+	struct uri *uri = NULL;
 	char *filename;
-	struct uri *uri;
 
-	/* Fetch next-server setting */
-	fetch_ipv4_setting ( settings, &next_server_setting, &next_server );
-	if ( next_server.s_addr )
-		printf ( "Next server: %s\n", inet_ntoa ( next_server ) );
+	/* Determine settings block containing the filename, if any */
+	settings = fetch_setting_origin ( settings, &filename_setting );
 
-	/* Fetch filename setting */
-	fetch_string_setting ( settings, &filename_setting,
-			       buf, sizeof ( buf ) );
-	if ( buf[0] )
-		printf ( "Filename: %s\n", buf );
+	/* If we have a filename, fetch it along with next-server */
+	if ( settings ) {
+		fetch_ipv4_setting ( settings, &next_server_setting,
+				     &next_server );
+		if ( fetch_string_setting_copy ( settings, &filename_setting,
+						 &raw_filename ) < 0 )
+			goto err_fetch;
+	}
 
 	/* Expand filename setting */
-	filename = expand_settings ( buf );
+	filename = expand_settings ( raw_filename ? raw_filename : "" );
 	if ( ! filename )
-		return NULL;
+		goto err_expand;
 
 	/* Parse next server and filename */
+	if ( next_server.s_addr )
+		printf ( "Next server: %s\n", inet_ntoa ( next_server ) );
+	if ( filename[0] )
+		printf ( "Filename: %s\n", filename );
 	uri = parse_next_server_and_filename ( next_server, filename );
+	if ( ! uri )
+		goto err_parse;
 
+ err_parse:
 	free ( filename );
+ err_expand:
+	free ( raw_filename );
+ err_fetch:
 	return uri;
 }