http: support file handles for HTTP_KEEP_ERROR

HTTP_KEEP_ERROR makes it easy to debug HTTP transport errors. In order
to make HTTP_KEEP_ERROR enabled for all requests, file handles need to
be supported.

Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -1952,16 +1952,26 @@ static int http_request_reauth(const char *url,
 	/*
 	 * If we are using KEEP_ERROR, the previous request may have
 	 * put cruft into our output stream; we should clear it out before
-	 * making our next request. We only know how to do this for
+	 * making our next request.
-	 * the strbuf case, but that is enough to satisfy current callers.
 	 */
 	if (options && options->keep_error) {
 		switch (target) {
 		case HTTP_REQUEST_STRBUF:
 			strbuf_reset(result);
 			break;
+		case HTTP_REQUEST_FILE:
+			if (fflush(result)) {
+				error_errno("unable to flush a file");
+				return HTTP_START_FAILED;
+			}
+			rewind(result);
+			if (ftruncate(fileno(result), 0) < 0) {
+				error_errno("unable to truncate a file");
+				return HTTP_START_FAILED;
+			}
+			break;
 		default:
-			BUG("HTTP_KEEP_ERROR is only supported with strbufs");
+			BUG("Unknown http_request target");
 		}
 	}