http: add support for HTTP 429 rate limit retries

Add retry logic for HTTP 429 (Too Many Requests) responses to handle
server-side rate limiting gracefully. When Git's HTTP client receives
a 429 response, it can now automatically retry the request after an
appropriate delay, respecting the server's rate limits.

The implementation supports the RFC-compliant Retry-After header in
both delay-seconds (integer) and HTTP-date (RFC 2822) formats. If a
past date is provided, Git retries immediately without waiting.

Retry behavior is controlled by three new configuration options
(http.maxRetries, http.retryAfter, and http.maxRetryTime) which are
documented in git-config(1).

The retry logic implements a fail-fast approach: if any delay
(whether from server header or configuration) exceeds maxRetryTime,
Git fails immediately with a clear error message rather than capping
the delay. This provides better visibility into rate limiting issues.

The implementation includes extensive test coverage for basic retry
behavior, Retry-After header formats (integer and HTTP-date),
configuration combinations, maxRetryTime limits, invalid header
handling, environment variable overrides, and edge cases.

Signed-off-by: Vaidas Pilkauskas <vaidas.pilkauskas@shopify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/http.h b/http.h
index f9d4593404..f9ee888c3e 100644
--- a/http.h
+++ b/http.h
@@ -20,6 +20,7 @@ struct slot_results {
 	long http_code;
 	long auth_avail;
 	long http_connectcode;
+	long retry_after;
 };
 
 struct active_request_slot {
@@ -157,6 +158,13 @@ struct http_get_options {
 	 * request has completed.
 	 */
 	struct string_list *extra_headers;
+
+	/*
+	 * After a request completes, contains the Retry-After delay in seconds
+	 * if the server returned HTTP 429 with a Retry-After header (requires
+	 * libcurl 7.66.0 or later), or -1 if no such header was present.
+	 */
+	long retry_after;
 };
 
 /* Return values for http_get_*() */
@@ -167,6 +175,7 @@ struct http_get_options {
 #define HTTP_REAUTH	4
 #define HTTP_NOAUTH	5
 #define HTTP_NOMATCHPUBLICKEY	6
+#define HTTP_RATE_LIMITED	7
 
 /*
  * Requests a URL and stores the result in a strbuf.