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>

Author: vaidas-shopify

Documentation/config/http.adoc

@@ -315,6 +315,29 @@ http.keepAliveCount::
 	unset, curl's default value is used. Can be overridden by the
 	`GIT_HTTP_KEEPALIVE_COUNT` environment variable.
 
+http.retryAfter::
+	Default wait time in seconds before retrying when a server returns
+	HTTP 429 (Too Many Requests) without a Retry-After header.
+	Defaults to 0 (retry immediately). When a Retry-After header is
+	present, its value takes precedence over this setting. Can be
+	overridden by the `GIT_HTTP_RETRY_AFTER` environment variable.
+	See also `http.maxRetries` and `http.maxRetryTime`.
+
+http.maxRetries::
+	Maximum number of times to retry after receiving HTTP 429 (Too Many
+	Requests) responses. Set to 0 (the default) to disable retries.
+	Can be overridden by the `GIT_HTTP_MAX_RETRIES` environment variable.
+	See also `http.retryAfter` and `http.maxRetryTime`.
+
+http.maxRetryTime::
+	Maximum time in seconds to wait for a single retry attempt when
+	handling HTTP 429 (Too Many Requests) responses. If the server
+	requests a delay (via Retry-After header) or if `http.retryAfter`
+	is configured with a value that exceeds this maximum, Git will fail
+	immediately rather than waiting. Default is 300 seconds (5 minutes).
+	Can be overridden by the `GIT_HTTP_MAX_RETRY_TIME` environment
+	variable. See also `http.retryAfter` and `http.maxRetries`.
+
 http.noEPSV::
 	A boolean which disables using of EPSV ftp command by curl.
 	This can be helpful with some "poor" ftp servers which don't

git-curl-compat.h

@@ -37,6 +37,14 @@
 #define GIT_CURL_NEED_TRANSFER_ENCODING_HEADER
 #endif
 
+/**
+ * CURLINFO_RETRY_AFTER was added in 7.66.0, released in September 2019.
+ * It allows curl to automatically parse Retry-After headers.
+ */
+#if LIBCURL_VERSION_NUM >= 0x074200
+#define GIT_CURL_HAVE_CURLINFO_RETRY_AFTER 1
+#endif
+
 /**
  * CURLOPT_PROTOCOLS_STR and CURLOPT_REDIR_PROTOCOLS_STR were added in 7.85.0,
  * released in August 2022.