http: support sending custom HTTP headers

We introduce a way to send custom HTTP headers with all requests.

This allows us, for example, to send an extra token from build agents
for temporary access to private repositories. (This is the use case that
triggered this patch.)

This feature can be used like this:

	git -c http.extraheader='Secret: sssh!' fetch $URL $REF

Note that `curl_easy_setopt(..., CURLOPT_HTTPHEADER, ...)` takes only
a single list, overriding any previous call. This means we have to
collect _all_ of the headers we want to use into a single list, and
feed it to cURL in one shot. Since we already unconditionally set a
"pragma" header when initializing the curl handles, we can add our new
headers to that list.

For callers which override the default header list (like probe_rpc),
we provide `http_copy_default_headers()` so they can do the same
trick.

Big thanks to Jeff King and Junio Hamano for their outstanding help and
patient reviews.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 32 insertions, 3 deletions

diff --git a/http.c b/http.c
index 69da4454d8..aae9944cb9 100644
--- a/http.c
+++ b/http.c
@@ -114,6 +114,7 @@ static unsigned long http_auth_methods = CURLAUTH_ANY;
 
 static struct curl_slist *pragma_header;
 static struct curl_slist *no_pragma_header;
+static struct curl_slist *extra_http_headers;
 
 static struct active_request_slot *active_queue_head;
 
@@ -323,6 +324,19 @@ static int http_options(const char *var, const char *value, void *cb)
 #endif
 	}
 
+	if (!strcmp("http.extraheader", var)) {
+		if (!value) {
+			return config_error_nonbool(var);
+		} else if (!*value) {
+			curl_slist_free_all(extra_http_headers);
+			extra_http_headers = NULL;
+		} else {
+			extra_http_headers =
+				curl_slist_append(extra_http_headers, value);
+		}
+		return 0;
+	}
+
 	/* Fall back on the default ones */
 	return git_default_config(var, value, cb);
 }
@@ -675,8 +689,10 @@ void http_init(struct remote *remote, const char *url, int proactive_auth)
 	if (remote)
 		var_override(&http_proxy_authmethod, remote->http_proxy_authmethod);
 
-	pragma_header = curl_slist_append(pragma_header, "Pragma: no-cache");
-	no_pragma_header = curl_slist_append(no_pragma_header, "Pragma:");
+	pragma_header = curl_slist_append(http_copy_default_headers(),
+		"Pragma: no-cache");
+	no_pragma_header = curl_slist_append(http_copy_default_headers(),
+		"Pragma:");
 
 #ifdef USE_CURL_MULTI
 	{
@@ -762,6 +778,9 @@ void http_cleanup(void)
 #endif
 	curl_global_cleanup();
 
+	curl_slist_free_all(extra_http_headers);
+	extra_http_headers = NULL;
+
 	curl_slist_free_all(pragma_header);
 	pragma_header = NULL;
 
@@ -1160,6 +1179,16 @@ int run_one_slot(struct active_request_slot *slot,
 	return handle_curl_result(results);
 }
 
+struct curl_slist *http_copy_default_headers(void)
+{
+	struct curl_slist *headers = NULL, *h;
+
+	for (h = extra_http_headers; h; h = h->next)
+		headers = curl_slist_append(headers, h->data);
+
+	return headers;
+}
+
 static CURLcode curlinfo_strbuf(CURL *curl, CURLINFO info, struct strbuf *buf)
 {
 	char *ptr;
@@ -1377,7 +1406,7 @@ static int http_request(const char *url,
 {
 	struct active_request_slot *slot;
 	struct slot_results results;
-	struct curl_slist *headers = NULL;
+	struct curl_slist *headers = http_copy_default_headers();
 	struct strbuf buf = STRBUF_INIT;
 	const char *accept_language;
 	int ret;