http: add custom hostname to IP address resolutions

Libcurl has a CURLOPT_RESOLVE easy option that allows
the result of hostname resolution in the following
format to be passed:

	[+]HOST:PORT:ADDRESS[,ADDRESS]

This way, redirects and everything operating against the
HOST+PORT will use the provided ADDRESS(s).

The following format is also allowed to stop using
hostname resolutions that have already been passed:

	-HOST:PORT

See https://curl.se/libcurl/c/CURLOPT_RESOLVE.html for
more details.

Let's add a corresponding "http.curloptResolve" config
option that takes advantage of CURLOPT_RESOLVE.

Each value configured for the "http.curloptResolve" key
is passed "as is" to libcurl through CURLOPT_RESOLVE, so
it should be in one of the above 2 formats. This keeps
the implementation simple and makes us consistent with
libcurl's CURLOPT_RESOLVE, and with curl's corresponding
`--resolve` command line option.

The implementation uses CURLOPT_RESOLVE only in
get_active_slot() which is called by all the HTTP
request sending functions.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/http.txt

@@ -98,6 +98,22 @@ http.version::
 	- HTTP/2
 	- HTTP/1.1
 
+http.curloptResolve::
+	Hostname resolution information that will be used first by
+	libcurl when sending HTTP requests.  This information should
+	be in one of the following formats:
+
+	- [+]HOST:PORT:ADDRESS[,ADDRESS]
+	- -HOST:PORT
+
++
+The first format redirects all requests to the given `HOST:PORT`
+to the provided `ADDRESS`(s). The second format clears all
+previous config values for that `HOST:PORT` combination.  To
+allow easy overriding of all the settings inherited from the
+system config, an empty value will reset all resolution
+information to the empty list.
+
 http.sslVersion::
 	The SSL version to use when negotiating an SSL connection, if you
 	want to force the default.  The available and default version

http.c

@@ -128,6 +128,8 @@ static struct curl_slist *pragma_header;
 static struct curl_slist *no_pragma_header;
 static struct string_list extra_http_headers = STRING_LIST_INIT_DUP;
 
+static struct curl_slist *host_resolutions;
+
 static struct active_request_slot *active_queue_head;
 
 static char *cached_accept_language;
@@ -393,6 +395,18 @@ static int http_options(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp("http.curloptresolve", var)) {
+		if (!value) {
+			return config_error_nonbool(var);
+		} else if (!*value) {
+			curl_slist_free_all(host_resolutions);
+			host_resolutions = NULL;
+		} else {
+			host_resolutions = curl_slist_append(host_resolutions, value);
+		}
+		return 0;
+	}
+
 	if (!strcmp("http.followredirects", var)) {
 		if (value && !strcmp(value, "initial"))
 			http_follow_config = HTTP_FOLLOW_INITIAL;
@@ -1131,6 +1145,9 @@ void http_cleanup(void)
 	curl_slist_free_all(no_pragma_header);
 	no_pragma_header = NULL;
 
+	curl_slist_free_all(host_resolutions);
+	host_resolutions = NULL;
+
 	if (curl_http_proxy) {
 		free((void *)curl_http_proxy);
 		curl_http_proxy = NULL;
@@ -1211,6 +1228,7 @@ struct active_request_slot *get_active_slot(void)
 	if (curl_save_cookies)
 		curl_easy_setopt(slot->curl, CURLOPT_COOKIEJAR, curl_cookie_file);
 	curl_easy_setopt(slot->curl, CURLOPT_HTTPHEADER, pragma_header);
+	curl_easy_setopt(slot->curl, CURLOPT_RESOLVE, host_resolutions);
 	curl_easy_setopt(slot->curl, CURLOPT_ERRORBUFFER, curl_errorstr);
 	curl_easy_setopt(slot->curl, CURLOPT_CUSTOMREQUEST, NULL);
 	curl_easy_setopt(slot->curl, CURLOPT_READFUNCTION, NULL);

t/t5551-http-fetch-smart.sh

@@ -567,4 +567,11 @@ test_expect_success 'client falls back from v2 to v0 to match server' '
 	grep symref=HEAD:refs/heads/ trace
 '
 
+test_expect_success 'passing hostname resolution information works' '
+	BOGUS_HOST=gitbogusexamplehost.invalid &&
+	BOGUS_HTTPD_URL=$HTTPD_PROTO://$BOGUS_HOST:$LIB_HTTPD_PORT &&
+	test_must_fail git ls-remote "$BOGUS_HTTPD_URL/smart/repo.git" >/dev/null &&
+	git -c "http.curloptResolve=$BOGUS_HOST:$LIB_HTTPD_PORT:127.0.0.1" ls-remote "$BOGUS_HTTPD_URL/smart/repo.git" >/dev/null
+'
+
 test_done