add support for disabling SSL revocation checks in cURL

This adds support for a new http.schannel.checkRevoke config value.

This config value is only used if http.sslBackend is set to "schannel",
which forces cURL to use the Windows Certificate Store when validating
server certificates associated with a remote server.

This config value should only be set to "false" if you are in an
environment where revocation checks are blocked by the network, with
no alternative options.

This is only supported in cURL 7.44 or later.

Signed-off-by: Brendan Forster <github@brendanforster.com>

Documentation/config.txt

@@ -2124,6 +2124,14 @@ http.sslBackend::
 	This option is ignored if cURL lacks support for choosing the SSL
 	backend at runtime.
 
+http.schannel.checkRevoke::
+	Used to enforce or disable certificate revocation checks in cURL
+	when http.sslBackend is set to "schannel". Defaults to `true` if
+	unset. Only necessary to disable this if Git consistently errors
+	and the message is about checking the revocation status of a
+	certificate. This option is ignored if cURL lacks support for 
+	setting the relevant SSL option at runtime.
+
 http.pinnedpubkey::
 	Public key of the https service. It may either be the filename of
 	a PEM or DER encoded public key file or a string starting with

http.c

@@ -157,6 +157,8 @@ static char *cached_accept_language;
 
 static char *http_ssl_backend;
 
+static int http_schannel_check_revoke = 1;
+
 size_t fread_buffer(char *ptr, size_t eltsize, size_t nmemb, void *buffer_)
 {
 	size_t size = eltsize * nmemb;
@@ -310,6 +312,11 @@ static int http_options(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp("http.schannel.checkrevoke", var)) {
+		http_schannel_check_revoke = git_config_bool(var, value);
+		return 0;
+	}
+
 	if (!strcmp("http.minsessions", var)) {
 		min_curl_sessions = git_config_int(var, value);
 #ifndef USE_CURL_MULTI
@@ -811,6 +818,16 @@ static CURL *get_curl_handle(void)
 	}
 #endif
 
+	if (http_ssl_backend && !strcmp("schannel", http_ssl_backend) &&
+	    !http_schannel_check_revoke) {
+#if LIBCURL_VERSION_NUM >= 0x074400
+		curl_easy_setopt(result, CURLOPT_SSL_OPTIONS, CURLSSLOPT_NO_REVOKE);
+#else
+		warning("CURLSSLOPT_NO_REVOKE not applied to curl SSL options because\n"
+			"your curl version is too old (>= 7.44.0)");
+#endif
+	}
+
 	if (http_proactive_auth)
 		init_curl_http_auth(result);