Merge pull request git-for-windows#2535 from dscho/schannel-revoke-best-effort

Introduce and use the new "best effort" strategy for Secure Channel revoke checking

Author: dscho

Documentation/config/http.txt

@@ -152,11 +152,13 @@ http.sslBackend::
 
 http.schannelCheckRevoke::
 	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.
+	when http.sslBackend is set to "schannel" via "true" and "false",
+	respectively. Another accepted value is "best-effort" (the default)
+	in which case revocation checks are performed, but errors due to
+	revocation list distribution points that are offline are silently
+	ignored, as well as errors due to certificates missing revocation
+	list distribution points. This option is ignored if cURL lacks
+	support for setting the relevant SSL option at runtime.
 
 http.schannelUseSSLCAInfo::
 	As of cURL v7.60.0, the Secure Channel backend can use the

http.c

@@ -158,7 +158,13 @@ static char *cached_accept_language;
 
 static char *http_ssl_backend;
 
-static int http_schannel_check_revoke = 1;
+static int http_schannel_check_revoke_mode =
+#ifdef CURLSSLOPT_REVOKE_BEST_EFFORT
+	CURLSSLOPT_REVOKE_BEST_EFFORT;
+#else
+	CURLSSLOPT_NO_REVOKE;
+#endif
+
 /*
  * With the backend being set to `schannel`, setting sslCAinfo would override
  * the Certificate Store in cURL v7.60.0 and later, which is not what we want
@@ -323,7 +329,19 @@ static int http_options(const char *var, const char *value, void *cb)
 	}
 
 	if (!strcmp("http.schannelcheckrevoke", var)) {
-		http_schannel_check_revoke = git_config_bool(var, value);
+		if (value && !strcmp(value, "best-effort")) {
+			http_schannel_check_revoke_mode =
+#ifdef CURLSSLOPT_REVOKE_BEST_EFFORT
+				CURLSSLOPT_REVOKE_BEST_EFFORT;
+#else
+				CURLSSLOPT_NO_REVOKE;
+			warning(_("%s=%s unsupported by current cURL"),
+				var, value);
+#endif
+		} else
+			http_schannel_check_revoke_mode =
+				(git_config_bool(var, value) ?
+				 0 : CURLSSLOPT_NO_REVOKE);
 		return 0;
 	}
 
@@ -869,9 +887,9 @@ static CURL *get_curl_handle(void)
 #endif
 
 	if (http_ssl_backend && !strcmp("schannel", http_ssl_backend) &&
-	    !http_schannel_check_revoke) {
+	    http_schannel_check_revoke_mode) {
 #if LIBCURL_VERSION_NUM >= 0x072c00
-		curl_easy_setopt(result, CURLOPT_SSL_OPTIONS, CURLSSLOPT_NO_REVOKE);
+		curl_easy_setopt(result, CURLOPT_SSL_OPTIONS, http_schannel_check_revoke_mode);
 #else
 		warning(_("CURLSSLOPT_NO_REVOKE not supported with cURL < 7.44.0"));
 #endif