http: use new "best effort" strategy for Secure Channel revoke checking

The native Windows HTTPS backend is based on Secure Channel which lets
the caller decide how to handle revocation checking problems caused by
missing information in the certificate or offline CRL distribution
points.

Unfortunately, cURL chose to handle these problems differently than
OpenSSL by default: while OpenSSL happily ignores those problems
(essentially saying "¯\_(ツ)_/¯"), the Secure Channel backend will error
out instead.

As a remedy, the "no revoke" mode was introduced, which turns off
revocation checking altogether. This is a bit heavy-handed. We support
this via the `http.schannelCheckRevoke` setting.

In curl/curl#4981, we contributed an opt-in
"best effort" strategy that emulates what OpenSSL seems to do.

In Git for Windows, we actually want this to be the default. This patch
makes it so, introducing it as a new value for the
`http.schannelCheckRevoke" setting, which now becmes a tristate: it
accepts the values "false", "true" or "best-effort" (defaulting to the
last one).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/config/http.txt

@@ -173,11 +173,13 @@ 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

@@ -165,7 +165,13 @@ 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
@@ -330,7 +336,19 @@ 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;
 	}
 
@@ -913,9 +931,9 @@ 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