Merge branch 'js/mingw-http-ssl'

On platforms with recent cURL library, http.sslBackend configuration
variable can be used to choose a different SSL backend at runtime.
The Windows port uses this mechanism to switch between OpenSSL and
Secure Channel while talking over the HTTPS protocol.

* js/mingw-http-ssl:
  http: when using Secure Channel, ignore sslCAInfo by default
  http: add support for disabling SSL revocation checks in cURL
  http: add support for selecting SSL backends at runtime

Documentation/config.txt

@@ -2003,6 +2003,27 @@ http.sslCAPath::
 	with when fetching or pushing over HTTPS. Can be overridden
 	by the `GIT_SSL_CAPATH` environment variable.
 
+http.sslBackend::
+	Name of the SSL backend to use (e.g. "openssl" or "schannel").
+	This option is ignored if cURL lacks support for choosing the SSL
+	backend at runtime.
+
+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.
+
+http.schannelUseSSLCAInfo::
+	As of cURL v7.60.0, the Secure Channel backend can use the
+	certificate bundle provided via `http.sslCAInfo`, but that would
+	override the Windows Certificate Store. Since this is not desirable
+	by default, Git will tell cURL not to use that bundle by default
+	when the `schannel` backend was configured via `http.sslBackend`,
+	unless `http.schannelUseSSLCAInfo` overrides this behavior.
+
 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

@@ -155,6 +155,16 @@ static struct active_request_slot *active_queue_head;
 
 static char *cached_accept_language;
 
+static char *http_ssl_backend;
+
+static int http_schannel_check_revoke = 1;
+/*
+ * 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
+ * by default.
+ */
+static int http_schannel_use_ssl_cainfo;
+
 size_t fread_buffer(char *ptr, size_t eltsize, size_t nmemb, void *buffer_)
 {
 	size_t size = eltsize * nmemb;
@@ -302,6 +312,22 @@ static int http_options(const char *var, const char *value, void *cb)
 		curl_ssl_try = git_config_bool(var, value);
 		return 0;
 	}
+	if (!strcmp("http.sslbackend", var)) {
+		free(http_ssl_backend);
+		http_ssl_backend = xstrdup_or_null(value);
+		return 0;
+	}
+
+	if (!strcmp("http.schannelcheckrevoke", var)) {
+		http_schannel_check_revoke = git_config_bool(var, value);
+		return 0;
+	}
+
+	if (!strcmp("http.schannelusesslcainfo", var)) {
+		http_schannel_use_ssl_cainfo = git_config_bool(var, value);
+		return 0;
+	}
+
 	if (!strcmp("http.minsessions", var)) {
 		min_curl_sessions = git_config_int(var, value);
 #ifndef USE_CURL_MULTI
@@ -803,6 +829,16 @@ static CURL *get_curl_handle(void)
 	}
 #endif
 
+	if (http_ssl_backend && !strcmp("schannel", http_ssl_backend) &&
+	    !http_schannel_check_revoke) {
+#if LIBCURL_VERSION_NUM >= 0x072c00
+		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);
 
@@ -844,7 +880,13 @@ static CURL *get_curl_handle(void)
 	if (ssl_pinnedkey != NULL)
 		curl_easy_setopt(result, CURLOPT_PINNEDPUBLICKEY, ssl_pinnedkey);
 #endif
-	if (ssl_cainfo != NULL)
+	if (http_ssl_backend && !strcmp("schannel", http_ssl_backend) &&
+	    !http_schannel_use_ssl_cainfo) {
+		curl_easy_setopt(result, CURLOPT_CAINFO, NULL);
+#if LIBCURL_VERSION_NUM >= 0x073400
+		curl_easy_setopt(result, CURLOPT_PROXY_CAINFO, NULL);
+#endif
+	} else if (ssl_cainfo != NULL)
 		curl_easy_setopt(result, CURLOPT_CAINFO, ssl_cainfo);
 
 	if (curl_low_speed_limit > 0 && curl_low_speed_time > 0) {
@@ -995,6 +1037,33 @@ void http_init(struct remote *remote, const char *url, int proactive_auth)
 	git_config(urlmatch_config_entry, &config);
 	free(normalized_url);
 
+#if LIBCURL_VERSION_NUM >= 0x073800
+	if (http_ssl_backend) {
+		const curl_ssl_backend **backends;
+		struct strbuf buf = STRBUF_INIT;
+		int i;
+
+		switch (curl_global_sslset(-1, http_ssl_backend, &backends)) {
+		case CURLSSLSET_UNKNOWN_BACKEND:
+			strbuf_addf(&buf, _("Unsupported SSL backend '%s'. "
+					    "Supported SSL backends:"),
+					    http_ssl_backend);
+			for (i = 0; backends[i]; i++)
+				strbuf_addf(&buf, "\n\t%s", backends[i]->name);
+			die("%s", buf.buf);
+		case CURLSSLSET_NO_BACKENDS:
+			die(_("Could not set SSL backend to '%s': "
+			      "cURL was built without SSL backends"),
+			    http_ssl_backend);
+		case CURLSSLSET_TOO_LATE:
+			die(_("Could not set SSL backend to '%s': already set"),
+			    http_ssl_backend);
+		case CURLSSLSET_OK:
+			break; /* Okay! */
+		}
+	}
+#endif
+
 	if (curl_global_init(CURL_GLOBAL_ALL) != CURLE_OK)
 		die("curl_global_init failed");