Merge branch 'http-ssl-backend'

This topic branch brings support for choosing cURL's SSL backend at
runtime via http.sslBackend, based on patches already submitted to the
cURL project and backported to cURL 7.54.1 as used in Git for Windows'
SDK.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

Documentation/config.txt

@@ -2257,6 +2257,27 @@ 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");