http: add support for selecting SSL backends at runtime

As of version 7.56.0, curl supports being compiled with multiple SSL
backends.

This patch adds the Git side of that feature: by setting http.sslBackend
to "openssl" or "schannel", Git for Windows can now choose the SSL
backend at runtime.

This comes in handy on Windows because Secure Channel ("schannel") is
the native solution, accessing the Windows Credential Store, thereby
allowing for enterprise-wide management of certificates. For historical
reasons, Git for Windows needs to support OpenSSL still, as it has
previously been the only supported SSL backend in Git for Windows for
almost a decade.

The patch has been carried in Git for Windows for over a year, and is
considered mature.

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

Author: dscho

Documentation/config.txt

@@ -2236,6 +2236,11 @@ 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.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 +155,8 @@ static struct active_request_slot *active_queue_head;
 
 static char *cached_accept_language;
 
+static char *http_ssl_backend;
+
 size_t fread_buffer(char *ptr, size_t eltsize, size_t nmemb, void *buffer_)
 {
 	size_t size = eltsize * nmemb;
@@ -302,6 +304,12 @@ 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.minsessions", var)) {
 		min_curl_sessions = git_config_int(var, value);
 #ifndef USE_CURL_MULTI
@@ -995,6 +1003,33 @@ 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");