Support FTP-over-SSL/TLS for regular FTP

Add a boolean http.sslTry option which allows to enable AUTH SSL/TLS and
encrypted data transfers when connecting via regular FTP protocol.

Default is false since it might trigger certificate verification errors on
misconfigured servers.

Signed-off-by: Modestas Vainius <modestas@vainius.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1447,6 +1447,14 @@ http.sslCAPath::
 	with when fetching or pushing over HTTPS. Can be overridden
 	by the 'GIT_SSL_CAPATH' environment variable.
 
+http.sslTry::
+	Attempt to use AUTH SSL/TLS and encrypted data transfers
+	when connecting via regular FTP protocol. This might be needed
+	if the FTP server requires it for security reasons or you wish
+	to connect securely whenever remote FTP server supports it.
+	Default is false since it might trigger certificate verification
+	errors on misconfigured servers.
+
 http.maxRequests::
 	How many HTTP requests to launch in parallel. Can be overridden
 	by the 'GIT_HTTP_MAX_REQUESTS' environment variable. Default is 5.

http.c

@@ -30,6 +30,7 @@ static CURL *curl_default;
 char curl_errorstr[CURL_ERROR_SIZE];
 
 static int curl_ssl_verify = -1;
+static int curl_ssl_try;
 static const char *ssl_cert;
 #if LIBCURL_VERSION_NUM >= 0x070903
 static const char *ssl_key;
@@ -162,6 +163,10 @@ static int http_options(const char *var, const char *value, void *cb)
 			ssl_cert_password_required = 1;
 		return 0;
 	}
+	if (!strcmp("http.ssltry", var)) {
+		curl_ssl_try = git_config_bool(var, value);
+		return 0;
+	}
 	if (!strcmp("http.minsessions", var)) {
 		min_curl_sessions = git_config_int(var, value);
 #ifndef USE_CURL_MULTI
@@ -306,6 +311,11 @@ static CURL *get_curl_handle(void)
 	if (curl_ftp_no_epsv)
 		curl_easy_setopt(result, CURLOPT_FTP_USE_EPSV, 0);
 
+#ifdef CURLOPT_USE_SSL
+	if (curl_ssl_try)
+		curl_easy_setopt(result, CURLOPT_USE_SSL, CURLUSESSL_TRY);
+#endif
+
 	if (curl_http_proxy) {
 		curl_easy_setopt(result, CURLOPT_PROXY, curl_http_proxy);
 		curl_easy_setopt(result, CURLOPT_PROXYAUTH, CURLAUTH_ANY);

http.h

@@ -42,6 +42,15 @@
 #define NO_CURL_IOCTL
 #endif
 
+/*
+ * CURLOPT_USE_SSL was known as CURLOPT_FTP_SSL up to 7.16.4,
+ * and the constants were known as CURLFTPSSL_*
+*/
+#if !defined(CURLOPT_USE_SSL) && defined(CURLOPT_FTP_SSL)
+#define CURLOPT_USE_SSL CURLOPT_FTP_SSL
+#define CURLUSESSL_TRY CURLFTPSSL_TRY
+#endif
+
 struct slot_results {
 	CURLcode curl_result;
 	long http_code;