stream: add support for setting a proxy

If the stream claims to support this feature, we can let the transport
set the proxy.

include/git2/sys/stream.h

@@ -29,8 +29,10 @@ typedef struct git_stream {
 	int version;
 
 	int encrypted;
+	int proxy_support;
 	int (*connect)(struct git_stream *);
 	int (*certificate)(git_cert **, struct git_stream *);
+	int (*set_proxy)(struct git_stream *, const char *proxy_url);
 	ssize_t (*read)(struct git_stream *, void *, size_t);
 	ssize_t (*write)(struct git_stream *, const char *, size_t, int);
 	int (*close)(struct git_stream *);

src/curl_stream.c

@@ -64,6 +64,17 @@ static int curls_certificate(git_cert **out, git_stream *stream)
 	return 0;
 }
 
+static int curls_set_proxy(git_stream *stream, const char *proxy_url)
+{
+	CURLcode res;
+	curl_stream *s = (curl_stream *) stream;
+
+	if ((res = curl_easy_setopt(s->handle, CURLOPT_PROXY, proxy_url)) != CURLE_OK)
+		return seterr_curl(s);
+
+	return 0;
+}
+
 static int wait_for(curl_socket_t fd, bool reading)
 {
 	int ret;
@@ -189,8 +200,10 @@ int git_curl_stream_new(git_stream **out, const char *host, const char *port, in
 
 	st->parent.version = GIT_STREAM_VERSION;
 	st->parent.encrypted = encrypted;
+	st->parent.proxy_support = 1;
 	st->parent.connect = curls_connect;
 	st->parent.certificate = curls_certificate;
+	st->parent.set_proxy = curls_set_proxy;
 	st->parent.read = curls_read;
 	st->parent.write = curls_write;
 	st->parent.close = curls_close;

src/stream.h

@@ -30,6 +30,21 @@ GIT_INLINE(int) git_stream_certificate(git_cert **out, git_stream *st)
 	return st->certificate(out, st);
 }
 
+GIT_INLINE(int) git_stream_supports_proxy(git_stream *st)
+{
+	return st->proxy_support;
+}
+
+GIT_INLINE(int) git_stream_set_proxy(git_stream *st, const char *proxy_url)
+{
+	if (!st->proxy_support) {
+		giterr_set(GITERR_INVALID, "proxy not supported on this stream");
+		return -1;
+	}
+
+	return st->set_proxy(st, proxy_url);
+}
+
 GIT_INLINE(ssize_t) git_stream_read(git_stream *st, void *data, size_t len)
 {
 	return st->read(st, data, len);