transport: let the progress output return an error

There are any number of issues that can come up in the progress
callback, and we should let the user cancel at that point as well.

include/git2/remote.h

@@ -410,7 +410,7 @@ struct git_remote_callbacks {
 	 * progress side-band will be passed to this function (this is
 	 * the 'counting objects' output.
 	 */
-	void (*progress)(const char *str, int len, void *data);
+	int (*progress)(const char *str, int len, void *data);
 
 	/**
 	 * Completion is called when different parts of the download

include/git2/transport.h

@@ -170,7 +170,7 @@ typedef enum {
 	GIT_TRANSPORTFLAGS_NO_CHECK_CERT = 1
 } git_transport_flags_t;
 
-typedef void (*git_transport_message_cb)(const char *str, int len, void *data);
+typedef int (*git_transport_message_cb)(const char *str, int len, void *data);
 
 typedef struct git_transport git_transport;
 

src/transports/smart_protocol.c

@@ -509,7 +509,10 @@ int git_smart__download_pack(
 		if (pkt->type == GIT_PKT_PROGRESS) {
 			if (t->progress_cb) {
 				git_pkt_progress *p = (git_pkt_progress *) pkt;
-				t->progress_cb(p->data, p->len, t->message_cb_payload);
+				if (t->progress_cb(p->data, p->len, t->message_cb_payload)) {
+					giterr_set(GITERR_NET, "The fetch was cancelled by the user");
+					return GIT_EUSER;
+				}
 			}
 			git__free(pkt);
 		} else if (pkt->type == GIT_PKT_DATA) {