diff --git a/builtin-fetch-pack.c b/builtin-fetch-pack.c
index 871b7042e7..51d8a32791 100644
--- a/builtin-fetch-pack.c
+++ b/builtin-fetch-pack.c
@@ -457,42 +457,37 @@ static int everything_local(struct ref **refs, int nr_match, char **match)
 	return retval;
 }
 
-static pid_t setup_sideband(int fd[2], int xd[2])
+static int sideband_demux(int fd, void *data)
 {
-	pid_t side_pid;
+	int *xd = data;
 
+	close(xd[1]);
+	return recv_sideband("fetch-pack", xd[0], fd, 2);
+}
+
+static void setup_sideband(int fd[2], int xd[2], struct async *demux)
+{
 	if (!use_sideband) {
 		fd[0] = xd[0];
 		fd[1] = xd[1];
-		return 0;
+		return;
 	}
 	/* xd[] is talking with upload-pack; subprocess reads from
 	 * xd[0], spits out band#2 to stderr, and feeds us band#1
-	 * through our fd[0].
+	 * through demux->out.
 	 */
-	if (pipe(fd) < 0)
-		die("fetch-pack: unable to set up pipe");
-	side_pid = fork();
-	if (side_pid < 0)
+	demux->proc = sideband_demux;
+	demux->data = xd;
+	if (start_async(demux))
 		die("fetch-pack: unable to fork off sideband demultiplexer");
-	if (!side_pid) {
-		/* subprocess */
-		close(fd[0]);
-		if (xd[0] != xd[1])
-			close(xd[1]);
-		if (recv_sideband("fetch-pack", xd[0], fd[1], 2))
-			exit(1);
-		exit(0);
-	}
 	close(xd[0]);
-	close(fd[1]);
+	fd[0] = demux->out;
 	fd[1] = xd[1];
-	return side_pid;
 }
 
 static int get_pack(int xd[2], char **pack_lockfile)
 {
-	pid_t side_pid;
+	struct async demux;
 	int fd[2];
 	const char *argv[20];
 	char keep_arg[256];
@@ -501,7 +496,7 @@ static int get_pack(int xd[2], char **pack_lockfile)
 	int do_keep = args.keep_pack;
 	struct child_process cmd;
 
-	side_pid = setup_sideband(fd, xd);
+	setup_sideband(fd, xd, &demux);
 
 	memset(&cmd, 0, sizeof(cmd));
 	cmd.argv = argv;
@@ -556,6 +551,8 @@ static int get_pack(int xd[2], char **pack_lockfile)
 
 	if (finish_command(&cmd))
 		die("%s failed", argv[0]);
+	if (use_sideband && finish_async(&demux))
+		die("error in sideband demultiplexer");
 	return 0;
 }