run-command: support custom fd-set in async

This patch adds the possibility to supply a set of non-0 file
descriptors for async process communication instead of the
default-created pipe.

Additionally, we now support bi-directional communiction with the
async procedure, by giving the async function both read and write
file descriptors.

To retain compatiblity and similar "API feel" with start_command,
we require start_async callers to set .out = -1 to get a readable
file descriptor.  If either of .in or .out is 0, we supply no file
descriptor to the async process.

[sp: Note: Erik started this patch, and a huge bulk of it is
     his work.  All bugs were introduced later by Shawn.]

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -327,17 +327,51 @@ int run_command_v_opt_cd_env(const char **argv, int opt, const char *dir, const
 static unsigned __stdcall run_thread(void *data)
 {
 	struct async *async = data;
-	return async->proc(async->fd_for_proc, async->data);
+	return async->proc(async->proc_in, async->proc_out, async->data);
 }
 #endif
 
 int start_async(struct async *async)
 {
-	int pipe_out[2];
+	int need_in, need_out;
+	int fdin[2], fdout[2];
+	int proc_in, proc_out;
 
-	if (pipe(pipe_out) < 0)
-		return error("cannot create pipe: %s", strerror(errno));
-	async->out = pipe_out[0];
+	need_in = async->in < 0;
+	if (need_in) {
+		if (pipe(fdin) < 0) {
+			if (async->out > 0)
+				close(async->out);
+			return error("cannot create pipe: %s", strerror(errno));
+		}
+		async->in = fdin[1];
+	}
+
+	need_out = async->out < 0;
+	if (need_out) {
+		if (pipe(fdout) < 0) {
+			if (need_in)
+				close_pair(fdin);
+			else if (async->in)
+				close(async->in);
+			return error("cannot create pipe: %s", strerror(errno));
+		}
+		async->out = fdout[0];
+	}
+
+	if (need_in)
+		proc_in = fdin[0];
+	else if (async->in)
+		proc_in = async->in;
+	else
+		proc_in = -1;
+
+	if (need_out)
+		proc_out = fdout[1];
+	else if (async->out)
+		proc_out = async->out;
+	else
+		proc_out = -1;
 
 #ifndef WIN32
 	/* Flush stdio before fork() to avoid cloning buffers */
@@ -346,24 +380,47 @@ int start_async(struct async *async)
 	async->pid = fork();
 	if (async->pid < 0) {
 		error("fork (async) failed: %s", strerror(errno));
-		close_pair(pipe_out);
-		return -1;
+		goto error;
 	}
 	if (!async->pid) {
-		close(pipe_out[0]);
-		exit(!!async->proc(pipe_out[1], async->data));
+		if (need_in)
+			close(fdin[1]);
+		if (need_out)
+			close(fdout[0]);
+		exit(!!async->proc(proc_in, proc_out, async->data));
 	}
-	close(pipe_out[1]);
+
+	if (need_in)
+		close(fdin[0]);
+	else if (async->in)
+		close(async->in);
+
+	if (need_out)
+		close(fdout[1]);
+	else if (async->out)
+		close(async->out);
 #else
-	async->fd_for_proc = pipe_out[1];
+	async->proc_in = proc_in;
+	async->proc_out = proc_out;
 	async->tid = (HANDLE) _beginthreadex(NULL, 0, run_thread, async, 0, NULL);
 	if (!async->tid) {
 		error("cannot create thread: %s", strerror(errno));
-		close_pair(pipe_out);
-		return -1;
+		goto error;
 	}
 #endif
 	return 0;
+
+error:
+	if (need_in)
+		close_pair(fdin);
+	else if (async->in)
+		close(async->in);
+
+	if (need_out)
+		close_pair(fdout);
+	else if (async->out)
+		close(async->out);
+	return -1;
 }
 
 int finish_async(struct async *async)