Teach run_command how to setup a stdin pipe

Sometimes callers trying to use run_command to execute a child
process will want to setup a pipe or file descriptor to redirect
into the child's stdin.

This idea is completely stolen from builtin-bundle's fork_with_pipe,
written by Johannes Schindelin.  All credit (and blame) should lie
with Dscho.  ;-)

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

run-command.c

@@ -4,15 +4,39 @@
 
 int start_command(struct child_process *cmd)
 {
+	int need_in = !cmd->no_stdin && cmd->in < 0;
+	int fdin[2];
+
+	if (need_in) {
+		if (pipe(fdin) < 0)
+			return -ERR_RUN_COMMAND_PIPE;
+		cmd->in = fdin[1];
+		cmd->close_in = 1;
+	}
+
 	cmd->pid = fork();
-	if (cmd->pid < 0)
+	if (cmd->pid < 0) {
+		if (need_in) {
+			close(fdin[0]);
+			close(fdin[1]);
+		}
 		return -ERR_RUN_COMMAND_FORK;
+	}
+
 	if (!cmd->pid) {
 		if (cmd->no_stdin) {
 			int fd = open("/dev/null", O_RDWR);
 			dup2(fd, 0);
 			close(fd);
+		} else if (need_in) {
+			dup2(fdin[0], 0);
+			close(fdin[0]);
+			close(fdin[1]);
+		} else if (cmd->in) {
+			dup2(cmd->in, 0);
+			close(cmd->in);
 		}
+
 		if (cmd->stdout_to_stderr)
 			dup2(2, 1);
 		if (cmd->git_cmd) {
@@ -22,11 +46,20 @@ int start_command(struct child_process *cmd)
 		}
 		die("exec %s failed.", cmd->argv[0]);
 	}
+
+	if (need_in)
+		close(fdin[0]);
+	else if (cmd->in)
+		close(cmd->in);
+
 	return 0;
 }
 
 int finish_command(struct child_process *cmd)
 {
+	if (cmd->close_in)
+		close(cmd->in);
+
 	for (;;) {
 		int status, code;
 		pid_t waiting = waitpid(cmd->pid, &status, 0);

run-command.h

@@ -4,6 +4,7 @@
 enum {
 	ERR_RUN_COMMAND_FORK = 10000,
 	ERR_RUN_COMMAND_EXEC,
+	ERR_RUN_COMMAND_PIPE,
 	ERR_RUN_COMMAND_WAITPID,
 	ERR_RUN_COMMAND_WAITPID_WRONG_PID,
 	ERR_RUN_COMMAND_WAITPID_SIGNAL,
@@ -13,6 +14,8 @@ enum {
 struct child_process {
 	const char **argv;
 	pid_t pid;
+	int in;
+	unsigned close_in:1;
 	unsigned no_stdin:1;
 	unsigned git_cmd:1; /* if this is to be git sub-command */
 	unsigned stdout_to_stderr:1;