Avoid a dup2(2) in apply_filter() - start_command() can do it for us.

When apply_filter() runs the external (clean or smudge) filter program, it
needs to pass the writable end of a pipe as its stdout. For this purpose,
it used to dup2(2) the file descriptor explicitly to stdout. Now we use
the facilities of start_command() to do it for us.

Furthermore, the path argument of a subordinate function, filter_buffer(),
was not used, so here we replace it to pass the fd instead.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

convert.c

@@ -192,7 +192,7 @@ static int crlf_to_worktree(const char *path, const char *src, size_t len,
 	return 1;
 }
 
-static int filter_buffer(const char *path, const char *src,
+static int filter_buffer(int fd, const char *src,
 			 unsigned long size, const char *cmd)
 {
 	/*
@@ -205,6 +205,7 @@ static int filter_buffer(const char *path, const char *src,
 	memset(&child_process, 0, sizeof(child_process));
 	child_process.argv = argv;
 	child_process.in = -1;
+	child_process.out = fd;
 
 	if (start_command(&child_process))
 		return error("cannot fork to run external filter %s", cmd);
@@ -254,10 +255,8 @@ static int apply_filter(const char *path, const char *src, size_t len,
 		return 0;
 	}
 	if (!child_process.pid) {
-		dup2(pipe_feed[1], 1);
 		close(pipe_feed[0]);
-		close(pipe_feed[1]);
-		exit(filter_buffer(path, src, len, cmd));
+		exit(filter_buffer(pipe_feed[1], src, len, cmd));
 	}
 	close(pipe_feed[1]);