Re: [RFH] plumber's puzzle

Linus Torvalds [off-list ref] writes:

On Sun, 22 Apr 2007, Junio C Hamano wrote:

quoted

This shows the single "diff --git" header line without anything,
to show that the path is not stat-clean, but the contents are
unchanged, which is what is expected.

Actually, I think the "good" case is the broken one.

Do an "strace -f" on the two cases, and you'll see an EBADF in the case 
that you think is good: the missing output *is* there, it's just that you 
closed the file descriptor so you don't see it.

So if the output you want is with the close(1) (ie with the output 
discarded), then you have some other bug there.

I think I figured it out.  The extra EBADF output comes from the
process that calls finish_command() in filter_buffer().

That is because the caller is diff_flush() which prepares its
output using stdio, and when apply_filter -> filter_bufer
callchain forks, the unflushed stdout hangs around in the
child.  Then we call exit() in apply_filter() to terminate the
child we spawned to do the filtering.  It flushes its copy of
stdio buffer.

Yuck.

I should be happy that I figured out what is going on, but I am
not very happy with this patch.

diff --git a/convert.c b/convert.c
index 845825b..35bb8cf 100644
--- a/convert.c
+++ b/convert.c

@@ -233,7 +233,6 @@ static int filter_buffer(const char *path, const char *src,
 		return 1;
 	}
 	close(pipe_feed[0]);
-	close(1);
 
 	write_err = (write_in_full(pipe_feed[1], src, size) < 0);
 	if (close(pipe_feed[1]))

@@ -273,6 +272,7 @@ static char *apply_filter(const char *path, const char *src,
 		return NULL;
 	}
 
+	fflush(stdout);
 	child_process.pid = fork();
 	if (child_process.pid < 0) {
 		error("cannot fork to run external filter %s", cmd);