Re: [PATCH] git-gui: drain the cat-file pipe before closing it

> From: chib <chib@foxmail.com>
> 
> commit_committree opens "git cat-file commit <parent>" to read the tree
> line for the empty-commit check, reads only the first line, and then
> closes the pipe while the rest of the commit object (often several
> kilobytes of commit message) is still unread.
> 
> On Linux this is harmless: the child process dies of SIGPIPE when it
> keeps writing, and that is not reported as an error when the pipe is
> closed. On Windows there is no SIGPIPE: the native git.exe gets a
> broken-pipe error when writing and exits with a non-zero status. Tcl's
> [close] then surfaces that as "child process exited abnormally", the
> commit is aborted, and the index lock is released with nothing
> committed. The failure only shows up once the parent commit's object is
> larger than the pipe buffer: in testing with Git for Windows 2.52,
> objects up to ~6.5 KiB always succeed while objects of ~9 KiB and up
> fail 10 out of 10 times (the threshold is around the 8 KiB pipe
> buffer). Amending a commit with a long message therefore triggers it
> reliably while short commits slip through.

> Reading the pipe to EOF
> before closing fixes it 10 out of 10 times, and is harmless on POSIX
> platforms where the same test succeeds either way.
> 
> Read the rest of the pipe before closing it, mirroring what the amend
> path already does when loading the parent commit's message.