[ISSUE] mpick: multi redir test hangs on Alpine

[nmeum <nmeum@users.noreply.github.com>]

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New issue by nmeum on mblaze repository

https://github.com/leahneukirchen/mblaze/issues/203

Description:
While trying to get the mblaze test suite running on Alpine Linux, I noticed that the `multi redir` test from `2000-mpick.t` does not terminate, even with current git HEAD. To reproduce:

```
$ cat /tmp/input
inbox/cur/1:2,S
inbox/cur/2:2,ST
inbox/cur/3:2,SRT
inbox/cur/4:2,SRFT
inbox/cur/5:2,T
inbox/cur/6:2,SRF
inbox/cur/7:2,SR
inbox/cur/8:2,SF
inbox/cur/9:2,
$ cat expr
let foo = from.addr == "peter@example.org"
let bar = from.disp == "Peter Example"
in
  foo |"sed ""s/^/1:&/""" && bar |"sed ""s/^/2:&/""" && skip
$ mpick -F ./expr < /tmp/input
1:inbox/cur/1:2,S
1:inbox/cur/9:2,
2:inbox/cur/1:2,S
2:inbox/cur/9:2,
<process does not terminate>
```

I debugged this a bit with `strace(1)`. The output of the 3 processes (mpick, sed, and sed) is available here:

* [sed.25493.txt](https://github.com/leahneukirchen/mblaze/files/6135400/sed.25493.txt)
* [sed.25494.txt](https://github.com/leahneukirchen/mblaze/files/6135401/sed.25494.txt)
* [sed.25495.txt](https://github.com/leahneukirchen/mblaze/files/6135402/sed.25495.txt)

I am a bit confused as to what is happening here. The mpick process (PID 25493) blocks because it waits for the first sed process (PID 25494) to terminate (since mpick uses `pclose(3)`). The sed process on the other hand doesn't terminate because it is still reading from the reading end of the pipe. However, I would expect that read to return with an EOF because the mpick process already closed the writing end of the pipe. Yet for some reason, it is stuck in a read system call. Maybe I am missing something obvious here, any idea what might be causing this?