pipe hang issue when running as SYSTEM

Takashi Yano takashi.yano@nifty.ne.jp
Tue Mar 22 10:18:07 GMT 2022

On Mon, 21 Mar 2022 11:45:57 -0700 (PDT)
Jeremy Drake wrote:
> This issue was reported to MSYS2 as a hang when trying to build libxml2 in
> a Windows docker container.  Another user was able to come up with a
> simple reproducer and a reasonable theory as to why it happens.  The msys2
> issue is https://github.com/msys2/msys2-runtime/issues/77, and I will
> quote the reproducer and analysis here:
> I am having the same issue when building mingw. I managed to reduce the
> issue to the following shell script:
> #!/bin/sh
> seq 1 99999 > big_file
> eval '$(eval cmd.exe /c "type big_file" | : )'
> When running as a normal user this completes immediately, but when run as
> a system service it hangs forever. The issue appears to be that when
> running under the SYSTEM account, the third sh process holds open the read
> end of the pipe. Since cmd has too much output to write all at once, it
> waits until the pipe's buffer has room to write more, but since sh isn't
> actually reading from the pipe, this hangs forever. When running as a
> normal user the read end of the pipe is not kept open, and so cmd.exe gets
> an error when attempting to write and exits immediately.
> My suspicion is that this is caused by f79a461 (which keeps the read end
> of the pipe open) and b531d6b (which changes the behavior depending on
> whether or not the program is running as a service).

Thanks for the report. This is expected problem as mentioned
in b531d6b commit message. However, I could not imagin the
situation that the service has multiple writer for the pipe
and one of them is a non-cygwin app.

Question is: Does the docker invoke the command using SYSTEM
account? Or is the processes in docker determined as running
as a service?

Takashi Yano <takashi.yano@nifty.ne.jp>

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