Re: [PATCH 6/8] run-command: add an asynchronous parallel child processor

> This allows to run external commands in parallel with ordered output
> on stderr.
>
> If we run external commands in parallel we cannot pipe the output directly
> to the our stdout/err as it would mix up. So each process's output will
> flow through a pipe, which we buffer. One subprocess can be directly
> piped to out stdout/err for a low latency feedback to the user.
>
> Example:
> Let's assume we have 5 submodules A,B,C,D,E and each fetch takes a
> different amount of time as the different submodules vary in size, then
> the output of fetches in sequential order might look like this:
>
>   time -->
>   output: |---A---| |-B-| |-------C-------| |-D-| |-E-|
>
> When we schedule these submodules into maximal two parallel processes,
> a schedule and sample output over time may look like this:
>
> process 1: |---A---| |-D-| |-E-|
>
> process 2: |-B-| |-------C-------|
>
> output:    |---A---|B|---C-------|DE
>
> So A will be perceived as it would run normally in the single child
> version. As B has finished by the time A is done, we can dump its whole
> progress buffer on stderr, such that it looks like it finished in no
> time. Once that is done, C is determined to be the visible child and
> its progress will be reported in real time.
>
> So this way of output is really good for human consumption, as it only
> changes the timing, not the actual output.
>
> For machine consumption the output needs to be prepared in the tasks,
> by either having a prefix per line or per block to indicate whose tasks
> output is displayed, because the output order may not follow the
> original sequential ordering:
>
>   |----A----| |--B--| |-C-|
>
> will be scheduled to be all parallel:
>
> process 1: |----A----|
> process 2: |--B--|
> process 3: |-C-|
> output:    |----A----|CB
>
> This happens because C finished before B did, so it will be queued for
> output before B.
>
> The detection when a child has finished executing is done the same way as
> two fold. First we check regularly if the stderr pipe still exists in an
> interleaved manner with other actions such as checking other children
> for their liveliness or starting new children. Once a child closed their
> stderr stream, we assume it is stopping very soon, such that we can use
> the `finish_command` code borrowed from the single external process
> execution interface.

>
> By maintaining the strong assumption of stderr being open until the
> very end of a child process, we can avoid other hassle such as an
> implementation using `waitpid(-1)`, which is not implemented in Windows.
>
> Signed-off-by: Stefan Beller <sbeller@google.com>