GAP.jl hangs sometimes

[fingolfin]

We've been debugging an issue where GAP.jl sometimes just hangs in CI jobs. I just couldn't find an issue for this, so I've decided to open one now; if there is already an issue, please add a comment here pointing to it.

@ThomasBreuer narrowed it down in PR #875 to a call to AUTODOC_AbsolutePath while running makedoc.g. That function hangs inside Process(Directory(dir), pwd, InputTextNone(), OutputTextString(result, true), []);, where the program it tries to execute is /bin/pwd.

The latest AutoDoc release 2023.06.19 (from today) actually gets rid of this function, so once we switch to that (by upgrading to a newer GAP package distro or some other means) this particular hang won't occur again.

But the underlying issue is still there: Process hangs. But I so far couldn't reproduce it locally :-(. I tried something naive (on both macOS and Linux), no success:

mypwd = GAP.evalstr("""
function()
    local pwd, result;
    pwd := Filename( DirectoriesSystemPrograms(), "pwd" );
    if pwd = fail then
        Error("failed to locate 'pwd' tool");
    fi;
    result := "";
    Process(DirectoryCurrent(), pwd, InputTextNone(), OutputTextString(result, true), []);
    return Chomp(result);
end
""")
for i in 1:1000 println(i, ": ", mypwd()) end

I am guessing for the hang to occur, certain other circumstances must align.

Earlier today I thought this was an issue with GAP's child process handling interfering with that of Julia. But I now realized that this is nonsense, because Process is implemented using ExecuteProcess, and we already are replacing that function by a Julia implementation! So if at all it seems like the replacement Julia code may have an issue?

Without a good way to reproduce the hang locally, it may be difficult to tackler this properly :-(

[fingolfin]

Resolved by PR #906 for now.