should libgap be fork(2)-safe?

[dimpase]

We have a scenario where an application which uses libgap calls fork on itself. It appears to work, except in the case where the corresponding installation of GAP has Browse package installed. In this case fork call leads to a lock-up, involving libgap calling ncurses.so GAP kernel module from Browse, which in turns calls libncurses or its equvalent, and, oops, it all freezes. Hitting Ctrl-C produces a dump showing that just mentioned components involved. See e.g. gap-system/homebrew-gap#23 (comment)

As there is no Unload equivalent to LoadPackage, the most obvious workaround is not having Browse installed. Which is kind of silly. Should work be put into getting it fixed?

[ChrisJefferson]

I think this is more complex than 'libgap is fork safe', I mean, unless there is a specific bug in GAP, I've auto-loaded Browse and used IO_fork from the IO package for years, many people have, and everything works fine. Can you produce a minimal example which just uses libgap (not the entirety of sage) and show the problem? I suppose we might be somehow specially catching browse some how, but I really don't want to debug this via sage.

[dimpase]

The problem comes from using python3 multiprocessing with fork option on python code with a libgap extension. The quickest way to have a minimal reproducer might be to write it in C, something I'm not quite looking forward to - hopefully it's not needed.

I understand that Browse does have code testing whether it outputs to tty, and (probably?) just doesn't do anything if this is not the case, right? So perhaps the question is about setting the appropriate environment for Browse to get this across. Do you know whether IO_fork does something non-obvious to deal with this?

[dimpase]

OK, Sage shoots itself in the foot by disabling SIGCHLD signal in its libgap initialization code:
https://github.com/sagemath/sage/blob/80e56eaeb7089cfc5e4e0cc495ba8da43ee0a573/src/sage/libs/gap/util.pyx#L343

It makes it more complicated that also cysignals is involved in the whole thing. @orlitzky - you touched libgap calling recently...

Also, the docs of IO_InstallSIGCHLDHandler are a bit cryptic, saying

 This function is automatically called by any  function  which  creates  new processes

What's any function here? Does it cover calling fork(2) on a process which loaded libgap/IO?

[fingolfin]

"Any function" here refers of course to any function defined by the IO package, i.e., the IO_* functions such as IO_fork, etc.

If you search the documentation for IO_InstallSIGCHLDHandler you'll also see it being mentioned explicitly in e.g. IO_Popen, IO_Popen2 etc.