Hi Sam,
Thanks for a delightfully illuminating analysis.
I hope you enjoyed the sleuthing, even if it did cost you a month !
> The TLDR of the above: make reads the job server's file descriptors
> from the MAKEFLAGS environment variable, then checks here if they
> actually exist. If they don't exist, make will create the job server
> pipe. Important: by default they will be file descriptors 3 and 4.
> This becomes a key player in this mystery, a little bit later.
>
> When make spawns a child job (other than a recursive make) the job
> server file descriptors get closed (marked O_CLOEXEC before the actual
> execve), but MAKEFLAGS remains in the environment.
Sounds to me like that's a bug: when the descriptors are closed, the
part of MAKEFLAGS that claims they're make's jobserver file descriptors
should be removed, since that's when the claim stops being true.
Eddy.
