Sporadic GNU make jobserver unavailable

Hi,

We're using Cygwin make to build several projects in parallel mode, e.g.
with -j 4 or sometimes higher settings, depending on the core count of
the machine.

Sporadically, and then I'm talking about "once a week" or so, random
jobs will fail with a non-repeatable error, and in _every_ case these
failures are preceded by a warning from make:

  make[4]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.

Obviously the "Add '+' to parent rule" is nonsensical, since simply
running exactly the same job again almost always works fine. But there
is something flaky about make's jobserver under Windows.

FWIW one machine I just observed this on has the make-4.4.1-2 package,
which should use the "pipe" jobserver, if I interpret
https://cygwin.com/pipermail/cygwin-announce/2023-March/010972.html
correctly.

Does anybody know a way to further debug this issue? It must be some
rare race condition...

-Dimitry


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