[CMake] cmake on multicore interix'en

[Brad King]

Markus Duft wrote:
> cmakes implementation of how child processes are handled doesn't work
> reliably on multicore interix. it seems that every other SIGCHLD is lost

Is this a known problem on that platform, independent of CMake?

The ProcessUNIX.c implementation is for POSIX platforms, which clearly
define SIGCHLD semantics.

> somewhere on the way. i (printf-)debugged cmake a little during
> bootstrap, and it seems that at random points in time, SIGCHLD is lost,

Can you print out the state of signal masks?

> and cmake locks up in a select() call on the signal pipe (SIGCHLD is
> lost, so nobody will write on the signal pipe).

The "signal pipe" approach is a standard way to implement race-free
handling of SIGCHLD while blocking in select().

> i thought of introducing some lame timeout when select()ing the signal
> pipe, then checking whether the process is still alive (wait()), and
> again selecting if it is. what do you think?

If select() is broken (your second problem) then there is no point
in pursuing this code path further.  Instead modify the polling
code path to use a non-blocking waitpid() instead of looking at
the signal pipe.

> the second problem i have is regarding a broken select(). i tried to
> work around it by setting KWSYSPE_USE_SELECT, which initially didn't
> work, because the code seems b0rked. it seems that there is a wrong
> timeout check in that code path.

IIRC that path was contributed for BeOS support which AFAIK is not
really tested anymore.  However, it looks correct at a quick glance.

> first kwsysProcessGetTimeoutLeft is
> called, like in the select() code path, but directly after that, the
> timeoutLength members are checkd seperately once more.

The call to GetTimeoutLeft fills the members of timeoutLength.
It also returns whether or not the timeout has already expired.
The caller is supposed to use timeoutLength after the call.

> with this check it seems that all sub-processes "time out" immediately.

At process start time we store an absolute TimeoutTime using the
starting wall clock time plus the process timeout length.  Later
the GetTimeoutLeft subtracts the current time from the TimeoutTime.
Print out the starting time, the computed TimeoutTime, and the
timeoutLength that gets computed for each poll.

-Brad