Improvement suggestion: listen to signals to adjust number of jobs

[Henrik Carlqvist]

Many times after starting make I have wished that I started it with more
parallel jobs than I did. A few time I have also wished that I started
make with fewer parallel jobs.

The attached patch allows you to send SIGUSR2 to your top level make
process to add one more parallel job.

Once a SIGUSR2 is received the functionality of SIGUSR1 also changes from
toggling debug output to decrease the number of jobs by 1. No running job
will be killed, but when the next target is finished one less job will be
started again.

If make was started with parallel jobs SIGUSR2 will immediately increase
the number of jobs with one. If make was started without -j SIGUSR2
will not cause make to work in parallel until the current target is
finished.

The patch was written against latest from git, but also works with latest
stable version 4.2.1. My intention has been to make the patch work with
both Unix and Windows, but unfortunately I have no Windows machine to test
with. I'm not even sure if Windows supports bsd signals. If not, my
changes to w32/w32os.c should better be undone.

I hope that you will find my patch useful and that it somehow will make it
into upcoming stable releases of make.

Please feel free to discuss improvements of the patch, I have subscribed
to this mailing list.

regards Henrik

signal_num_jobs2.patch
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