use -j; all make's in a recursive build cooperate to limit the total
number of jobs, using a token-passing scheme.
The current token passing algorithm is similar to the one implemented
by gmake; there is a single pipe which is inherited through the entire
process hierarchy; tokens are obtained by reading a byte from the
"read end" of the pipe, and are returned by writing them to the "write
end". This exact algorithm is likely to change in the future.
Implementation details:
- Use the new trace facility to allow measurement of the
effectiveness of different token-passing schemes
- Get a token in MakeStartJobs(), return it in Make_Update()
- Eliminate Job_Full() and the jobFull global since they are
redundant with token system.
- Add an "internal" -J option (to pass the token pipe fd's down to
submakes) and a -T option for tracing.
- Change how compatMake is forced so that -j means something when
inherited by submakes.
- When waiting for a token, poll the token-passing pipe as well as
the output pipes of existing jobs.
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