Breakage ahead

Unfortunately, the jobserver protocol is broken.

Job tokens are passed down via inherited file descriptors -

Caveat 1: If the called program is not $(MAKE) (verbatim), the file descriptors are closed on exec (FD_CLOEXEC), and the chain ends there. You can opt-in by adding a '+' in front of your command, but then you have to do that on every elegible command. Hint - none of the common Makefile generators does that.

Caveat 2: If you happen to call make indirectly (e.g. via a script called from the Makefile), unfortunately the '--jobserver-auth=<readfd>,<writefd>' will still be in the MAKEFLAGS env var although make had just closed the corresponding file descriptors. Two things can happen:

a) The fds no longer refer to valid file descriptors - make will detect this
b) The fds have been reused in the meantime - make will find valid file descriptors. If the fd is not readable (e.g. referring to a write end of a pipe), make will start busy looping. If the fd is readable, it will spawn a new job for every byte read (now figure what will happen if it points to a 2 kByte source file ...)