The automatic parallelism in cmd/go is wonderful when the command is used interactively. However, it interacts poorly when another build system is driving cmd/go. The driving build system cannot safely start multiple Go builds without overloading the system, and it cannot detect when a Go build is doing very little work so it should start other work.

GNU make has an elegant solution for this, the "jobserver." It is much simpler than the name suggests:

On POSIX systems the jobserver is implemented as a simple UNIX pipe. The pipe will be pre-loaded with one single-character token for each available job. To obtain an extra slot you must read a single character from the jobserver pipe; to release a slot you must write a single character back into the jobserver pipe.

https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html

I would like to propose that cmd/go parses the environment variable MAKEFLAGS for --jobserver-auth=R,W, and if it finds valid FDs R and W then it uses tokens from the pipe to control its parallelism. This will make cmd/go play nicely with GNU make and other build systems that use the jobserver, like redo.