Here's the doc comment from the commit:
For one example of why this is handy, consider the case of building musl libc.
We keep a lock open for each of the object files in the form of a file descriptor
until they are finally put into an archive file. This is to allow a zig-cache
garbage collector to run concurrently to zig processes, and to allow multiple
zig processes to run concurrently with each other, without clobbering each other.
This code is disabled until #6361 is implemented (getrlimit/setrlimit
are not yet added to the standard library).
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