feat: Make Client::from_env
safe to call for any number of times
#57
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So that it can be used in different crates without causing UB.
Motivation
opencv-rust
usesjobserver::Client::from_env
in itsbuild.rs
to parallelize generation of bindings, then callcc::Build::compile
with featurescc/parallel
enabled it will also calljobserver::Client::from_env
and creates a UB rust-lang/cc-rs#844 (comment)Workaround for this will require either wrapping
jobserver::Client
withstd::mem::ManuallyDrop
, or switching to my forkjobslot
which supports callingClient::from_env
unlimited number of times.This PR thus port the change to upstream so that any crate using
jobserver
can benefit from this and can stop worrying about UB caused by callingClieng::from_env
multiple times in different crates.