Summary:
The Rust upstream took the "set F_CLOEXEC on every opened file" approach and
provided no support for closing fds at spawn time to make spawn lightweight [1].
However, that does not play well in our case:
- On Windows:
- stdin/stdout/stderr are not created by Rust, and inheritable by
default (other process like `cargo`, or `dotslash` might leak them too).
- a few other handles like "Null", "Afd" are inheritable. It's
unclear how they get created, though.
- Fortunately, files opened by Python or C in edenscm (ex. packfiles) seem to
be not inheritable and do not require special handling.
- On Linux:
- Files opened by Python or C are likely lack of F_CLOEXEC and need special
handling.
Implement logic to close file handlers (or set F_CLOEXEC) explicitly.
[1]: https://github.com/rust-lang/rust/issues/12148
Reviewed By: DurhamG
Differential Revision: D23124167
fbshipit-source-id: 32f3a1b9e3ae3a9475609df282151c9d6c4badd4
