fuse3: Set FD_CLOEXEC on duplicated fuse session fd

[gimaker]

The duplicated file descriptor returned by dup will not have FD_CLOEXEC flag set:

The two file descriptors do not share file descriptor flags (the close-on-exec flag). The close-on-exec flag (FD_CLOEXEC; see fcntl(2)) for the duplicate descriptor is off.

(The file descriptor returned by libfuse3 has FD_CLOEXEC set as of libfuse/libfuse@4a6fb67)

It's good practice to set the close-on-exec flag on file descriptors to avoid "leaking" fds when calling exec*().

[gimaker]

Turns out this contributes to test flakiness for us - with enough test concurrency and leaking enough fds things go boom. There are other ways to fix our particular issue, but it seems apt to set FD_CLOEXEC on this fd.

(In addition from the "leaking" fd from fuser, libfuse3 also leaks another fd - 🙏 for setting FD_CLOEXEC for all fds in fuse_pure.rs!)

[cberner]

lgtm, thanks!

[gimaker]

Thanks for the fast response time, appreciated!