adb: win32: file descriptor reliability improvements
When repeatedly opening and closing a file descriptor, the sequence of
fds returned was: 100...227,100,100,100,100,100... Basically, the first
wave was constantly increasing fds, but after the entire fd table was
traversed once, the alloc algorithm would switch to returning the first
free fd. This is sub-optimal for reliability because use-after-free bugs
would be more likely to be hit because right after a close, the same fd
would be given out next.
This change makes the alloc algorithm use a persistent clock hand that
walks forward through the fd table (wrapping around if necessary),
searching for a free fd.
This change adds locking for fd closing:
- This prevents multiple concurrent closes of the same fd.
- There was a race between alloc and close that wasn't guaranteed to be
correct: close would set f->clazz to NULL last, but without any
preceding memory barrier/fence, then the alloc thread would check for
NULL. It probably worked out ok in practice, but it is probably best
to fix this up with a lock (as in this change) or a memory barrier/fence
(but this code isn't about performance, so why go with a complicated
Also in this change:
- Use errno = EMFILE for the out of fds case.
- Clear FH->name
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>
1 file changed