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

Change-Id: Ic11d2a1a9d53996edfc1ca13566a2f46de4a4316
Signed-off-by: Spencer Low <>
1 file changed