tree 5d57a34a041e58ea5a6723da74f72919dad4a463
parent e6c5973eaab5e5531c7e3b872be0da64d593a61c
author James Tucker <raggi@google.com> 1544481547 -0800
committer CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org> 1544568091 +0000

[fdio] only accept sockets when there's a free fd

This change introduces a reservation mechanism throughout fdio that can be
used in the intermediate time between a callers request for a new fd, and
when that request is fulfilled by some remote. This reservation mechanism is
then used in the socket accept path to prevent client programs from pulling
sockets from the listen queue if they do not have an fd with which they can
service the socket.

A prior change was reverted in bbdb4a51d5849d91c73945994be75db397528dd5 which
instead attempted to simply hold the fdtab mutex for the duration. That
approach was flawed because it creates a deadlock in the (common) socket
scenario of:
- listen
- thread(connect)
- accept
If the accept call races connect.

This approach likely should also be used in open and connect among other
places to avoid creating new remote resources that can not be serviced.

Test: garnet package: netstack_tests
Bug: PKG-371 #comment accept does not pull sockets it can't service
Change-Id: Ic7745bca80c6cec3bad468bfc1d923a275e4159b