Previously, recvmsg() on a unix stream socket with its peer closed will
never return, with goroutine call trace like this:
...
2 in gvisor.dev/gvisor/pkg/sentry/kernel.(*Task).block
at pkg/sentry/kernel/task_block.go:124
3 in gvisor.dev/gvisor/pkg/sentry/kernel.(*Task).BlockWithDeadline
at pkg/sentry/kernel/task_block.go:69
4 in gvisor.dev/gvisor/pkg/sentry/socket/unix.(*SocketOperations).RecvMsg
at pkg/sentry/socket/unix/unix.go:612
5 in gvisor.dev/gvisor/pkg/sentry/syscalls/linux.recvFrom
at pkg/sentry/syscalls/linux/sys_socket.go:885
6 in gvisor.dev/gvisor/pkg/sentry/syscalls/linux.RecvFrom
at pkg/sentry/syscalls/linux/sys_socket.go:910
...
The issue is caused by that ErrClosedForReceive returned by
unix/transport.queue is turned into nil in
unix.(*EndpointReader).ReadToBlocks():
err.ToError()
As a result, in unix.(*SocketOperations).RecvMsg():
n == 0 and err == nil
We shall differentiate it from another case - no data to read where
ErrWouldBlock shall be returned; and return 0 immediately.
Fixes: #734
Reported-by: chenglang.hy <chenglang.hy@antfin.com>
Signed-off-by: Jianfeng Tan <henry.tjf@antfin.com>
96f78e2466