net: use closesocket when closing socket os.File's on Windows
The WSASocket documentation states that the returned socket must be
closed by calling closesocket instead of CloseHandle. The different
File methods on the net package return an os.File that is not aware
that it should use closesocket. Ideally, os.NewFile should detect that
the passed handle is a socket and use the appropriate close function,
but there is no reliable way to detect that a handle is a socket on
Windows (see CL 671455).
To work around this, we add a hidden function to the os package that
can be used to return an os.File that uses closesocket. This approach
is the same as used on Unix, which also uses a hidden function for other
purposes.
While here, fix a potential issue with FileConn, which was using File.Fd
rather than File.SyscallConn to get the handle. This could result in the
File being closed and garbage collected before the syscall was made.
Fixes #73683.
Change-Id: I179405f34c63cbbd555d8119e0f77157c670eb3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/672195 Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>