Fix decoding of abstract unix sockets (#661)

* test_unix_msg: Remove thread synchronisation

This refactors the test_unix_msg() test a bit. No change in behaviour is
intended.

Previously, two threads were started in parallel. The server thread used
a mutex and a condition variable to indicate that it set up its
listening socket. The client thread waited for this signal before it
attempted to connect.

This commit makes the code instead bind the server socket before
starting the worker threads. That way, no synchronisation is necessary.

Signed-off-by: Uli Schlachter <[email protected]>

* Add test for abstract unix sockets

This commit duplicates the existing test_unix_msg() tests. The
duplicated version uses an abstract unix socket instead of a path based
one.

To make the code-duplication not too bad, a helper function
do_test_unix_msg() is introduced that does "the actual work" and just
needs as input a socket address.

This new test currently fails. This reproduces
#660.

Signed-off-by: Uli Schlachter <[email protected]>

* Fix recvmsg() on abstract unix sockets

This case was just not implemented at all. Abstract unix sockets are not
null-terminated and are indicated by sun_path beginning with a null
byte.

Fixes: #660
Signed-off-by: Uli Schlachter <[email protected]>

* Strengthen recvmsg() test

The fix in the previous commit added some parsing for abstract unix
sockets. I wasn't sure whether I got all the indicies right, so this
commit extends the existing test to check that the result from recvmsg()
contains the expected socket address.

Signed-off-by: Uli Schlachter <[email protected]>

* Fix recvmsg() on abstract unix sockets on libc backend

The previous commit for this only fixed the linux_raw backend. This
commit applies the same fix to the libc backend.

Fixes: #660
Signed-off-by: Uli Schlachter <[email protected]>

* Restrict test_abstract_unix_msg() test to Linux

Abstract unix sockets are a feature of the Linux kernel that is not
available elsewhere.

Signed-off-by: Uli Schlachter <[email protected]>

* Skip the test extension on FreeBSD

Commit "Strengthen recvmsg() test" added a new assertion to this test.
For unknown reasons, this test failed in Currus CI on
x86_64-unknown-freebsd-13 as follows:

---- unix::test_unix_msg stdout ----
thread 'client' panicked at 'assertion failed: `(left == right)`
  left: `Some("/tmp/.tmpy5Fj4e/scp_4804")`,
 right: `Some("(unnamed)")`', tests/net/unix.rs:243:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'unix::test_unix_msg' panicked at 'called `Result::unwrap()` on an `Err` value: Any { .. }', tests/net/unix.rs:276:19

The text "(unnamed)" comes from the Debug impl of SocketAddrUnix. This
text is generated when SocketAddrUnix::path() returns None.

I do not know why this happens. I am just trying to get CI not to
complain. A random guess would be that recvmsg() does not return the
endpoint for connected sockets. This is because the "recvmsg" man page
for FreeBSD says:

> Here msg_name and msg_namelen specify the source address if the socket
> is unconnected;

Signed-off-by: Uli Schlachter <[email protected]>

---------

Signed-off-by: Uli Schlachter <[email protected]>