[libc] Prevent changing ownership of the port once opened

[jhuber6]

Summary

The Port type has stipuations that the same exact mask used to open it
needs to close it. This can currently be violated by calling its move
constructor to put it somewhere else. We still need the move constructor
to handle the open and closing functions. So, we simply make these
constructors private and only allow a few classes to have move
priviledges on it.

[JonChesterfield]

Let's put port.close in the destructor in the same commit as that's available branch-free for non-movable types. Also write a comment to the effect of "move only modulo optional" or similar as otherwise it's not obvious why the move ctors are not deleted.

I'm assuming that optional doesn't call the destructor on a value that isn't there but that's not a safe assumption. Let's go with this patch as-is and work out what to do with raii separately.

[lntue]

Ok after @JonChesterfield 's comments are addressed.

[jhuber6]

In D150118#4333153, @lntue wrote:

Ok after @JonChesterfield 's comments are addressed.

I tried implementing the destructor as an RAII interface, but it had some issues I didn't delve too far into at the time.

[JonChesterfield]

Diff is ok by me as currently written

libc/src/__support/RPC/rpc.h

261

Guessing we can't have delete here while using an optional that requires move?

[jhuber6]

libc/src/__support/RPC/rpc.h

261

Yep, so next best thing is making it private.

libc/src/__support/RPC/rpc.h

LIBC_INLINE void invoke_rpc(cpp::function<void(Buffer *, uint32_t)> fn,
     }
   }
 };
 
 /// The port provides the interface to communicate between the multiple
 /// processes. A port is conceptually an index into the memory provided by the
 /// underlying process that is guarded by a lock bit.
 template <bool T> struct Port {
-  // TODO: This should be move-only.
   LIBC_INLINE Port(Process<T> &process, uint64_t lane_mask, uint64_t index,
                    uint32_t out)
       : process(process), lane_mask(lane_mask), index(index), out(out) {}
+  LIBC_INLINE ~Port() = default;
+
+private:
   LIBC_INLINE Port(const Port &) = delete;
   LIBC_INLINE Port &operator=(const Port &) = delete;
   LIBC_INLINE Port(Port &&) = default;
   LIBC_INLINE Port &operator=(Port &&) = default;
-  LIBC_INLINE ~Port() = default;
 
+  friend struct Client;
+  friend struct Server;
+  friend class cpp::optional<Port<T>>;
+
+public:
   template <typename U> LIBC_INLINE void recv(U use);
   template <typename F> LIBC_INLINE void send(F fill);
   template <typename F, typename U>
   LIBC_INLINE void send_and_recv(F fill, U use);
   template <typename W> LIBC_INLINE void recv_and_send(W work);
   LIBC_INLINE void send_n(const void *src, uint64_t size);
   template <typename A> LIBC_INLINE void recv_n(A alloc);