[llvm] Add raw_socket_stream (PR #73603)

[Michael Spencer via llvm-commits]

================
@@ -630,6 +630,31 @@ class raw_fd_stream : public raw_fd_ostream {
   static bool classof(const raw_ostream *OS);
 };
 
+//===----------------------------------------------------------------------===//
+// Socket Streams
+//===----------------------------------------------------------------------===//
+
+/// A raw stream for sockets reading/writing
+
+class raw_socket_stream : public raw_fd_ostream {
+  StringRef SocketPath;
+  bool ShouldUnlink;
+
+  uint64_t current_pos() const override { return 0; }
+
+public:
+  int get_socket() { return get_fd(); }
+
+  static int MakeServerSocket(StringRef SocketPath, unsigned int MaxBacklog,
+                              std::error_code &EC);
+
+  raw_socket_stream(int SocketFD, StringRef SockPath, std::error_code &EC);
+  raw_socket_stream(StringRef SockPath, std::error_code &EC);
+  ~raw_socket_stream();
+
+  Expected<std::string> read_impl();
+};
----------------
Bigcheese wrote:

I think a better interface would be something like:

```suggestion
class raw_socket_stream;

class ListeningSocket {
  ListeningSocket(int SocketFD);
  // ...
public:
  static Expected<ListeningSocket> createUnix(StringRef SocketPath);
  static Expected<raw_socket_stream> accept();
  ~ListeningSocket();
};

class raw_socket_stream : public raw_fd_stream {
  raw_socket_stream(int SocketFD);

  uint64_t current_pos() const override { return 0; }

public:
  /// Create a \p raw_socket_stream connected to the Unix domain socket at \p
  /// SocketPath.
  static Expected<raw_socket_stream> createConnectedUnix(StringRef SocketPath);

  ~raw_socket_stream();
};
```

This separates out the listening socket into its own type, and makes the constructors of `raw_socket_stream`  much less confusing about what type of socket it's creating. It also means API users never need to deal with raw `int` FDs. 

This is different from how `raw_fd_ostream` handles errors, but that was written a long time ago, this is the preferred way now.

Both classes should be made move only, and `ListeningSocket`'s destructor can handle removing the socket file.

https://github.com/llvm/llvm-project/pull/73603