net/SocketDescriptor: decouple from FileDescriptor on Windows

On Windows, a socket is not a file descriptor; it is a different beast
(and anyway, Windows doesn't have file descriptors).

src/net/SocketDescriptor.cxx

@@ -8,7 +8,6 @@
 #include "IPv6Address.hxx"
 
 #ifdef _WIN32
-#include <winsock2.h>
 #include <ws2tcpip.h>
 #else
 #include <sys/socket.h>

src/net/SocketDescriptor.hxx

@@ -5,9 +5,17 @@
 #define SOCKET_DESCRIPTOR_HXX
 
 #include "Features.hxx"
+
+#ifndef _WIN32
 #include "io/FileDescriptor.hxx"
+#endif
 
 #include <type_traits>
+#include <utility>
+
+#ifdef _WIN32
+#include <winsock2.h> // for SOCKET, INVALID_SOCKET
+#endif
 
 class SocketAddress;
 class StaticSocketAddress;
@@ -15,24 +23,44 @@ class IPv4Address;
 class IPv6Address;
 
 /**
- * An OO wrapper for a UNIX socket descriptor.
+ * An OO wrapper for a Berkeley or WinSock socket descriptor.
  */
-class SocketDescriptor : protected FileDescriptor {
+class SocketDescriptor
+#ifndef _WIN32
+/* Berkeley sockets are represented as file descriptors */
+	: protected FileDescriptor
+#endif
+{
 protected:
+#ifdef _WIN32
+	/* WinSock sockets are not file descriptors, they are a
+	   special type */
+	SOCKET fd;
+#else // !_WIN32
 	explicit constexpr SocketDescriptor(FileDescriptor _fd) noexcept
 		:FileDescriptor(_fd) {}
+#endif // !_WIN32
 
 public:
 	SocketDescriptor() = default;
 
+#ifdef _WIN32
+	explicit constexpr SocketDescriptor(SOCKET _fd) noexcept
+		:fd(_fd) {}
+#else // !_WIN32
 	explicit constexpr SocketDescriptor(int _fd) noexcept
 		:FileDescriptor(_fd) {}
+#endif // !_WIN32
 
 	constexpr bool operator==(SocketDescriptor other) const noexcept {
 		return fd == other.fd;
 	}
 
-#ifndef _WIN32
+#ifdef _WIN32
+	constexpr bool IsDefined() const noexcept {
+		return fd != INVALID_SOCKET;
+	}
+#else // !_WIN32
 	/**
 	 * Convert a #FileDescriptor to a #SocketDescriptor.  This is only
 	 * possible on operating systems where socket descriptors are the
@@ -52,13 +80,11 @@ public:
 	constexpr const FileDescriptor &ToFileDescriptor() const noexcept {
 		return *this;
 	}
-#endif
 
 	using FileDescriptor::IsDefined;
-#ifndef _WIN32
 	using FileDescriptor::IsValid;
 	using FileDescriptor::IsSocket;
-#endif
+#endif // !_WIN32
 
 	/**
 	 * Determine the socket type, i.e. SOCK_STREAM, SOCK_DGRAM or
@@ -73,25 +99,48 @@ public:
 	[[gnu::pure]]
 	bool IsStream() const noexcept;
 
+	static constexpr SocketDescriptor Undefined() noexcept {
+#ifdef _WIN32
+		return SocketDescriptor{INVALID_SOCKET};
+#else // !_WIN32
+		return SocketDescriptor(FileDescriptor::Undefined());
+#endif // !_WIN32
+	}
+
+#ifndef _WIN32
 	using FileDescriptor::Get;
 	using FileDescriptor::Set;
 	using FileDescriptor::Steal;
 	using FileDescriptor::SetUndefined;
 
-	static constexpr SocketDescriptor Undefined() noexcept {
-		return SocketDescriptor(FileDescriptor::Undefined());
-	}
-
 	using FileDescriptor::EnableCloseOnExec;
 	using FileDescriptor::DisableCloseOnExec;
 
-#ifndef _WIN32
 	using FileDescriptor::SetNonBlocking;
 	using FileDescriptor::SetBlocking;
 	using FileDescriptor::Duplicate;
 	using FileDescriptor::CheckDuplicate;
 	using FileDescriptor::Close;
 #else
+	constexpr SOCKET Get() const noexcept {
+		return fd;
+	}
+
+	constexpr void Set(SOCKET _fd) noexcept {
+		fd = _fd;
+	}
+
+	constexpr void SetUndefined() noexcept {
+		fd = INVALID_SOCKET;
+	}
+
+	constexpr SOCKET Steal() noexcept {
+		return std::exchange(fd, INVALID_SOCKET);
+	}
+
+	void EnableCloseOnExec() const noexcept {}
+	void DisableCloseOnExec() const noexcept {}
+
 	bool SetNonBlocking() const noexcept;
 
 	/**

src/net/UniqueSocketDescriptor.hxx

@@ -20,13 +20,20 @@ public:
 
 	explicit UniqueSocketDescriptor(SocketDescriptor _fd) noexcept
 		:SocketDescriptor(_fd) {}
+#ifndef _WIN32
 	explicit UniqueSocketDescriptor(FileDescriptor _fd) noexcept
 		:SocketDescriptor(_fd) {}
+#endif // !_WIN32
 	explicit UniqueSocketDescriptor(int _fd) noexcept
 		:SocketDescriptor(_fd) {}
 
+#ifdef _WIN32
+	UniqueSocketDescriptor(UniqueSocketDescriptor &&other) noexcept
+		:SocketDescriptor(std::exchange(other.fd, INVALID_SOCKET)) {}
+#else // !_WIN32
 	UniqueSocketDescriptor(UniqueSocketDescriptor &&other) noexcept
 		:SocketDescriptor(std::exchange(other.fd, -1)) {}
+#endif // !_WIN32
 
 	~UniqueSocketDescriptor() noexcept {
 		if (IsDefined())