net/SocketAddress: add GetSteadyPart()

src/net/SocketAddress.cxx

@@ -107,4 +107,41 @@ SocketAddress::GetPort() const noexcept
 	}
 }
 
+static constexpr ConstBuffer<void>
+GetSteadyPart(const struct sockaddr_in &address) noexcept
+{
+	return {&address.sin_addr, sizeof(address.sin_addr)};
+}
+
+static constexpr ConstBuffer<void>
+GetSteadyPart(const struct sockaddr_in6 &address) noexcept
+{
+	return {&address.sin6_addr, sizeof(address.sin6_addr)};
+}
+
 #endif
+
+ConstBuffer<void>
+SocketAddress::GetSteadyPart() const noexcept
+{
+	if (IsNull())
+		return nullptr;
+
+	switch (GetFamily()) {
+#ifdef HAVE_UN
+	case AF_LOCAL:
+		return GetLocalRaw().ToVoid();
+#endif
+
+#ifdef HAVE_TCP
+	case AF_INET:
+		return ::GetSteadyPart(*(const struct sockaddr_in *)(const void *)GetAddress());
+
+	case AF_INET6:
+		return ::GetSteadyPart(*(const struct sockaddr_in6 *)(const void *)GetAddress());
+#endif
+
+	default:
+		return nullptr;
+	}
+}

src/net/SocketAddress.hxx

@@ -41,6 +41,7 @@
 #include <sys/socket.h>
 #endif
 
+template<typename T> struct ConstBuffer;
 struct StringView;
 
 /**
@@ -127,6 +128,16 @@ public:
 	unsigned GetPort() const noexcept;
 #endif
 
+	/**
+	 * Return a buffer pointing to the "steady" portion of the
+	 * address, i.e. without volatile parts like the port number.
+	 * This buffer is useful for hashing the address, but not so
+	 * much for anything else.  Returns nullptr if the address is
+	 * not supported.
+	 */
+	gcc_pure
+	ConstBuffer<void> GetSteadyPart() const noexcept;
+
 	gcc_pure
 	bool operator==(const SocketAddress other) const noexcept;