posix: Support partial reads in unix sockets readSome

posix/subsystem/src/un-socket.cpp

@@ -123,13 +123,14 @@ struct OpenFile : File {
 
 		// TODO: Truncate packets (for SOCK_DGRAM) here.
 		auto packet = &_recvQueue.front();
-		assert(!packet->offset);
 		assert(packet->files.empty());
-		auto size = packet->buffer.size();
-		assert(max_length >= size);
-		memcpy(data, packet->buffer.data(), size);
-		_recvQueue.pop_front();
-		co_return size;
+
+		auto chunk = std::min(packet->buffer.size() - packet->offset, max_length);
+		memcpy(data, packet->buffer.data() + packet->offset, chunk);
+		packet->offset += chunk;
+		if(packet->offset == packet->buffer.size())
+			_recvQueue.pop_front();
+		co_return chunk;
 	}
 
 	async::result<frg::expected<Error, size_t>>