LibIPC+LibWeb: Add an IPC helper to transfer an IPC message buffer

This large block of code is repeated nearly verbatim in LibWeb. Move it
to a helper function that both LibIPC and LibWeb can defer to. This will
let us make changes to this method in a singular location going forward.

Note this is a bit of a regression for the MessagePort. It now suffers
from the same performance issue that IPC messages face - we prepend the
meessage size to the message buffer. This degredation is very temporary
though, as a fix is imminent, and this change makes that fix easier.

Userland/Libraries/LibIPC/Message.cpp

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2024, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibCore/Socket.h>
+#include <LibIPC/Message.h>
+#include <sched.h>
+
+namespace IPC {
+
+using MessageSizeType = u32;
+
+ErrorOr<void> MessageBuffer::transfer_message(Core::LocalSocket& fd_passing_socket, Core::LocalSocket& data_socket)
+{
+    MessageSizeType message_size = data.size();
+    TRY(data.try_prepend(reinterpret_cast<u8 const*>(&message_size), sizeof(message_size)));
+
+    for (auto const& fd : fds)
+        TRY(fd_passing_socket.send_fd(fd->value()));
+
+    ReadonlyBytes bytes_to_write { data.span() };
+    size_t writes_done = 0;
+
+    while (!bytes_to_write.is_empty()) {
+        auto maybe_nwritten = data_socket.write_some(bytes_to_write);
+        ++writes_done;
+
+        if (maybe_nwritten.is_error()) {
+            if (auto error = maybe_nwritten.release_error(); error.is_errno()) {
+                // FIXME: This is a hacky way to at least not crash on large messages
+                // The limit of 100 writes is arbitrary, and there to prevent indefinite spinning on the EventLoop
+                if (error.code() == EAGAIN && writes_done < 100) {
+                    sched_yield();
+                    continue;
+                }
+
+                switch (error.code()) {
+                case EPIPE:
+                    return Error::from_string_literal("IPC::transfer_message: Disconnected from peer");
+                case EAGAIN:
+                    return Error::from_string_literal("IPC::transfer_message: Peer buffer overflowed");
+                default:
+                    return Error::from_syscall("IPC::transfer_message write"sv, -error.code());
+                }
+            } else {
+                return error;
+            }
+        }
+
+        bytes_to_write = bytes_to_write.slice(maybe_nwritten.value());
+    }
+
+    if (writes_done > 1) {
+        dbgln("LibIPC::transfer_message FIXME Warning, needed {} writes needed to send message of size {}B, this is pretty bad, as it spins on the EventLoop", writes_done, data.size());
+    }
+
+    return {};
+}
+
+}