LibIPC: Return TransportHandle directly from create_paired()

Previously, `create_paired()` returned two full Transport objects, and
callers would immediately call `from_transport()` on the remote side to
extract its underlying fd. This wasted resources: the remote
Transport's IO thread, wakeup pipes, and send queue were initialized
only to be torn down without ever sending or receiving a message.

Now `create_paired()` returns `{Transport, TransportHandle}` — the
remote side is born as a lightweight handle containing just the raw fd,
skipping all unnecessary initialization.

Also replace `release_underlying_transport_for_transfer()` (which
returned a raw int fd) with `release_for_transfer()` (which returns a
TransportHandle directly), hiding the socket implementation detail
from callers including MessagePort.

Libraries/LibIPC/TransportHandle.cpp

@@ -18,16 +18,10 @@ TransportHandle::TransportHandle(File file)
 {
 }
 
-ErrorOr<TransportHandle> TransportHandle::from_transport(Transport& transport)
-{
-    auto fd = TRY(transport.release_underlying_transport_for_transfer());
-    return TransportHandle { File::adopt_fd(fd) };
-}
-
 ErrorOr<NonnullOwnPtr<Transport>> TransportHandle::create_transport() const
 {
     auto socket = TRY(Core::LocalSocket::adopt_fd(m_file.take_fd()));
-    TRY(socket->set_blocking(true));
+    TRY(socket->set_blocking(false));
     return make<Transport>(move(socket));
 }
 

Libraries/LibIPC/TransportHandle.h

@@ -31,13 +31,11 @@ public:
     TransportHandle(TransportHandle&&) = default;
     TransportHandle& operator=(TransportHandle&&) = default;
 
-    static ErrorOr<TransportHandle> from_transport(Transport& transport);
-
     ErrorOr<NonnullOwnPtr<Transport>> create_transport() const;
 
-private:
     explicit TransportHandle(File);
 
+private:
     template<typename U>
     friend ErrorOr<void> encode(Encoder&, U const&);
 

Libraries/LibIPC/TransportSocket.cpp

@@ -12,7 +12,9 @@
 #include <LibCore/Socket.h>
 #include <LibCore/System.h>
 #include <LibIPC/Attachment.h>
+#include <LibIPC/File.h>
 #include <LibIPC/Limits.h>
+#include <LibIPC/TransportHandle.h>
 #include <LibIPC/TransportSocket.h>
 #include <LibThreading/Thread.h>
 
@@ -31,14 +33,13 @@ ErrorOr<TransportSocket::Paired> TransportSocket::create_paired()
     TRY(socket0->set_close_on_exec(true));
     TRY(socket0->set_blocking(false));
 
-    auto socket1 = TRY(Core::LocalSocket::adopt_fd(fds[1]));
+    TRY(Core::System::set_close_on_exec(fds[1], true));
     guard_fd_1.disarm();
-    TRY(socket1->set_close_on_exec(true));
-    TRY(socket1->set_blocking(false));
 
+    // Local side gets a full transport; remote side is just a handle containing the raw fd for transfer to another process.
     return Paired {
         make<TransportSocket>(move(socket0)),
-        make<TransportSocket>(move(socket1)),
+        TransportHandle { File::adopt_fd(fds[1]) },
     };
 }
 
@@ -504,10 +505,11 @@ TransportSocket::ShouldShutdown TransportSocket::read_as_many_messages_as_possib
     return m_peer_eof ? ShouldShutdown::Yes : ShouldShutdown::No;
 }
 
-ErrorOr<int> TransportSocket::release_underlying_transport_for_transfer()
+ErrorOr<TransportHandle> TransportSocket::release_for_transfer()
 {
     stop_io_thread(IOThreadState::SendPendingMessagesAndStop);
-    return m_socket->release_fd();
+    auto fd = TRY(m_socket->release_fd());
+    return TransportHandle { File::adopt_fd(fd) };
 }
 
 }

Libraries/LibIPC/TransportSocket.h

@@ -12,7 +12,7 @@
 #include <LibCore/Socket.h>
 #include <LibIPC/Attachment.h>
 #include <LibIPC/AutoCloseFileDescriptor.h>
-#include <LibIPC/Forward.h>
+#include <LibIPC/TransportHandle.h>
 #include <LibThreading/ConditionVariable.h>
 #include <LibThreading/Forward.h>
 
@@ -43,7 +43,7 @@ public:
 
     struct Paired {
         NonnullOwnPtr<TransportSocket> local;
-        NonnullOwnPtr<TransportSocket> remote;
+        TransportHandle remote_handle;
     };
     static ErrorOr<Paired> create_paired();
 
@@ -70,8 +70,7 @@ public:
     };
     ShouldShutdown read_as_many_messages_as_possible_without_blocking(Function<void(Message&&)>&&);
 
-    // Obnoxious name to make it clear that this is a dangerous operation.
-    ErrorOr<int> release_underlying_transport_for_transfer();
+    ErrorOr<TransportHandle> release_for_transfer();
 
 private:
     enum class TransferState {

Libraries/LibIPC/TransportSocketWindows.cpp

@@ -10,8 +10,10 @@
 #include <AK/ScopeGuard.h>
 #include <AK/Types.h>
 #include <LibCore/System.h>
+#include <LibIPC/File.h>
 #include <LibIPC/HandleType.h>
 #include <LibIPC/Limits.h>
+#include <LibIPC/TransportHandle.h>
 #include <LibIPC/TransportSocketWindows.h>
 
 #include <AK/Windows.h>
@@ -31,14 +33,12 @@ ErrorOr<TransportSocketWindows::Paired> TransportSocketWindows::create_paired()
     TRY(socket0->set_close_on_exec(true));
     TRY(socket0->set_blocking(false));
 
-    auto socket1 = TRY(Core::LocalSocket::adopt_fd(fds[1]));
+    TRY(Core::System::set_close_on_exec(fds[1], true));
     guard_fd_1.disarm();
-    TRY(socket1->set_close_on_exec(true));
-    TRY(socket1->set_blocking(false));
 
     return Paired {
         make<TransportSocketWindows>(move(socket0)),
-        make<TransportSocketWindows>(move(socket1)),
+        TransportHandle { File::adopt_fd(fds[1]) },
     };
 }
 
@@ -272,9 +272,10 @@ TransportSocketWindows::ShouldShutdown TransportSocketWindows::read_as_many_mess
     return should_shutdown;
 }
 
-ErrorOr<int> TransportSocketWindows::release_underlying_transport_for_transfer()
+ErrorOr<TransportHandle> TransportSocketWindows::release_for_transfer()
 {
-    return m_socket->release_fd();
+    auto fd = TRY(m_socket->release_fd());
+    return TransportHandle { File::adopt_fd(fd) };
 }
 
 }

Libraries/LibIPC/TransportSocketWindows.h

@@ -10,6 +10,7 @@
 #include <AK/Queue.h>
 #include <LibCore/Socket.h>
 #include <LibIPC/Attachment.h>
+#include <LibIPC/TransportHandle.h>
 
 namespace IPC {
 
@@ -20,7 +21,7 @@ class TransportSocketWindows {
 public:
     struct Paired {
         NonnullOwnPtr<TransportSocketWindows> local;
-        NonnullOwnPtr<TransportSocketWindows> remote;
+        TransportHandle remote_handle;
     };
     static ErrorOr<Paired> create_paired();
 
@@ -46,8 +47,7 @@ public:
     };
     ShouldShutdown read_as_many_messages_as_possible_without_blocking(Function<void(Message&&)>&&);
 
-    // Obnoxious name to make it clear that this is a dangerous operation.
-    ErrorOr<int> release_underlying_transport_for_transfer();
+    ErrorOr<TransportHandle> release_for_transfer();
 
 private:
     ErrorOr<void> duplicate_handles(Bytes, Vector<size_t> const& handle_offsets);

Libraries/LibWeb/HTML/MessagePort.cpp

@@ -8,13 +8,12 @@
 
 #include <AK/ByteReader.h>
 #include <AK/MemoryStream.h>
-#include <LibCore/Socket.h>
 #include <LibCore/System.h>
 #include <LibGC/WeakHashSet.h>
 #include <LibIPC/Decoder.h>
 #include <LibIPC/Encoder.h>
-#include <LibIPC/File.h>
 #include <LibIPC/Transport.h>
+#include <LibIPC/TransportHandle.h>
 #include <LibWeb/Bindings/ExceptionOrUtils.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/Bindings/MessagePortPrototype.h>
@@ -107,13 +106,12 @@ WebIDL::ExceptionOr<void> MessagePort::transfer_steps(HTML::TransferDataEncoder&
         if (m_remote_port)
             m_remote_port->m_has_been_shipped = true;
 
-        auto fd = MUST(m_transport->release_underlying_transport_for_transfer());
+        auto handle = MUST(m_transport->release_for_transfer());
         m_transport.clear();
 
         // 2. Set dataHolder.[[RemotePort]] to remotePort.
-        // TODO: Mach IPC
         data_holder.encode(IPC_FILE_TAG);
-        data_holder.encode(IPC::File::adopt_fd(fd));
+        data_holder.encode(handle);
     }
     // 4. Otherwise, set dataHolder.[[RemotePort]] to null.
     else {
@@ -135,9 +133,8 @@ WebIDL::ExceptionOr<void> MessagePort::transfer_receiving_steps(HTML::TransferDa
     // 3. If dataHolder.[[RemotePort]] is not null, then entangle dataHolder.[[RemotePort]] and value.
     //     (This will disentangle dataHolder.[[RemotePort]] from the original port that was transferred.)
     if (auto fd_tag = data_holder.decode<u8>(); fd_tag == IPC_FILE_TAG) {
-        // TODO: Mach IPC
-        auto fd = data_holder.decode<IPC::File>();
-        m_transport = make<IPC::Transport>(MUST(Core::LocalSocket::adopt_fd(fd.take_fd())));
+        auto handle = data_holder.decode<IPC::TransportHandle>();
+        m_transport = MUST(handle.create_transport());
 
         m_transport->set_up_read_hook([strong_this = GC::make_root(this)]() {
             if (strong_this->m_enabled)
@@ -185,7 +182,7 @@ void MessagePort::entangle_with(MessagePort& remote_port)
 
     auto paired = MUST(IPC::Transport::create_paired());
     m_transport = move(paired.local);
-    m_remote_port->m_transport = move(paired.remote);
+    m_remote_port->m_transport = MUST(paired.remote_handle.create_transport());
 
     m_transport->set_up_read_hook([strong_this = GC::make_root(this)]() {
         if (strong_this->m_enabled)

Libraries/LibWebView/WebContentClient.cpp

@@ -791,7 +791,7 @@ Messages::WebContentClient::RequestWorkerAgentResponse WebContentClient::request
         auto request_server_handle = MUST(connect_new_request_server_client());
         auto image_decoder_handle = MUST(connect_new_image_decoder_client());
         auto worker_client = MUST(WebView::launch_web_worker_process(worker_type));
-        auto worker_handle = MUST(IPC::TransportHandle::from_transport(worker_client->transport()));
+        auto worker_handle = MUST(worker_client->transport().release_for_transfer());
         return { move(worker_handle), move(request_server_handle), move(image_decoder_handle) };
     }
 

Libraries/LibWebView/WebUI.cpp

@@ -17,7 +17,7 @@ template<typename WebUIType>
 static ErrorOr<NonnullRefPtr<WebUIType>> create_web_ui(WebContentClient& client, String host)
 {
     auto paired = TRY(IPC::Transport::create_paired());
-    auto handle = TRY(IPC::TransportHandle::from_transport(*paired.remote));
+    auto handle = move(paired.remote_handle);
 
     auto web_ui = WebUIType::create(client, move(paired.local), move(host));
     client.async_connect_to_web_ui(0, move(handle));

Services/ImageDecoder/ConnectionFromClient.cpp

@@ -58,7 +58,7 @@ Messages::ImageDecoderServer::InitTransportResponse ConnectionFromClient::init_t
 ErrorOr<IPC::TransportHandle> ConnectionFromClient::connect_new_client()
 {
     auto paired = TRY(IPC::Transport::create_paired());
-    auto handle = TRY(IPC::TransportHandle::from_transport(*paired.remote));
+    auto handle = move(paired.remote_handle);
 
     // Note: A ref is stored in the static s_connections map
     auto client = adopt_ref(*new ConnectionFromClient(move(paired.local)));

Services/RequestServer/ConnectionFromClient.cpp

@@ -141,7 +141,7 @@ Messages::RequestServer::ConnectNewClientsResponse ConnectionFromClient::connect
 ErrorOr<IPC::TransportHandle> ConnectionFromClient::create_client_socket()
 {
     auto paired = TRY(IPC::Transport::create_paired());
-    auto handle = TRY(IPC::TransportHandle::from_transport(*paired.remote));
+    auto handle = move(paired.remote_handle);
 
     // Note: A ref is stored in the m_connections map
     auto client = adopt_ref(*new ConnectionFromClient(move(paired.local), IsPrimaryConnection::No, m_connections, m_disk_cache));