Implement FileChooser portal proxy with virtiofsd mount

flake.nix

@@ -15,7 +15,13 @@
         pkgs = import nixpkgs {
           inherit system overlays;
         };
-        
+
+        buildEnvVars = {
+          BIN_XDG_PERMISSION_STORE = "${pkgs.xdg-desktop-portal}/libexec/xdg-permission-store";
+          BIN_XDG_DOCUMENT_PORTAL = "${pkgs.xdg-desktop-portal}/libexec/xdg-document-portal";
+          BIN_VIRTIOFSD = "${pkgs.virtiofsd}/bin/virtiofsd";
+        };
+
         rustToolchain = pkgs.pkgsBuildHost.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;
         rustPlatform = pkgs.makeRustPlatform {
           cargo = rustToolchain;
@@ -32,11 +38,13 @@
               "busd-0.4.0" = "sha256-UzTclEJ8lRMmiuLJJi+gsm7vkx+MLfnDdi5s9OVT1HE=";
             };
             buildAndTestSubdir = crate;
+            env = buildEnvVars;
           };
       in
       {
         devShells.default = pkgs.mkShell {
           buildInputs = [ rustToolchain ];
+          env = buildEnvVars;
         };
 
         packages.sidebus-agent = rustPackage "sidebus-agent";
@@ -58,6 +66,14 @@
             };
             documentation = [ "https://git.clan.lol/valpackett/sidebus" ];
           };
+          systemd.mounts = [
+            {
+              type = "virtiofs";
+              what = "vm-doc-portal";
+              where = "/run/vm-doc-portal";
+              wantedBy = [ "multi-user.target" ];
+            }
+          ];
         };
       }
     );

sidebus-broker/src/main.rs

@@ -1,10 +1,13 @@
 mod bus;
+mod portal;
 mod vsock;
 
 use bus::SharedHostedBus;
 use clap::Parser;
-use std::sync::Arc;
-use tokio::sync::Mutex;
+use std::{path::PathBuf, sync::Arc};
+use tokio::{net::UnixListener, process::Command, sync::Mutex};
+use tracing::error;
+use zbus::names::WellKnownName;
 
 // https://github.com/rust-lang/rfcs/issues/2407#issuecomment-385291238
 macro_rules! enclose {
@@ -18,39 +21,140 @@ macro_rules! enclose {
 
 #[derive(Parser)]
 #[command(version, about, long_about = None)]
-struct BrokerCli {}
+struct BrokerCli {
+    /// Create unix socket listeners for all internal busses in the provided directory
+    #[clap(long)]
+    debug_access: Option<PathBuf>,
+
+    /// Private XDG_RUNTIME_DIR for the VM ('./doc' under it will be used as the document portal FS mountpoint, './fs.sock' will be the FS)
+    #[clap(long)]
+    runtime_dir: PathBuf,
+
+    /// Absolute path where the './doc' under the runtime-dir would be mounted over virtiofs in the guest
+    #[clap(long, default_value = "/run/vm-doc-portal")]
+    guest_mountpoint: PathBuf,
+
+    /// Vsock port number to listen on
+    #[clap(long)]
+    vsock_port: u32,
+}
+
+async fn new_hosted_bus() -> eyre::Result<(
+    Arc<Mutex<bus::HostedBus>>,
+    zbus::OwnedGuid,
+    bus::NameOwnerStream,
+)> {
+    let mut bus = bus::HostedBus::new().await?;
+    let guid = bus.server_guid().to_owned().into();
+    let owner_stream = bus.name_owner_changes().await?;
+
+    Ok((Arc::new(Mutex::new(bus)), guid, owner_stream))
+}
 
 #[tokio::main]
 async fn main() -> eyre::Result<()> {
     tracing_subscriber::fmt::init();
 
-    let _cli = BrokerCli::parse();
+    let cli = BrokerCli::parse();
 
-    let vm_bus = bus::HostedBus::new().await?;
-    let vm_bus_guid: zbus::OwnedGuid = vm_bus.server_guid().to_owned().into();
-    let vm_bus = Arc::new(Mutex::new(vm_bus));
+    let (vm_bus, vm_bus_guid, _) = new_hosted_bus().await?;
+    let (priv_bus, _, mut priv_lst) = new_hosted_bus().await?;
 
-    // Direct access for the host (just trying things out)
-    tokio::spawn(
-        vm_bus
-            .clone()
-            .run_unix_listener(tokio::net::UnixListener::bind("vmbus.sock")?),
-    );
+    if let Some(dir_path) = cli.debug_access {
+        if !dir_path.is_dir() {
+            error!(path = %dir_path.display(), "--debug-access path is not an existing directory");
+            std::process::exit(1);
+        }
+        let vm_dbg_listener = UnixListener::bind(dir_path.join("vm.sock"))?;
+        let _vm_dbg_task = tokio::spawn(vm_bus.clone().run_unix_listener(vm_dbg_listener));
+        let priv_dbg_listener = UnixListener::bind(dir_path.join("priv.sock"))?;
+        let _priv_dbg_task = tokio::spawn(priv_bus.clone().run_unix_listener(priv_dbg_listener));
+        // TODO: unlink sockets on exit
+    }
+
+    std::fs::create_dir_all(&cli.runtime_dir)?;
+
+    let _xps = priv_bus
+        .clone()
+        .spawn_external_client(
+            Command::new(env!("BIN_XDG_PERMISSION_STORE"))
+                .env("XDG_RUNTIME_DIR", cli.runtime_dir.as_os_str())
+                .kill_on_drop(true),
+        )
+        .await?;
+
+    let impl_permission_store =
+        WellKnownName::from_static_str("org.freedesktop.impl.portal.PermissionStore")?.into();
+    priv_lst.wait_for_acquisition(impl_permission_store).await?;
+
+    let _xdp = priv_bus
+        .clone()
+        .spawn_external_client(
+            Command::new(env!("BIN_XDG_DOCUMENT_PORTAL"))
+                .env("XDG_RUNTIME_DIR", cli.runtime_dir.as_os_str())
+                .kill_on_drop(true),
+        )
+        .await?;
+
+    let portal_documents =
+        WellKnownName::from_static_str("org.freedesktop.portal.Documents")?.into();
+    priv_lst.wait_for_acquisition(portal_documents).await?;
+
+    let _vfs = Command::new(env!("BIN_VIRTIOFSD"))
+        .args(&[
+            "--shared-dir",
+            cli.runtime_dir.join("doc").to_str().unwrap(),
+            "--socket-path",
+            cli.runtime_dir.join("fs.sock").to_str().unwrap(),
+            "--uid-map",
+            ":1000:1001:1:",
+            "--gid-map",
+            ":100:100:1:",
+            "--log-level",
+            "debug",
+        ])
+        .env("XDG_RUNTIME_DIR", cli.runtime_dir.as_os_str())
+        .kill_on_drop(true)
+        .spawn();
+    // TODO: die when it exits
+
+    let vm_bus_conn = vm_bus.lock().await.connect_channel(false).await?;
+    let priv_bus_conn = priv_bus.lock().await.connect_channel(false).await?;
+    let host_session_conn = zbus::connection::Builder::session()?.build().await?;
+    let file_chooser_imp = portal::file_chooser::FileChooser::new(
+        &host_session_conn,
+        &priv_bus_conn,
+        cli.guest_mountpoint,
+    )
+    .await?;
+    vm_bus_conn
+        .request_name("org.freedesktop.portal.Desktop")
+        .await?;
+    let true = vm_bus_conn
+        .object_server()
+        .at("/org/freedesktop/portal/desktop", file_chooser_imp)
+        .await?
+    else {
+        unreachable!("our own fresh bus can't have interfaces already provided");
+    };
 
     // TODO: modprobe vhost_vsock first!
     // NOTE: Every individual D-Bus client inside of the VM is a new client here!
-    vsock::ListenerBuilder::new(vsock::VsockAddr::new(vsock::VMADDR_CID_HOST, 4269))
-        .with_label("VM Bus")
-        .listen(move |client| {
-            enclose! { (vm_bus, vm_bus_guid) async move {
-                // TODO: Not necessary to go through the channel, add vsock support to the Peer too
-                let client_conn = client.build((&vm_bus_guid).into()).await?;
-                let vmbus_conn = vm_bus.lock().await.connect_channel(true).await?;
-                sidebus_common::raw::splice_conns(client_conn, vmbus_conn).await;
-                Ok(())
-            } }
-        })
-        .await?;
+    vsock::ListenerBuilder::new(vsock::VsockAddr::new(
+        vsock::VMADDR_CID_HOST,
+        cli.vsock_port,
+    ))
+    .with_label("VM Bus")
+    .listen(move |client| {
+        enclose! { (vm_bus, vm_bus_guid) async move {
+            // TODO: Not necessary to go through the channel, add vsock support to the Peer too
+            let client_conn = client.build((&vm_bus_guid).into()).await?;
+            let vmbus_conn = vm_bus.lock().await.connect_channel(true).await?;
+            sidebus_common::raw::splice_conns(client_conn, vmbus_conn).await;
+            Ok(())
+        } }
+    })
+    .await?;
 
     Ok(())
 }

sidebus-broker/src/portal/documents.rs

@@ -0,0 +1,85 @@
+use std::collections::HashMap;
+
+use zbus::{proxy, zvariant};
+
+#[proxy(
+    interface = "org.freedesktop.portal.Documents",
+    default_service = "org.freedesktop.portal.Documents",
+    default_path = "/org/freedesktop/portal/documents"
+)]
+pub trait Documents {
+    /// Add method
+    fn add(
+        &self,
+        o_path_fd: zvariant::Fd<'_>,
+        reuse_existing: bool,
+        persistent: bool,
+    ) -> zbus::Result<String>;
+
+    /// AddFull method
+    fn add_full(
+        &self,
+        o_path_fds: &[zvariant::Fd<'_>],
+        flags: u32,
+        app_id: &str,
+        permissions: &[&str],
+    ) -> zbus::Result<(Vec<String>, HashMap<String, zvariant::OwnedValue>)>;
+
+    /// AddNamed method
+    fn add_named(
+        &self,
+        o_path_parent_fd: zvariant::Fd<'_>,
+        filename: &[u8],
+        reuse_existing: bool,
+        persistent: bool,
+    ) -> zbus::Result<String>;
+
+    /// AddNamedFull method
+    #[allow(clippy::too_many_arguments)]
+    fn add_named_full(
+        &self,
+        o_path_fd: zvariant::Fd<'_>,
+        filename: &[u8],
+        flags: u32,
+        app_id: &str,
+        permissions: &[&str],
+    ) -> zbus::Result<(String, HashMap<String, zvariant::OwnedValue>)>;
+
+    /// Delete method
+    fn delete(&self, doc_id: &str) -> zbus::Result<()>;
+
+    /// GetHostPaths method
+    fn get_host_paths(&self, doc_ids: &[&str]) -> zbus::Result<HashMap<String, Vec<u8>>>;
+
+    /// GetMountPoint method
+    fn get_mount_point(&self) -> zbus::Result<Vec<u8>>;
+
+    /// GrantPermissions method
+    fn grant_permissions(
+        &self,
+        doc_id: &str,
+        app_id: &str,
+        permissions: &[&str],
+    ) -> zbus::Result<()>;
+
+    /// Info method
+    fn info(&self, doc_id: &str) -> zbus::Result<(Vec<u8>, HashMap<String, Vec<String>>)>;
+
+    /// List method
+    fn list(&self, app_id: &str) -> zbus::Result<HashMap<String, Vec<u8>>>;
+
+    /// Lookup method
+    fn lookup(&self, filename: &[u8]) -> zbus::Result<String>;
+
+    /// RevokePermissions method
+    fn revoke_permissions(
+        &self,
+        doc_id: &str,
+        app_id: &str,
+        permissions: &[&str],
+    ) -> zbus::Result<()>;
+
+    /// version property
+    #[zbus(property, name = "version")]
+    fn version(&self) -> zbus::Result<u32>;
+}

sidebus-broker/src/portal/file_chooser.rs

@@ -0,0 +1,301 @@
+use std::collections::HashMap;
+use std::os::{fd::AsFd as _, unix::ffi::OsStrExt as _};
+use std::path::PathBuf;
+
+use tracing::{debug, error, warn};
+use zbus::{Connection, ObjectServer, fdo::Result, zvariant};
+
+use super::documents::DocumentsProxy;
+use super::request::{RESPONSE_SUCCESS, ReqHandler, ResultTransformer};
+
+pub struct FileChooser {
+    host: FileChooserProxy<'static>,
+    docs: DocumentsProxy<'static>,
+    guest_root: PathBuf,
+}
+
+impl FileChooser {
+    pub async fn new(
+        host_session_conn: &Connection,
+        priv_conn: &Connection,
+        guest_root: PathBuf,
+    ) -> Result<Self> {
+        let host = FileChooserProxy::builder(host_session_conn).build().await?;
+        let docs = DocumentsProxy::builder(priv_conn).build().await?;
+        Ok(FileChooser {
+            host,
+            docs,
+            guest_root,
+        })
+    }
+}
+
+#[zbus::interface(
+    name = "org.freedesktop.portal.FileChooser",
+    proxy(
+        default_service = "org.freedesktop.portal.Desktop",
+        default_path = "/org/freedesktop/portal/desktop"
+    )
+)]
+impl FileChooser {
+    async fn open_file(
+        &self,
+        #[zbus(header)] hdr: zbus::message::Header<'_>,
+        #[zbus(object_server)] server: &ObjectServer,
+        #[zbus(connection)] conn: &zbus::Connection,
+        parent_window: &str,
+        title: &str,
+        options: HashMap<&str, zvariant::Value<'_>>,
+    ) -> Result<zvariant::OwnedObjectPath> {
+        ReqHandler::prepare(&self.host, hdr, server, conn, &options)
+            .with_transform(FileTransformer {
+                docs: self.docs.clone(),
+                guest_root: self.guest_root.clone(),
+                for_save: false,
+                directory: options.get_as("directory")?.unwrap_or(false),
+            })
+            .perform(async || self.host.open_file(parent_window, title, options).await)
+            .await
+    }
+
+    async fn save_file(
+        &self,
+        #[zbus(header)] hdr: zbus::message::Header<'_>,
+        #[zbus(object_server)] server: &ObjectServer,
+        #[zbus(connection)] conn: &zbus::Connection,
+        parent_window: &str,
+        title: &str,
+        options: HashMap<&str, zvariant::Value<'_>>,
+    ) -> Result<zvariant::OwnedObjectPath> {
+        ReqHandler::prepare(&self.host, hdr, server, conn, &options)
+            .with_transform(FileTransformer {
+                docs: self.docs.clone(),
+                guest_root: self.guest_root.clone(),
+                for_save: true,
+                directory: false,
+            })
+            .perform(async || self.host.save_file(parent_window, title, options).await)
+            .await
+    }
+
+    async fn save_files(
+        &self,
+        #[zbus(header)] hdr: zbus::message::Header<'_>,
+        #[zbus(object_server)] server: &ObjectServer,
+        #[zbus(connection)] conn: &zbus::Connection,
+        parent_window: &str,
+        title: &str,
+        options: HashMap<&str, zvariant::Value<'_>>,
+    ) -> Result<zvariant::OwnedObjectPath> {
+        ReqHandler::prepare(&self.host, hdr, server, conn, &options)
+            .with_transform(FileTransformer {
+                docs: self.docs.clone(),
+                guest_root: self.guest_root.clone(),
+                for_save: true,
+                directory: false,
+            })
+            .perform(async || self.host.save_files(parent_window, title, options).await)
+            .await
+    }
+
+    /// version property
+    #[zbus(property, name = "version")]
+    fn version(&self) -> Result<u32> {
+        Ok(5)
+    }
+}
+
+struct FileTransformer {
+    docs: DocumentsProxy<'static>,
+    guest_root: PathBuf,
+    for_save: bool,
+    directory: bool,
+}
+
+// ref: send_response_in_thread_func
+// https://github.com/flatpak/xdg-desktop-portal/blob/d037b5c3f91b68ca208a9a41b6e18e6a3a659e05/src/file-chooser.c#L70C1-L70C29
+
+impl ResultTransformer for FileTransformer {
+    async fn apply<'a>(
+        self,
+        response: u32,
+        mut results: HashMap<&'a str, zvariant::Value<'a>>,
+    ) -> Result<(u32, HashMap<&'a str, zvariant::Value<'a>>)> {
+        if response != RESPONSE_SUCCESS {
+            debug!(?response, ?results, "non-success, not transforming");
+            return Ok((response, results));
+        }
+
+        let guest_uris = results
+            .get_required_as::<zvariant::Array>("uris")?
+            .into_iter()
+            .flat_map(uri_to_path)
+            .async_map(|u| self.add_path_as_doc(u))
+            .await
+            .flatten()
+            .collect::<Vec<_>>();
+
+        results.insert("uris", guest_uris.into());
+        Ok((response, results))
+    }
+}
+
+const REUSE_EXISTING: u32 = 1 << 0;
+const PERSISTENT: u32 = 1 << 1;
+const AS_NEEDED_BY_APP: u32 = 1 << 2;
+const DIRECTORY: u32 = 1 << 3;
+
+// ref: xdp_register_document
+// https://github.com/flatpak/xdg-desktop-portal/blob/10e712e06aa8eb9cd0e59c73c5be62ba53e981a4/src/xdp-documents.c#L71
+
+impl FileTransformer {
+    async fn add_path_as_doc(&self, path: PathBuf) -> Option<String> {
+        use rustix::fs::{Mode, OFlags};
+
+        let o_path_fd = match rustix::fs::open(
+            if self.for_save { path.parent()? } else { &path },
+            OFlags::CLOEXEC | OFlags::PATH,
+            Mode::empty(),
+        ) {
+            Ok(fd) => fd,
+            Err(err) => {
+                warn!(%err, ?path, "could not open path descriptor");
+                return None;
+            }
+        };
+
+        let flags = REUSE_EXISTING
+            | PERSISTENT
+            | AS_NEEDED_BY_APP
+            | if self.directory { DIRECTORY } else { 0 };
+
+        // XXX: portal impl can return writable=false but host frontend does not pass that back..
+        // https://github.com/flatpak/xdg-desktop-portal/discussions/1763
+        let permissions = &["read", "write", "grant-permissions"][..];
+
+        let filename = path.file_name()?;
+        debug!(
+            ?path,
+            ?filename,
+            ?o_path_fd,
+            ?flags,
+            ?permissions,
+            "adding path to doc portal"
+        );
+        let app_id = ""; // host
+        let doc_id = match if self.for_save {
+            let filename_c = std::ffi::CString::new(filename.as_bytes()).ok()?;
+            self.docs
+                .add_named_full(
+                    o_path_fd.as_fd().into(),
+                    filename_c.as_bytes_with_nul(),
+                    flags,
+                    app_id,
+                    permissions,
+                )
+                .await
+                .map(|(doc_id, m)| (Some(doc_id), m))
+        } else {
+            self.docs
+                .add_full(&[o_path_fd.as_fd().into()], flags, app_id, permissions)
+                .await
+                .map(|(mut doc_ids, m)| (doc_ids.pop(), m))
+        } {
+            Ok((Some(v), _)) => v,
+            Ok((None, _)) => {
+                warn!(?filename, "adding doc to portal gave no ids");
+                return None;
+            }
+            Err(err) => {
+                warn!(?err, ?filename, "could not add doc to portal");
+                return None;
+            }
+        };
+        let path = self.guest_root.join(doc_id).join(filename);
+        match url::Url::from_file_path(&path) {
+            Ok(url) => Some(url.to_string()),
+            Err(err) => {
+                warn!(?err, ?path, "could not make url from returned path");
+                None
+            }
+        }
+    }
+}
+
+fn uri_to_path(v: &zvariant::Value<'_>) -> Option<PathBuf> {
+    let url_str = match v.downcast_ref::<zvariant::Str>() {
+        Ok(sv) => sv,
+        Err(err) => {
+            warn!(%err, ?v, "option 'uris' contains non-string?");
+            return None;
+        }
+    };
+    let url = match url::Url::parse(url_str.as_str()) {
+        Ok(u) => u,
+        Err(err) => {
+            warn!(%err, %url_str, "option 'uris' contains non-parseable uri");
+            return None;
+        }
+    };
+    if url.scheme() != "file" {
+        warn!(%url, "skipping non-file uri");
+        return None;
+    }
+    Some(PathBuf::from(url.path()))
+}
+
+trait MapExt<'a> {
+    fn get_as<T>(&'a self, key: &'a str) -> Result<Option<T>>
+    where
+        T: TryFrom<&'a zvariant::Value<'a>>,
+        <T as TryFrom<&'a zvariant::Value<'a>>>::Error: std::fmt::Display;
+
+    fn get_required_as<T>(&'a self, key: &'a str) -> Result<T>
+    where
+        T: TryFrom<&'a zvariant::Value<'a>>,
+        <T as TryFrom<&'a zvariant::Value<'a>>>::Error: std::fmt::Display,
+    {
+        self.get_as(key).and_then(|o| {
+            o.ok_or_else(|| {
+                error!(%key, "options get_as, missing");
+                zbus::fdo::Error::Failed(format!("option '{key}' missing"))
+            })
+        })
+    }
+}
+
+impl<'a> MapExt<'a> for HashMap<&'a str, zvariant::Value<'a>> {
+    fn get_as<T>(&'a self, key: &str) -> Result<Option<T>>
+    where
+        T: TryFrom<&'a zvariant::Value<'a>>,
+        <T as TryFrom<&'a zvariant::Value<'a>>>::Error: std::fmt::Display,
+    {
+        self.get(key)
+            .map(|v| {
+                // inlined downcast_ref
+                if let zvariant::Value::Value(v) = v {
+                    <T>::try_from(v)
+                } else {
+                    <T>::try_from(v)
+                }
+            })
+            .transpose()
+            .map_err(|err| {
+                error!(%err, %key, "options get_as");
+                zbus::fdo::Error::Failed(format!("option '{key}' type mismatch"))
+            })
+    }
+}
+
+trait IterAsyncExt: Iterator {
+    async fn async_map<B, F, FU>(self, f: F) -> impl Iterator<Item = B>
+    where
+        Self: Sized,
+        F: FnMut(Self::Item) -> FU,
+        FU: Future<Output = B>,
+    {
+        futures::future::join_all(self.map(f)).await.into_iter()
+    }
+}
+
+impl<T: Iterator> IterAsyncExt for T {}

sidebus-broker/src/portal/mod.rs

@@ -0,0 +1,3 @@
+pub mod documents;
+pub mod file_chooser;
+pub mod request;

sidebus-broker/src/portal/request.rs

@@ -0,0 +1,183 @@
+use std::{collections::HashMap, ops::Deref as _};
+
+use tokio_stream::StreamExt as _;
+use tracing::{Instrument, debug, debug_span, error, trace, warn};
+use zbus::{
+    Connection, ObjectServer,
+    fdo::Result,
+    names::OwnedUniqueName,
+    object_server::SignalEmitter,
+    proxy::ProxyImpl,
+    zvariant::{self, OwnedObjectPath},
+};
+
+pub const RESPONSE_SUCCESS: u32 = 0;
+// pub const RESPONSE_CANCELLED: u32 = 1;
+pub const RESPONSE_OTHER: u32 = 2;
+
+/// A handler for the org.freedesktop.portal.Request interface, proxying to another
+/// instance of the same interface.
+struct Request {
+    host: RequestProxy<'static>,
+}
+
+#[zbus::interface(
+    name = "org.freedesktop.portal.Request",
+    proxy(default_service = "org.freedesktop.portal.Desktop")
+)]
+impl Request {
+    #[zbus(signal)]
+    async fn response(
+        signal_emitter: &SignalEmitter<'_>,
+        response: u32,
+        results: HashMap<&str, zvariant::Value<'_>>,
+    ) -> zbus::Result<()>;
+
+    async fn close(&self) -> Result<()> {
+        self.host.close().await
+    }
+}
+
+pub trait ResultTransformer {
+    fn apply<'a>(
+        self,
+        response: u32,
+        results: HashMap<&'a str, zvariant::Value<'a>>,
+    ) -> impl std::future::Future<Output = Result<(u32, HashMap<&'a str, zvariant::Value<'a>>)>>
+    + std::marker::Send;
+}
+
+impl ResultTransformer for () {
+    async fn apply<'a>(
+        self,
+        response: u32,
+        results: HashMap<&'a str, zvariant::Value<'a>>,
+    ) -> Result<(u32, HashMap<&'a str, zvariant::Value<'a>>)> {
+        Ok((response, results))
+    }
+}
+
+pub struct ReqHandler<T: ResultTransformer> {
+    token: String,
+    sender: Option<OwnedUniqueName>,
+    conn: Connection,
+    server: ObjectServer,
+    host_conn: Connection,
+    transformer: T,
+}
+
+impl ReqHandler<()> {
+    pub fn prepare<'a>(
+        host: &impl ProxyImpl<'a>,
+        hdr: zbus::message::Header<'_>,
+        server: &ObjectServer,
+        conn: &Connection,
+        options: &HashMap<&str, zvariant::Value<'_>>,
+    ) -> Self {
+        ReqHandler {
+            token: get_token(options),
+            sender: hdr.sender().map(|s| s.to_owned().into()),
+            conn: conn.to_owned(),
+            server: server.to_owned(),
+            host_conn: host.inner().connection().to_owned(),
+            transformer: (),
+        }
+    }
+}
+
+impl<T: ResultTransformer> ReqHandler<T> {
+    pub fn with_transform<T1: ResultTransformer>(self, transformer: T1) -> ReqHandler<T1> {
+        ReqHandler {
+            transformer,
+            token: self.token,
+            sender: self.sender,
+            conn: self.conn,
+            server: self.server,
+            host_conn: self.host_conn,
+        }
+    }
+}
+
+impl<T: ResultTransformer + Send + 'static> ReqHandler<T> {
+    pub async fn perform(
+        self,
+        call: impl AsyncFnOnce() -> Result<OwnedObjectPath>,
+    ) -> Result<OwnedObjectPath> {
+        let sender = self.sender.ok_or_else(|| zbus::Error::MissingField)?;
+        let sender_str = sender.trim_start_matches(':').replace('.', "_");
+        let token = self.token;
+        let path = zvariant::ObjectPath::try_from(format!(
+            "/org/freedesktop/portal/desktop/request/{sender_str}/{token}"
+        ))
+        .map_err(zbus::Error::from)?;
+
+        let host_path = call().await?;
+        let imp = Request {
+            host: RequestProxy::builder(&self.host_conn)
+                .path(host_path)?
+                .build()
+                .await?,
+        };
+        let stream = imp.host.receive_response().await?;
+        if !self.server.at(&path, imp).await? {
+            return Err(zbus::fdo::Error::Failed(
+                "Duplicate request object path".to_owned(),
+            ));
+        }
+
+        let path_1: OwnedObjectPath = path.clone().into();
+        let sender = sender.to_owned().into();
+        tokio::spawn(
+            forward_response(stream, self.conn.clone(), path_1, sender, self.transformer)
+                .instrument(debug_span!("response proxy", ?path)),
+        );
+        Ok(path.into())
+    }
+}
+
+fn get_token(options: &HashMap<&str, zvariant::Value<'_>>) -> String {
+    match options.get("handle_token") {
+        Some(zvariant::Value::Str(str)) => {
+            trace!("extracted token from handle_token option");
+            return String::from(str.deref());
+        }
+        Some(value) => warn!(?value, "handle_token option provided but not a string"),
+        None => trace!("handle_token not provided"),
+    };
+    use rand::distr::{Alphanumeric, SampleString};
+    Alphanumeric.sample_string(&mut rand::rng(), 16)
+}
+
+async fn forward_response(
+    mut stream: ResponseStream,
+    conn: Connection,
+    path: zvariant::OwnedObjectPath,
+    sender: zbus::names::OwnedUniqueName,
+    transform: impl ResultTransformer,
+) -> Result<()> {
+    let signal_emitter = SignalEmitter::new(&conn, path)?
+        .set_destination(zbus::names::BusName::Unique(sender.into()))
+        .into_owned();
+    let Some(resp) = stream.next().await else {
+        debug!("response stream gone");
+        return Ok(());
+    };
+    debug!(?resp, "got resp");
+    let (response, results) = match resp.0.deserialize() {
+        Ok((response, results)) => match transform.apply(response, results).await {
+            Ok(res) => res,
+            Err(err) => {
+                error!(%err, "transform error");
+                (RESPONSE_OTHER, HashMap::new())
+            }
+        },
+        Err(err) => {
+            error!(%err, "signal body type mismatch");
+            (RESPONSE_OTHER, HashMap::new())
+        }
+    };
+    if let Err(err) = Request::response(&signal_emitter, response, results).await {
+        error!(%err, "signal forwarding failed");
+    }
+    Ok(())
+}