microsandbox_agentd/

fs.rs

//! Guest-side filesystem operation handlers.
//!
//! Handles `core.fs.*` protocol messages by performing filesystem operations
//! using `std::fs` and `tokio::fs`, then sending responses back to the host.

use std::collections::HashMap;
use std::ffi::CString;
use std::io::IoSlice;
use std::os::fd::AsRawFd;
use std::os::unix::ffi::OsStrExt;
use std::os::unix::fs::{MetadataExt, OpenOptionsExt, PermissionsExt};
use std::path::Path;
use std::sync::Arc;

use bytes::Bytes;
use microsandbox_protocol::bulk::{
    BULK_FLOW_MASK_GUEST_TO_HOST, BULK_FLOW_MASK_HOST_TO_GUEST, BULK_PROTOCOL_VERSION,
    BulkAccepted, BulkCredit, BulkFinish, BulkFlow, BulkKind, BulkOffer, BulkReceiveState,
    BulkRecord, BulkSendState, DEFAULT_BULK_WINDOW, DEFAULT_FILESYSTEM_BULK_RECORD_PAYLOAD,
};
use microsandbox_protocol::codec;
use microsandbox_protocol::fs::{
    FS_CHUNK_SIZE, FsData, FsEntryInfo, FsOp, FsOpenOptions, FsRequest, FsResponse, FsResponseData,
    FsSetAttrs,
};
use microsandbox_protocol::message::{Message, MessageType};
use microsandbox_protocol::transport::relay_client_slot;
use serde::Serialize;
use tokio::io::{AsyncReadExt, AsyncSeekExt, AsyncWriteExt};
use tokio::sync::{Mutex, watch};
use tokio::task::JoinHandle;

use crate::session::{
    BulkSessionOutput, RawActivity, RawSessionCompletion, RawSessionOutput, SessionOutput,
    SessionOutputSender,
};

//--------------------------------------------------------------------------------------------------
// Constants
//--------------------------------------------------------------------------------------------------

/// Default maximum number of entries returned by one `ReadDir` request.
const DEFAULT_READ_DIR_LIMIT: u32 = 128;

/// Maximum number of open filesystem handles owned by one relay client.
const MAX_OPEN_HANDLES_PER_OWNER: usize = 1024;

//--------------------------------------------------------------------------------------------------
// Types
//--------------------------------------------------------------------------------------------------

/// Mutable filesystem protocol state held by agentd.
#[derive(Default)]
pub struct FsState {
    next_handle: u64,
    handles: HashMap<u64, FsHandleEntry>,
}

/// Tracks an in-progress streaming write operation.
pub struct FsWriteSession {
    owner_id: u32,
    handle: u64,
    file: Arc<Mutex<tokio::fs::File>>,
    offset: u64,
    append: bool,
    expected_len: Option<u64>,
    written: u64,
    bulk: Option<BulkReceiveState>,
}

/// Tracks an in-progress streaming read operation.
pub struct FsReadSession {
    owner_id: u32,
    handle: u64,
    task: JoinHandle<()>,
    credit_tx: Option<watch::Sender<Option<BulkCredit>>>,
}

/// A filesystem stream session started by a request.
pub enum FsStreamSession {
    /// Read stream task.
    Read(FsReadSession),

    /// Write stream awaiting `FsData` chunks.
    Write(FsWriteSession),
}

enum FsHandleEntry {
    File {
        owner_id: u32,
        file: Arc<Mutex<tokio::fs::File>>,
        read: bool,
        write: bool,
        append: bool,
        path: String,
    },
    Dir {
        owner_id: u32,
        dir: Arc<Mutex<tokio::fs::ReadDir>>,
        path: String,
    },
}

//--------------------------------------------------------------------------------------------------
// Methods
//--------------------------------------------------------------------------------------------------

impl FsState {
    /// Close handles opened by a disconnected relay client.
    pub fn close_owner_range(&mut self, id_start: u32, id_end_exclusive: u32) {
        self.handles.retain(|_, handle| {
            let owner_id = handle.owner_id();
            owner_id < id_start || owner_id >= id_end_exclusive
        });
    }

    /// Close all handles.
    pub fn clear(&mut self) {
        self.handles.clear();
    }

    fn insert_file(
        &mut self,
        owner_id: u32,
        file: tokio::fs::File,
        read: bool,
        write: bool,
        append: bool,
        path: String,
    ) -> Result<u64, String> {
        self.enforce_owner_limit(owner_id)?;
        let handle = self.alloc_handle();
        self.handles.insert(
            handle,
            FsHandleEntry::File {
                owner_id,
                file: Arc::new(Mutex::new(file)),
                read,
                write,
                append,
                path,
            },
        );
        Ok(handle)
    }

    fn insert_dir(
        &mut self,
        owner_id: u32,
        dir: tokio::fs::ReadDir,
        path: String,
    ) -> Result<u64, String> {
        self.enforce_owner_limit(owner_id)?;
        let handle = self.alloc_handle();
        self.handles.insert(
            handle,
            FsHandleEntry::Dir {
                owner_id,
                dir: Arc::new(Mutex::new(dir)),
                path,
            },
        );
        Ok(handle)
    }

    fn close_handle(&mut self, caller_id: u32, handle: u64) -> Result<FsHandleEntry, String> {
        if let Some(entry) = self.handles.get(&handle) {
            entry.ensure_owner(handle, caller_id)?;
        }
        self.handles
            .remove(&handle)
            .ok_or_else(|| format!("invalid handle: {handle}"))
    }

    fn file(
        &self,
        caller_id: u32,
        handle: u64,
        need_read: bool,
        need_write: bool,
    ) -> Result<(Arc<Mutex<tokio::fs::File>>, bool, String), String> {
        match self.handles.get(&handle) {
            Some(FsHandleEntry::File {
                file,
                read,
                write,
                append,
                path,
                ..
            }) => {
                self.handles
                    .get(&handle)
                    .expect("entry just matched")
                    .ensure_owner(handle, caller_id)?;
                if need_read && !read {
                    return Err(format!("handle {handle} is not open for reading"));
                }
                if need_write && !write && !append {
                    return Err(format!("handle {handle} is not open for writing"));
                }
                Ok((Arc::clone(file), *append, path.clone()))
            }
            Some(FsHandleEntry::Dir { .. }) => Err(format!("handle {handle} is a directory")),
            None => Err(format!("invalid handle: {handle}")),
        }
    }

    fn dir(
        &self,
        caller_id: u32,
        handle: u64,
    ) -> Result<(Arc<Mutex<tokio::fs::ReadDir>>, String), String> {
        match self.handles.get(&handle) {
            Some(FsHandleEntry::Dir { dir, path, .. }) => {
                self.handles
                    .get(&handle)
                    .expect("entry just matched")
                    .ensure_owner(handle, caller_id)?;
                Ok((Arc::clone(dir), path.clone()))
            }
            Some(FsHandleEntry::File { .. }) => Err(format!("handle {handle} is a file")),
            None => Err(format!("invalid handle: {handle}")),
        }
    }

    fn alloc_handle(&mut self) -> u64 {
        self.next_handle = self.next_handle.wrapping_add(1).max(1);
        while self.handles.contains_key(&self.next_handle) {
            self.next_handle = self.next_handle.wrapping_add(1).max(1);
        }
        self.next_handle
    }

    fn enforce_owner_limit(&self, owner_id: u32) -> Result<(), String> {
        let count = self
            .handles
            .values()
            .filter(|entry| same_relay_client(entry.owner_id(), owner_id))
            .count();
        if count >= MAX_OPEN_HANDLES_PER_OWNER {
            return Err(format!(
                "too many open filesystem handles for relay client: {count}"
            ));
        }
        Ok(())
    }
}

impl FsHandleEntry {
    fn owner_id(&self) -> u32 {
        match self {
            Self::File { owner_id, .. } | Self::Dir { owner_id, .. } => *owner_id,
        }
    }

    fn ensure_owner(&self, handle: u64, caller_id: u32) -> Result<(), String> {
        if same_relay_client(self.owner_id(), caller_id) {
            Ok(())
        } else {
            Err(format!(
                "handle {handle} is owned by a different relay client"
            ))
        }
    }
}

impl FsReadSession {
    /// Correlation ID whose relay client owns this read stream.
    pub fn owner_id(&self) -> u32 {
        self.owner_id
    }

    /// Filesystem handle being read.
    pub fn handle(&self) -> u64 {
        self.handle
    }

    /// Abort the background read task.
    pub fn abort(self) {
        self.task.abort();
    }

    /// Delivers the latest absolute credit update to a generation-8 read task.
    pub fn apply_credit(&self, credit: BulkCredit) -> Result<(), String> {
        let Some(tx) = &self.credit_tx else {
            return Err("filesystem read session is not using raw bulk".into());
        };
        if credit.kind != BulkKind::Filesystem || credit.flow != BulkFlow::GuestToHost {
            return Err("filesystem read received credit for another kind or flow".into());
        }
        tx.send_replace(Some(credit));
        Ok(())
    }

    /// Whether this read session negotiated generation-8 raw bulk.
    pub fn is_bulk(&self) -> bool {
        self.credit_tx.is_some()
    }
}

impl FsWriteSession {
    /// Correlation ID whose relay client owns this write stream.
    pub fn owner_id(&self) -> u32 {
        self.owner_id
    }

    /// Filesystem handle being written.
    pub fn handle(&self) -> u64 {
        self.handle
    }

    /// Whether this write session negotiated generation-8 raw bulk.
    pub fn is_bulk(&self) -> bool {
        self.bulk.is_some()
    }
}

//--------------------------------------------------------------------------------------------------
// Functions
//--------------------------------------------------------------------------------------------------

fn same_relay_client(left: u32, right: u32) -> bool {
    relay_client_slot(left).is_some_and(|left| Some(left) == relay_client_slot(right))
}

/// Handles an incoming `FsRequest` message.
pub async fn handle_fs_request(
    id: u32,
    protocol_version: u8,
    req: FsRequest,
    state: &mut FsState,
    out_buf: &mut Vec<u8>,
    session_tx: &SessionOutputSender,
) -> Result<Option<FsStreamSession>, String> {
    let FsRequest { op, bulk } = req;
    if bulk.is_some() && protocol_version < BULK_PROTOCOL_VERSION {
        encode_response(
            id,
            error_response(format!(
                "raw bulk offer requires protocol generation {BULK_PROTOCOL_VERSION}"
            )),
            out_buf,
        )?;
        return Ok(None);
    }
    if bulk.is_some() && !matches!(&op, FsOp::Read { .. } | FsOp::Write { .. }) {
        encode_response(
            id,
            error_response("raw bulk is only valid for streaming reads and writes".into()),
            out_buf,
        )?;
        return Ok(None);
    }

    match op {
        FsOp::RealPath { path } => {
            let resp = handle_realpath(&path).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::Stat {
            path,
            follow_symlink,
        } => {
            let resp = handle_stat(&path, follow_symlink).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::SetStat {
            path,
            follow_symlink,
            attrs,
        } => {
            let resp = handle_setstat(&path, follow_symlink, attrs).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::List { path } => {
            let resp = handle_list(&path).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::ReadLink { path } => {
            let resp = handle_readlink(&path).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::Symlink {
            target,
            link_path,
            user,
        } => {
            let resp = handle_symlink(&target, &link_path, user).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::Mkdir { path, mode, user } => {
            let resp = handle_mkdir(&path, mode, user).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::Remove { path } => {
            let resp = handle_remove(&path).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::RemoveDir { path, recursive } => {
            let resp = handle_remove_dir(&path, recursive).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::Copy { src, dst } => {
            let resp = handle_copy(&src, &dst).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::Rename { src, dst } => {
            let resp = handle_rename(&src, &dst).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::OpenFile { path, options } => {
            let resp = handle_open_file(id, state, &path, options).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::OpenDir { path } => {
            let resp = handle_open_dir(id, state, &path).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::CloseHandle { handle } => {
            let resp = handle_close_handle(id, state, handle).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::Read {
            handle,
            offset,
            len,
        } => match state.file(id, handle, true, false) {
            Ok((file, _, _)) => {
                let tx = session_tx.clone();
                let (task, credit_tx) = match bulk {
                    Some(offer) => {
                        let accepted = accept_fs_read_offer(offer)?;
                        encode_control(MessageType::BulkAccepted, id, &accepted, out_buf)?;
                        let sender = BulkSendState::new(
                            BulkKind::Filesystem,
                            BulkFlow::GuestToHost,
                            accepted.max_record_payload,
                            accepted.guest_to_host_credit_limit,
                        )
                        .map_err(|error| format!("accept read bulk state: {error}"))?;
                        let (credit_tx, credit_rx) = watch::channel(None);
                        let task = tokio::spawn(async move {
                            handle_bulk_read_stream(id, file, offset, len, sender, credit_rx, &tx)
                                .await;
                        });
                        (task, Some(credit_tx))
                    }
                    None => {
                        let task = tokio::spawn(async move {
                            handle_read_stream(id, file, offset, len, &tx).await;
                        });
                        (task, None)
                    }
                };
                Ok(Some(FsStreamSession::Read(FsReadSession {
                    owner_id: id,
                    handle,
                    task,
                    credit_tx,
                })))
            }
            Err(e) => {
                encode_response(id, error_response(format!("read: {e}")), out_buf)?;
                Ok(None)
            }
        },
        FsOp::Write {
            handle,
            offset,
            len,
        } => match state.file(id, handle, false, true) {
            Ok((file, append, _)) => {
                let bulk = match bulk {
                    Some(offer) => {
                        let accepted = accept_fs_write_offer(offer)?;
                        encode_control(MessageType::BulkAccepted, id, &accepted, out_buf)?;
                        Some(
                            BulkReceiveState::new(
                                BulkKind::Filesystem,
                                BulkFlow::HostToGuest,
                                accepted.max_record_payload,
                                accepted.host_to_guest_credit_limit,
                                DEFAULT_BULK_WINDOW,
                            )
                            .map_err(|error| format!("accept write bulk state: {error}"))?,
                        )
                    }
                    None => None,
                };
                Ok(Some(FsStreamSession::Write(FsWriteSession {
                    owner_id: id,
                    handle,
                    file,
                    offset,
                    append,
                    expected_len: len,
                    written: 0,
                    bulk,
                })))
            }
            Err(e) => {
                encode_response(id, error_response(format!("write: {e}")), out_buf)?;
                Ok(None)
            }
        },
        FsOp::ReadDir { handle, limit } => {
            let resp = handle_read_dir(id, state, handle, limit).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::FStat { handle } => {
            let resp = handle_fstat(id, state, handle).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
        FsOp::FSetStat { handle, attrs } => {
            let resp = handle_fsetstat(id, state, handle, attrs).await;
            encode_response(id, resp, out_buf)?;
            Ok(None)
        }
    }
}

/// Handles an incoming `FsData` message for a streaming write session.
///
/// If `data` is empty, the file is flushed and a terminal `FsResponse` is sent.
/// Returns `true` if the session should be removed (EOF received).
pub async fn handle_fs_data(
    id: u32,
    data: FsData,
    session: &mut FsWriteSession,
    out_buf: &mut Vec<u8>,
) -> Result<bool, String> {
    if session.bulk.is_some() {
        encode_response(
            id,
            error_response("CBOR filesystem data is invalid after raw bulk acceptance".into()),
            out_buf,
        )?;
        return Ok(true);
    }
    if data.data.is_empty() {
        if let Some(expected) = session.expected_len
            && session.written != expected
        {
            let resp = error_response(format!(
                "write length mismatch: expected {expected}, wrote {}",
                session.written
            ));
            encode_response(id, resp, out_buf)?;
            return Ok(true);
        }

        if let Some(expected) = session.expected_len {
            let next_written = session.written.saturating_add(data.data.len() as u64);
            if next_written > expected {
                let resp = error_response(format!(
                    "write length mismatch: expected {expected}, received at least {next_written}"
                ));
                encode_response(id, resp, out_buf)?;
                return Ok(true);
            }
        }

        let mut file = session.file.lock().await;
        if let Err(e) = file.flush().await {
            encode_response(id, error_response(format!("flush: {e}")), out_buf)?;
            return Ok(true);
        }

        encode_response(id, ok_response(None), out_buf)?;
        Ok(true)
    } else {
        let mut file = session.file.lock().await;
        if !session.append
            && let Err(e) = file.seek(std::io::SeekFrom::Start(session.offset)).await
        {
            encode_response(id, error_response(format!("seek: {e}")), out_buf)?;
            return Ok(true);
        }
        if let Err(e) = file.write_all(&data.data).await {
            encode_response(id, error_response(format!("write: {e}")), out_buf)?;
            return Ok(true);
        }
        session.offset = session.offset.saturating_add(data.data.len() as u64);
        session.written = session.written.saturating_add(data.data.len() as u64);
        Ok(false)
    }
}

/// Handles one generation-8 raw record for a filesystem write correlation.
pub async fn handle_fs_bulk_record(
    id: u32,
    record: &BulkRecord,
    session: &mut FsWriteSession,
    out_buf: &mut Vec<u8>,
) -> Result<bool, String> {
    handle_fs_bulk_records(id, std::slice::from_ref(record), session, out_buf).await
}

/// Handles a bounded contiguous batch of generation-8 filesystem records.
///
/// The protocol still validates and accounts for each wire record independently. Coalescing only
/// changes the local filesystem effect: one lock, one seek and a vectored write replace multiple
/// blocking-file tasks when a peer negotiates records below the current filesystem default.
pub async fn handle_fs_bulk_records(
    id: u32,
    records: &[BulkRecord],
    session: &mut FsWriteSession,
    out_buf: &mut Vec<u8>,
) -> Result<bool, String> {
    if records.is_empty() {
        return Err("filesystem bulk record batch is empty".into());
    }

    let mut final_end = session.written;
    let mut payload_bytes = 0usize;
    {
        let Some(receiver) = session.bulk.as_mut() else {
            return Err("raw bulk record sent to a generation-6 filesystem write".into());
        };
        for record in records {
            final_end = receiver
                .accept_record(record)
                .map_err(|error| format!("invalid filesystem bulk record: {error}"))?;
            payload_bytes = payload_bytes
                .checked_add(record.payload.len())
                .ok_or_else(|| "filesystem bulk batch byte count overflowed usize".to_string())?;

            if let Some(expected) = session.expected_len
                && final_end > expected
            {
                encode_response(
                    id,
                    error_response(format!(
                        "write length mismatch: expected {expected}, received at least {final_end}"
                    )),
                    out_buf,
                )?;
                return Ok(true);
            }
        }
    }

    let mut file = session.file.lock().await;
    if !session.append
        && let Err(error) = file.seek(std::io::SeekFrom::Start(session.offset)).await
    {
        encode_response(id, error_response(format!("seek: {error}")), out_buf)?;
        return Ok(true);
    }
    if let Err(error) = write_bulk_payloads_vectored(&mut file, records).await {
        encode_response(id, error_response(format!("write: {error}")), out_buf)?;
        return Ok(true);
    }
    drop(file);

    session.offset = session.offset.saturating_add(payload_bytes as u64);
    session.written = final_end;
    let receiver = session
        .bulk
        .as_mut()
        .expect("bulk receiver was validated before the filesystem write");
    if let Some(credit) = receiver
        .consume(final_end)
        .map_err(|error| format!("advance filesystem bulk credit: {error}"))?
    {
        encode_control(MessageType::BulkCredit, id, &credit, out_buf)?;
    }
    Ok(false)
}

/// Completes a vectored Tokio file write even when the kernel accepts only a prefix of the batch.
async fn write_bulk_payloads_vectored(
    file: &mut tokio::fs::File,
    records: &[BulkRecord],
) -> std::io::Result<()> {
    if records.len() == 1 {
        return file.write_all(&records[0].payload).await;
    }

    let mut record_index = 0usize;
    let mut record_offset = 0usize;

    while record_index < records.len() {
        let mut slices = Vec::with_capacity(records.len() - record_index);
        slices.push(IoSlice::new(
            &records[record_index].payload[record_offset..],
        ));
        slices.extend(
            records[record_index + 1..]
                .iter()
                .map(|record| IoSlice::new(&record.payload)),
        );

        let written = file.write_vectored(&slices).await?;
        if written == 0 {
            return Err(std::io::Error::new(
                std::io::ErrorKind::WriteZero,
                "failed to write filesystem bulk batch",
            ));
        }

        let mut remaining = written;
        while record_index < records.len() {
            let record_remaining = records[record_index].payload.len() - record_offset;
            if remaining < record_remaining {
                record_offset += remaining;
                break;
            }
            remaining -= record_remaining;
            record_index += 1;
            record_offset = 0;
            if remaining == 0 {
                break;
            }
        }
    }
    Ok(())
}

/// Handles the exact end marker for a generation-8 filesystem write.
pub async fn handle_fs_bulk_finish(
    id: u32,
    finish: BulkFinish,
    session: &mut FsWriteSession,
    out_buf: &mut Vec<u8>,
) -> Result<bool, String> {
    let Some(receiver) = session.bulk.as_mut() else {
        return Err("bulk finish sent to a generation-6 filesystem write".into());
    };
    receiver
        .accept_finish(finish)
        .map_err(|error| format!("invalid filesystem bulk finish: {error}"))?;

    if let Some(expected) = session.expected_len
        && session.written != expected
    {
        encode_response(
            id,
            error_response(format!(
                "write length mismatch: expected {expected}, wrote {}",
                session.written
            )),
            out_buf,
        )?;
        return Ok(true);
    }

    let mut file = session.file.lock().await;
    if let Err(error) = file.flush().await {
        encode_response(id, error_response(format!("flush: {error}")), out_buf)?;
        return Ok(true);
    }
    encode_response(id, ok_response(None), out_buf)?;
    Ok(true)
}

//--------------------------------------------------------------------------------------------------
// Functions: Handlers
//--------------------------------------------------------------------------------------------------

async fn handle_realpath(path: &str) -> FsResponse {
    match realpath(path).await {
        Ok(path) => ok_response(Some(FsResponseData::Path(path))),
        Err(e) => error_response(format!("realpath: {e}")),
    }
}

async fn handle_stat(path: &str, follow_symlink: bool) -> FsResponse {
    let result = if follow_symlink {
        tokio::fs::metadata(path).await
    } else {
        tokio::fs::symlink_metadata(path).await
    };

    match result {
        Ok(meta) => ok_response(Some(FsResponseData::Stat(metadata_to_entry_info(
            path, &meta,
        )))),
        Err(e) => error_response(format!("stat: {e}")),
    }
}

async fn handle_setstat(path: &str, follow_symlink: bool, attrs: FsSetAttrs) -> FsResponse {
    match apply_path_attrs(path, follow_symlink, attrs).await {
        Ok(()) => ok_response(None),
        Err(e) => error_response(format!("setstat: {e}")),
    }
}

async fn handle_list(path: &str) -> FsResponse {
    match read_all_dir(path).await {
        Ok(entries) => ok_response(Some(FsResponseData::List(entries))),
        Err(e) => error_response(format!("readdir: {e}")),
    }
}

async fn handle_readlink(path: &str) -> FsResponse {
    match tokio::fs::read_link(path).await {
        Ok(target) => ok_response(Some(FsResponseData::Path(
            target.to_string_lossy().to_string(),
        ))),
        Err(e) => error_response(format!("readlink: {e}")),
    }
}

async fn handle_symlink(target: &str, link_path: &str, user: Option<String>) -> FsResponse {
    let target = target.to_string();
    let link_path = link_path.to_string();
    match run_as_user(user, move || std::os::unix::fs::symlink(target, link_path)).await {
        Ok(Ok(())) => ok_response(None),
        Ok(Err(e)) => error_response(format!("symlink: {e}")),
        Err(e) => error_response(e),
    }
}

async fn handle_open_file(
    id: u32,
    state: &mut FsState,
    path: &str,
    options: FsOpenOptions,
) -> FsResponse {
    let mut open_options = std::fs::OpenOptions::new();
    open_options
        .read(options.read)
        .write(options.write)
        .append(options.append)
        .create(options.create)
        .truncate(options.truncate)
        .create_new(options.create_new);
    if let Some(mode) = options.mode {
        open_options.mode(mode);
    }

    let open_path = path.to_string();
    match run_as_user(options.user, move || open_options.open(open_path)).await {
        Ok(Ok(file)) => match state.insert_file(
            id,
            tokio::fs::File::from_std(file),
            options.read,
            options.write,
            options.append,
            path.to_string(),
        ) {
            Ok(handle) => ok_response(Some(FsResponseData::Handle(handle))),
            Err(e) => error_response(format!("open: {e}")),
        },
        Ok(Err(e)) => error_response(format!("open: {e}")),
        Err(e) => error_response(e),
    }
}

async fn handle_open_dir(id: u32, state: &mut FsState, path: &str) -> FsResponse {
    match tokio::fs::read_dir(path).await {
        Ok(dir) => match state.insert_dir(id, dir, path.to_string()) {
            Ok(handle) => ok_response(Some(FsResponseData::Handle(handle))),
            Err(e) => error_response(format!("opendir: {e}")),
        },
        Err(e) => error_response(format!("opendir: {e}")),
    }
}

async fn handle_close_handle(id: u32, state: &mut FsState, handle: u64) -> FsResponse {
    match state.close_handle(id, handle) {
        Ok(FsHandleEntry::File { file, .. }) => {
            let mut file = file.lock().await;
            match file.flush().await {
                Ok(()) => ok_response(None),
                Err(e) => error_response(format!("close: {e}")),
            }
        }
        Ok(FsHandleEntry::Dir { .. }) => ok_response(None),
        Err(e) => error_response(format!("close: {e}")),
    }
}

async fn handle_read_dir(id: u32, state: &FsState, handle: u64, limit: Option<u32>) -> FsResponse {
    let (dir, path) = match state.dir(id, handle) {
        Ok(v) => v,
        Err(e) => return error_response(format!("readdir: {e}")),
    };

    let limit = limit.unwrap_or(DEFAULT_READ_DIR_LIMIT).max(1);
    let mut dir = dir.lock().await;
    let mut entries = Vec::new();

    for _ in 0..limit {
        match dir.next_entry().await {
            Ok(Some(entry)) => {
                let entry_path = entry.path();
                let path_str = entry_path.to_string_lossy().to_string();
                match tokio::fs::symlink_metadata(&entry_path).await {
                    Ok(meta) => entries.push(metadata_to_entry_info(&path_str, &meta)),
                    Err(_) => entries.push(unknown_entry_info(&path_str)),
                }
            }
            Ok(None) => break,
            Err(e) => return error_response(format!("readdir {path}: {e}")),
        }
    }

    ok_response(Some(FsResponseData::List(entries)))
}

async fn handle_fstat(id: u32, state: &FsState, handle: u64) -> FsResponse {
    match state.handles.get(&handle) {
        Some(FsHandleEntry::File { file, path, .. }) => {
            if let Err(e) = state
                .handles
                .get(&handle)
                .expect("entry just matched")
                .ensure_owner(handle, id)
            {
                return error_response(format!("fstat: {e}"));
            }
            let file = file.lock().await;
            match file.metadata().await {
                Ok(meta) => ok_response(Some(FsResponseData::Stat(metadata_to_entry_info(
                    path, &meta,
                )))),
                Err(e) => error_response(format!("fstat: {e}")),
            }
        }
        Some(FsHandleEntry::Dir { path, .. }) => {
            if let Err(e) = state
                .handles
                .get(&handle)
                .expect("entry just matched")
                .ensure_owner(handle, id)
            {
                return error_response(format!("fstat: {e}"));
            }
            match tokio::fs::metadata(path).await {
                Ok(meta) => ok_response(Some(FsResponseData::Stat(metadata_to_entry_info(
                    path, &meta,
                )))),
                Err(e) => error_response(format!("fstat: {e}")),
            }
        }
        None => error_response(format!("fstat: invalid handle: {handle}")),
    }
}

async fn handle_fsetstat(id: u32, state: &FsState, handle: u64, attrs: FsSetAttrs) -> FsResponse {
    let (file, _, path) = match state.file(id, handle, false, false) {
        Ok(v) => v,
        Err(e) => return error_response(format!("fsetstat: {e}")),
    };

    let mut file = file.lock().await;
    match apply_file_attrs(&mut file, &path, attrs).await {
        Ok(()) => ok_response(None),
        Err(e) => error_response(format!("fsetstat: {e}")),
    }
}

async fn handle_mkdir(path: &str, mode: Option<u32>, user: Option<String>) -> FsResponse {
    let path = path.to_string();
    match run_as_user(user, move || {
        std::fs::create_dir_all(&path).map_err(|e| format!("mkdir: {e}"))?;
        if let Some(mode) = mode {
            std::fs::set_permissions(&path, std::fs::Permissions::from_mode(mode))
                .map_err(|e| format!("chmod: {e}"))?;
        }
        Ok(())
    })
    .await
    {
        Ok(Ok(())) => ok_response(None),
        Ok(Err(e)) => error_response(e),
        Err(e) => error_response(e),
    }
}

/// Runs a blocking filesystem call with the filesystem identity of `user`.
///
/// The identity is thread-local (`setfsuid`/`setfsgid` and the raw `setgroups` syscall), so it never
/// leaks into other requests or exec sessions, and the kernel enforces the user's permissions and
/// assigns ownership of anything the call creates. `None` runs the call unchanged, as root.
async fn run_as_user<T, F>(user: Option<String>, f: F) -> Result<T, String>
where
    T: Send + 'static,
    F: FnOnce() -> T + Send + 'static,
{
    let Some(user) = user else {
        return tokio::task::spawn_blocking(f)
            .await
            .map_err(|e| format!("filesystem task: {e}"));
    };
    let (uid, gid, groups) =
        crate::session::resolve_user_groups(&user).map_err(|e| e.to_string())?;
    tokio::task::spawn_blocking(move || {
        let _identity =
            FsIdentity::switch(uid, gid, &groups).map_err(|e| format!("switch to {user}: {e}"))?;
        Ok(f())
    })
    .await
    .map_err(|e| format!("filesystem task: {e}"))?
}

/// Restores the thread's filesystem identity on drop, including when the call panics.
struct FsIdentity {
    uid: libc::c_int,
    gid: libc::c_int,
    groups: Vec<libc::gid_t>,
}

impl FsIdentity {
    fn switch(uid: u32, gid: u32, groups: &[libc::gid_t]) -> std::io::Result<Self> {
        let previous = current_groups()?;
        set_groups(groups)?;
        // Each call returns the previous id even when it fails, so a second call confirms the change.
        let identity = unsafe {
            Self {
                gid: libc::setfsgid(gid),
                uid: libc::setfsuid(uid),
                groups: previous,
            }
        };
        if unsafe { libc::setfsuid(uid) } != uid as libc::c_int
            || unsafe { libc::setfsgid(gid) } != gid as libc::c_int
        {
            return Err(std::io::Error::from(std::io::ErrorKind::PermissionDenied));
        }
        Ok(identity)
    }
}

impl Drop for FsIdentity {
    fn drop(&mut self) {
        unsafe {
            libc::setfsuid(self.uid as libc::uid_t);
            libc::setfsgid(self.gid as libc::gid_t);
        }
        let _ = set_groups(&self.groups);
    }
}

fn current_groups() -> std::io::Result<Vec<libc::gid_t>> {
    let count = unsafe { libc::getgroups(0, std::ptr::null_mut()) };
    let mut groups = vec![0; count.max(0) as usize];
    let count = unsafe { libc::getgroups(groups.len() as libc::c_int, groups.as_mut_ptr()) };
    if count < 0 {
        return Err(std::io::Error::last_os_error());
    }
    groups.truncate(count as usize);
    Ok(groups)
}

/// Sets the calling thread's supplementary groups. `libc::setgroups` would change every thread in
/// the process, including concurrent root requests, so this calls the syscall directly.
fn set_groups(groups: &[libc::gid_t]) -> std::io::Result<()> {
    if unsafe { libc::syscall(libc::SYS_setgroups, groups.len(), groups.as_ptr()) } != 0 {
        return Err(std::io::Error::last_os_error());
    }
    Ok(())
}

async fn handle_remove(path: &str) -> FsResponse {
    match tokio::fs::remove_file(path).await {
        Ok(()) => ok_response(None),
        Err(e) => error_response(format!("remove: {e}")),
    }
}

async fn handle_remove_dir(path: &str, recursive: bool) -> FsResponse {
    let result = if recursive {
        tokio::fs::remove_dir_all(path).await
    } else {
        tokio::fs::remove_dir(path).await
    };
    match result {
        Ok(()) => ok_response(None),
        Err(e) => error_response(format!("remove_dir: {e}")),
    }
}

async fn handle_copy(src: &str, dst: &str) -> FsResponse {
    match tokio::fs::copy(src, dst).await {
        Ok(_) => ok_response(None),
        Err(e) => error_response(format!("copy: {e}")),
    }
}

async fn handle_rename(src: &str, dst: &str) -> FsResponse {
    match tokio::fs::rename(src, dst).await {
        Ok(()) => ok_response(None),
        Err(e) => error_response(format!("rename: {e}")),
    }
}

fn accept_fs_read_offer(offer: BulkOffer) -> Result<BulkAccepted, String> {
    let offer = offer
        .validate()
        .map_err(|error| format!("invalid filesystem read bulk offer: {error}"))?;
    if offer.guest_to_host_credit_limit == 0 {
        return Err("filesystem read bulk offer must grant guest-to-host credit".into());
    }
    Ok(BulkAccepted {
        kind: BulkKind::Filesystem,
        flows: BULK_FLOW_MASK_GUEST_TO_HOST,
        format: offer.format,
        max_record_payload: offer
            .max_record_payload
            .min(DEFAULT_FILESYSTEM_BULK_RECORD_PAYLOAD),
        host_to_guest_credit_limit: 0,
        guest_to_host_credit_limit: offer.guest_to_host_credit_limit,
    })
}

fn accept_fs_write_offer(offer: BulkOffer) -> Result<BulkAccepted, String> {
    let offer = offer
        .validate()
        .map_err(|error| format!("invalid filesystem write bulk offer: {error}"))?;
    if offer.guest_to_host_credit_limit != 0 {
        return Err("filesystem write bulk offer must not grant guest-to-host credit".into());
    }
    Ok(BulkAccepted {
        kind: BulkKind::Filesystem,
        flows: BULK_FLOW_MASK_HOST_TO_GUEST,
        format: offer.format,
        max_record_payload: offer
            .max_record_payload
            .min(DEFAULT_FILESYSTEM_BULK_RECORD_PAYLOAD),
        host_to_guest_credit_limit: DEFAULT_BULK_WINDOW,
        guest_to_host_credit_limit: 0,
    })
}

async fn handle_bulk_read_stream(
    id: u32,
    file: Arc<Mutex<tokio::fs::File>>,
    offset: u64,
    len: Option<u64>,
    mut sender: BulkSendState,
    mut credit_rx: watch::Receiver<Option<BulkCredit>>,
    tx: &SessionOutputSender,
) {
    let mut file = file.lock().await;
    if let Err(error) = file.seek(std::io::SeekFrom::Start(offset)).await {
        send_raw_response(id, false, Some(format!("seek: {error}")), None, tx).await;
        return;
    }

    let mut remaining = len;
    loop {
        if remaining == Some(0) {
            break;
        }
        while sender.available_credit() == 0 {
            if credit_rx.changed().await.is_err() {
                return;
            }
            let Some(credit) = *credit_rx.borrow_and_update() else {
                continue;
            };
            if let Err(error) = sender.apply_credit(credit) {
                send_raw_response(
                    id,
                    false,
                    Some(format!("invalid filesystem bulk credit: {error}")),
                    None,
                    tx,
                )
                .await;
                return;
            }
        }

        let read_len = sender
            .available_credit()
            .min(sender.max_record_payload() as u64)
            .min(remaining.unwrap_or(u64::MAX)) as usize;
        let Some(permit) = tx.reserve_bulk(read_len).await else {
            return;
        };
        let mut payload = vec![0u8; read_len];
        match file.read(&mut payload).await {
            Ok(0) => break,
            Ok(read) => {
                payload.truncate(read);
                if let Some(remaining) = &mut remaining {
                    *remaining = remaining.saturating_sub(read as u64);
                }
                let record_offset = match sender.admit(read) {
                    Ok(offset) => offset,
                    Err(error) => {
                        send_raw_response(
                            id,
                            false,
                            Some(format!("admit filesystem bulk record: {error}")),
                            None,
                            tx,
                        )
                        .await;
                        return;
                    }
                };
                let record = BulkRecord {
                    id,
                    kind: BulkKind::Filesystem,
                    flow: BulkFlow::GuestToHost,
                    offset: record_offset,
                    payload: Bytes::from(payload),
                };
                let output = BulkSessionOutput::new(record, RawActivity::fs_bytes(read));
                if !tx
                    .send_reserved(id, SessionOutput::Bulk(output), permit)
                    .await
                {
                    return;
                }
            }
            Err(error) => {
                send_raw_response(id, false, Some(format!("read: {error}")), None, tx).await;
                return;
            }
        }
    }

    let finish = match sender.finish() {
        Ok(finish) => finish,
        Err(error) => {
            send_raw_response(
                id,
                false,
                Some(format!("finish filesystem bulk read: {error}")),
                None,
                tx,
            )
            .await;
            return;
        }
    };
    if !send_raw_control(id, MessageType::BulkFinish, &finish, None, tx).await {
        return;
    }
    send_raw_response(id, true, None, None, tx).await;
}

async fn handle_read_stream(
    id: u32,
    file: Arc<Mutex<tokio::fs::File>>,
    offset: u64,
    len: Option<u64>,
    tx: &SessionOutputSender,
) {
    let mut file = file.lock().await;
    if let Err(e) = file.seek(std::io::SeekFrom::Start(offset)).await {
        send_raw_response(id, false, Some(format!("seek: {e}")), None, tx).await;
        return;
    }

    let mut remaining = len;
    let mut chunk = vec![0u8; FS_CHUNK_SIZE];
    let mut buf = Vec::new();

    loop {
        // Reserve before the file read and CBOR materialization so concurrent streams cannot
        // each create an uncharged maximum-sized frame while aggregate output is saturated.
        let Some(permit) = tx.reserve(codec::MAX_FRAME_SIZE as usize + 4).await else {
            return;
        };
        let read_len = match remaining {
            Some(0) => break,
            Some(n) => chunk.len().min(n as usize),
            None => chunk.len(),
        };

        match file.read(&mut chunk[..read_len]).await {
            Ok(0) => break,
            Ok(n) => {
                if let Some(ref mut remaining) = remaining {
                    *remaining = remaining.saturating_sub(n as u64);
                }
                let data = FsData {
                    data: chunk[..n].to_vec(),
                };
                let msg = match Message::with_payload(MessageType::FsData, id, &data) {
                    Ok(msg) => msg,
                    Err(e) => {
                        send_raw_response(id, false, Some(format!("encode chunk: {e}")), None, tx)
                            .await;
                        return;
                    }
                };
                buf.clear();
                if let Err(e) = codec::encode_to_buf(&msg, &mut buf) {
                    send_raw_response(
                        id,
                        false,
                        Some(format!("encode chunk frame: {e}")),
                        None,
                        tx,
                    )
                    .await;
                    return;
                }
                let output =
                    RawSessionOutput::new(std::mem::take(&mut buf), RawActivity::fs_bytes(n), None);
                if !tx
                    .send_reserved(id, SessionOutput::Raw(output), permit)
                    .await
                {
                    return;
                }
            }
            Err(e) => {
                send_raw_response(id, false, Some(format!("read: {e}")), None, tx).await;
                return;
            }
        }
    }

    send_raw_response(id, true, None, None, tx).await;
}

//--------------------------------------------------------------------------------------------------
// Functions: Attribute Helpers
//--------------------------------------------------------------------------------------------------

async fn apply_path_attrs(
    path: &str,
    follow_symlink: bool,
    attrs: FsSetAttrs,
) -> Result<(), String> {
    if let Some(size) = attrs.size {
        let file = tokio::fs::OpenOptions::new()
            .write(true)
            .open(path)
            .await
            .map_err(|e| format!("open for truncate: {e}"))?;
        file.set_len(size)
            .await
            .map_err(|e| format!("set_len: {e}"))?;
    }

    if let Some(mode) = attrs.mode {
        if !follow_symlink
            && tokio::fs::symlink_metadata(path)
                .await
                .map_err(|e| format!("lstat before chmod: {e}"))?
                .file_type()
                .is_symlink()
        {
            return Err("chmod on symlink without following is not supported".into());
        }
        tokio::fs::set_permissions(path, std::fs::Permissions::from_mode(mode))
            .await
            .map_err(|e| format!("chmod: {e}"))?;
    }

    if attrs.uid.is_some() || attrs.gid.is_some() {
        chown_path(path, follow_symlink, attrs.uid, attrs.gid)?;
    }

    if attrs.atime.is_some() || attrs.mtime.is_some() {
        set_times_path(path, follow_symlink, attrs.atime, attrs.mtime).await?;
    }

    Ok(())
}

async fn apply_file_attrs(
    file: &mut tokio::fs::File,
    path: &str,
    attrs: FsSetAttrs,
) -> Result<(), String> {
    if let Some(size) = attrs.size {
        file.set_len(size)
            .await
            .map_err(|e| format!("set_len: {e}"))?;
    }

    if let Some(mode) = attrs.mode {
        file.set_permissions(std::fs::Permissions::from_mode(mode))
            .await
            .map_err(|e| format!("chmod: {e}"))?;
    }

    if attrs.uid.is_some() || attrs.gid.is_some() {
        let uid = attrs.uid.map(|v| v as libc::uid_t).unwrap_or(!0);
        let gid = attrs.gid.map(|v| v as libc::gid_t).unwrap_or(!0);
        let rc = unsafe { libc::fchown(file.as_raw_fd(), uid, gid) };
        if rc != 0 {
            return Err(format!("fchown: {}", std::io::Error::last_os_error()));
        }
    }

    if attrs.atime.is_some() || attrs.mtime.is_some() {
        set_times_fd(file.as_raw_fd(), path, attrs.atime, attrs.mtime).await?;
    }

    Ok(())
}

fn chown_path(
    path: &str,
    follow_symlink: bool,
    uid: Option<u32>,
    gid: Option<u32>,
) -> Result<(), String> {
    let c_path = cstring_path(path)?;
    let uid = uid.map(|v| v as libc::uid_t).unwrap_or(!0);
    let gid = gid.map(|v| v as libc::gid_t).unwrap_or(!0);
    let rc = unsafe {
        if follow_symlink {
            libc::chown(c_path.as_ptr(), uid, gid)
        } else {
            libc::lchown(c_path.as_ptr(), uid, gid)
        }
    };
    if rc != 0 {
        return Err(format!("chown: {}", std::io::Error::last_os_error()));
    }
    Ok(())
}

async fn set_times_path(
    path: &str,
    follow_symlink: bool,
    atime: Option<i64>,
    mtime: Option<i64>,
) -> Result<(), String> {
    let meta = if follow_symlink {
        tokio::fs::metadata(path).await
    } else {
        tokio::fs::symlink_metadata(path).await
    }
    .map_err(|e| format!("stat before utimensat: {e}"))?;
    let times = timespecs(atime.unwrap_or(meta.atime()), mtime.unwrap_or(meta.mtime()));
    let c_path = cstring_path(path)?;
    let flags = if follow_symlink {
        0
    } else {
        libc::AT_SYMLINK_NOFOLLOW
    };
    let rc = unsafe { libc::utimensat(libc::AT_FDCWD, c_path.as_ptr(), times.as_ptr(), flags) };
    if rc != 0 {
        return Err(format!("utimensat: {}", std::io::Error::last_os_error()));
    }
    Ok(())
}

async fn set_times_fd(
    fd: std::os::fd::RawFd,
    path: &str,
    atime: Option<i64>,
    mtime: Option<i64>,
) -> Result<(), String> {
    let meta = tokio::fs::metadata(path)
        .await
        .map_err(|e| format!("stat before futimens: {e}"))?;
    let times = timespecs(atime.unwrap_or(meta.atime()), mtime.unwrap_or(meta.mtime()));
    let rc = unsafe { libc::futimens(fd, times.as_ptr()) };
    if rc != 0 {
        return Err(format!("futimens: {}", std::io::Error::last_os_error()));
    }
    Ok(())
}

fn timespecs(atime: i64, mtime: i64) -> [libc::timespec; 2] {
    [
        libc::timespec {
            tv_sec: atime as _,
            tv_nsec: 0,
        },
        libc::timespec {
            tv_sec: mtime as _,
            tv_nsec: 0,
        },
    ]
}

//--------------------------------------------------------------------------------------------------
// Functions: Helpers
//--------------------------------------------------------------------------------------------------

fn encode_response(id: u32, resp: FsResponse, out_buf: &mut Vec<u8>) -> Result<(), String> {
    let msg = Message::with_payload(MessageType::FsResponse, id, &resp)
        .map_err(|e| format!("encode fs response: {e}"))?;
    codec::encode_to_buf(&msg, out_buf).map_err(|e| format!("encode fs response frame: {e}"))?;
    Ok(())
}

fn encode_control<T: Serialize>(
    message_type: MessageType,
    id: u32,
    payload: &T,
    out_buf: &mut Vec<u8>,
) -> Result<(), String> {
    let message = Message::with_payload(message_type, id, payload)
        .map_err(|error| format!("encode {}: {error}", message_type.as_str()))?;
    codec::encode_to_buf(&message, out_buf)
        .map_err(|error| format!("encode {} frame: {error}", message_type.as_str()))
}

async fn send_raw_control<T: Serialize>(
    id: u32,
    message_type: MessageType,
    payload: &T,
    completion: Option<RawSessionCompletion>,
    tx: &SessionOutputSender,
) -> bool {
    let mut frame = Vec::new();
    if let Err(error) = encode_control(message_type, id, payload, &mut frame) {
        eprintln!("failed to {error}");
        return false;
    }
    tx.send(
        id,
        SessionOutput::Raw(RawSessionOutput::new(
            frame,
            RawActivity::guest_message(),
            completion,
        )),
    )
    .await
}

async fn send_raw_response(
    id: u32,
    ok: bool,
    error: Option<String>,
    data: Option<FsResponseData>,
    tx: &SessionOutputSender,
) {
    let resp = FsResponse { ok, error, data };
    match Message::with_payload(MessageType::FsResponse, id, &resp) {
        Ok(msg) => {
            let mut buf = Vec::new();
            match codec::encode_to_buf(&msg, &mut buf) {
                Ok(()) => {
                    let output = RawSessionOutput::new(
                        buf,
                        RawActivity::guest_message(),
                        Some(RawSessionCompletion::FsRead),
                    );
                    let _ = tx.send(id, SessionOutput::Raw(output)).await;
                }
                Err(e) => {
                    eprintln!("failed to encode fs response frame for {id}: {e}");
                }
            }
        }
        Err(e) => {
            eprintln!("failed to encode fs response for {id}: {e}");
        }
    }
}

async fn realpath(path: &str) -> Result<String, String> {
    match tokio::fs::canonicalize(path).await {
        Ok(path) => Ok(path.to_string_lossy().to_string()),
        Err(original_error) => {
            let path = Path::new(path);
            let Some(parent) = path.parent() else {
                return Err(original_error.to_string());
            };
            let parent = tokio::fs::canonicalize(parent)
                .await
                .map_err(|_| original_error.to_string())?;
            let resolved = match path.file_name() {
                Some(name) => parent.join(name),
                None => parent,
            };
            Ok(resolved.to_string_lossy().to_string())
        }
    }
}

async fn read_all_dir(path: &str) -> Result<Vec<FsEntryInfo>, String> {
    let mut dir = tokio::fs::read_dir(path)
        .await
        .map_err(|e| format!("opendir: {e}"))?;
    let mut entries = Vec::new();

    loop {
        match dir.next_entry().await {
            Ok(Some(entry)) => {
                let entry_path = entry.path();
                let path_str = entry_path.to_string_lossy().to_string();
                match tokio::fs::symlink_metadata(&entry_path).await {
                    Ok(meta) => entries.push(metadata_to_entry_info(&path_str, &meta)),
                    Err(_) => entries.push(unknown_entry_info(&path_str)),
                }
            }
            Ok(None) => break,
            Err(e) => return Err(e.to_string()),
        }
    }

    Ok(entries)
}

fn ok_response(data: Option<FsResponseData>) -> FsResponse {
    FsResponse {
        ok: true,
        error: None,
        data,
    }
}

fn error_response(error: String) -> FsResponse {
    FsResponse {
        ok: false,
        error: Some(error),
        data: None,
    }
}

fn metadata_to_entry_info(path: &str, meta: &std::fs::Metadata) -> FsEntryInfo {
    let kind = if meta.is_file() {
        "file"
    } else if meta.is_dir() {
        "dir"
    } else if meta.is_symlink() {
        "symlink"
    } else {
        "other"
    };

    let mtime = Some(meta.mtime());
    let atime = Some(meta.atime());

    FsEntryInfo {
        path: path.to_string(),
        kind: kind.to_string(),
        size: meta.len(),
        mode: meta.mode(),
        modified: mtime,
        uid: meta.uid(),
        gid: meta.gid(),
        atime,
        mtime,
    }
}

fn unknown_entry_info(path: &str) -> FsEntryInfo {
    FsEntryInfo {
        path: path.to_string(),
        kind: "other".to_string(),
        size: 0,
        mode: 0,
        modified: None,
        uid: 0,
        gid: 0,
        atime: None,
        mtime: None,
    }
}

fn cstring_path(path: impl AsRef<Path>) -> Result<CString, String> {
    CString::new(path.as_ref().as_os_str().as_bytes())
        .map_err(|e| format!("path contains NUL: {e}"))
}

//--------------------------------------------------------------------------------------------------
// Tests
//--------------------------------------------------------------------------------------------------

#[cfg(test)]
mod tests {
    use std::time::{SystemTime, UNIX_EPOCH};

    use super::*;

    #[test]
    fn filesystem_offer_preserves_an_older_hosts_smaller_record_limit() {
        assert_eq!(
            DEFAULT_FILESYSTEM_BULK_RECORD_PAYLOAD as usize,
            FS_CHUNK_SIZE
        );
        let old_offer = BulkOffer {
            max_record_payload: microsandbox_protocol::bulk::DEFAULT_BULK_RECORD_PAYLOAD,
            ..BulkOffer::filesystem_write()
        };

        let accepted = accept_fs_write_offer(old_offer).unwrap();
        assert_eq!(
            accepted.max_record_payload,
            microsandbox_protocol::bulk::DEFAULT_BULK_RECORD_PAYLOAD
        );
    }

    #[tokio::test]
    async fn raw_bulk_write_requires_exact_offsets_and_finish_length() {
        let path = test_path("bulk-write");
        let file = tokio::fs::File::create(&path).await.unwrap();
        let mut session = FsWriteSession {
            owner_id: 1,
            handle: 1,
            file: Arc::new(Mutex::new(file)),
            offset: 0,
            append: false,
            expected_len: Some(7),
            written: 0,
            bulk: Some(
                BulkReceiveState::new(
                    BulkKind::Filesystem,
                    BulkFlow::HostToGuest,
                    microsandbox_protocol::bulk::DEFAULT_BULK_RECORD_PAYLOAD,
                    DEFAULT_BULK_WINDOW,
                    DEFAULT_BULK_WINDOW,
                )
                .unwrap(),
            ),
        };
        let mut out = Vec::new();

        let wrong_offset = BulkRecord {
            id: 1,
            kind: BulkKind::Filesystem,
            flow: BulkFlow::HostToGuest,
            offset: 1,
            payload: Bytes::from_static(b"ignored"),
        };
        assert!(
            handle_fs_bulk_record(1, &wrong_offset, &mut session, &mut out)
                .await
                .unwrap_err()
                .contains("does not match expected")
        );

        let first = BulkRecord {
            offset: 0,
            payload: Bytes::from_static(b"ign"),
            ..wrong_offset
        };
        let second = BulkRecord {
            offset: 3,
            payload: Bytes::from_static(b"ored"),
            ..first.clone()
        };
        assert!(
            !handle_fs_bulk_records(1, &[first, second], &mut session, &mut out)
                .await
                .unwrap()
        );
        assert!(
            handle_fs_bulk_finish(
                1,
                BulkFinish {
                    kind: BulkKind::Filesystem,
                    flow: BulkFlow::HostToGuest,
                    final_offset: 7,
                },
                &mut session,
                &mut out,
            )
            .await
            .unwrap()
        );
        drop(session);

        let response = codec::try_decode_from_buf(&mut out).unwrap().unwrap();
        assert_eq!(response.t, MessageType::FsResponse);
        assert!(response.payload::<FsResponse>().unwrap().ok);
        assert_eq!(tokio::fs::read(&path).await.unwrap(), b"ignored");
        tokio::fs::remove_file(path).await.unwrap();
    }

    #[tokio::test]
    async fn raw_bulk_read_emits_payload_exact_finish_then_terminal_response() {
        let path = test_path("bulk-read");
        tokio::fs::write(&path, b"raw-read-payload").await.unwrap();
        let file = tokio::fs::File::open(&path).await.unwrap();
        let sender = BulkSendState::new(
            BulkKind::Filesystem,
            BulkFlow::GuestToHost,
            microsandbox_protocol::bulk::DEFAULT_BULK_RECORD_PAYLOAD,
            DEFAULT_BULK_WINDOW,
        )
        .unwrap();
        let (_credit_tx, credit_rx) = watch::channel(None);
        let (session_tx, mut session_rx) = SessionOutputSender::channel();

        handle_bulk_read_stream(
            2,
            Arc::new(Mutex::new(file)),
            0,
            None,
            sender,
            credit_rx,
            &session_tx,
        )
        .await;

        let first = session_rx.recv().await.unwrap();
        let SessionOutput::Bulk(first) = first.output else {
            panic!("expected raw filesystem record");
        };
        assert_eq!(first.record.offset, 0);
        assert_eq!(
            first.record.payload,
            Bytes::from_static(b"raw-read-payload")
        );

        let finish = decode_raw_output(session_rx.recv().await.unwrap().output);
        assert_eq!(finish.t, MessageType::BulkFinish);
        let finish: BulkFinish = finish.payload().unwrap();
        assert_eq!(finish.final_offset, b"raw-read-payload".len() as u64);
        let response = decode_raw_output(session_rx.recv().await.unwrap().output);
        assert_eq!(response.t, MessageType::FsResponse);
        assert!(response.payload::<FsResponse>().unwrap().ok);
        tokio::fs::remove_file(path).await.unwrap();
    }

    #[tokio::test]
    async fn requested_user_owns_created_entries_and_is_bound_by_permissions() {
        // Switching the filesystem identity needs root.
        if unsafe { libc::geteuid() } != 0 {
            return;
        }
        let user = Some("nobody".to_string());
        let nobody = crate::session::resolve_default_user(user.as_deref()).unwrap();
        let owner = |path: &std::path::Path| {
            let meta = std::fs::symlink_metadata(path).unwrap();
            (meta.uid(), meta.gid())
        };
        let dir = test_path("as-user");
        std::fs::create_dir(&dir).unwrap();
        std::fs::set_permissions(&dir, std::fs::Permissions::from_mode(0o777)).unwrap();
        let mut state = FsState::default();
        let open = |user: Option<String>| FsOpenOptions {
            write: true,
            create: true,
            user,
            ..Default::default()
        };

        let file = dir.join("file");
        let resp =
            handle_open_file(1, &mut state, file.to_str().unwrap(), open(user.clone())).await;
        assert!(resp.ok, "{:?}", resp.error);
        assert_eq!(owner(&file), nobody);

        let nested = dir.join("a/b");
        let resp = handle_mkdir(nested.to_str().unwrap(), Some(0o755), user.clone()).await;
        assert!(resp.ok, "{:?}", resp.error);
        assert_eq!(owner(&dir.join("a")), nobody);
        assert_eq!(owner(&nested), nobody);

        let link = dir.join("link");
        let resp = handle_symlink("file", link.to_str().unwrap(), user.clone()).await;
        assert!(resp.ok, "{:?}", resp.error);
        assert_eq!(owner(&link), nobody);

        // Entries created without a user stay root-owned, and the user cannot open them.
        let root_file = dir.join("root-file");
        let resp = handle_open_file(2, &mut state, root_file.to_str().unwrap(), open(None)).await;
        assert!(resp.ok, "{:?}", resp.error);
        std::fs::set_permissions(&root_file, std::fs::Permissions::from_mode(0o600)).unwrap();
        assert_eq!(owner(&root_file), (0, 0));
        let resp = handle_open_file(
            3,
            &mut state,
            root_file.to_str().unwrap(),
            open(user.clone()),
        )
        .await;
        assert!(!resp.ok);

        // The identity is confined to the request.
        let after = dir.join("after");
        let resp = handle_open_file(4, &mut state, after.to_str().unwrap(), open(None)).await;
        assert!(resp.ok, "{:?}", resp.error);
        assert_eq!(owner(&after), (0, 0));

        // The rest edits /etc/group, so it runs only where the caller opted in (a throwaway container).
        if std::env::var_os("MSB_TEST_EDIT_ETC_GROUP").is_none() {
            std::fs::remove_dir_all(dir).unwrap();
            return;
        }
        // A directory writable only through a supplementary group of the user.
        struct RestoreGroupFile(Vec<u8>);
        impl Drop for RestoreGroupFile {
            fn drop(&mut self) {
                std::fs::write("/etc/group", &self.0).unwrap();
            }
        }
        let group_gid = 54321;
        let original = std::fs::read("/etc/group").unwrap();
        let _restore = RestoreGroupFile(original.clone());
        let mut entries = original;
        entries.extend_from_slice(format!("msb-shared:x:{group_gid}:nobody\n").as_bytes());
        std::fs::write("/etc/group", entries).unwrap();
        let shared = dir.join("shared");
        std::fs::create_dir(&shared).unwrap();
        std::os::unix::fs::chown(&shared, None, Some(group_gid)).unwrap();
        std::fs::set_permissions(&shared, std::fs::Permissions::from_mode(0o770)).unwrap();
        let root_groups = current_groups().unwrap();
        let in_shared = shared.join("file");
        let resp = handle_open_file(
            5,
            &mut state,
            in_shared.to_str().unwrap(),
            open(user.clone()),
        )
        .await;
        assert!(resp.ok, "{:?}", resp.error);
        assert_eq!(owner(&in_shared), nobody);
        let resp = handle_mkdir(shared.join("sub").to_str().unwrap(), None, user.clone()).await;
        assert!(resp.ok, "{:?}", resp.error);
        // Only the calling thread takes the user's groups: a concurrent thread keeps root's.
        let (ping, pinged) = std::sync::mpsc::channel::<()>();
        let (pong, ponged) = std::sync::mpsc::channel();
        std::thread::spawn(move || {
            while pinged.recv().is_ok() {
                pong.send(current_groups().unwrap()).unwrap();
            }
        });
        let concurrent = run_as_user(user.clone(), move || {
            ping.send(()).unwrap();
            ponged.recv().unwrap()
        })
        .await
        .unwrap();
        assert_eq!(concurrent, root_groups);
        // The user's groups do not outlive the call, on this thread or on reused blocking threads.
        assert_eq!(current_groups().unwrap(), root_groups);
        let checks: Vec<_> = (0..64)
            .map(|_| tokio::task::spawn_blocking(|| current_groups().unwrap()))
            .collect();
        for check in checks {
            assert_eq!(check.await.unwrap(), root_groups);
        }

        std::fs::remove_dir_all(dir).unwrap();
    }

    fn decode_raw_output(output: SessionOutput) -> Message {
        let SessionOutput::Raw(mut output) = output else {
            panic!("expected raw control frame");
        };
        codec::try_decode_from_buf(&mut output.frame)
            .unwrap()
            .unwrap()
    }

    fn test_path(name: &str) -> std::path::PathBuf {
        let unique = SystemTime::now()
            .duration_since(UNIX_EPOCH)
            .unwrap()
            .as_nanos();
        std::env::temp_dir().join(format!("msb-agentd-{name}-{}-{unique}", std::process::id()))
    }
}