//! FUSE ↔ 9P2000 translation: the request loop that turns kernel FUSE requests
//! into synchronous 9P calls on a `nine.Session` and sends the replies back.
//!
//! Everything here is single-threaded and one request at a time. State is three
//! tables: inodes (nodeid → fid/qid, deduplicated by qid.path), open handles
//! (fh → fid plus a cached directory listing), and the reverse qid map.
const std = @import("std");
const cloud9 = @import("cloud9");
const fuse = @import("fuse.zig");
const nine = @import("nine.zig");
const linux = std.os.linux;

pub const Options = struct {
    /// Reported owner of every file.
    uid: u32,
    gid: u32,
    /// attr/entry cache validity (0 = none).
    attr_timeout_ns: u64 = 1_000_000_000,
    /// FOPEN_DIRECT_IO on every regular file.
    direct_io: bool = true,
    /// Trace every request, reply and 9P call to stderr.
    debug: bool = false,
};

/// Largest single READ/WRITE payload we accept from the kernel.
pub const max_write: u32 = 1 << 20;
/// Upper bound on the raw bytes of one directory listing (about a million entries);
/// past it the listing fails with EIO instead of eating memory.
pub const max_dir_bytes: u64 = 64 << 20;
/// The kernel refuses dirents longer than this (FUSE_NAME_MAX) with EIO.
pub const max_name_len: usize = 1024;
/// Request buffer: `max_write` plus room for the header and the largest in-struct.
pub const request_buf_len: usize = max_write + 4096;

const Inode = struct {
    fid: u32,
    qid: cloud9.Qid,
    nlookup: u64,
    /// nodeid of the directory this inode was looked up in (root: itself). Used for "..".
    parent: u64,
};

pub const Entry = struct { name: []u8, ino: u64, dtype: u32 };

pub const DirList = struct {
    entries: std.ArrayList(Entry) = .empty,

    pub fn deinit(d: *DirList, gpa: std.mem.Allocator) void {
        for (d.entries.items) |e| gpa.free(e.name);
        d.entries.deinit(gpa);
    }
};

const Handle = struct {
    fid: u32,
    nodeid: u64,
    dir: ?DirList,
};

/// Errors a request handler may surface. Policy failures are ordinary errors
/// that the dispatcher maps to an errno; `FuseIo` means the kernel side is broken.
const HandlerError = nine.Session.Error || error{
    BadRequest,
    NoEntry,
    BadHandle,
    Exdev,
    Perm,
    NotSup,
    /// A directory listing the server sent could not be parsed (EIO, not fatal).
    BadDir,
    FuseIo,
};

/// Runs until the FUSE fd reports ENODEV, a DESTROY arrives, or `stop_fd`
/// becomes readable (also while a 9P reply is outstanding). Returns
/// `error.Closed` if the 9P server went away.
pub fn serve(gpa: std.mem.Allocator, fuse_fd: i32, session: *nine.Session, root_fid: u32, stop_fd: i32, opts: Options) !void {
    var effective = opts;
    // With page caching on, a nonzero attr cache lets the kernel trust a stale
    // (often zero) size and truncate reads: 9P sizes are authoritative and change
    // under us. direct_io ignores the cached size, so the cache is safe only there.
    if (!effective.direct_io) effective.attr_timeout_ns = 0;
    var b: Bridge = .{
        .gpa = gpa,
        .fuse_fd = fuse_fd,
        .nine = session,
        .opts = effective,
    };
    defer b.deinit();

    b.req_buf = try gpa.alignedAlloc(u8, .@"8", request_buf_len);
    b.data_buf = try gpa.alloc(u8, max_write);

    // Abandon any pending 9P reply once the child is gone (stop_fd readable),
    // including the initial root stat below: a silent server must not pin us.
    session.stop_fd = stop_fd;
    defer session.stop_fd = -1;

    // Node 1 is the root; its qid comes from a stat so lookups resolving back to
    // it (e.g. via a walk) dedupe onto node 1.
    var root_qid: cloud9.Qid = .{ .type = cloud9.qtdir, .version = 0, .path = 0 };
    if (b.stat(root_fid)) |st| {
        root_qid = st.qid;
        b.root_path = st.qid.path;
        try b.by_qid.put(gpa, st.qid.path, fuse.root_id);
    } else |e| switch (e) {
        error.Nine => {},
        error.Stopped => return,
        else => return error.Closed,
    }
    try b.inodes.put(gpa, fuse.root_id, .{ .fid = root_fid, .qid = root_qid, .nlookup = 1, .parent = fuse.root_id });

    var pfds = [_]linux.pollfd{
        .{ .fd = fuse_fd, .events = linux.POLL.IN, .revents = 0 },
        .{ .fd = stop_fd, .events = linux.POLL.IN, .revents = 0 },
    };
    while (true) {
        pfds[0].revents = 0;
        pfds[1].revents = 0;
        const rc = linux.poll(&pfds, pfds.len, -1);
        switch (linux.errno(rc)) {
            .SUCCESS => {},
            .INTR, .AGAIN => continue,
            else => return error.Io,
        }
        if (pfds[1].revents != 0) {
            b.trace("stop_fd readable; leaving serve loop", .{});
            return;
        }
        if (pfds[0].revents == 0) continue;
        const req = (fuse.readRequest(fuse_fd, b.req_buf) catch |e| switch (e) {
            error.Protocol => return error.FuseProtocol,
            else => return error.FuseIo,
        }) orelse {
            b.trace("fuse fd reports ENODEV; unmounted", .{});
            return;
        };
        if (!try b.dispatch(req)) return;
    }
}

const Bridge = struct {
    gpa: std.mem.Allocator,
    fuse_fd: i32,
    nine: *nine.Session,
    opts: Options,
    req_buf: []align(8) u8 = &.{},
    data_buf: []u8 = &.{},
    inodes: std.AutoHashMapUnmanaged(u64, Inode) = .empty,
    by_qid: std.AutoHashMapUnmanaged(u64, u64) = .empty,
    handles: std.AutoHashMapUnmanaged(u64, Handle) = .empty,
    next_node: u64 = 2,
    next_fh: u64 = 1,
    /// qid.path of the root, reported as ino 1 wherever it shows up.
    root_path: u64 = 0,
    /// errno of the most recent Rerror that a handler did not swallow. Kept here
    /// because `Session.rpc` clears its ename on every call, and error paths
    /// clunk (an rpc) before the dispatcher maps the failure to an errno.
    last_err: linux.E = .IO,

    fn deinit(b: *Bridge) void {
        var it = b.handles.valueIterator();
        while (it.next()) |h| if (h.dir) |*d| d.deinit(b.gpa);
        b.handles.deinit(b.gpa);
        b.inodes.deinit(b.gpa);
        b.by_qid.deinit(b.gpa);
        if (b.req_buf.len != 0) b.gpa.free(b.req_buf);
        if (b.data_buf.len != 0) b.gpa.free(b.data_buf);
    }

    fn trace(b: *const Bridge, comptime fmt: []const u8, args: anytype) void {
        if (b.opts.debug) std.debug.print("9player: " ++ fmt ++ "\n", args);
    }

    // -- dispatch --------------------------------------------------------------------

    /// Handles one request. Returns false when the loop should stop (DESTROY).
    /// Fatal errors (dead 9P session, broken FUSE fd) propagate.
    fn dispatch(b: *Bridge, req: fuse.Request) !bool {
        const h = req.header;
        const op = h.op();
        b.trace("<- {s} unique={d} nodeid={d} len={d} (fids={d} inodes={d} handles={d})", .{ opName(op), h.unique, h.nodeid, h.len, b.nine.fidsInUse(), b.inodes.count(), b.handles.count() });
        const wants_reply = switch (op) {
            .forget, .batch_forget, .interrupt => false,
            else => true,
        };
        if (op == .destroy) {
            b.reply(h.unique, &.{}) catch {};
            return false;
        }
        b.handle(req) catch |e| {
            const code: linux.E = switch (e) {
                error.Nine => b.last_err,
                error.BadRequest => .INVAL,
                error.NoEntry => .NOENT,
                error.BadHandle => .BADF,
                error.Exdev => .XDEV,
                error.Perm => .PERM,
                error.NotSup => .NOSYS,
                error.OutOfMemory => .NOMEM,
                error.TooLarge => .NAMETOOLONG,
                error.BadDir => .IO,
                error.Closed, error.Protocol, error.Io, error.Stopped => .IO,
                error.FuseIo => return error.FuseIo,
            };
            if (wants_reply) try b.replyError(h.unique, code);
            switch (e) {
                error.Closed, error.Protocol, error.Io => return error.Closed,
                error.Stopped => return false, // the child is gone; the mount is being torn down
                else => {},
            }
        };
        return true;
    }

    fn handle(b: *Bridge, req: fuse.Request) HandlerError!void {
        const u = req.header.unique;
        switch (req.header.op()) {
            .init => {
                const in = try body(fuse.InitIn, req);
                const out = fuse.initReply(in, max_write);
                try b.reply(u, &.{std.mem.asBytes(&out)});
            },
            .lookup => {
                const name = try nameAfter(void, req);
                const entry = try b.lookupEntry(req.header.nodeid, name);
                try b.reply(u, &.{std.mem.asBytes(&entry)});
            },
            .forget => {
                const in = try body(fuse.ForgetIn, req);
                try b.forget(req.header.nodeid, in.nlookup);
            },
            .batch_forget => {
                const in = try body(fuse.BatchForgetIn, req);
                const rest = req.body[@sizeOf(fuse.BatchForgetIn)..];
                const count: usize = in.count;
                if (rest.len < count * @sizeOf(fuse.ForgetOne)) return error.BadRequest;
                for (0..count) |i| {
                    const one = std.mem.bytesToValue(fuse.ForgetOne, rest[i * @sizeOf(fuse.ForgetOne) ..][0..@sizeOf(fuse.ForgetOne)]);
                    try b.forget(one.nodeid, one.nlookup);
                }
            },
            .getattr => {
                const ino = b.inodes.get(req.header.nodeid) orelse return error.NoEntry;
                const st = try b.stat(ino.fid);
                const out = b.attrOut(st, b.inoOf(req.header.nodeid, ino.qid));
                try b.reply(u, &.{std.mem.asBytes(&out)});
            },
            .setattr => try b.setattr(req),
            .open => try b.openFile(req, false),
            .opendir => try b.openFile(req, true),
            .read => {
                const in = try body(fuse.ReadIn, req);
                const h = b.handles.get(in.fh) orelse return error.BadHandle;
                const want: usize = @min(in.size, max_write);
                const n = try b.read(h.fid, in.offset, b.data_buf[0..want]);
                try b.reply(u, &.{b.data_buf[0..n]});
            },
            .write => {
                const in = try body(fuse.WriteIn, req);
                const h = b.handles.get(in.fh) orelse return error.BadHandle;
                const rest = req.body[@sizeOf(fuse.WriteIn)..];
                if (rest.len < in.size) return error.BadRequest;
                const n = try b.write(h.fid, in.offset, rest[0..in.size]);
                const out = fuse.WriteOut{ .size = @intCast(n) };
                try b.reply(u, &.{std.mem.asBytes(&out)});
            },
            .readdir => try b.readdir(req),
            .release, .releasedir => {
                const in = try body(fuse.ReleaseIn, req);
                const kv = b.handles.fetchRemove(in.fh) orelse return error.BadHandle;
                var h = kv.value;
                if (h.dir) |*d| d.deinit(b.gpa);
                try b.clunk(h.fid);
                try b.reply(u, &.{});
            },
            .flush, .fsync, .fsyncdir => try b.reply(u, &.{}),
            .create => try b.create(req),
            .mkdir => {
                const in = try body(fuse.MkdirIn, req);
                const name = try nameAfter(fuse.MkdirIn, req);
                const parent = b.inodes.get(req.header.nodeid) orelse return error.NoEntry;
                const fid = try b.clone(parent.fid);
                _ = b.create9(fid, name, cloud9.dmdir | (in.mode & 0o777), cloud9.oread) catch |e| {
                    b.clunkQuiet(fid);
                    return e;
                };
                try b.clunk(fid);
                const entry = try b.lookupEntry(req.header.nodeid, name);
                try b.reply(u, &.{std.mem.asBytes(&entry)});
            },
            .unlink, .rmdir => {
                const name = try nameAfter(void, req);
                const parent = b.inodes.get(req.header.nodeid) orelse return error.NoEntry;
                const tmp = try b.walkName(parent.fid, name);
                try b.remove(tmp);
                try b.reply(u, &.{});
            },
            .rename => {
                const in = try body(fuse.RenameIn, req);
                const old = try nameAfter(fuse.RenameIn, req);
                const new = try secondName(req, old, @sizeOf(fuse.RenameIn));
                try b.rename(req.header.nodeid, in.newdir, old, new, 0);
                try b.reply(u, &.{});
            },
            .rename2 => {
                const in = try body(fuse.Rename2In, req);
                const old = try nameAfter(fuse.Rename2In, req);
                const new = try secondName(req, old, @sizeOf(fuse.Rename2In));
                try b.rename(req.header.nodeid, in.newdir, old, new, in.flags);
                try b.reply(u, &.{});
            },
            .statfs => {
                const out = fuse.StatfsOut{ .st = .{ .bsize = 4096, .namelen = 255, .frsize = 4096 } };
                try b.reply(u, &.{std.mem.asBytes(&out)});
            },
            .interrupt => {},
            .destroy => unreachable, // handled in dispatch
            .access => return error.NotSup,
            else => return error.NotSup,
        }
    }

    // -- handlers ----------------------------------------------------------------------

    /// walk(parent → new fid, [name]) + stat, deduplicated by qid.path. Bumps nlookup.
    fn lookupEntry(b: *Bridge, parent_id: u64, name: []const u8) HandlerError!fuse.EntryOut {
        const parent = b.inodes.get(parent_id) orelse return error.NoEntry;
        const newfid = try b.walkName(parent.fid, name);
        const st = b.stat(newfid) catch |e| {
            b.clunkQuiet(newfid);
            return e;
        };
        const qid = st.qid;
        var nodeid: u64 = undefined;
        if (b.by_qid.get(qid.path)) |existing| {
            // A directory and a file sharing a qid.path (a server bug) must not
            // share a node: the kernel would mark the inode bad, and for the
            // root that is fatal for the whole mount.
            const merge = if (b.inodes.getPtr(existing)) |ino| (ino.qid.type & cloud9.qtdir) == (qid.type & cloud9.qtdir) else false;
            if (merge) {
                const ino = b.inodes.getPtr(existing).?;
                ino.nlookup += 1;
                ino.qid = qid;
                nodeid = existing;
                if (existing == fuse.root_id) {
                    b.clunkQuiet(newfid);
                } else {
                    // Keep the fresh fid (it is bound to the current file at this
                    // name) and retire the older one.
                    const stale = ino.fid;
                    ino.fid = newfid;
                    b.clunkQuiet(stale);
                }
            } else {
                // Stale reverse entry, or a type clash: bind a fresh node to it.
                nodeid = try b.newInode(newfid, qid, parent_id);
            }
        } else {
            nodeid = try b.newInode(newfid, qid, parent_id);
        }
        var out = fuse.EntryOut{
            .nodeid = nodeid,
            .generation = 0,
            .attr = b.attrFrom(st, b.inoOf(nodeid, qid)),
        };
        out.entry_valid = b.opts.attr_timeout_ns / 1_000_000_000;
        out.entry_valid_nsec = @intCast(b.opts.attr_timeout_ns % 1_000_000_000);
        out.attr_valid = out.entry_valid;
        out.attr_valid_nsec = out.entry_valid_nsec;
        return out;
    }

    fn newInode(b: *Bridge, fid: u32, qid: cloud9.Qid, parent: u64) HandlerError!u64 {
        const nodeid = b.next_node;
        b.inodes.put(b.gpa, nodeid, .{ .fid = fid, .qid = qid, .nlookup = 1, .parent = parent }) catch |e| {
            b.clunkQuiet(fid);
            return e;
        };
        b.by_qid.put(b.gpa, qid.path, nodeid) catch |e| {
            _ = b.inodes.remove(nodeid);
            b.clunkQuiet(fid);
            return e;
        };
        b.next_node += 1;
        return nodeid;
    }

    fn forget(b: *Bridge, nodeid: u64, n: u64) HandlerError!void {
        if (nodeid == fuse.root_id) return;
        const ino = b.inodes.getPtr(nodeid) orelse return;
        if (ino.nlookup > n) {
            ino.nlookup -= n;
            return;
        }
        const fid = ino.fid;
        const path = ino.qid.path;
        _ = b.inodes.remove(nodeid);
        if (b.by_qid.get(path)) |mapped| {
            if (mapped == nodeid) _ = b.by_qid.remove(path);
        }
        b.clunk(fid) catch |e| switch (e) {
            error.Nine => {},
            else => return e,
        };
    }

    fn setattr(b: *Bridge, req: fuse.Request) HandlerError!void {
        const in = try body(fuse.SetattrIn, req);
        const ino = b.inodes.get(req.header.nodeid) orelse return error.NoEntry;
        const old = try b.stat(ino.fid);
        const old_mode = old.mode;

        var st = nine.dontcare;
        var changed = false;
        if (in.valid & fuse.FATTR_UID != 0 and in.uid != b.opts.uid) return error.Perm;
        if (in.valid & fuse.FATTR_GID != 0 and in.gid != b.opts.gid) return error.Perm;
        if (in.valid & fuse.FATTR_SIZE != 0) {
            st.length = in.size;
            changed = true;
        }
        if (in.valid & fuse.FATTR_MODE != 0) {
            st.mode = (old_mode & ~@as(u32, 0o777)) | (in.mode & 0o777);
            changed = true;
        }
        if (in.valid & fuse.FATTR_MTIME_NOW != 0) {
            st.mtime = nowSeconds();
            changed = true;
        } else if (in.valid & fuse.FATTR_MTIME != 0) {
            st.mtime = @truncate(in.mtime);
            changed = true;
        }
        if (changed) try b.wstat(ino.fid, st);
        const fresh = try b.stat(ino.fid);
        const out = b.attrOut(fresh, b.inoOf(req.header.nodeid, ino.qid));
        try b.reply(req.header.unique, &.{std.mem.asBytes(&out)});
    }

    fn openFile(b: *Bridge, req: fuse.Request, is_dir: bool) HandlerError!void {
        const in = try body(fuse.OpenIn, req);
        const ino = b.inodes.get(req.header.nodeid) orelse return error.NoEntry;
        const mode: u8 = if (is_dir) cloud9.oread else openMode(in.flags);
        const fid = try b.clone(ino.fid);
        _ = b.open9(fid, mode) catch |e| {
            b.clunkQuiet(fid);
            return e;
        };
        const fh = try b.newHandle(fid, req.header.nodeid);
        const out = fuse.OpenOut{
            .fh = fh,
            .open_flags = if (!is_dir and b.opts.direct_io) fuse.FOPEN_DIRECT_IO else 0,
        };
        try b.reply(req.header.unique, &.{std.mem.asBytes(&out)});
    }

    fn newHandle(b: *Bridge, fid: u32, nodeid: u64) HandlerError!u64 {
        const fh = b.next_fh;
        b.handles.put(b.gpa, fh, .{ .fid = fid, .nodeid = nodeid, .dir = null }) catch |e| {
            b.clunkQuiet(fid);
            return e;
        };
        b.next_fh += 1;
        return fh;
    }

    fn create(b: *Bridge, req: fuse.Request) HandlerError!void {
        const in = try body(fuse.CreateIn, req);
        const name = try nameAfter(fuse.CreateIn, req);
        const parent = b.inodes.get(req.header.nodeid) orelse return error.NoEntry;
        // The created fid becomes the open file.
        const fid = try b.clone(parent.fid);
        _ = b.create9(fid, name, in.mode & 0o777, openMode(in.flags)) catch |e| {
            b.clunkQuiet(fid);
            return e;
        };
        const entry = b.lookupEntry(req.header.nodeid, name) catch |e| {
            b.clunkQuiet(fid);
            return e;
        };
        const fh = try b.newHandle(fid, entry.nodeid);
        const oo = fuse.OpenOut{
            .fh = fh,
            .open_flags = if (b.opts.direct_io) fuse.FOPEN_DIRECT_IO else 0,
        };
        try b.reply(req.header.unique, &.{ std.mem.asBytes(&entry), std.mem.asBytes(&oo) });
    }

    fn rename(b: *Bridge, parent_id: u64, newdir: u64, old: []const u8, new: []const u8, flags: u32) HandlerError!void {
        if (newdir != parent_id) return error.Exdev;
        const rf: linux.RENAME = @bitCast(flags);
        if (rf.EXCHANGE or rf.WHITEOUT) return error.BadRequest;
        const parent = b.inodes.get(parent_id) orelse return error.NoEntry;
        const tmp = try b.walkName(parent.fid, old);
        defer b.clunkQuiet(tmp);
        var st = nine.dontcare;
        st.name = new;
        b.wstat(tmp, st) catch |e| {
            // 9P2000 rename never replaces an existing name; POSIX rename does.
            if (e != error.Nine or rf.NOREPLACE or b.nine.errno() != .EXIST) return e;
            try b.renameOver(parent.fid, tmp, new);
        };
    }

    /// Replace `new` with the file behind `src`. An (empty) directory target is
    /// removed first: it holds no data and the VFS already ruled out mismatched
    /// types. A file target is parked under a temporary name so that a failing
    /// second rename can put it back instead of having destroyed it.
    fn renameOver(b: *Bridge, parent_fid: u32, src: u32, new: []const u8) HandlerError!void {
        const victim = try b.walkName(parent_fid, new);
        const vst = b.stat(victim) catch |e| {
            b.clunkQuiet(victim);
            return e;
        };
        var st = nine.dontcare;
        st.name = new;
        if (vst.mode & cloud9.dmdir != 0) {
            b.trace("   rename target is a directory; removing it and retrying", .{});
            try b.remove(victim);
            return b.wstat(src, st);
        }
        var park_buf: [48]u8 = undefined;
        const park = std.fmt.bufPrint(&park_buf, ".9player-rename-{x}", .{randomU64()}) catch unreachable;
        b.trace("   rename target exists; parking it as {s} and retrying", .{park});
        var pst = nine.dontcare;
        pst.name = park;
        b.wstat(victim, pst) catch |e| {
            b.clunkQuiet(victim);
            return e;
        };
        b.wstat(src, st) catch |e| {
            b.trace("   rename still failed; restoring the target", .{});
            const saved = b.last_err;
            b.wstat(victim, st) catch {};
            b.last_err = saved;
            b.clunkQuiet(victim);
            return e;
        };
        b.remove(victim) catch b.trace("   could not remove the parked target {s}", .{park});
    }

    fn readdir(b: *Bridge, req: fuse.Request) HandlerError!void {
        const in = try body(fuse.ReadIn, req);
        const h = b.handles.getPtr(in.fh) orelse return error.BadHandle;
        if (in.offset == 0 or h.dir == null) {
            if (h.dir) |*d| d.deinit(b.gpa);
            h.dir = null;
            h.dir = try b.loadDir(h.fid, h.nodeid);
        }
        const dir = &h.dir.?;
        const size: usize = @min(in.size, max_write);
        const used = packDirents(dir.entries.items, in.offset, b.data_buf[0..size]);
        try b.reply(req.header.unique, &.{b.data_buf[0..used]});
    }

    /// Reads the whole directory and builds its listing, "." and ".." first.
    fn loadDir(b: *Bridge, fid: u32, nodeid: u64) HandlerError!DirList {
        var list: DirList = .{};
        errdefer list.deinit(b.gpa);
        const self_ino = b.inoOfNode(nodeid);
        const parent_ino = if (b.inodes.get(nodeid)) |ino| b.inoOfNode(ino.parent) else self_ino;
        try list.entries.append(b.gpa, .{ .name = try b.gpa.dupe(u8, "."), .ino = self_ino, .dtype = fuse.DT_DIR });
        try list.entries.append(b.gpa, .{ .name = try b.gpa.dupe(u8, ".."), .ino = parent_ino, .dtype = fuse.DT_DIR });

        var offset: u64 = 0;
        while (true) {
            // A server that ignores the offset would otherwise feed us forever.
            if (offset >= max_dir_bytes) return error.BadDir;
            const n = try b.read(fid, offset, b.data_buf);
            if (n == 0) break;
            try parseDirRecords(b.gpa, b.data_buf[0..n], &list);
            offset += n;
        }
        // Entries carrying the root's own qid.path get the root's ino (1), as GETATTR would report it.
        for (list.entries.items[2..]) |*e| if (e.ino == b.root_path) {
            e.ino = fuse.root_id;
        };
        return list;
    }

    // -- 9P wrappers (tracing) -----------------------------------------------------------

    fn stat(b: *Bridge, fid: u32) nine.Session.Error!cloud9.Stat {
        const st = b.nine.stat(fid) catch |e| return b.nineErr("stat", fid, e);
        b.trace("   9p stat fid={d} -> name={s} mode={o} len={d} qid={x}", .{ fid, st.name, st.mode, st.length, st.qid.path });
        return st;
    }

    fn walkName(b: *Bridge, fid: u32, name: []const u8) nine.Session.Error!u32 {
        const newfid = b.nine.allocFid();
        _ = b.nine.walk(fid, newfid, &.{name}) catch |e| {
            b.nine.freeFid(newfid);
            return b.nineErr("walk", fid, e);
        };
        b.trace("   9p walk fid={d} newfid={d} name={s} -> ok", .{ fid, newfid, name });
        return newfid;
    }

    fn clone(b: *Bridge, fid: u32) nine.Session.Error!u32 {
        const newfid = b.nine.clone(fid) catch |e| return b.nineErr("clone", fid, e);
        b.trace("   9p walk fid={d} newfid={d} (clone) -> ok", .{ fid, newfid });
        return newfid;
    }

    fn open9(b: *Bridge, fid: u32, mode: u8) nine.Session.Error!nine.Session.Open {
        const o = b.nine.open(fid, mode) catch |e| return b.nineErr("open", fid, e);
        b.trace("   9p open fid={d} mode={d} -> iounit={d}", .{ fid, mode, o.iounit });
        return o;
    }

    fn create9(b: *Bridge, fid: u32, name: []const u8, perm: u32, mode: u8) nine.Session.Error!nine.Session.Open {
        const o = b.nine.create(fid, name, perm, mode) catch |e| return b.nineErr("create", fid, e);
        b.trace("   9p create fid={d} name={s} perm={o} mode={d} -> iounit={d}", .{ fid, name, perm, mode, o.iounit });
        return o;
    }

    fn read(b: *Bridge, fid: u32, offset: u64, buf: []u8) nine.Session.Error!usize {
        const n = b.nine.read(fid, offset, buf) catch |e| return b.nineErr("read", fid, e);
        b.trace("   9p read fid={d} offset={d} count={d} -> {d}", .{ fid, offset, buf.len, n });
        return n;
    }

    fn write(b: *Bridge, fid: u32, offset: u64, data: []const u8) nine.Session.Error!usize {
        const n = b.nine.write(fid, offset, data) catch |e| return b.nineErr("write", fid, e);
        b.trace("   9p write fid={d} offset={d} count={d} -> {d}", .{ fid, offset, data.len, n });
        return n;
    }

    fn wstat(b: *Bridge, fid: u32, st: cloud9.Stat) nine.Session.Error!void {
        b.nine.wstat(fid, st) catch |e| return b.nineErr("wstat", fid, e);
        b.trace("   9p wstat fid={d} name={s} mode={x} len={x} mtime={x} -> ok", .{ fid, st.name, st.mode, st.length, st.mtime });
    }

    fn clunk(b: *Bridge, fid: u32) nine.Session.Error!void {
        b.nine.clunk(fid) catch |e| return b.nineErr("clunk", fid, e);
        b.trace("   9p clunk fid={d} -> ok", .{fid});
    }

    /// Best-effort clunk during error unwinding; a dead session surfaces on the
    /// next call. Does not disturb the errno of the failure being unwound.
    fn clunkQuiet(b: *Bridge, fid: u32) void {
        const saved = b.last_err;
        defer b.last_err = saved;
        b.clunk(fid) catch {};
    }

    fn remove(b: *Bridge, fid: u32) nine.Session.Error!void {
        b.nine.remove(fid) catch |e| return b.nineErr("remove", fid, e);
        b.trace("   9p remove fid={d} -> ok", .{fid});
    }

    fn nineErr(b: *Bridge, what: []const u8, fid: u32, e: nine.Session.Error) nine.Session.Error {
        if (e == error.Nine) {
            b.last_err = b.nine.errno();
            b.trace("   9p {s} fid={d} -> Rerror \"{s}\" ({s})", .{ what, fid, b.nine.ename[0..b.nine.ename_len], @tagName(b.nine.errno()) });
        } else {
            b.trace("   9p {s} fid={d} -> {s}", .{ what, fid, @errorName(e) });
        }
        return e;
    }

    // -- FUSE wrappers (tracing) --------------------------------------------------------

    fn reply(b: *Bridge, unique: u64, payloads: []const []const u8) error{FuseIo}!void {
        var total: usize = 0;
        for (payloads) |p| total += p.len;
        b.trace("-> unique={d} ok ({d} bytes)", .{ unique, total });
        fuse.reply(b.fuse_fd, unique, payloads) catch return error.FuseIo;
    }

    fn replyError(b: *Bridge, unique: u64, code: linux.E) error{FuseIo}!void {
        b.trace("-> unique={d} error E{s}", .{ unique, @tagName(code) });
        fuse.replyError(b.fuse_fd, unique, code) catch return error.FuseIo;
    }

    // -- attrs ---------------------------------------------------------------------------

    fn inoOf(b: *const Bridge, nodeid: u64, qid: cloud9.Qid) u64 {
        return if (nodeid == fuse.root_id or qid.path == b.root_path) fuse.root_id else qid.path;
    }

    fn inoOfNode(b: *const Bridge, nodeid: u64) u64 {
        if (nodeid == fuse.root_id) return fuse.root_id;
        const ino = b.inodes.get(nodeid) orelse return nodeid;
        return b.inoOf(nodeid, ino.qid);
    }

    fn attrFrom(b: *const Bridge, st: cloud9.Stat, ino: u64) fuse.Attr {
        return attrFromStat(st, ino, b.opts.uid, b.opts.gid);
    }

    fn attrOut(b: *const Bridge, st: cloud9.Stat, ino: u64) fuse.AttrOut {
        return .{
            .attr_valid = b.opts.attr_timeout_ns / 1_000_000_000,
            .attr_valid_nsec = @intCast(b.opts.attr_timeout_ns % 1_000_000_000),
            .attr = b.attrFrom(st, ino),
        };
    }
};

// -- pure helpers (unit-tested) ------------------------------------------------------------

/// Attr from a 9P Stat: DMDIR → S_IFDIR else S_IFREG, low 9 permission bits kept.
pub fn attrFromStat(st: cloud9.Stat, ino: u64, uid: u32, gid: u32) fuse.Attr {
    const ftype: u32 = if (st.mode & cloud9.dmdir != 0) fuse.S_IFDIR else fuse.S_IFREG;
    return .{
        .ino = ino,
        // The kernel marks an inode bad when size > LLONG_MAX; clamp hostile lengths.
        .size = @min(st.length, std.math.maxInt(i64)),
        // Saturating: a hostile length of 2^64-1 must not overflow.
        .blocks = st.length / 512 + @intFromBool(st.length % 512 != 0),
        .atime = st.atime,
        .mtime = st.mtime,
        .ctime = st.mtime,
        .mode = ftype | (st.mode & 0o777),
        .nlink = 1,
        .uid = uid,
        .gid = gid,
        .blksize = 4096,
    };
}

/// Kernel open(2) flags → 9P open mode. O_APPEND has no 9P equivalent and is ignored.
pub fn openMode(flags: u32) u8 {
    const o: linux.O = @bitCast(flags);
    var mode: u8 = switch (o.ACCMODE) {
        .RDONLY => cloud9.oread,
        .WRONLY => cloud9.owrite,
        .RDWR => cloud9.ordwr,
    };
    if (o.TRUNC) mode |= cloud9.otrunc;
    return mode;
}

/// Parses consecutive 9P directory records (2-byte size + Stat) and appends entries.
pub fn parseDirRecords(gpa: std.mem.Allocator, bytes: []const u8, list: *DirList) error{ OutOfMemory, BadDir }!void {
    var pos: usize = 0;
    while (pos < bytes.len) {
        if (bytes.len - pos < 2) return error.BadDir;
        const size: usize = std.mem.readInt(u16, bytes[pos..][0..2], .little);
        if (bytes.len - pos < 2 + size) return error.BadDir;
        const st = cloud9.Stat.decode(bytes[pos..][0 .. 2 + size]) catch return error.BadDir;
        pos += 2 + size;
        // The kernel rejects a whole READDIR reply (EIO) over one bad name, and
        // "." and ".." are synthesised by loadDir: drop such records instead.
        if (!validDirentName(st.name)) continue;
        const name = try gpa.dupe(u8, st.name);
        errdefer gpa.free(name);
        try list.entries.append(gpa, .{
            .name = name,
            .ino = st.qid.path,
            .dtype = if (st.mode & cloud9.dmdir != 0) fuse.DT_DIR else fuse.DT_REG,
        });
    }
}

/// A name the kernel will accept in a dirent and that does not duplicate the synthetic "." / "..".
pub fn validDirentName(name: []const u8) bool {
    if (name.len == 0 or name.len > max_name_len) return false;
    if (std.mem.indexOfAny(u8, name, "/\x00") != null) return false;
    if (std.mem.eql(u8, name, ".") or std.mem.eql(u8, name, "..")) return false;
    return true;
}

/// Packs dirents from `entries[offset..]` into `buf`; each record's `off` is its index + 1.
/// Returns the number of bytes used.
pub fn packDirents(entries: []const Entry, offset: u64, buf: []u8) usize {
    var used: usize = 0;
    var i: usize = @intCast(@min(offset, entries.len));
    while (i < entries.len) : (i += 1) {
        const e = entries[i];
        if (!fuse.addDirent(buf, &used, e.ino, @as(u64, i) + 1, e.dtype, e.name)) break;
    }
    return used;
}

fn randomU64() u64 {
    var bytes: [8]u8 = undefined;
    if (linux.errno(linux.getrandom(&bytes, bytes.len, 0)) == .SUCCESS) return std.mem.readInt(u64, &bytes, .little);
    var ts: linux.timespec = undefined;
    _ = linux.clock_gettime(.MONOTONIC, &ts);
    return @as(u64, @bitCast(ts.nsec)) ^ (@as(u64, @bitCast(ts.sec)) << 32);
}

fn nowSeconds() u32 {
    var ts: linux.timespec = undefined;
    if (linux.errno(linux.clock_gettime(.REALTIME, &ts)) != .SUCCESS) return 0;
    return @intCast(@as(u64, @intCast(ts.sec)) & 0xFFFF_FFFF);
}

fn opName(op: fuse.Opcode) []const u8 {
    return switch (op) {
        _ => "unknown",
        else => @tagName(op),
    };
}

// Thin adapters so fuse.zig's parse errors become HandlerError.BadRequest.
fn body(comptime T: type, req: fuse.Request) error{BadRequest}!*const T {
    return fuse.body(T, req) catch error.BadRequest;
}

fn nameAfter(comptime T: type, req: fuse.Request) error{BadRequest}![]const u8 {
    return fuse.nameAfter(T, req) catch error.BadRequest;
}

fn secondName(req: fuse.Request, first: []const u8, offset: usize) error{BadRequest}![]const u8 {
    return fuse.secondName(req, first, offset) catch error.BadRequest;
}

// -- tests ------------------------------------------------------------------------------

const testing = std.testing;

test {
    // Force semantic analysis of `serve` and the whole dispatch path, which no
    // unit test can exercise without a FUSE mount.
    testing.refAllDecls(@This());
}

fn testStat(name: []const u8, mode: u32, length: u64, path: u64) cloud9.Stat {
    return .{
        .type = 0,
        .dev = 0,
        .qid = .{ .type = if (mode & cloud9.dmdir != 0) cloud9.qtdir else 0, .version = 0, .path = path },
        .mode = mode,
        .atime = 100,
        .mtime = 200,
        .length = length,
        .name = name,
        .uid = "u",
        .gid = "g",
        .muid = "u",
    };
}

test "attr mapping: DMDIR → S_IFDIR|perm, length → size/blocks" {
    const d = attrFromStat(testStat("d", cloud9.dmdir | 0o755, 0, 9), 9, 1000, 1001);
    try testing.expectEqual(fuse.S_IFDIR | 0o755, d.mode);
    try testing.expectEqual(@as(u64, 9), d.ino);
    try testing.expectEqual(@as(u64, 0), d.size);
    try testing.expectEqual(@as(u64, 0), d.blocks);
    try testing.expectEqual(@as(u32, 1000), d.uid);
    try testing.expectEqual(@as(u32, 1001), d.gid);
    try testing.expectEqual(@as(u32, 1), d.nlink);

    const f = attrFromStat(testStat("f", 0o640 | cloud9.dmappend, 1025, 4), 4, 0, 0);
    try testing.expectEqual(fuse.S_IFREG | 0o640, f.mode); // dmappend bit not leaked
    try testing.expectEqual(@as(u64, 1025), f.size);
    try testing.expectEqual(@as(u64, 3), f.blocks);
    try testing.expectEqual(@as(u32, 4096), f.blksize);
    try testing.expectEqual(@as(u64, 100), f.atime);
    try testing.expectEqual(@as(u64, 200), f.mtime);
    try testing.expectEqual(@as(u64, 200), f.ctime);

    try testing.expectEqual(@as(u64, 1), attrFromStat(testStat("f", 0o600, 512, 4), 4, 0, 0).blocks);
    try testing.expectEqual(@as(u64, 2), attrFromStat(testStat("f", 0o600, 513, 4), 4, 0, 0).blocks);
}

test "open flag → 9P mode mapping" {
    const rdonly: u32 = @bitCast(linux.O{ .ACCMODE = .RDONLY });
    const wronly: u32 = @bitCast(linux.O{ .ACCMODE = .WRONLY });
    const rdwr: u32 = @bitCast(linux.O{ .ACCMODE = .RDWR });
    const trunc: u32 = @bitCast(linux.O{ .TRUNC = true });
    const append: u32 = @bitCast(linux.O{ .APPEND = true });
    const creat: u32 = @bitCast(linux.O{ .CREAT = true });
    try testing.expectEqual(cloud9.oread, openMode(rdonly));
    try testing.expectEqual(cloud9.owrite, openMode(wronly));
    try testing.expectEqual(cloud9.ordwr, openMode(rdwr));
    try testing.expectEqual(cloud9.owrite | cloud9.otrunc, openMode(wronly | trunc));
    try testing.expectEqual(cloud9.ordwr | cloud9.otrunc, openMode(rdwr | trunc | creat));
    try testing.expectEqual(cloud9.owrite, openMode(wronly | append)); // O_APPEND ignored
}

test "dirlist parsing from two hand-encoded Stat records" {
    var buf: [512]u8 = undefined;
    const a = try cloud9.Stat.encode(testStat("alpha", 0o644, 10, 0x11), &buf);
    const bb = try cloud9.Stat.encode(testStat("beta", cloud9.dmdir | 0o755, 0, 0x22), buf[a.len..]);
    const bytes = buf[0 .. a.len + bb.len];
    // Sanity: the record is prefixed by its own 2-byte size.
    try testing.expectEqual(a.len - 2, std.mem.readInt(u16, bytes[0..2], .little));

    var list: DirList = .{};
    defer list.deinit(testing.allocator);
    try parseDirRecords(testing.allocator, bytes, &list);
    try testing.expectEqual(@as(usize, 2), list.entries.items.len);
    try testing.expectEqualStrings("alpha", list.entries.items[0].name);
    try testing.expectEqual(@as(u64, 0x11), list.entries.items[0].ino);
    try testing.expectEqual(fuse.DT_REG, list.entries.items[0].dtype);
    try testing.expectEqualStrings("beta", list.entries.items[1].name);
    try testing.expectEqual(@as(u64, 0x22), list.entries.items[1].ino);
    try testing.expectEqual(fuse.DT_DIR, list.entries.items[1].dtype);

    // Truncated input is a protocol error and leaves earlier entries intact.
    try testing.expectError(error.BadDir, parseDirRecords(testing.allocator, bytes[0 .. bytes.len - 1], &list));
    try testing.expectEqual(@as(usize, 3), list.entries.items.len);
}

test "readdir packing and offset resumption" {
    const names = [_][]const u8{ ".", "..", "one", "two", "three" };
    var entries: [names.len]Entry = undefined;
    for (&entries, names, 0..) |*e, n, i| e.* = .{ .name = @constCast(n), .ino = 100 + i, .dtype = if (i < 2) fuse.DT_DIR else fuse.DT_REG };

    // Everything fits: five records, off = index + 1.
    var big: [1024]u8 = undefined;
    const used = packDirents(&entries, 0, &big);
    var pos: usize = 0;
    var idx: usize = 0;
    while (pos < used) : (idx += 1) {
        const d = std.mem.bytesToValue(fuse.Dirent, big[pos..][0..@sizeOf(fuse.Dirent)]);
        try testing.expectEqual(@as(u64, 100 + idx), d.ino);
        try testing.expectEqual(@as(u64, idx + 1), d.off);
        try testing.expectEqualStrings(names[idx], big[pos + @sizeOf(fuse.Dirent) ..][0..d.namelen]);
        pos += (@sizeOf(fuse.Dirent) + d.namelen + 7) & ~@as(usize, 7);
    }
    try testing.expectEqual(names.len, idx);

    // A buffer that fits exactly two records ("." = 32, ".." = 32) stops there…
    var small: [64]u8 = undefined;
    const first_used = packDirents(&entries, 0, &small);
    try testing.expectEqual(@as(usize, 64), first_used);
    const last = std.mem.bytesToValue(fuse.Dirent, small[32..][0..@sizeOf(fuse.Dirent)]);
    try testing.expectEqual(@as(u64, 2), last.off);
    // …and resuming at the last `off` yields "one" next.
    const second_used = packDirents(&entries, last.off, &small);
    const next = std.mem.bytesToValue(fuse.Dirent, small[0..@sizeOf(fuse.Dirent)]);
    try testing.expectEqualStrings("one", small[@sizeOf(fuse.Dirent)..][0..next.namelen]);
    try testing.expectEqual(@as(u64, 3), next.off);
    try testing.expect(second_used > 0);

    // Past the end: nothing (EOF for the kernel).
    try testing.expectEqual(@as(usize, 0), packDirents(&entries, names.len, &big));
    try testing.expectEqual(@as(usize, 0), packDirents(&entries, 1000, &big));
}

test "attr mapping saturates hostile lengths instead of overflowing" {
    const a = attrFromStat(testStat("f", 0o600, std.math.maxInt(u64), 4), 4, 0, 0);
    try testing.expectEqual(@as(u64, std.math.maxInt(i64)), a.size);
    try testing.expectEqual(@as(u64, std.math.maxInt(u64) / 512 + 1), a.blocks);
    const b = attrFromStat(testStat("f", 0o600, 1024, 4), 4, 0, 0);
    try testing.expectEqual(@as(u64, 2), b.blocks);
    try testing.expectEqual(@as(u64, 1024), b.size);
}

test "dirent names the kernel would reject are dropped from listings" {
    try testing.expect(validDirentName("a"));
    try testing.expect(validDirentName("x" ** 1024));
    try testing.expect(!validDirentName(""));
    try testing.expect(!validDirentName("a/b"));
    try testing.expect(!validDirentName("a\x00b"));
    try testing.expect(!validDirentName("."));
    try testing.expect(!validDirentName(".."));
    try testing.expect(!validDirentName("x" ** 1025));

    var buf: [4096]u8 = undefined;
    var n: usize = 0;
    for ([_][]const u8{ ".", "..", "", "a/b", "keep", "x" ** 1025, "also" }) |name| {
        n += (try cloud9.Stat.encode(testStat(name, 0o644, 1, 0x30), buf[n..])).len;
    }
    var list: DirList = .{};
    defer list.deinit(testing.allocator);
    try parseDirRecords(testing.allocator, buf[0..n], &list);
    try testing.expectEqual(@as(usize, 2), list.entries.items.len);
    try testing.expectEqualStrings("keep", list.entries.items[0].name);
    try testing.expectEqualStrings("also", list.entries.items[1].name);
}

test "DirList frees its names" {
    var list: DirList = .{};
    try list.entries.append(testing.allocator, .{ .name = try testing.allocator.dupe(u8, "x"), .ino = 1, .dtype = fuse.DT_REG });
    list.deinit(testing.allocator);
}