v86/lib/filesystem.js

// -------------------------------------------------
// ----------------- FILESYSTEM---------------------
// -------------------------------------------------
// Implementation of a unix filesystem in memory.

"use strict";

var S_IRWXUGO = 0x1FF;
var S_IFMT = 0xF000;
var S_IFSOCK = 0xC000;
var S_IFLNK = 0xA000;
var S_IFREG = 0x8000;
var S_IFBLK = 0x6000;
var S_IFDIR = 0x4000;
var S_IFCHR = 0x2000;

//var S_IFIFO  0010000
//var S_ISUID  0004000
//var S_ISGID  0002000
//var S_ISVTX  0001000

var O_RDONLY = 0x0000; // open for reading only
var O_WRONLY = 0x0001; // open for writing only
var O_RDWR = 0x0002; // open for reading and writing
var O_ACCMODE = 0x0003; // mask for above modes

var STATUS_INVALID = -0x1;
var STATUS_OK = 0x0;
var STATUS_ON_STORAGE = 0x2;
var STATUS_UNLINKED = 0x4;
var STATUS_FORWARDING = 0x5;


/** @const */ var JSONFS_VERSION = 3;


/** @const */ var JSONFS_IDX_NAME = 0;
/** @const */ var JSONFS_IDX_SIZE = 1;
/** @const */ var JSONFS_IDX_MTIME = 2;
/** @const */ var JSONFS_IDX_MODE = 3;
/** @const */ var JSONFS_IDX_UID = 4;
/** @const */ var JSONFS_IDX_GID = 5;
/** @const */ var JSONFS_IDX_TARGET = 6;
/** @const */ var JSONFS_IDX_SHA256 = 6;


/**
 * @constructor
 * @param {!FileStorageInterface} storage
 * @param {{ last_qidnumber: number }=} qidcounter Another fs's qidcounter to synchronise with.
 */
function FS(storage, qidcounter) {
    /** @type {Array.<!Inode>} */
    this.inodes = [];
    this.events = [];

    this.storage = storage;

    this.qidcounter = qidcounter || { last_qidnumber: 0 };

    //this.tar = new TAR(this);

    this.inodedata = {};

    this.total_size = 256 * 1024 * 1024 * 1024;
    this.used_size = 0;

    /** @type {!Array<!FSMountInfo>} */
    this.mounts = [];

    //RegisterMessage("LoadFilesystem", this.LoadFilesystem.bind(this) );
    //RegisterMessage("MergeFile", this.MergeFile.bind(this) );
    //RegisterMessage("tar",
    //    function(data) {
    //        SendToMaster("tar", this.tar.Pack(data));
    //    }.bind(this)
    //);
    //RegisterMessage("sync",
    //    function(data) {
    //        SendToMaster("sync", this.tar.Pack(data));
    //    }.bind(this)
    //);

    // root entry
    this.CreateDirectory("", -1);
}

FS.prototype.get_state = function()
{
    let state = [];

    state[0] = this.inodes;
    state[1] = this.qidcounter.last_qidnumber;
    state[2] = [];
    for(const [id, data] of Object.entries(this.inodedata))
    {
        if((this.inodes[id].mode & S_IFDIR) === 0)
        {
            state[2].push([id, data]);
        }
    }
    state[3] = this.total_size;
    state[4] = this.used_size;
    state = state.concat(this.mounts);

    return state;
};

FS.prototype.set_state = function(state)
{
    this.inodes = state[0].map(state => { const inode = new Inode(0); inode.set_state(state); return inode; });
    this.qidcounter.last_qidnumber = state[1];
    this.inodedata = {};
    for(let [key, value] of state[2])
    {
        if(value.buffer.byteLength !== value.byteLength)
        {
            // make a copy if we didn't get one
            value = value.slice();
        }

        this.inodedata[key] = value;
    }
    this.total_size = state[3];
    this.used_size = state[4];
    this.mounts = state.slice(5);
};


// -----------------------------------------------------

FS.prototype.AddEvent = function(id, OnEvent) {
    var inode = this.inodes[id];
    if (inode.status == STATUS_OK || inode.status == STATUS_ON_STORAGE) {
        OnEvent();
    }
    else if(this.is_forwarder(inode))
    {
        this.follow_fs(inode).AddEvent(inode.foreign_id, OnEvent);
    }
    else
    {
        this.events.push({id: id, OnEvent: OnEvent});
    }
};

FS.prototype.HandleEvent = function(id) {
    const inode = this.inodes[id];
    if(this.is_forwarder(inode))
    {
        this.follow_fs(inode).HandleEvent(inode.foreign_id);
    }
    //message.Debug("number of events: " + this.events.length);
    var newevents = [];
    for(var i=0; i<this.events.length; i++) {
        if (this.events[i].id == id) {
            this.events[i].OnEvent();
        } else {
            newevents.push(this.events[i]);
        }
    }
    this.events = newevents;
};

FS.prototype.load_from_json = function(fs, done)
{
    dbg_assert(fs, "Invalid fs passed to load_from_json");

    if(fs["version"] !== JSONFS_VERSION)
    {
        throw "The filesystem JSON format has changed. " +
              "Please update your fs2json (https://github.com/copy/fs2json) and recreate the filesystem JSON.";
    }

    var fsroot = fs["fsroot"];
    this.used_size = fs["size"];

    for(var i = 0; i < fsroot.length; i++) {
        this.LoadRecursive(fsroot[i], 0);
    }

    //if(DEBUG)
    //{
    //    this.Check();
    //}

    done && done();
};

FS.prototype.LoadRecursive = function(data, parentid)
{
    var inode = this.CreateInode();

    const name = data[JSONFS_IDX_NAME];
    inode.size = data[JSONFS_IDX_SIZE];
    inode.mtime = data[JSONFS_IDX_MTIME];
    inode.ctime = inode.mtime;
    inode.atime = inode.mtime;
    inode.mode = data[JSONFS_IDX_MODE];
    inode.uid = data[JSONFS_IDX_UID];
    inode.gid = data[JSONFS_IDX_GID];

    var ifmt = inode.mode & S_IFMT;

    if(ifmt === S_IFDIR)
    {
        this.PushInode(inode, parentid, name);
        this.LoadDir(this.inodes.length - 1, data[JSONFS_IDX_TARGET]);
    }
    else if(ifmt === S_IFREG)
    {
        inode.status = STATUS_ON_STORAGE;
        inode.sha256sum = data[JSONFS_IDX_SHA256];
        dbg_assert(inode.sha256sum);
        this.PushInode(inode, parentid, name);
    }
    else if(ifmt === S_IFLNK)
    {
        inode.symlink = data[JSONFS_IDX_TARGET];
        this.PushInode(inode, parentid, name);
    }
    else if(ifmt === S_IFSOCK)
    {
        // socket: ignore
    }
    else
    {
        dbg_log("Unexpected ifmt: " + h(ifmt) + " (" + name + ")");
    }
};

FS.prototype.LoadDir = function(parentid, children)
{
    for(var i = 0; i < children.length; i++) {
        this.LoadRecursive(children[i], parentid);
    }
};


// -----------------------------------------------------

/**
 * @private
 * @param {Inode} inode
 * @return {boolean}
 */
FS.prototype.should_be_linked = function(inode)
{
    // Note: Non-root forwarder inode could still have a non-forwarder parent, so don't use
    // parent inode to check.
    return !this.is_forwarder(inode) || inode.foreign_id === 0;
};

/**
 * @private
 * @param {number} parentid
 * @param {number} idx
 * @param {string} name
 */
FS.prototype.link_under_dir = function(parentid, idx, name)
{
    const inode = this.inodes[idx];
    const parent_inode = this.inodes[parentid];

    dbg_assert(!this.is_forwarder(parent_inode),
        "Filesystem: Shouldn't link under fowarder parents");
    dbg_assert(this.IsDirectory(parentid),
        "Filesystem: Can't link under non-directories");
    dbg_assert(this.should_be_linked(inode),
        "Filesystem: Can't link across filesystems apart from their root");
    dbg_assert(inode.nlinks >= 0,
        "Filesystem: Found negative nlinks value of " + inode.nlinks);
    dbg_assert(!parent_inode.direntries.has(name),
        "Filesystem: Name '" + name + "' is already taken");

    parent_inode.direntries.set(name, idx);
    inode.nlinks++;

    if(this.IsDirectory(idx))
    {
        dbg_assert(!inode.direntries.has(".."),
            "Filesystem: Cannot link a directory twice");

        if(!inode.direntries.has(".")) inode.nlinks++;
        inode.direntries.set(".", idx);

        inode.direntries.set("..", parentid);
        parent_inode.nlinks++;
    }
};

/**
 * @private
 * @param {number} parentid
 * @param {string} name
 */
FS.prototype.unlink_from_dir = function(parentid, name)
{
    const idx = this.Search(parentid, name);
    const inode = this.inodes[idx];
    const parent_inode = this.inodes[parentid];

    dbg_assert(!this.is_forwarder(parent_inode), "Filesystem: Can't unlink from forwarders");
    dbg_assert(this.IsDirectory(parentid), "Filesystem: Can't unlink from non-directories");

    const exists = parent_inode.direntries.delete(name);
    if(!exists)
    {
        dbg_assert(false, "Filesystem: Can't unlink non-existent file: " + name);
        return;
    }

    inode.nlinks--;

    if(this.IsDirectory(idx))
    {
        dbg_assert(inode.direntries.get("..") === parentid,
            "Filesystem: Found directory with bad parent id");

        inode.direntries.delete("..");
        parent_inode.nlinks--;
    }

    dbg_assert(inode.nlinks >= 0,
        "Filesystem: Found negative nlinks value of " + inode.nlinks);
};

FS.prototype.PushInode = function(inode, parentid, name) {
    if (parentid != -1) {
        this.inodes.push(inode);
        inode.fid = this.inodes.length - 1;
        this.link_under_dir(parentid, inode.fid, name);
        return;
    } else {
        if (this.inodes.length == 0) { // if root directory
            this.inodes.push(inode);
            inode.direntries.set(".", 0);
            inode.direntries.set("..", 0);
            inode.nlinks = 2;
            return;
        }
    }

    message.Debug("Error in Filesystem: Pushed inode with name = "+ name + " has no parent");
    message.Abort();

};

/** @constructor */
function Inode(qidnumber)
{
    this.direntries = new Map(); // maps filename to inode id
    this.status = 0;
    this.size = 0x0;
    this.uid = 0x0;
    this.gid = 0x0;
    this.fid = 0;
    this.ctime = 0;
    this.atime = 0;
    this.mtime = 0;
    this.major = 0x0;
    this.minor = 0x0;
    this.symlink = "";
    this.mode = 0x01ED;
    this.qid = {
        type: 0,
        version: 0,
        path: qidnumber,
    };
    this.caps = undefined;
    this.nlinks = 0;
    this.sha256sum = "";

    /** @type{!Array<!FSLockRegion>} */
    this.locks = []; // lock regions applied to the file, sorted by starting offset.

    // For forwarders:
    this.mount_id = -1; // which fs in this.mounts does this inode forward to?
    this.foreign_id = -1; // which foreign inode id does it represent?

    //this.qid_type = 0;
    //this.qid_version = 0;
    //this.qid_path = qidnumber;
}

Inode.prototype.get_state = function()
{
    const state = [];
    state[0] = this.mode;

    if((this.mode & S_IFMT) === S_IFDIR)
    {
        state[1] = [...this.direntries];
    }
    else if((this.mode & S_IFMT) === S_IFREG)
    {
        state[1] = this.sha256sum;
    }
    else if((this.mode & S_IFMT) === S_IFLNK)
    {
        state[1] = this.symlink;
    }
    else if((this.mode & S_IFMT) === S_IFSOCK)
    {
        state[1] = [this.minor, this.major];
    }
    else
    {
        state[1] = null;
    }

    state[2] = this.locks;
    state[3] = this.status;
    state[4] = this.size;
    state[5] = this.uid;
    state[6] = this.gid;
    state[7] = this.fid;
    state[8] = this.ctime;
    state[9] = this.atime;
    state[10] = this.mtime;
    state[11] = this.qid.version;
    state[12] = this.qid.path;
    state[13] = this.nlinks;

    //state[23] = this.mount_id;
    //state[24] = this.foreign_id;
    //state[25] = this.caps; // currently not writable
    return state;
};

Inode.prototype.set_state = function(state)
{
    this.mode = state[0];

    if((this.mode & S_IFMT) === S_IFDIR)
    {
        this.direntries = new Map();
        for(const [name, entry] of state[1])
        {
            this.direntries.set(name, entry);
        }
    }
    else if((this.mode & S_IFMT) === S_IFREG)
    {
        this.sha256sum = state[1];
    }
    else if((this.mode & S_IFMT) === S_IFLNK)
    {
        this.symlink = state[1];
    }
    else if((this.mode & S_IFMT) === S_IFSOCK)
    {
        [this.minor, this.major] = state[1];
    }
    else
    {
        // Nothing
    }

    this.locks = [];
    for(const lock_state of state[2])
    {
        const lock = new FSLockRegion();
        lock.set_state(lock_state);
        this.locks.push(lock);
    }
    this.status = state[3];
    this.size = state[4];
    this.uid = state[5];
    this.gid = state[6];
    this.fid = state[7];
    this.ctime = state[8];
    this.atime = state[9];
    this.mtime = state[10];
    this.qid.type = (this.mode & S_IFMT) >> 8;
    this.qid.version = state[11];
    this.qid.path = state[12];
    this.nlinks = state[13];

    //this.mount_id = state[23];
    //this.foreign_id = state[24];
    //this.caps = state[20];
};

/**
 * Clones given inode to new idx, effectively diverting the inode to new idx value.
 * Hence, original idx value is now free to use without losing the original information.
 * @private
 * @param {number} parentid Parent of target to divert.
 * @param {string} filename Name of target to divert.
 * @return {number} New idx of diversion.
 */
FS.prototype.divert = function(parentid, filename)
{
    const old_idx = this.Search(parentid, filename);
    const old_inode = this.inodes[old_idx];
    const new_inode = new Inode(-1);

    dbg_assert(old_inode, "Filesystem divert: name (" + filename + ") not found");
    dbg_assert(this.IsDirectory(old_idx) || old_inode.nlinks <= 1,
        "Filesystem: can't divert hardlinked file '" + filename + "' with nlinks=" +
        old_inode.nlinks);

    // Shallow copy is alright.
    Object.assign(new_inode, old_inode);

    const idx = this.inodes.length;
    this.inodes.push(new_inode);
    new_inode.fid = idx;

    // Relink references
    if(this.is_forwarder(old_inode))
    {
        this.mounts[old_inode.mount_id].backtrack.set(old_inode.foreign_id, idx);
    }
    if(this.should_be_linked(old_inode))
    {
        this.unlink_from_dir(parentid, filename);
        this.link_under_dir(parentid, idx, filename);
    }

    // Update children
    if(this.IsDirectory(old_idx) && !this.is_forwarder(old_inode))
    {
        for(const [name, child_id] of new_inode.direntries)
        {
            if(name === "." || name === "..") continue;
            if(this.IsDirectory(child_id))
            {
                this.inodes[child_id].direntries.set("..", idx);
            }
        }
    }

    // Relocate local data if any.
    this.inodedata[idx] = this.inodedata[old_idx];
    delete this.inodedata[old_idx];

    // Retire old reference information.
    old_inode.direntries = new Map();
    old_inode.nlinks = 0;

    return idx;
};

/**
 * Copy all non-redundant info.
 * References left untouched: local idx value and links
 * @private
 * @param {!Inode} src_inode
 * @param {!Inode} dest_inode
 */
FS.prototype.copy_inode = function(src_inode, dest_inode)
{
    Object.assign(dest_inode, src_inode, {
        fid: dest_inode.fid,
        direntries: dest_inode.direntries,
        nlinks: dest_inode.nlinks,
    });
};

FS.prototype.CreateInode = function() {
    //console.log("CreateInode", Error().stack);
    const now = Math.round(Date.now() / 1000);
    const inode = new Inode(++this.qidcounter.last_qidnumber);
    inode.atime = inode.ctime = inode.mtime = now;
    return inode;
};


// Note: parentid = -1 for initial root directory.
FS.prototype.CreateDirectory = function(name, parentid) {
    const parent_inode = this.inodes[parentid];
    if(parentid >= 0 && this.is_forwarder(parent_inode))
    {
        const foreign_parentid = parent_inode.foreign_id;
        const foreign_id = this.follow_fs(parent_inode).CreateDirectory(name, foreign_parentid);
        return this.create_forwarder(parent_inode.mount_id, foreign_id);
    }
    var x = this.CreateInode();
    x.mode = 0x01FF | S_IFDIR;
    if (parentid >= 0) {
        x.uid = this.inodes[parentid].uid;
        x.gid = this.inodes[parentid].gid;
        x.mode = (this.inodes[parentid].mode & 0x1FF) | S_IFDIR;
    }
    x.qid.type = S_IFDIR >> 8;
    this.PushInode(x, parentid, name);
    this.NotifyListeners(this.inodes.length-1, 'newdir');
    return this.inodes.length-1;
};

FS.prototype.CreateFile = function(filename, parentid) {
    const parent_inode = this.inodes[parentid];
    if(this.is_forwarder(parent_inode))
    {
        const foreign_parentid = parent_inode.foreign_id;
        const foreign_id = this.follow_fs(parent_inode).CreateFile(filename, foreign_parentid);
        return this.create_forwarder(parent_inode.mount_id, foreign_id);
    }
    var x = this.CreateInode();
    x.uid = this.inodes[parentid].uid;
    x.gid = this.inodes[parentid].gid;
    x.qid.type = S_IFREG >> 8;
    x.mode = (this.inodes[parentid].mode & 0x1B6) | S_IFREG;
    this.PushInode(x, parentid, filename);
    this.NotifyListeners(this.inodes.length-1, 'newfile');
    return this.inodes.length-1;
};


FS.prototype.CreateNode = function(filename, parentid, major, minor) {
    const parent_inode = this.inodes[parentid];
    if(this.is_forwarder(parent_inode))
    {
        const foreign_parentid = parent_inode.foreign_id;
        const foreign_id =
            this.follow_fs(parent_inode).CreateNode(filename, foreign_parentid, major, minor);
        return this.create_forwarder(parent_inode.mount_id, foreign_id);
    }
    var x = this.CreateInode();
    x.major = major;
    x.minor = minor;
    x.uid = this.inodes[parentid].uid;
    x.gid = this.inodes[parentid].gid;
    x.qid.type = S_IFSOCK >> 8;
    x.mode = (this.inodes[parentid].mode & 0x1B6);
    this.PushInode(x, parentid, filename);
    return this.inodes.length-1;
};

FS.prototype.CreateSymlink = function(filename, parentid, symlink) {
    const parent_inode = this.inodes[parentid];
    if(this.is_forwarder(parent_inode))
    {
        const foreign_parentid = parent_inode.foreign_id;
        const foreign_id =
            this.follow_fs(parent_inode).CreateSymlink(filename, foreign_parentid, symlink);
        return this.create_forwarder(parent_inode.mount_id, foreign_id);
    }
    var x = this.CreateInode();
    x.uid = this.inodes[parentid].uid;
    x.gid = this.inodes[parentid].gid;
    x.qid.type = S_IFLNK >> 8;
    x.symlink = symlink;
    x.mode = S_IFLNK;
    this.PushInode(x, parentid, filename);
    return this.inodes.length-1;
};

FS.prototype.CreateTextFile = async function(filename, parentid, str) {
    const parent_inode = this.inodes[parentid];
    if(this.is_forwarder(parent_inode))
    {
        const foreign_parentid = parent_inode.foreign_id;
        const foreign_id = await
            this.follow_fs(parent_inode).CreateTextFile(filename, foreign_parentid, str);
        return this.create_forwarder(parent_inode.mount_id, foreign_id);
    }
    var id = this.CreateFile(filename, parentid);
    var x = this.inodes[id];
    var data = new Uint8Array(str.length);
    x.size = str.length;
    for (var j = 0; j < str.length; j++) {
        data[j] = str.charCodeAt(j);
    }
    await this.set_data(id, data);
    return id;
};

/**
 * @param {Uint8Array} buffer
 */
FS.prototype.CreateBinaryFile = async function(filename, parentid, buffer) {
    const parent_inode = this.inodes[parentid];
    if(this.is_forwarder(parent_inode))
    {
        const foreign_parentid = parent_inode.foreign_id;
        const foreign_id = await
            this.follow_fs(parent_inode).CreateBinaryFile(filename, foreign_parentid, buffer);
        return this.create_forwarder(parent_inode.mount_id, foreign_id);
    }
    var id = this.CreateFile(filename, parentid);
    var x = this.inodes[id];
    var data = new Uint8Array(buffer.length);
    data.set(buffer);
    await this.set_data(id, data);
    x.size = buffer.length;
    return id;
};


FS.prototype.OpenInode = function(id, mode) {
    var inode = this.inodes[id];
    if(this.is_forwarder(inode))
    {
        return this.follow_fs(inode).OpenInode(inode.foreign_id, mode);
    }
    if ((inode.mode&S_IFMT) == S_IFDIR) {
        this.FillDirectory(id);
    }
    /*
    var type = "";
    switch(inode.mode&S_IFMT) {
        case S_IFREG: type = "File"; break;
        case S_IFBLK: type = "Block Device"; break;
        case S_IFDIR: type = "Directory"; break;
        case S_IFCHR: type = "Character Device"; break;
    }
    */
    //message.Debug("open:" + this.GetFullPath(id) +  " type: " + inode.mode + " status:" + inode.status);
    return true;
};

FS.prototype.CloseInode = async function(id) {
    //message.Debug("close: " + this.GetFullPath(id));
    var inode = this.inodes[id];
    if(this.is_forwarder(inode))
    {
        return await this.follow_fs(inode).CloseInode(inode.foreign_id);
    }
    if(inode.status === STATUS_ON_STORAGE)
    {
        this.storage.uncache(inode.sha256sum);
    }
    if (inode.status == STATUS_UNLINKED) {
        //message.Debug("Filesystem: Delete unlinked file");
        inode.status = STATUS_INVALID;
        await this.DeleteData(id);
    }
};

/**
 * @return {!Promise<number>} 0 if success, or -errno if failured.
 */
FS.prototype.Rename = async function(olddirid, oldname, newdirid, newname) {
    // message.Debug("Rename " + oldname + " to " + newname);
    if ((olddirid == newdirid) && (oldname == newname)) {
        return 0;
    }
    var oldid = this.Search(olddirid, oldname);
    if(oldid === -1)
    {
        return -ENOENT;
    }

    // For event notification near end of method.
    var oldpath = this.GetFullPath(olddirid) + "/" + oldname;

    var newid = this.Search(newdirid, newname);
    if (newid != -1) {
        const ret = this.Unlink(newdirid, newname);
        if(ret < 0) return ret;
    }

    var idx = oldid; // idx contains the id which we want to rename
    var inode = this.inodes[idx];
    const olddir = this.inodes[olddirid];
    const newdir = this.inodes[newdirid];

    if(!this.is_forwarder(olddir) && !this.is_forwarder(newdir))
    {
        // Move inode within current filesystem.

        this.unlink_from_dir(olddirid, oldname);
        this.link_under_dir(newdirid, idx, newname);

        inode.qid.version++;
    }
    else if(this.is_forwarder(olddir) && olddir.mount_id === newdir.mount_id)
    {
        // Move inode within the same child filesystem.

        const ret = await
            this.follow_fs(olddir).Rename(olddir.foreign_id, oldname, newdir.foreign_id, newname);

        if(ret < 0) return ret;
    }
    else if(this.is_a_root(idx))
    {
        // The actual inode is a root of some descendant filesystem.
        // Moving mountpoint across fs not supported - needs to update all corresponding forwarders.
        dbg_log("XXX: Attempted to move mountpoint (" + oldname + ") - skipped", LOG_9P);
        return -EPERM;
    }
    else if(!this.IsDirectory(idx) && this.GetInode(idx).nlinks > 1)
    {
        // Move hardlinked inode vertically in mount tree.
        dbg_log("XXX: Attempted to move hardlinked file (" + oldname + ") " +
                "across filesystems - skipped", LOG_9P);
        return -EPERM;
    }
    else
    {
        // Jump between filesystems.

        // Can't work with both old and new inode information without first diverting the old
        // information into a new idx value.
        const diverted_old_idx = this.divert(olddirid, oldname);
        const old_real_inode = this.GetInode(idx);

        const data = await this.Read(diverted_old_idx, 0, old_real_inode.size);

        if(this.is_forwarder(newdir))
        {
            // Create new inode.
            const foreign_fs = this.follow_fs(newdir);
            const foreign_id = this.IsDirectory(diverted_old_idx) ?
                foreign_fs.CreateDirectory(newname, newdir.foreign_id) :
                foreign_fs.CreateFile(newname, newdir.foreign_id);

            const new_real_inode = foreign_fs.GetInode(foreign_id);
            this.copy_inode(old_real_inode, new_real_inode);

            // Point to this new location.
            this.set_forwarder(idx, newdir.mount_id, foreign_id);
        }
        else
        {
            // Replace current forwarder with real inode.
            this.delete_forwarder(inode);
            this.copy_inode(old_real_inode, inode);

            // Link into new location in this filesystem.
            this.link_under_dir(newdirid, idx, newname);
        }

        // Rewrite data to newly created destination.
        await this.ChangeSize(idx, old_real_inode.size);
        if(data && data.length)
        {
            await this.Write(idx, 0, data.length, data);
        }

        // Move children to newly created destination.
        if(this.IsDirectory(idx))
        {
            for(const child_filename of this.GetChildren(diverted_old_idx))
            {
                const ret = await this.Rename(diverted_old_idx, child_filename, idx, child_filename);
                if(ret < 0) return ret;
            }
        }

        // Perform destructive changes only after migration succeeded.
        await this.DeleteData(diverted_old_idx);
        const ret = this.Unlink(olddirid, oldname);
        if(ret < 0) return ret;
    }

    this.NotifyListeners(idx, "rename", {oldpath: oldpath});

    return 0;
};

FS.prototype.Write = async function(id, offset, count, buffer) {
    this.NotifyListeners(id, 'write');
    var inode = this.inodes[id];

    if(this.is_forwarder(inode))
    {
        const foreign_id = inode.foreign_id;
        await this.follow_fs(inode).Write(foreign_id, offset, count, buffer);
        return;
    }

    var data = await this.get_buffer(id);

    if (!data || data.length < (offset+count)) {
        await this.ChangeSize(id, Math.floor(((offset+count)*3)/2));
        inode.size = offset + count;
        data = await this.get_buffer(id);
    } else
    if (inode.size < (offset+count)) {
        inode.size = offset + count;
    }
    if(buffer)
    {
        data.set(buffer.subarray(0, count), offset);
    }
    await this.set_data(id, data);
};

FS.prototype.Read = async function(inodeid, offset, count)
{
    const inode = this.inodes[inodeid];
    if(this.is_forwarder(inode))
    {
        const foreign_id = inode.foreign_id;
        return await this.follow_fs(inode).Read(foreign_id, offset, count);
    }

    return await this.get_data(inodeid, offset, count);
};

FS.prototype.Search = function(parentid, name) {
    const parent_inode = this.inodes[parentid];

    if(this.is_forwarder(parent_inode))
    {
        const foreign_parentid = parent_inode.foreign_id;
        const foreign_id = this.follow_fs(parent_inode).Search(foreign_parentid, name);
        if(foreign_id === -1) return -1;
        return this.get_forwarder(parent_inode.mount_id, foreign_id);
    }

    const childid = parent_inode.direntries.get(name);
    return childid === undefined ? -1 : childid;
};

FS.prototype.CountUsedInodes = function()
{
    let count = this.inodes.length;
    for(const { fs, backtrack } of this.mounts)
    {
        count += fs.CountUsedInodes();

        // Forwarder inodes don't count.
        count -=  backtrack.size;
    }
    return count;
};

FS.prototype.CountFreeInodes = function()
{
    let count = 1024 * 1024;
    for(const { fs } of this.mounts)
    {
        count += fs.CountFreeInodes();
    }
    return count;
};

FS.prototype.GetTotalSize = function() {
    let size = this.used_size;
    for(const { fs } of this.mounts)
    {
        size += fs.GetTotalSize();
    }
    return size;
    //var size = 0;
    //for(var i=0; i<this.inodes.length; i++) {
    //    var d = this.inodes[i].data;
    //    size += d ? d.length : 0;
    //}
    //return size;
};

FS.prototype.GetSpace = function() {
    let size = this.total_size;
    for(const { fs } of this.mounts)
    {
        size += fs.GetSpace();
    }
    return this.total_size;
};

/**
 * XXX: Not ideal.
 * @param {number} idx
 * @return {string}
 */
FS.prototype.GetDirectoryName = function(idx)
{
    const parent_inode = this.inodes[this.GetParent(idx)];

    if(this.is_forwarder(parent_inode))
    {
        return this.follow_fs(parent_inode).GetDirectoryName(this.inodes[idx].foreign_id);
    }

    // Root directory.
    if(!parent_inode) return "";

    for(const [name, childid] of parent_inode.direntries)
    {
        if(childid === idx) return name;
    }

    dbg_assert(false, "Filesystem: Found directory inode whose parent doesn't link to it");
    return "";
};

FS.prototype.GetFullPath = function(idx) {
    dbg_assert(this.IsDirectory(idx), "Filesystem: Cannot get full path of non-directory inode");

    var path = "";

    while(idx != 0) {
        path = "/" + this.GetDirectoryName(idx) + path;
        idx = this.GetParent(idx);
    }
    return path.substring(1);
};

/**
 * @param {number} parentid
 * @param {number} targetid
 * @param {string} name
 * @return {number} 0 if success, or -errno if failured.
 */
FS.prototype.Link = function(parentid, targetid, name)
{
    if(this.IsDirectory(targetid))
    {
        return -EPERM;
    }

    const parent_inode = this.inodes[parentid];
    const inode = this.inodes[targetid];

    if(this.is_forwarder(parent_inode))
    {
        if(!this.is_forwarder(inode) || inode.mount_id !== parent_inode.mount_id)
        {
            dbg_log("XXX: Attempted to hardlink a file into a child filesystem - skipped", LOG_9P);
            return -EPERM;
        }
        return this.follow_fs(parent_inode).Link(parent_inode.foreign_id, inode.foreign_id, name);
    }

    if(this.is_forwarder(inode))
    {
        dbg_log("XXX: Attempted to hardlink file across filesystems - skipped", LOG_9P);
        return -EPERM;
    }

    this.link_under_dir(parentid, targetid, name);
    return 0;
};

FS.prototype.Unlink = function(parentid, name) {
    if(name === "." || name === "..")
    {
        // Also guarantees that root cannot be deleted.
        return -EPERM;
    }
    const idx = this.Search(parentid, name);
    const inode = this.inodes[idx];
    const parent_inode = this.inodes[parentid];
    //message.Debug("Unlink " + inode.name);

    // forward if necessary
    if(this.is_forwarder(parent_inode))
    {
        dbg_assert(this.is_forwarder(inode), "Children of forwarders should be forwarders");

        const foreign_parentid = parent_inode.foreign_id;
        return this.follow_fs(parent_inode).Unlink(foreign_parentid, name);

        // Keep the forwarder dangling - file is still accessible.
    }

    if(this.IsDirectory(idx) && !this.IsEmpty(idx))
    {
        return -ENOTEMPTY;
    }

    this.unlink_from_dir(parentid, name);

    if(inode.nlinks === 0)
    {
        // don't delete the content. The file is still accessible
        inode.status = STATUS_UNLINKED;
        this.NotifyListeners(idx, 'delete');
    }
    return 0;
};

FS.prototype.DeleteData = async function(idx)
{
    const inode = this.inodes[idx];
    if(this.is_forwarder(inode))
    {
        await this.follow_fs(inode).DeleteData(inode.foreign_id);
        return;
    }
    inode.size = 0;
    delete this.inodedata[idx];
};

/**
 * @private
 * @param {number} idx
 * @return {!Promise<Uint8Array>} The buffer that contains the file contents, which may be larger
 *      than the data itself. To ensure that any modifications done to this buffer is reflected
 *      to the file, call set_data with the modified buffer.
 */
FS.prototype.get_buffer = async function(idx)
{
    const inode = this.inodes[idx];
    dbg_assert(inode, `Filesystem get_buffer: idx ${idx} does not point to an inode`);

    if(this.inodedata[idx])
    {
        return this.inodedata[idx];
    }
    else if(inode.status === STATUS_ON_STORAGE)
    {
        dbg_assert(inode.sha256sum, "Filesystem get_data: found inode on server without sha256sum");
        return await this.storage.read(inode.sha256sum, 0, inode.size);
    }
    else
    {
        return null;
    }
};

/**
 * @private
 * @param {number} idx
 * @param {number} offset
 * @param {number} count
 * @return {!Promise<Uint8Array>}
 */
FS.prototype.get_data = async function(idx, offset, count)
{
    const inode = this.inodes[idx];
    dbg_assert(inode, `Filesystem get_data: idx ${idx} does not point to an inode`);

    if(this.inodedata[idx])
    {
        return this.inodedata[idx].subarray(offset, offset + count);
    }
    else if(inode.status === STATUS_ON_STORAGE)
    {
        dbg_assert(inode.sha256sum, "Filesystem get_data: found inode on server without sha256sum");
        return await this.storage.read(inode.sha256sum, offset, count);
    }
    else
    {
        return null;
    }
};

/**
 * @private
 * @param {number} idx
 * @param {Uint8Array} buffer
 */
FS.prototype.set_data = async function(idx, buffer)
{
    // Current scheme: Save all modified buffers into local inodedata.
    this.inodedata[idx] = buffer;
    if(this.inodes[idx].status === STATUS_ON_STORAGE)
    {
        this.inodes[idx].status = STATUS_OK;
        this.storage.uncache(this.inodes[idx].sha256sum);
    }
};

/**
 * @param {number} idx
 * @return {!Inode}
 */
FS.prototype.GetInode = function(idx)
{
    dbg_assert(!isNaN(idx), "Filesystem GetInode: NaN idx");
    dbg_assert(idx >= 0 && idx < this.inodes.length, "Filesystem GetInode: out of range idx:" + idx);

    const inode = this.inodes[idx];
    if(this.is_forwarder(inode))
    {
        return this.follow_fs(inode).GetInode(inode.foreign_id);
    }

    return inode;
};

FS.prototype.ChangeSize = async function(idx, newsize)
{
    var inode = this.GetInode(idx);
    var temp = await this.get_data(idx, 0, inode.size);
    //message.Debug("change size to: " + newsize);
    if (newsize == inode.size) return;
    var data = new Uint8Array(newsize);
    inode.size = newsize;
    if(temp)
    {
        var size = Math.min(temp.length, inode.size);
        data.set(temp.subarray(0, size), 0);
    }
    await this.set_data(idx, data);
};

FS.prototype.SearchPath = function(path) {
    //path = path.replace(/\/\//g, "/");
    path = path.replace("//", "/");
    var walk = path.split("/");
    if (walk.length > 0 && walk[walk.length - 1].length === 0) walk.pop();
    if (walk.length > 0 && walk[0].length === 0) walk.shift();
    const n = walk.length;

    var parentid = -1;
    var id = 0;
    let forward_path = null;
    for(var i=0; i<n; i++) {
        parentid = id;
        id = this.Search(parentid, walk[i]);
        if(!forward_path && this.is_forwarder(this.inodes[parentid]))
        {
            forward_path = "/" + walk.slice(i).join("/");
        }
        if (id == -1) {
            if (i < n-1) return {id: -1, parentid: -1, name: walk[i], forward_path }; // one name of the path cannot be found
            return {id: -1, parentid: parentid, name: walk[i], forward_path}; // the last element in the path does not exist, but the parent
        }
    }
    return {id: id, parentid: parentid, name: walk[i], forward_path};
};
// -----------------------------------------------------

/**
 * @param {number} dirid
 * @param {Array<{parentid: number, name: string}>} list
 */
FS.prototype.GetRecursiveList = function(dirid, list) {
    if(this.is_forwarder(this.inodes[dirid]))
    {
        const foreign_fs = this.follow_fs(this.inodes[dirid]);
        const foreign_dirid = this.inodes[dirid].foreign_id;
        const mount_id = this.inodes[dirid].mount_id;

        const foreign_start = list.length;
        foreign_fs.GetRecursiveList(foreign_dirid, list);
        for(let i = foreign_start; i < list.length; i++)
        {
            list[i].parentid = this.get_forwarder(mount_id, list[i].parentid);
        }
        return;
    }
    for(const [name, id] of this.inodes[dirid].direntries)
    {
        if(name !== "." && name !== "..")
        {
            list.push({ parentid: dirid, name });
            if(this.IsDirectory(id))
            {
                this.GetRecursiveList(id, list);
            }
        }
    }
};

FS.prototype.RecursiveDelete = function(path) {
    var toDelete = [];
    var ids = this.SearchPath(path);
    if(ids.id === -1) return;

    this.GetRecursiveList(ids.id, toDelete);

    for(var i=toDelete.length-1; i>=0; i--)
    {
        const ret = this.Unlink(toDelete[i].parentid, toDelete[i].name);
        dbg_assert(ret === 0, "Filesystem RecursiveDelete failed at parent=" + toDelete[i].parentid +
            ", name='" + toDelete[i].name + "' with error code: " + (-ret));
    }
};

FS.prototype.DeleteNode = function(path) {
    var ids = this.SearchPath(path);
    if (ids.id == -1) return;

    if ((this.inodes[ids.id].mode&S_IFMT) == S_IFREG){
        const ret = this.Unlink(ids.parentid, ids.name);
        dbg_assert(ret === 0, "Filesystem DeleteNode failed with error code: " + (-ret));
        return;
    }
    if ((this.inodes[ids.id].mode&S_IFMT) == S_IFDIR){
        this.RecursiveDelete(path);
        const ret = this.Unlink(ids.parentid, ids.name);
        dbg_assert(ret === 0, "Filesystem DeleteNode failed with error code: " + (-ret));
        return;
    }
};

/** @param {*=} info */
FS.prototype.NotifyListeners = function(id, action, info) {
    //if(info==undefined)
    //    info = {};

    //var path = this.GetFullPath(id);
    //if (this.watchFiles[path] == true && action=='write') {
    //  message.Send("WatchFileEvent", path);
    //}
    //for (var directory of this.watchDirectories) {
    //    if (this.watchDirectories.hasOwnProperty(directory)) {
    //        var indexOf = path.indexOf(directory)
    //        if(indexOf == 0 || indexOf == 1)
    //            message.Send("WatchDirectoryEvent", {path: path, event: action, info: info});
    //    }
    //}
};


FS.prototype.Check = function() {
    for(var i=1; i<this.inodes.length; i++)
    {
        if (this.inodes[i].status == STATUS_INVALID) continue;

        var inode = this.GetInode(i);
        if (inode.nlinks < 0) {
            message.Debug("Error in filesystem: negative nlinks=" + inode.nlinks + " at id =" + i);
        }

        if(this.IsDirectory(i))
        {
            const inode = this.GetInode(i);
            if(this.IsDirectory(i) && this.GetParent(i) < 0) {
                message.Debug("Error in filesystem: negative parent id " + i);
            }
            for(const [name, id] of inode.direntries)
            {
                if(name.length === 0) {
                    message.Debug("Error in filesystem: inode with no name and id " + id);
                }

                for (const c of name) {
                    if (c < 32) {
                        message.Debug("Error in filesystem: Unallowed char in filename");
                    }
                }
            }
        }
    }

};


FS.prototype.FillDirectory = function(dirid) {
    const inode = this.inodes[dirid];
    if(this.is_forwarder(inode))
    {
        // XXX: The ".." of a mountpoint should point back to an inode in this fs.
        // Otherwise, ".." gets the wrong qid and mode.
        this.follow_fs(inode).FillDirectory(inode.foreign_id);
        return;
    }

    let size = 0;
    for(const name of inode.direntries.keys())
    {
        size += 13 + 8 + 1 + 2 + UTF8.UTF8Length(name);
    }
    const data = this.inodedata[dirid] = new Uint8Array(size);
    inode.size = size;

    let offset = 0x0;
    for(const [name, id] of inode.direntries)
    {
        const child = this.GetInode(id);
        offset += marshall.Marshall(
            ["Q", "d", "b", "s"],
            [child.qid,
            offset+13+8+1+2+UTF8.UTF8Length(name),
            child.mode >> 12,
            name],
            data, offset);
    }
};

FS.prototype.RoundToDirentry = function(dirid, offset_target)
{
    const data = this.inodedata[dirid];
    dbg_assert(data, `FS directory data for dirid=${dirid} should be generated`);
    dbg_assert(data.length, "FS directory should have at least an entry");

    if(offset_target >= data.length)
    {
        return data.length;
    }

    let offset = 0;
    while(true)
    {
        const next_offset = marshall.Unmarshall(["Q", "d"], data, { offset })[1];
        if(next_offset > offset_target) break;
        offset = next_offset;
    }

    return offset;
};

/**
 * @param {number} idx
 * @return {boolean}
 */
FS.prototype.IsDirectory = function(idx)
{
    const inode = this.inodes[idx];
    if(this.is_forwarder(inode))
    {
        return this.follow_fs(inode).IsDirectory(inode.foreign_id);
    }
    return (inode.mode & S_IFMT) === S_IFDIR;
};

/**
 * @param {number} idx
 * @return {boolean}
 */
FS.prototype.IsEmpty = function(idx)
{
    const inode = this.inodes[idx];
    if(this.is_forwarder(inode))
    {
        return this.follow_fs(inode).IsDirectory(inode.foreign_id);
    }
    for(const name of inode.direntries.keys())
    {
        if(name !== "." && name !== "..") return false;
    }
    return true;
};

/**
 * @param {number} idx
 * @return {!Array<string>} List of children names
 */
FS.prototype.GetChildren = function(idx)
{
    dbg_assert(this.IsDirectory(idx), "Filesystem: cannot get children of non-directory inode");
    const inode = this.inodes[idx];
    if(this.is_forwarder(inode))
    {
        return this.follow_fs(inode).GetChildren(inode.foreign_id);
    }
    const children = [];
    for(const name of inode.direntries.keys())
    {
        if(name !== "." && name !== "..")
        {
            children.push(name);
        }
    }
    return children;
};

/**
 * @param {number} idx
 * @return {number} Local idx of parent
 */
FS.prototype.GetParent = function(idx)
{
    dbg_assert(this.IsDirectory(idx), "Filesystem: cannot get parent of non-directory inode");

    const inode = this.inodes[idx];

    if(this.should_be_linked(inode))
    {
        return inode.direntries.get("..");
    }
    else
    {
        const foreign_dirid = this.follow_fs(inode).GetParent(inode.foreign_id);
        dbg_assert(foreign_dirid !== -1, "Filesystem: should not have invalid parent ids");
        return this.get_forwarder(inode.mount_id, foreign_dirid);
    }
};


// -----------------------------------------------------

// only support for security.capabilities
// should return a  "struct vfs_cap_data" defined in
// linux/capability for format
// check also:
//   sys/capability.h
//   http://lxr.free-electrons.com/source/security/commoncap.c#L376
//   http://man7.org/linux/man-pages/man7/capabilities.7.html
//   http://man7.org/linux/man-pages/man8/getcap.8.html
//   http://man7.org/linux/man-pages/man3/libcap.3.html
FS.prototype.PrepareCAPs = function(id) {
    var inode = this.GetInode(id);
    if (inode.caps) return inode.caps.length;
    inode.caps = new Uint8Array(20);
    // format is little endian
    // note: getxattr returns -EINVAL if using revision 1 format.
    // note: getxattr presents revision 3 as revision 2 when revision 3 is not needed.
    // magic_etc (revision=0x02: 20 bytes)
    inode.caps[0]  = 0x00;
    inode.caps[1]  = 0x00;
    inode.caps[2]  = 0x00;
    inode.caps[3]  = 0x02;

    // lower
    // permitted (first 32 capabilities)
    inode.caps[4]  = 0xFF;
    inode.caps[5]  = 0xFF;
    inode.caps[6]  = 0xFF;
    inode.caps[7]  = 0xFF;
    // inheritable (first 32 capabilities)
    inode.caps[8]  = 0xFF;
    inode.caps[9]  = 0xFF;
    inode.caps[10] = 0xFF;
    inode.caps[11] = 0xFF;

    // higher
    // permitted (last 6 capabilities)
    inode.caps[12] = 0x3F;
    inode.caps[13] = 0x00;
    inode.caps[14] = 0x00;
    inode.caps[15] = 0x00;
    // inheritable (last 6 capabilities)
    inode.caps[16] = 0x3F;
    inode.caps[17] = 0x00;
    inode.caps[18] = 0x00;
    inode.caps[19] = 0x00;

    return inode.caps.length;
};

// -----------------------------------------------------

/**
 * @constructor
 * @param {FS} filesystem
 */
function FSMountInfo(filesystem)
{
    /** @type {FS}*/
    this.fs = filesystem;

    /**
     * Maps foreign inode id back to local inode id.
     * @type {!Map<number,number>}
     */
    this.backtrack = new Map();
}

FSMountInfo.prototype.get_state = function()
{
    const state = [];

    state[0] = this.fs;
    state[1] = [...this.backtrack];

    return state;
};

FSMountInfo.prototype.set_state = function(state)
{
    this.fs = state[0];
    this.backtrack = new Map(state[1]);
};

/**
 * @private
 * @param {number} idx Local idx of inode.
 * @param {number} mount_id Mount number of the destination fs.
 * @param {number} foreign_id Foreign idx of destination inode.
 */
FS.prototype.set_forwarder = function(idx, mount_id, foreign_id)
{
    const inode = this.inodes[idx];

    dbg_assert(inode.nlinks === 0,
        "Filesystem: attempted to convert an inode into forwarder before unlinking the inode");

    if(this.is_forwarder(inode))
    {
        this.mounts[inode.mount_id].backtrack.delete(inode.foreign_id);
    }

    inode.status = STATUS_FORWARDING;
    inode.mount_id = mount_id;
    inode.foreign_id = foreign_id;

    this.mounts[mount_id].backtrack.set(foreign_id, idx);
};

/**
 * @private
 * @param {number} mount_id Mount number of the destination fs.
 * @param {number} foreign_id Foreign idx of destination inode.
 * @return {number} Local idx of newly created forwarder.
 */
FS.prototype.create_forwarder = function(mount_id, foreign_id)
{
    const inode = this.CreateInode();

    const idx = this.inodes.length;
    this.inodes.push(inode);
    inode.fid = idx;

    this.set_forwarder(idx, mount_id, foreign_id);
    return idx;
};

/**
 * @private
 * @param {Inode} inode
 * @return {boolean}
 */
FS.prototype.is_forwarder = function(inode)
{
    return inode.status === STATUS_FORWARDING;
};

/**
 * Whether the inode it points to is a root of some filesystem.
 * @private
 * @param {number} idx
 * @return {boolean}
 */
FS.prototype.is_a_root = function(idx)
{
    return this.GetInode(idx).fid === 0;
};

/**
 * Ensures forwarder exists, and returns such forwarder, for the described foreign inode.
 * @private
 * @param {number} mount_id
 * @param {number} foreign_id
 * @return {number} Local idx of a forwarder to described inode.
 */
FS.prototype.get_forwarder = function(mount_id, foreign_id)
{
    const mount = this.mounts[mount_id];

    dbg_assert(foreign_id >= 0, "Filesystem get_forwarder: invalid foreign_id: " + foreign_id);
    dbg_assert(mount, "Filesystem get_forwarder: invalid mount number: " + mount_id);

    const result = mount.backtrack.get(foreign_id);

    if(result === undefined)
    {
        // Create if not already exists.
        return this.create_forwarder(mount_id, foreign_id);
    }

    return result;
};

/**
 * @private
 * @param {Inode} inode
 */
FS.prototype.delete_forwarder = function(inode)
{
    dbg_assert(this.is_forwarder(inode), "Filesystem delete_forwarder: expected forwarder");

    inode.status = STATUS_INVALID;
    this.mounts[inode.mount_id].backtrack.delete(inode.foreign_id);
};

/**
 * @private
 * @param {Inode} inode
 * @return {FS}
 */
FS.prototype.follow_fs = function(inode)
{
    const mount = this.mounts[inode.mount_id];

    dbg_assert(this.is_forwarder(inode),
        "Filesystem follow_fs: inode should be a forwarding inode");
    dbg_assert(mount, "Filesystem follow_fs: inode<id=" + inode.fid +
        "> should point to valid mounted FS");

    return mount.fs;
};

/**
 * Mount another filesystem to given path.
 * @param {string} path
 * @param {FS} fs
 * @return {number} inode id of mount point if successful, or -errno if mounting failed.
 */
FS.prototype.Mount = function(path, fs)
{
    dbg_assert(fs.qidcounter === this.qidcounter,
        "Cannot mount filesystem whose qid numbers aren't synchronised with current filesystem.");

    const path_infos = this.SearchPath(path);

    if(path_infos.parentid === -1)
    {
        dbg_log("Mount failed: parent for path not found: " + path, LOG_9P);
        return -ENOENT;
    }
    if(path_infos.id !== -1)
    {
        dbg_log("Mount failed: file already exists at path: " + path, LOG_9P);
        return -EEXIST;
    }
    if(path_infos.forward_path)
    {
        const parent = this.inodes[path_infos.parentid];
        const ret = this.follow_fs(parent).Mount(path_infos.forward_path, fs);
        if(ret < 0) return ret;
        return this.get_forwarder(parent.mount_id, ret);
    }

    const mount_id = this.mounts.length;
    this.mounts.push(new FSMountInfo(fs));

    const idx = this.create_forwarder(mount_id, 0);
    this.link_under_dir(path_infos.parentid, idx, path_infos.name);

    return idx;
};

/**
 * @constructor
 */
function FSLockRegion()
{
    this.type = P9_LOCK_TYPE_UNLCK;
    this.start = 0;
    this.length = Infinity;
    this.proc_id = -1;
    this.client_id = "";
}

FSLockRegion.prototype.get_state = function()
{
    const state = [];

    state[0] = this.type;
    state[1] = this.start;
    // Infinity is not JSON.stringify-able
    state[2] = this.length === Infinity ? 0 : this.length;
    state[3] = this.proc_id;
    state[4] = this.client_id;

    return state;
};

FSLockRegion.prototype.set_state = function(state)
{
    this.type = state[0];
    this.start = state[1];
    this.length = state[2] === 0 ? Infinity : state[2];
    this.proc_id = state[3];
    this.client_id = state[4];
};

/**
 * @return {FSLockRegion}
 */
FSLockRegion.prototype.clone = function()
{
    const new_region = new FSLockRegion();
    new_region.set_state(this.get_state());
    return new_region;
};

/**
 * @param {FSLockRegion} region
 * @return {boolean}
 */
FSLockRegion.prototype.conflicts_with = function(region)
{
    if(this.proc_id === region.proc_id && this.client_id === region.client_id) return false;
    if(this.type === P9_LOCK_TYPE_UNLCK || region.type === P9_LOCK_TYPE_UNLCK) return false;
    if(this.type !== P9_LOCK_TYPE_WRLCK && region.type !== P9_LOCK_TYPE_WRLCK) return false;
    if(this.start + this.length <= region.start) return false;
    if(region.start + region.length <= this.start) return false;
    return true;
};

/**
 * @param {FSLockRegion} region
 * @return {boolean}
 */
FSLockRegion.prototype.is_alike = function(region)
{
    return region.proc_id === this.proc_id &&
        region.client_id === this.client_id &&
        region.type === this.type;
};

/**
 * @param {FSLockRegion} region
 * @return {boolean}
 */
FSLockRegion.prototype.may_merge_after = function(region)
{
    return this.is_alike(region) && region.start + region.length === this.start;
};

/**
 * @param {number} type
 * @param {number} start
 * @param {number} length
 * @param {number} proc_id
 * @param {string} client_id
 * @return {!FSLockRegion}
 */
FS.prototype.DescribeLock = function(type, start, length, proc_id, client_id)
{
    dbg_assert(type === P9_LOCK_TYPE_RDLCK ||
        type === P9_LOCK_TYPE_WRLCK ||
        type === P9_LOCK_TYPE_UNLCK,
        "Filesystem: Invalid lock type: " + type);
    dbg_assert(start >= 0, "Filesystem: Invalid negative lock starting offset: " + start);
    dbg_assert(length > 0, "Filesystem: Invalid non-positive lock length: " + length);

    const lock = new FSLockRegion();
    lock.type = type;
    lock.start = start;
    lock.length = length;
    lock.proc_id = proc_id;
    lock.client_id = client_id;

    return lock;
};

/**
 * @param {number} id
 * @param {FSLockRegion} request
 * @return {FSLockRegion} The first conflicting lock found, or null if requested lock is possible.
 */
FS.prototype.GetLock = function(id, request)
{
    const inode = this.inodes[id];

    if(this.is_forwarder(inode))
    {
        const foreign_id = inode.foreign_id;
        return this.follow_fs(inode).GetLock(foreign_id, request);
    }

    for(const region of inode.locks)
    {
        if(request.conflicts_with(region))
        {
            return region.clone();
        }
    }
    return null;
};

/**
 * @param {number} id
 * @param {FSLockRegion} request
 * @param {number} flags
 * @return {number} One of P9_LOCK_SUCCESS / P9_LOCK_BLOCKED / P9_LOCK_ERROR / P9_LOCK_GRACE.
 */
FS.prototype.Lock = function(id, request, flags)
{
    const inode = this.inodes[id];

    if(this.is_forwarder(inode))
    {
        const foreign_id = inode.foreign_id;
        return this.follow_fs(inode).Lock(foreign_id, request, flags);
    }

    request = request.clone();

    // (1) Check whether lock is possible before any modification.
    if(request.type !== P9_LOCK_TYPE_UNLCK && this.GetLock(id, request))
    {
        return P9_LOCK_BLOCKED;
    }

    // (2) Subtract requested region from locks of the same owner.
    for(let i = 0; i < inode.locks.length; i++)
    {
        const region = inode.locks[i];

        dbg_assert(region.length > 0,
            "Filesystem: Found non-positive lock region length: " + region.length);
        dbg_assert(region.type === P9_LOCK_TYPE_RDLCK || region.type === P9_LOCK_TYPE_WRLCK,
            "Filesystem: Found invalid lock type: " + region.type);
        dbg_assert(!inode.locks[i-1] || inode.locks[i-1].start <= region.start,
            "Filesystem: Locks should be sorted by starting offset");

        // Skip to requested region.
        if(region.start + region.length <= request.start) continue;

        // Check whether we've skipped past the requested region.
        if(request.start + request.length <= region.start) break;

        // Skip over locks of different owners.
        if(region.proc_id !== request.proc_id || region.client_id !== request.client_id)
        {
            dbg_assert(!region.conflicts_with(request),
                "Filesytem: Found conflicting lock region, despite already checked for conflicts");
            continue;
        }

        // Pretend region would be split into parts 1 and 2.
        const start1 = region.start;
        const start2 = request.start + request.length;
        const length1 = request.start - start1;
        const length2 = region.start + region.length - start2;

        if(length1 > 0 && length2 > 0 && region.type === request.type)
        {
            // Requested region is already locked with the required type.
            // Return early - no need to modify anything.
            return P9_LOCK_SUCCESS;
        }

        if(length1 > 0)
        {
            // Shrink from right / first half of the split.
            region.length = length1;
        }

        if(length1 <= 0 && length2 > 0)
        {
            // Shrink from left.
            region.start = start2;
            region.length = length2;
        }
        else if(length2 > 0)
        {
            // Add second half of the split.

            // Fast-forward to correct location.
            while(i < inode.locks.length && inode.locks[i].start < start2) i++;

            inode.locks.splice(i, 0,
                this.DescribeLock(region.type, start2, length2, region.proc_id, region.client_id));
        }
        else if(length1 <= 0)
        {
            // Requested region completely covers this region. Delete.
            inode.locks.splice(i, 1);
            i--;
        }
    }

    // (3) Insert requested lock region as a whole.
    // No point in adding the requested lock region as fragmented bits in the above loop
    // and having to merge them all back into one.
    if(request.type !== P9_LOCK_TYPE_UNLCK)
    {
        let new_region = request;
        let has_merged = false;
        let i = 0;

        // Fast-forward to requested position, and try merging with previous region.
        for(; i < inode.locks.length; i++)
        {
            if(new_region.may_merge_after(inode.locks[i]))
            {
                inode.locks[i].length += request.length;
                new_region = inode.locks[i];
                has_merged = true;
            }
            if(request.start <= inode.locks[i].start) break;
        }

        if(!has_merged)
        {
            inode.locks.splice(i, 0, new_region);
            i++;
        }

        // Try merging with the subsequent alike region.
        for(; i < inode.locks.length; i++)
        {
            if(!inode.locks[i].is_alike(new_region)) continue;

            if(inode.locks[i].may_merge_after(new_region))
            {
                new_region.length += inode.locks[i].length;
                inode.locks.splice(i, 1);
            }

            // No more mergable regions after this.
            break;
        }
    }

    return P9_LOCK_SUCCESS;
};

FS.prototype.read_dir = function(path)
{
    const p = this.SearchPath(path);

    if(p.id === -1)
    {
        return undefined;
    }

    const dir = this.GetInode(p.id);

    return Array.from(dir.direntries.keys()).filter(path => path !== "." && path !== "..");
};

FS.prototype.read_file = function(file)
{
    const p = this.SearchPath(file);

    if(p.id === -1)
    {
        return Promise.resolve(null);
    }

    const inode = this.GetInode(p.id);

    return this.Read(p.id, 0, inode.size);
};