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Request for feedback: Filesystem API

From: Jonas Sicking <jonas@sicking.cc>
Date: Fri, 9 Aug 2013 14:02:49 -0700
Message-ID: <CA+c2ei9d0M0d9W=RVnb6=_Q3reYvwZSYy0wGPVYgEWuwGVjAXA@mail.gmail.com>
To: "public-script-coord@w3.org" <public-script-coord@w3.org>
Over the past few months a few of us at mozilla, with input from a lot
of other people, has been iterating on a filesystem API. The goal of
this filesystem API is first and foremost to expose a sandboxed
filesystem to webpages. This filesystem would be origin-specific and
would not allow accessing the user's OS filesystem. This avoids a lot
of the security concerns around filesystem APIs.

However it is expected that this API will eventually also be used for
accessing real filesystems eventually, but there are a lot of security
concerns that needs to be solved before we can create a real standard
for that. Hence that is not the topic of this email.

API summary:

The proposed API introduces two new abstractions: A Directory object
which allows manipulating files and directories within it, and a
FileHandle object which allows holding an exclusive lock on a file
while performing multiple read/write operations on it.

The API intentionally reuses the already existing File abstraction as
defined by [1] as we didn't want to have two different primitives for
"a file". The File object has already been shipping in browsers for a
while, so it's not an API that we expect to be able to make backwards
incompatible changes to, which somewhat limits the design of the
proposed filesystem API.

Only adding two new abstractions was very intentional. We wanted to
keep the API as small and simple as possible. So for example there is
no abstraction for "a filesystem". Instead we simply let the root
directory represent the filesystem.

The API is entirely asynchronous since we don't expect implementations
to be able to keep the whole filesystem in memory, and we don't want
to force synchronous IO. But we've still tried to keep the API as
friendly as possible.

Detailed API:

Apologies for using WebIDL here. I know it's not very popular with a
lot of people on this list. And it's especially unfortunate in this
API since the use of WebIDL to describe the API results in a lot of
extra syntax in the description which doesn't actually affect the
javascript that developers would write.

Unfortunately I don't know of any other formal way of describing the
API without spending tons of time typing up long descriptions of each

partial interface Navigator {
  // This is what provides access to the sandboxed filesystem root.
  Promise<Directory> getFilesystem(optional FilesystemParameters parameters);

interface Directory {
  readonly attribute DOMString name;

  Promise<File> createFile(DOMString path,
                           CreateFileOptions options);
  Promise<Directory> createDirectory(DOMString path);

  Promise<(File or Directory)> get(DOMString path);

    move((DOMString or File or Directory) path,
         (DOMString or Directory or DestinationDict) dest);
    copy((DOMString or File or Directory) path,
         (DOMString or Directory or DestinationDict) dest);
  Promise<boolean> remove((DOMString or File or Directory) path);
  Promise<boolean> removeDeep((DOMString or File or Directory) path);

  Promise<FileHandle> openRead((DOMString or File) path);
  Promise<FileHandleWritable> openWrite((DOMString or File) path,
        OpenWriteOptions options);

  EventStream<(File or Directory)> enumerate(optional DOMString path);
  EventStream<File> enumerateDeep(optional DOMString path);

interface FileHandle
  readonly attribute FileOpenMode mode;
  readonly attribute boolean active;

  attribute long long? offset;

  Promise<File> getFile();
ArrayBuffer> read(unsigned long long size);
  AbortableProgressPromise<DOMString> readText(unsigned long long
size, optional DOMString encoding = "utf-8");

  void abort();

interface FileHandleWritable : FileHandle
  AbortableProgressPromise<void> write((DOMString or ArrayBuffer or
ArrayBufferView or Blob) value);

  Promise<void> setSize(optional unsigned long long size);

  Promise<void> flush();

partial interface URL {
  static DOMString? getPersistentURL(File file);

// WebIDL cruft that's largely transparent
enum StorageType { "temporary", "persistent" };
dictionary FilesystemParameters {
  StorageType storage = "temporary";

dictionary CreateFileOptions {
  CreateIfExistsMode ifExists = "fail";
  (DOMString or Blob or ArrayBuffer or ArrayBufferView) data;

dictionary OpenWriteOptions {
  OpenIfNotExistsMode ifNotExists = "create";
  OpenIfExistsMode ifExists = "open";

enum CreateIfExistsMode { "replace", "fail" };
enum OpenIfExistsMode { "open", "fail" };
enum OpenIfNotExistsMode { "create", "fail" };

dictionary DestinationDict {
  Directory dir;
  DOMString name;

enum FileOpenMode { "readonly", "readwrite" };

API Description:

I won't go into the details about each function as it's hopefully
mostly obvious. A few general comments:

The functions on Directory that accept DOMString arguments for
filenames allow names like "path/to/file.txt". If the function creates a
file, then it creates the intermediate directories. Such paths are
always interpreted as relative to the directory itself, never relative
to the root.

We were thinking of *not* allowing paths that walk up the directory
tree. So paths like "../foo", "..", "/foo/bar" or "foo/../bar" are not
allowed. This to keep things simple and avoid security issues for the
page. Attempting to use a path that contains a segment that is equal
to ".." or ".", or any path which starts with "/" will cause an error.
This way we can add support for this later if desired.

Likewise, passing a File object to an operation of Directory where the
File object isn't contained in that directory or its descendents also
results in an error.

One thing that is probably not obvious is how the FileHandle.location
attribute works. This attribute is used by the read/readText/write
functions to select where the read or write operation starts. When
.read is called, it uses the current value of .location to determine
where the reading starts. It then fires off an asynchronous read
operation. It finally synchronously increases .location by the amount
of the 'size' argument before returning. Same thing for .write() and

This means that the caller can simply set .location and then fire off
multiple read or write operations which automatically will happen
staggered in the file. It also means that the caller can set the
location for next operation by simply setting .location, or can check
the current location by simply getting .location.

Setting .offset to null means "go to the end". This is why there is no
openAppend function. Calling openWrite and then setting .offset to
null before writing results in an append.

Note that getting or setting .offset does not need to synchronously
call seek, or do any IO operations, in the implementation. Instead the
implementation simply tracks .offset in the API implementation.
Whenever a read or write operation is scheduled, the current .offset
is sent along with the operation information to the IO thread and the
seek can happen there. Many times the implementation can optimize out
the seek entirely.

The FileHandle class automatically closes itself as soon as the page
stops posting further calls to .read/.readBinary/.write to it. This
happens once the last Promise returned from one of those operations
has been resolved, without further calls to .read/.readBinary/.write
having happened. This is similar to IDB transactions, though obviously
there are no transactional semantics here. I.e. there is no way to
roll back any changes.

Open Questions:

There are a few things that we did have disagreements on and which
would be worth debating.

Is the setup around the FileHandle.offset attribute a good idea? Some
people found it confusingly different from posix.

Can we get rid of the the non-recursive remove() function. The
removeRecusive() function has the same capabilities, except that
removeRecusive doesn't produce an error if you attempt to delete a
non-empty directory.

Can we get rid of the copy() function? Copy operations are certainly
common to expose in UIs, but they can be easily implemented
programmatically, so having it in the API isn't strictly needed.

Should we add an openAppend function which always appends for all
writes. Note that since FileHandle always holds an exclusive lock on
the file, there is no risk that other actors will append to the file
as long as a FileHandle is being used.

Finally, should we remove the Directory abstraction? It's not needed
given that you can directly interact with files in subdirectories. But
it does provide the ability to do some capability management. I.e.
holding a Directory object enables you to interact with the files in
that directory and its subdirectories, but there is no way to reach
out to a parent directory. Directory objects also is a familiar
concept in filesystem APIs, so it seems natural to have it even though
it's not strictly needed.

[1] http://dev.w3.org/2006/webapi/FileAPI/

/ Jonas
Received on Friday, 9 August 2013 21:03:51 UTC

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