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Re: Request for feedback: Filesystem API

From: Jonas Sicking <jonas@sicking.cc>
Date: Fri, 9 Aug 2013 15:15:20 -0700
Message-ID: <CA+c2ei-YHQ0BG4QDeVuzZwmPsUaEjaAQkg8Ysr2VpBTOQEiDZw@mail.gmail.com>
To: "public-script-coord@w3.org" <public-script-coord@w3.org>
On Fri, Aug 9, 2013 at 2:02 PM, Jonas Sicking <jonas@sicking.cc> wrote:
> 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
> function.
>
> 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);
>
>   AbortableProgressPromise<void>
>     move((DOMString or File or Directory) path,
>          (DOMString or Directory or DestinationDict) dest);
>   AbortableProgressPromise<void>
>     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();
>   AbortableProgressPromise<
> 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
> .readText().
>
> 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/

After all that, of course I forgot to include examples of what the API
looks like when used.

// Save some downloaded data into a new file:
navigator.getFilesystem().then(function(root) {
  root.createFile("myfile.txt", { data: xhr.response });
});

// Append 5 bytes to the end of a large existing file:
navigator.getFilesystem().then(function(root) {
  return root.openWrite("largefile.dat");
}).then(function(handle) {
  handle.offset = null;
  return handle.write(new Uint8Array([1, 1, 2, 3, 5]));
});

// Increase the 100th byte in large existing file:
var fileHandle;
navigator.getFilesystem().then(function(root) {
  return root.openWrite("dir/highscores");
}).then(function(handle) {
  fileHandle = handle;
  fileHandle.offset = 100;
  return fileHandle.read(1);
}).then(function(buffer) {
  assert(buffer.byteLength === 1);
  var view = new Uint8Array(buffer);
  view[0]++;
  fileHandle.location--;
  return handle.write(buffer);
});

/ Jonas
Received on Friday, 9 August 2013 22:16:18 UTC

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