I don't think we should worry about underlying file changes. If the app wants to cut a file into parts and copy them separately, then perhaps the app should first copy the file into a private area. (I'm presuming that one day, we'll have the concept of a chroot'd private file storage area for a web app.)
I think we should avoid solutions that involve file locking since it is bad for the user (loss of control) if their files are locked by the browser on behalf of a web app. It might be reasonable, however, to lock a file while sending it. -Darin On Thu, Jan 14, 2010 at 2:41 PM, Jian Li <jia...@chromium.org> wrote: > It seems that we feel that when a File object is sent via either Form or > XHR, the latest underlying version should be used. When we get a slice via > Blob.slice, we assume that the underlying file data is stable since then. > > So for uploader scenario, we need to cut a big file into multiple pieces. > With current File API spec, we will have to do something like the following > to make sure that all pieces are cut from a stable file. > var file = myInputElement.files[0]; > var blob = file.slice(0, file.size); > var piece1 = blob.slice(0, 1000); > var piece2 = blob.slice(1001, 1000); > ... > > The above seems a bit ugly. If we want to make it clean, what Dmitry > proposed above seems to be reasonable. But it would require non-trivial spec > change. > > > On Wed, Jan 13, 2010 at 11:28 AM, Dmitry Titov <dim...@chromium.org>wrote: > >> Atomic read is obviously a nice thing - it would be hard to program >> against API that behaves as unpredictably as a single read operation that >> reads half of old content and half of new content. >> >> At the same note, it would be likely very hard to program against Blob >> objects if they could change underneath unpredictably. Imagine that we need >> to build an uploader that cuts a big file in multiple pieces and sends those >> pieces to the servers so they will be stitched together later. If during >> this operation the underlying file changes and this changes all the pieces >> that Blobs refer to (due to clamping and just silent change of content), all >> the slicing/stitching assumptions are invalid and it's hard to even notice >> since blobs are simply 'clamped' silently. Some degree of mess is possible >> then. >> >> Another use case could be a JPEG image processor that uses slice() to cut >> the headers from the image file and then uses info from the headers to cut >> further JFIF fields from the file (reading EXIF and populating local >> database of images for example). Changing the file in the middle of that is >> bad. >> >> It seems the typical use cases that will need Blob.slice() functionality >> form 'units of work' where Blob.slice() is used with likely assumption that >> underlying data is stable and does not change silently. Such a 'unit of >> work' should fail as a whole if underlying file changes. One way to achieve >> that is to reliably fail operations with 'derived' Blobs and even perhaps >> have a 'isValid' property on it. 'Derived' Blobs are those obtained via >> slice(), as opposite to 'original' Blobs that are also File. >> >> One disadvantage of this approach is that it implies that the same Blob >> has 2 possible behaviors - when it is obtained via Blob.slice() (or other >> methods) vs is a File. >> >> It all could be a bit cleaner if File did not derive from Blob, but >> instead had getAsBlob() method - then it would be possible to say that Blobs >> are always immutable but may become 'invalid' over time if underlying data >> changes. The FileReader can then be just a BlobReader and have cleaner >> semantics. >> >> If that was the case, then xhr.send(file) would capture the state of file >> at the moment of sending, while xhr.send(blob) would fail with exception if >> the blob is 'invalid' at the moment of send() operation. This would keep >> compatibility with current behavior and avoid duplicity of Blob behavior. >> Quite a change to the spec though... >> >> Dmitry >> >> On Wed, Jan 13, 2010 at 2:38 AM, Jonas Sicking <jo...@sicking.cc> wrote: >> >>> On Tue, Jan 12, 2010 at 5:28 PM, Chris Prince <cpri...@google.com> >>> wrote: >>> >> For the record, I'd like to make the read "atomic", such that you can >>> >> never get half a file before a change, and half after. But it likely >>> >> depends on what OSs can enforce here. >>> > >>> > I think *enforcing* atomicity is difficult across all OSes. >>> > >>> > But implementations can get nearly the same effect by checking the >>> > file's last modification time at the start + end of the API call. If >>> > it has changed, the read operation can throw an exception. >>> >>> I'm talking about during the actual read. I.e. not related to the >>> lifetime of the File object, just related to the time between the >>> first 'progress' event, and the 'loadend' event. If the file changes >>> during this time there is no way to fake atomicity since the partial >>> file has already been returned. >>> >>> / Jonas >>> >> >> >
