From pete: we should simply delete the saved page if we detect that we cancelled in the middle.

Perhaps handling duplicate IDs could be another way to solve this. Once started, it's hard to cleanly and reliably prevent the save from happening.

To clarify a bit:  My suggestion is that we remember we got a cancel request while in the middle of saving a page, and then when the save page call returns, we should delete the archived page then.

This bug has unclear initial description - what exactly are the main issues?

Today if a user cancels an offlining request via a notification, and the downloading has completed, but the request is in the saving stage, the request to cancel is dropped, and the request is not actually canceled.  Since we can't actually stop the saving while it is in progress, we should remember that the user asked us to cancel, and when the saving to a MHTML file step is done, we should delete the file.

I see. Interesting that since save happens async and some parts are done on file thread, the cancelable save would have to do a very similar trick - try to cancel, then delete the file (and perhaps a DB record) if the cancel did not catch.

Note that we should be able to make changes to MHTMLGenerationManager and the saving call chain to allow for cancellations, especially as for now we are sending one IPC per frame to the renderer(s). I can help with context and ideas on how to do so.

If/when we move to one IPC per renderer (that may hold many frames) this cancellation will get trickier (require at least one more IPC tot he current renderer serializing its contents).

I think short term solution should be we delete the page that was saved if cancel was called in the middle. In the background loading case, this will allow the request to save subsequently without running into ALREADY_EXISTS errors

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2915191f0a64f34fcdf8c4e4a0a5d89b2271d816

commit 2915191f0a64f34fcdf8c4e4a0a5d89b2271d816
Author: petewil <petewil@chromium.org>
Date: Tue May 02 00:51:09 2017

If MHTML saving is cancelled, delete the page afterwards.

BUG= 705097 

Review-Url: https://codereview.chromium.org/2850943002
Cr-Commit-Position: refs/heads/master@{#468515}

[modify] https://crrev.com/2915191f0a64f34fcdf8c4e4a0a5d89b2271d816/chrome/browser/android/offline_pages/background_loader_offliner.cc
[modify] https://crrev.com/2915191f0a64f34fcdf8c4e4a0a5d89b2271d816/chrome/browser/android/offline_pages/background_loader_offliner.h
[modify] https://crrev.com/2915191f0a64f34fcdf8c4e4a0a5d89b2271d816/chrome/browser/android/offline_pages/background_loader_offliner_unittest.cc