Enable quick and complete cancellation of running searches

[Jaifroid]

This is proof-of-concept code for the first part of the PR title (cancellation of running searches). It implements #637 and is in draft, for testing.

Because I believe the techniques and solution are the same, I intend to address #426 in this PR as well.

The code currently contains console log reports to prove that it cancels title search completely. They will be removed at a later stage.

[Jaifroid]

Here is the output of console log running the above on two title searches in succession. The first is for "unesco world heritage site"; after a short time this is quickly replaced with "unesco world heritage". The former search is complex with many variations searched, and it is replaced by a simpler search.

Asterisks indicate where the first search was cancelled, and where the cancellation was registered - note that it is after the global search object has been changed, which proves that the running search object in zimArchive.js updates its status property to "cancelled" when it is modified in the global search object. When the global search object is completely replaced, the zimArchive search continues to point to the old search object that it was launched with (which has a modified status of "cancelled").

Hence, we have an effective signalling mechanism without recourse to "super-global" objects, and it is sufficient to pass all the needed information to zimArchive in a single appstate.search object.

[Jaifroid]

Having looked back over this, I now believe it is too ambitious to address #426 in this PR. However, it implements #637 and could be merged as an easy PR (after removal of console.log entries).

[Jaifroid]

I'm listing here the stages I mentioned in #637. I believe they have all been implemented, but I'll tick them off as I check through them before removing Draft status for this PR:

• Move globalstate object from init.js to app.js; possibly rename appstate;
• Pass the appstate.search object to zimArchive.js;
• When a search is cancelled, set the appstate.search.status property to 'cancelled';
• When a new search is undertaken, also set appstate.search.status to 'cancelled': this will be reflected in the search object passed to zimArchive.js because they both still point to the same underlying object;
• Instantiate a new search object, and set appstate.search to this new object; the one passed to zimArchive.js will still point to the old object according to Javascript Pointer Reference craziness;
• Insert an early cancellation at the very start of the binary search loop: if a search status is cancelled, return 0 instead of -1 or +1 -- this causes the loop to exit early;
• Once all Promises have returned, the old search object is no longer referenced, and should be garbage collected automatically in due course.

[Jaifroid]

@mossroy I have checked over the code (see checkboxes above) and added a short technical explanation of how the object assignment works between the different scopes, as it may be useful to my future self or other future devs encountering the code. But otherwise, this is ready for review. It effectively completes #617 (which was merged despite the known issue) by implementing #637.

NB I have left in (for now) console.log output that verifies precisely what is happening and that the old search is cancelled in its own scope as soon as possible. If you wish to test, follow this procedure: open a large ZIM like Wikipedia English all, and search for something that will take a fairly long time to search for in all its permutations, e.g. "unesco world heritage site". (use all lowercase) After a few seconds quickly backspace in the search field to delete "site" so that a new search is launched. Observe console.log entries as per screenshot above.

I will delete diagnostic console logging before squash/merge once the code is approved.

[mossroy]

I'm not forgetting this PR, and will review it as soon as I can.
But it's hard for me to find some time

[Jaifroid]

No rush, I sympathize -- things going crazy here too.

[mossroy]

It seems to be working well on my FirefoxOS device.
I added a few minor remarks