iOS14-indexeddb.md

This is an explanation of the root cause for WebKit bug 216962.

Okay, so WebKit wants to move from per-ObjectStore Index IDs to global Index IDs.

Our current indices table (in iOS 13) looks like this:

id	name	objectStoreID
1	uuid	1
2	date	1
1	uuid	2
2	time	2
3	object_uuid	2
4	created_at	2
1	uuid	3
2	name	3
1	uuid	4
2	sort_order	4

This code loops over all the existing indices and picks global IDs for them: https://github.com/WebKit/webkit/blob/37a90a973bb2dbfe5f55e8df2db2ac5a06423a87/Source/WebCore/Modules/indexeddb/server/SQLiteIDBBackingStore.cpp#L822-L828

So, in our case, that map looks something like this:

{ 1, 1 } = 1
{ 1, 2 } = 2
{ 2, 1 } = 3
{ 2, 2 } = 4
{ 2, 3 } = 5
{ 2, 4 } = 6
{ 3, 1 } = 7
{ 3, 2 } = 8
{ 4, 1 } = 9
{ 4, 2 } = 10

Now, we loop over each entry in that map and run the following:

UPDATE IndexInfo SET id = {map.value} WHERE id = {map.key[0]} AND objectStoreID = {map.key[1]};

Let's walk through this...

First Loop

Basically no change.

UPDATE IndexInfo SET id = 1 WHERE id = 1 AND objectStoreID = 1;

	id	name	objectStoreID
🟊	1	uuid	1
	2	date	1
	1	uuid	2
	2	time	2
	3	object_uuid	2
	4	created_at	2
	1	uuid	3
	2	name	3
	1	uuid	4
	2	sort_order	4

Second Loop

Also basically no change.

UPDATE IndexInfo SET id = 2 WHERE id = 2 AND objectStoreID = 1;

	id	name	objectStoreID
	1	uuid	1
🟊	2	date	1
	1	uuid	2
	2	time	2
	3	object_uuid	2
	4	created_at	2
	1	uuid	3
	2	name	3
	1	uuid	4
	2	sort_order	4

Third Loop

This is where things start changing...

UPDATE IndexInfo SET id = 3 WHERE id = 1 AND objectStoreID = 2;

	id	name	objectStoreID
	1	uuid	1
	2	date	1
🟊	3	uuid	2
	2	time	2
	3	object_uuid	2
	4	created_at	2
	1	uuid	3
	2	name	3
	1	uuid	4
	2	sort_order	4

Fourth Loop

Things are still working as expected here...

UPDATE IndexInfo SET id = 4 WHERE id = 2 AND objectStoreID = 2;

	id	name	objectStoreID
	1	uuid	1
	2	date	1
	3	uuid	2
🟊	4	time	2
	3	object_uuid	2
	4	created_at	2
	1	uuid	3
	2	name	3
	1	uuid	4
	2	sort_order	4

Fifth Loop

This is where we run into our first bit of index corruption

UPDATE IndexInfo SET id = 5 WHERE id = 3 AND objectStoreID = 2;

	id	name	objectStoreID
	1	uuid	1
	2	date	1
🟊	5	uuid	2
	4	time	2
🟊	5	object_uuid	2
	4	created_at	2
	1	uuid	3
	2	name	3
	1	uuid	4
	2	sort_order	4

Sixth Loop

This is where the second index gets corrupted

UPDATE IndexInfo SET id = 6 WHERE id = 4 AND objectStoreID = 2;

	id	name	objectStoreID
	1	uuid	1
	2	date	1
	5	uuid	2
🟊	6	time	2
	5	object_uuid	2
🟊	6	created_at	2
	1	uuid	3
	2	name	3
	1	uuid	4
	2	sort_order	4

Seventh Loop

Now it continues on normally

UPDATE IndexInfo SET id = 7 WHERE id = 1 AND objectStoreID = 3;

	id	name	objectStoreID
	1	uuid	1
	2	date	1
	5	uuid	2
	6	time	2
	5	object_uuid	2
	6	created_at	2
🟊	7	uuid	3
	2	name	3
	1	uuid	4
	2	sort_order	4

End Result

We have corrupted the index IDs and now our objectStore is broken.

	id	name	objectStoreID
	1	uuid	1
	2	date	1
🡂	5	uuid	2
🡂	6	time	2
	5	object_uuid	2
	6	created_at	2
	7	uuid	3
	8	name	3
	9	uuid	4
	10	sort_order	4