[core] Tile prefetching no longer works

[tmpsantos]

Tile prefetch (prefetching and rendering lower resolution tiles until we get tiles for the current zoom level, avoiding blank spots on the map) is no longer working. I bisect the regression to 10a4405.

This is probably why the Map.PrefetchTiles is failing. Sadly seems like the test doesn't always fail, so it would have detected the regression when it was introduced (my bad).

It is easy to reproduce and inspect visually, as one commit before the regression we see that lower resolution tiles are loaded first:

Mapbox GL at 10a4405:

Mapbox GL at bfa4cea:

/cc @asheemmamoowala @Guardiola31337 @kkaefer

[ChrisLoer]

@asheemmamoowala I just saw two CI failures in Map.PrefetchTiles off of master -- any ideas?

A 'heap-buffer-overflow' on clang4-sanitize-address:

https://circleci.com/gh/mapbox/mapbox-gl-native/76253?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

An unexpected empty tile set on gcc 4.9:

https://circleci.com/gh/mapbox/mapbox-gl-native/76255?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

[asheemmamoowala]

I debugged the Map.PrefetchTiles test, and don't see it hitting any of the code introduced in 10a4405.

I'm seeing a few issues:

1. Map.PrefetchTiles sets the map mode to Static, but TilePyramid's prefetch-ing is conditional on map mode being Continuous. [core] Allow prefetching tiles for all source types #10548 added the check in TilePyramid. This test should not have passed since that change.

2. The std::is_permutation check seems to be passing even when it shouldn't. According to the docs:

Returns true if there exists a permutation of the elements in the range [first1, last1) that makes that range equal to the range [first2,last2), where last2 denotes first2 + (last1 - first1) if it was not given.

mapbox-gl-native/test/map/prefetch.test.cpp

Line 62 in 25cfb58

ASSERT_TRUE(std::is_permutation(tiles.begin(), tiles.end(), expected.begin()));

In the cases where a parent tile should be prefetched, expected.size() > tiles.size(). It turns out that the permutation test matches because the expected parent zoom is always the last value in expected, and isn't being considered in the permutation check.

A naive fix for #2 is to move the parent tile zoom value to the beginning of the expected vector. With this change, the test passed before #10548 merged, and consistently fails after.

[asheemmamoowala]

Moving to -> #11326