[core] Implement an eviction policy for OfflineDatabase

When inserting an cached resource, or removing a region, remove least-recently used resources and tiles, not used by offline regions, until the used database size, as calculated by multiplying the number of in-use pages by the page size, is less than the maximum cache size minus 5 times the page size.

In addition, OfflineDatabase may be configured to ignore cache puts of individual resources larger than a certain size.

This policy is similar but not identical to the former SQLiteCache policy:

* It accounts for offline, by exempting resources required by offline regions from eviction.
* It must delete from two tables (resources and tiles), rather than one. Currently the strategy is naive: evict 50 rows at a time from each table.
* It makes maximumCacheSize and maximumCacheEntrySize completely independent. The SQLiteCache implementation evicted when `usedSize > maximumCacheSize - 2 * maximumCacheEntrySize`. This evicts when `usedSize > maximumCacheSize - 5 * pageSize`.
* It uses a non-unlimited default value for maximumCacheSize: 50 MB. We should have always had a limit in place; "a cache without an eviction policy is a resource leak".

1 files changed, 0 insertions, 2 deletions

diff --git a/platform/linux/main.cpp b/platform/linux/main.cpp
index f90b5de849..98fb32075e 100644
--- a/platform/linux/main.cpp
+++ b/platform/linux/main.cpp
@@ -106,8 +106,6 @@ int main(int argc, char *argv[]) {
     view = std::make_unique<GLFWView>(fullscreen, benchmark);
 
     mbgl::DefaultFileSource fileSource("/tmp/mbgl-cache.db", ".");
-    fileSource.setMaximumCacheEntrySize(1 * 1024 * 1024); // 1 MB
-    fileSource.setMaximumCacheSize(50 * 1024 * 1024); // 50 MB
 
     // Set access token if present
     const char *token = getenv("MAPBOX_ACCESS_TOKEN");