[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, 72 insertions, 0 deletions

diff --git a/test/storage/offline_database.cpp b/test/storage/offline_database.cpp
index 4af262deb1..680a14bb4f 100644
--- a/test/storage/offline_database.cpp
+++ b/test/storage/offline_database.cpp
@@ -4,11 +4,14 @@
 #include <mbgl/storage/resource.hpp>
 #include <mbgl/storage/response.hpp>
 #include <mbgl/util/io.hpp>
+#include <mbgl/util/string.hpp>
 
 #include <gtest/gtest.h>
 #include <sqlite3.h>
 #include <thread>
 
+using namespace std::literals::string_literals;
+
 namespace {
 
 void createDir(const char* name) {
@@ -480,3 +483,72 @@ TEST(OfflineDatabase, ConcurrentUse) {
     thread1.join();
     thread2.join();
 }
+
+TEST(OfflineDatabase, PutIgnoresOversizedResources) {
+    using namespace mbgl;
+
+    Log::setObserver(std::make_unique<FixtureLogObserver>());
+    OfflineDatabase db(":memory:", 1000, 1);
+
+    Resource resource = Resource::style("http://example.com/");
+    Response response;
+    response.data = std::make_shared<std::string>("data");
+
+    db.put(resource, response);
+    EXPECT_FALSE(bool(db.get(resource)));
+
+    auto observer = Log::removeObserver();
+    auto flo = dynamic_cast<FixtureLogObserver*>(observer.get());
+    EXPECT_EQ(1ul, flo->count({ EventSeverity::Warning, Event::Database, -1, "Entry too big for caching" }));
+}
+
+TEST(OfflineDatabase, PutRegionResourceDoesNotIgnoreOversizedResources) {
+    using namespace mbgl;
+
+    OfflineDatabase db(":memory:", 1000, 1);
+
+    OfflineRegionDefinition definition { "", LatLngBounds::world(), 0, INFINITY, 1.0 };
+    OfflineRegion region = db.createRegion(definition, OfflineRegionMetadata());
+
+    Resource resource = Resource::style("http://example.com/");
+    Response response;
+    response.data = std::make_shared<std::string>("data");
+
+    db.putRegionResource(region.getID(), resource, response);
+    EXPECT_TRUE(bool(db.get(resource)));
+}
+
+TEST(OfflineDatabase, PutEvictsLeastRecentlyUsedResources) {
+    using namespace mbgl;
+
+    OfflineDatabase db(":memory:", 1024 * 20);
+
+    Response response;
+    response.data = std::make_shared<std::string>(1024, '0');
+
+    for (uint32_t i = 1; i <= 20; i++) {
+        db.put(Resource::style("http://example.com/"s + util::toString(i)), response);
+    }
+
+    EXPECT_FALSE(bool(db.get(Resource::style("http://example.com/1"))));
+    EXPECT_TRUE(bool(db.get(Resource::style("http://example.com/20"))));
+}
+
+TEST(OfflineDatabase, PutRegionResourceDoesNotEvict) {
+    using namespace mbgl;
+
+    OfflineDatabase db(":memory:", 1024 * 20);
+
+    OfflineRegionDefinition definition { "", LatLngBounds::world(), 0, INFINITY, 1.0 };
+    OfflineRegion region = db.createRegion(definition, OfflineRegionMetadata());
+
+    Response response;
+    response.data = std::make_shared<std::string>(1024, '0');
+
+    for (uint32_t i = 1; i <= 20; i++) {
+        db.putRegionResource(region.getID(), Resource::style("http://example.com/"s + util::toString(i)), response);
+    }
+
+    EXPECT_TRUE(bool(db.get(Resource::style("http://example.com/1"))));
+    EXPECT_TRUE(bool(db.get(Resource::style("http://example.com/20"))));
+}