Schema migrations for LevelDB (#728)

* Implement schema versions

* Style fixes

* newlines, copyrights, assumptions

* Fix nullability

* Raw ptr -> shared_ptr

* kVersionTableGlobal -> kVersionGlobalTable

* Drop utils, move into static methods

* Drop extra include

* Add a few more comments

* Move version constant into migrations file

* formatting?

* Fix comment

3 files changed, 88 insertions, 2 deletions

diff --git a/Firestore/Example/Tests/Local/FSTLevelDBMigrationsTests.mm b/Firestore/Example/Tests/Local/FSTLevelDBMigrationsTests.mm
new file mode 100644
index 0000000..8ef0e94
--- /dev/null
+++ b/Firestore/Example/Tests/Local/FSTLevelDBMigrationsTests.mm
@@ -0,0 +1,75 @@
+/*
+ * Copyright 2018 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#import <XCTest/XCTest.h>
+#include <leveldb/db.h>
+
+#import "Firestore/Protos/objc/firestore/local/Target.pbobjc.h"
+#import "Firestore/Source/Local/FSTLevelDBMigrations.h"
+#import "Firestore/Source/Local/FSTLevelDBQueryCache.h"
+
+#import "Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+using leveldb::DB;
+using leveldb::Options;
+using leveldb::Status;
+
+@interface FSTLevelDBMigrationsTests : XCTestCase
+@end
+
+@implementation FSTLevelDBMigrationsTests {
+  std::shared_ptr<DB> _db;
+}
+
+- (void)setUp {
+  Options options;
+  options.error_if_exists = true;
+  options.create_if_missing = true;
+
+  NSString *dir = [FSTPersistenceTestHelpers levelDBDir];
+  DB *db;
+  Status status = DB::Open(options, [dir UTF8String], &db);
+  XCTAssert(status.ok(), @"Failed to create db: %s", status.ToString().c_str());
+  _db.reset(db);
+}
+
+- (void)tearDown {
+  _db.reset();
+}
+
+- (void)testAddsTargetGlobal {
+  FSTPBTargetGlobal *metadata = [FSTLevelDBQueryCache readTargetMetadataFromDB:_db];
+  XCTAssertNil(metadata, @"Not expecting metadata yet, we should have an empty db");
+  [FSTLevelDBMigrations runMigrationsOnDB:_db];
+  metadata = [FSTLevelDBQueryCache readTargetMetadataFromDB:_db];
+  XCTAssertNotNil(metadata, @"Migrations should have added the metadata");
+}
+
+- (void)testSetsVersionNumber {
+  FSTLevelDBSchemaVersion initial = [FSTLevelDBMigrations schemaVersionForDB:_db];
+  XCTAssertEqual(0, initial, "No version should be equivalent to 0");
+
+  // Pick an arbitrary high migration number and migrate to it.
+  [FSTLevelDBMigrations runMigrationsOnDB:_db];
+  FSTLevelDBSchemaVersion actual = [FSTLevelDBMigrations schemaVersionForDB:_db];
+  XCTAssertGreaterThan(actual, 0, @"Expected to migrate to a schema version > 0");
+}
+
+@end
+
+NS_ASSUME_NONNULL_END
diff --git a/Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.h b/Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.h
index 936bacf..5859d4b 100644
--- a/Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.h
+++ b/Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.h
@@ -24,6 +24,12 @@ NS_ASSUME_NONNULL_BEGIN
 @interface FSTPersistenceTestHelpers : NSObject
 
 /**
+ * @return The directory where a leveldb instance can store data files. Any files that existed
+ * there will be deleted first.
+ */
++ (NSString *)levelDBDir;
+
+/**
  * Creates and starts a new FSTLevelDB instance for testing, destroying any previous contents
  * if they existed.
  *
diff --git a/Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.m b/Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.m
index c773b12..e9e129d 100644
--- a/Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.m
+++ b/Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.m
@@ -26,10 +26,9 @@ NS_ASSUME_NONNULL_BEGIN
 
 @implementation FSTPersistenceTestHelpers
 
-+ (FSTLevelDB *)levelDBPersistence {
++ (NSString *)levelDBDir {
   NSError *error;
   NSFileManager *files = [NSFileManager defaultManager];
-
   NSString *dir =
       [NSTemporaryDirectory() stringByAppendingPathComponent:@"FSTPersistenceTestHelpers"];
   if ([files fileExistsAtPath:dir]) {
@@ -40,12 +39,18 @@ NS_ASSUME_NONNULL_BEGIN
                   format:@"Failed to clean up leveldb path %@: %@", dir, error];
     }
   }
+  return dir;
+}
+
++ (FSTLevelDB *)levelDBPersistence {
+  NSString *dir = [self levelDBDir];
 
   FSTDatabaseID *databaseID = [FSTDatabaseID databaseIDWithProject:@"p" database:@"d"];
   FSTSerializerBeta *remoteSerializer = [[FSTSerializerBeta alloc] initWithDatabaseID:databaseID];
   FSTLocalSerializer *serializer =
       [[FSTLocalSerializer alloc] initWithRemoteSerializer:remoteSerializer];
   FSTLevelDB *db = [[FSTLevelDB alloc] initWithDirectory:dir serializer:serializer];
+  NSError *error;
   BOOL success = [db start:&error];
   if (!success) {
     [NSException raise:NSInternalInconsistencyException