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

1 files changed, 95 insertions, 0 deletions

diff --git a/Firestore/Source/Local/FSTLevelDBMigrations.mm b/Firestore/Source/Local/FSTLevelDBMigrations.mm
new file mode 100644
index 0000000..49af893
--- /dev/null
+++ b/Firestore/Source/Local/FSTLevelDBMigrations.mm
@@ -0,0 +1,95 @@
+/*
+ * 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.
+ */
+
+#include "Firestore/Source/Local/FSTLevelDBMigrations.h"
+
+#include <leveldb/db.h>
+#include <leveldb/write_batch.h>
+
+#import "Firestore/Protos/objc/firestore/local/Target.pbobjc.h"
+#import "Firestore/Source/Local/FSTLevelDB.h"
+#import "Firestore/Source/Local/FSTLevelDBKey.h"
+#import "Firestore/Source/Local/FSTLevelDBQueryCache.h"
+#import "Firestore/Source/Local/FSTWriteGroup.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+// Current version of the schema defined in this file.
+static FSTLevelDBSchemaVersion kSchemaVersion = 1;
+
+using leveldb::DB;
+using leveldb::Status;
+using leveldb::Slice;
+using leveldb::WriteOptions;
+
+/**
+ * Ensures that the global singleton target metadata row exists in LevelDB.
+ * @param db The db in which to require the row.
+ */
+static void EnsureTargetGlobal(std::shared_ptr<DB> db, FSTWriteGroup *group) {
+  FSTPBTargetGlobal *targetGlobal = [FSTLevelDBQueryCache readTargetMetadataFromDB:db];
+  if (!targetGlobal) {
+    [group setMessage:[FSTPBTargetGlobal message] forKey:[FSTLevelDBTargetGlobalKey key]];
+  }
+}
+
+/**
+ * Save the given version number as the current version of the schema of the database.
+ * @param version The version to save
+ * @param group The transaction in which to save the new version number
+ */
+static void SaveVersion(FSTLevelDBSchemaVersion version, FSTWriteGroup *group) {
+  std::string key = [FSTLevelDBVersionKey key];
+  std::string version_string = std::to_string(version);
+  [group setData:version_string forKey:key];
+}
+
+@implementation FSTLevelDBMigrations
+
++ (FSTLevelDBSchemaVersion)schemaVersionForDB:(std::shared_ptr<DB>)db {
+  std::string key = [FSTLevelDBVersionKey key];
+  std::string version_string;
+  Status status = db->Get([FSTLevelDB standardReadOptions], key, &version_string);
+  if (status.IsNotFound()) {
+    return 0;
+  } else {
+    return stoi(version_string);
+  }
+}
+
++ (void)runMigrationsOnDB:(std::shared_ptr<DB>)db {
+  FSTWriteGroup *group = [FSTWriteGroup groupWithAction:@"Migrations"];
+  FSTLevelDBSchemaVersion currentVersion = [self schemaVersionForDB:db];
+  // Each case in this switch statement intentionally falls through. This lets us
+  // start at the current schema version and apply any migrations that have not yet
+  // been applied, to bring us up to current, as defined by the kSchemaVersion constant.
+  switch (currentVersion) {
+    case 0:
+      EnsureTargetGlobal(db, group);
+      // Fallthrough
+    default:
+      if (currentVersion < kSchemaVersion) {
+        SaveVersion(kSchemaVersion, group);
+      }
+  }
+  if (!group.isEmpty) {
+    [group writeToDB:db];
+  }
+}
+
+@end
+
+NS_ASSUME_NONNULL_END