Conditionally run post-attachment migrations

Introduce placeholder migrations for Backbone models so they never implicitly
run migrations whenever they are `fetch`ed. We prefer to run our migrations
explicitly upon app startup and then let Backbone models be (slightly) dumb(er)
models, without inadvertently triggering migrations.

js/modules/migrations/get_placeholder_migrations.js

@@ -0,0 +1,23 @@
+const Migrations0DatabaseWithAttachmentData =
+  require('./migrations_0_database_with_attachment_data');
+const Migrations1DatabaseWithoutAttachmentData =
+  require('./migrations_1_database_without_attachment_data');
+
+
+exports.getPlaceholderMigrations = () => {
+  const last0MigrationVersion =
+    Migrations0DatabaseWithAttachmentData.getLatestVersion();
+  const last1MigrationVersion =
+    Migrations1DatabaseWithoutAttachmentData.getLatestVersion();
+
+  const lastMigrationVersion = last1MigrationVersion || last0MigrationVersion;
+
+  return [{
+    version: lastMigrationVersion,
+    migrate() {
+      throw new Error('Unexpected invocation of placeholder migration!' +
+        '\n\nMigrations must explicitly be run upon application startup instead' +
+        ' of implicitly via Backbone IndexedDB adapter at any time.');
+    },
+  }];
+};

js/modules/migrations/migrations_0_database_with_attachment_data.js

@@ -154,3 +154,12 @@ exports.getDatabase = () => ({
   name: database.id,
   version: last(exports.migrations).version,
 });
+
+exports.getLatestVersion = () => {
+  const lastMigration = last(migrations);
+  if (!lastMigration) {
+    return null;
+  }
+
+  return lastMigration.version;
+};

js/modules/migrations/migrations_1_database_without_attachment_data.js

@@ -1,15 +1,50 @@
+const last = require('lodash/last');
+
+const db = require('../database');
+const settings = require('../settings');
 const { runMigrations } = require('./run_migrations');
 
 
-exports.migrations = [
+// NOTE: Add new migrations that need to traverse entire database, e.g. messages
+// store, here. These will only run after attachment migration has completed in
+// the background:
+const migrations = [
   // {
-  //   version: 18,
-  //   async migrate(transaction, next) {
-  //     console.log('Migration 18');
-  //     console.log('Attachments stored on disk');
+  //   version: 0,
+  //   migrate(transaction, next) {
   //     next();
   //   },
   // },
 ];
 
-exports.run = runMigrations;
+exports.run = async ({ Backbone, database } = {}) => {
+  const { canRun } = await exports.getStatus({ database });
+  if (!canRun) {
+    throw new Error('Cannot run migrations on database without attachment data');
+  }
+
+  await runMigrations({ Backbone, database });
+};
+
+exports.getStatus = async ({ database } = {}) => {
+  const connection = await db.open(database.id, database.version);
+  const isAttachmentMigrationComplete =
+    await settings.isAttachmentMigrationComplete(connection);
+  const hasMigrations = migrations.length > 0;
+
+  const canRun = isAttachmentMigrationComplete && hasMigrations;
+  return {
+    isAttachmentMigrationComplete,
+    hasMigrations,
+    canRun,
+  };
+};
+
+exports.getLatestVersion = () => {
+  const lastMigration = last(migrations);
+  if (!lastMigration) {
+    return null;
+  }
+
+  return lastMigration.version;
+};