Add an API interface for performing DB migrations as required

module/hooks/ready.mjs

@@ -11,6 +11,38 @@ Hooks.on(`ready`, () => {
 		CONFIG.stats.db = NilDatabase;
 	};
 
+	/*
+	Perform any required data migration if any is required for the version
+	jump that the user may have caused. This only migrates the data iff the
+	currently authenticated user is able to perform the full migration of
+	data.
+	*/
+	const db = CONFIG.stats.db;
+	const lastVersion = game.settings.get(__ID__, `lastVersion`);
+	const canDoMigration = db.canPerformMigration();
+	const requiresMigration = db.requiresMigrationFrom(lastVersion);
+	if (requiresMigration) {
+		if (canDoMigration) {
+			const notif = ui.notifications.info(
+				`${__TITLE__} | Performing data migration, please do not close the window`,
+				{ progress: true, permanent: true },
+			);
+
+			// Fire and forget
+			CONFIG.stats.db.migrateData(notif)
+				.then(() => {
+					game.settings.set(__ID__, __VERSION__);
+					setTimeout(() => ui.notifications.remove(notif), 500);
+				});
+		} else {
+			ui.notifications.error(
+				`The stat-tracker database is out of date, temporarily disabling the stat-tracker module's functionality until the migration can be performed by a GM user logging into the world.`,
+				{ console: false, permanent: true },
+			);
+			CONFIG.stats.db = NilDatabase;
+		};
+	};
+
 	/*
 	Prevent any run-time modifications to the CONFIG API so that users can't wreck
 	themselves nor their data by fooling around with the values.

module/settings/meta.mjs

@@ -23,4 +23,11 @@ export function registerMetaSettings() {
 		config: false,
 		requiresReload: false,
 	});
+
+	game.settings.register(__ID__, `lastVersion`, {
+		scope: `world`,
+		type: String,
+		config: false,
+		requiresReload: false,
+	});
 };

module/utils/databases/Database.mjs

@@ -220,6 +220,43 @@ export class Database {
 	static async triggerListeners() {};
 
 	static async unregisterListeners() {};
+
+	// MARK: Migrations
+	/**
+	 * Determines if the currently authenticated user is capable of running
+	 * the full migration on their own.
+	 *
+	 * @returns {boolean}
+	 */
+	static async canPerformMigration() {
+		// TODO: this *must* account for isActiveGM, because otherwise the
+		// world setting cannot be updated after the migration finishes.
+		return game.user.isActiveGM;
+	};
+
+	/**
+	 * Determines if the previous version of the plugin that was active
+	 * needs to be migrated in order to work with the new version.
+	 *
+	 * @param {string} lastVersion The version that was last active
+	 * @returns {boolean}
+	 */
+	static async requiresMigrationFrom(lastVersion) {
+		return foundry.utils.isNewerVersion(__VERSION__, lastVersion);
+	};
+
+	/**
+	 * This method migrates ALL of the database data from one version of
+	 * the module to the currently installed module. This is not guaranteed
+	 * to run only on one client, so it should be made to be either
+	 * idempotent, or have an operation locking mechanism that can prevent
+	 * other clients from executing it if there's a migration in-progress.
+	 *
+	 * @param {string} lastVersion The last version that the user had active
+	 * @param {Notification} notif The progress bar notification used for
+	 * user feedback while performing migrations.
+	 */
+	static async migrateData(lastVersion, notif) {};
 };
 
 /* eslint-enable no-unused-vars */

module/utils/databases/NilDatabase.mjs

@@ -31,4 +31,9 @@ export class NilDatabase extends Database {
 	static async registerListeners() {};
 	static async triggerListeners() {};
 	static async unregisterListeners() {};
+
+	// MARK: Migrations
+	static async canPerformMigration() { return true };
+	static async requiresMigrationFrom() { return false };
+	static async migrateData() {};
 };