session.ts

import { type Context } from "../context.ts";
import { type MiddlewareFn } from "../composer.ts";
import { debug as d } from "../platform.deno.ts";
const debug = d("grammy:session");

type MaybePromise<T> = Promise<T> | T;

/**
 * A session flavor is a context flavor that holds session data under
 * `ctx.session`.
 *
 * Session middleware will load the session data of a specific chat from your
 * storage solution, and make it available to you on the context object. Check
 * out the
 * [documentation](https://doc.deno.land/https://deno.land/x/grammy/mod.ts/~/session)
 * on session middleware to know more, and read the section about sessions on
 * the [website](https://grammy.dev/plugins/session.html).
 */
export interface SessionFlavor<S> {
    /**
     * Session data on the context object.
     *
     * **WARNING:** You have to make sure that your session data is not
     * undefined by _providing an initial value to the session middleware_, or by
     * making sure that `ctx.session` is assigned if it is empty! The type
     * system does not include `| undefined` because this is really annoying to
     * work with.
     *
     *  Accessing `ctx.session` by reading or writing will throw if
     * `getSessionKey(ctx) === undefined` for the respective context object
     * `ctx`.
     */
    get session(): S;
    set session(session: S | null | undefined);
}
/**
 * A lazy session flavor is a context flavor that holds a promise of some
 * session data under `ctx.session`.
 *
 * Lazy session middleware will provide this promise lazily on the context
 * object. Once you access `ctx.session`, the storage will be queried and the
 * session data becomes available. If you access `ctx.session` again for the
 * same context object, the cached value will be used. Check out the
 * [documentation](https://doc.deno.land/https://deno.land/x/grammy/mod.ts/~/lazySession)
 * on lazy session middleware to know more, and read the section about lazy
 * sessions on the
 * [website](https://grammy.dev/plugins/session.html#lazy-sessions).
 */
export interface LazySessionFlavor<S> {
    /**
     * Session data on the context object, potentially a promise.
     *
     * **WARNING:** You have to make sure that your session data is not
     * undefined by _providing a default value to the session middleware_, or by
     * making sure that `ctx.session` is assigned if it is empty! The type
     * system does not include `| undefined` because this is really annoying to
     * work with.
     *
     * Accessing `ctx.session` by reading or writing will throw iff
     * `getSessionKey(ctx) === undefined` holds for the respective context
     * object `ctx`.
     */
    get session(): MaybePromise<S>;
    set session(session: MaybePromise<S | null | undefined>);
}

/**
 * A storage adapter is an abstraction that provides read, write, and delete
 * access to a storage solution of any kind. Storage adapters are used to keep
 * session middleware independent of your database provider, and they allow you
 * to pass your own storage solution.
 */
export interface StorageAdapter<T> {
    /**
     * Reads a value for the given key from the storage. May return the value or
     * undefined, or a promise of either.
     */
    read: (key: string) => MaybePromise<T | undefined>;
    /**
     * Writes a value for the given key to the storage.
     */
    write: (key: string, value: T) => MaybePromise<void>;
    /**
     * Deletes a value for the given key from the storage.
     */
    delete: (key: string) => MaybePromise<void>;
}

/**
 * Options for session middleware.
 */
export interface SessionOptions<S> {
    /**
     * **Recommended to use.**
     *
     * A function that produces an initial value for `ctx.session`. This
     * function will be called every time the storage solution returns undefined
     * for a given session key. Make sure to create a new value every time, such
     * that different context objects do that accidentally share the same
     * session data.
     */
    initial?: () => S;
    /**
     * This option lets you generate your own session keys per context object.
     * The session key determines how to map the different session objects to
     * your chats and users. Check out the
     * [documentation](https://grammy.dev/plugins/session.html#how-to-use-sessions)
     * on the website about how to use session middleware to know how session
     * keys are used.
     *
     * The default implementation will store sessions per chat, as determined by
     * `ctx.chat?.id`.
     */
    getSessionKey?: (ctx: Context) => MaybePromise<string | undefined>;
    /**
     * A storage adapter to your storage solution. Provides read, write, and
     * delete access to the session middleware.
     *
     * Consider using a [known storage
     * adapter](https://grammy.dev/plugins/session.html#known-storage-adapters)
     * instead of rolling your own implementation of this.
     *
     * The default implementation will store session in memory. The data will be
     * lost whenever your bot restarts.
     */
    storage?: StorageAdapter<S>;
}
/**
 * Session middleware provides a persistent data storage for your bot. You can
 * use it to let your bot remember any data you want, for example the messages
 * it sent or received in the past. This is done by attaching _session data_ to
 * every chat. The stored data is then provided on the context object under
 * `ctx.session`.
 *
 * > **What is a session?** Simply put, the session of a chat is a little
 * > persistent storage that is attached to it. As an example, your bot can send
 * > a message to a chat and store the ID of that message in the corresponding
 * > session. The next time your bot receives an update from that chat, the
 * > session will still contain that ID.
 * >
 * > Session data can be stored in a database, in a file, or simply in memory.
 * > grammY only supports memory sessions out of the box, but you can use
 * > third-party session middleware to connect to other storage solutions. Note
 * > that memory sessions will be lost when you stop your bot and the process
 * > exits, so they are usually not useful in production.
 *
 * Whenever your bot receives an update, the first thing the session middleware
 * will do is to load the correct session from your storage solution. This
 * object is then provided on `ctx.session` while your other middleware is
 * running. As soon as your bot is done handling the update, the middleware
 * takes over again and writes back the session object to your storage. This
 * allows you to modify the session object arbitrarily in your middleware, and
 * to stop worrying about the database.
 *
 * ```ts
 * bot.use(session())
 *
 * bot.on('message', ctx => {
 *   // The session object is persisted across updates!
 *   const session = ctx.session
 * })
 * ```
 *
 * It is recommended to make use of the `inital` option in the configuration
 * object, which correctly initializes session objects for new chats.
 *
 * You can delete the session data by setting `ctx.session` to `null` or
 * `undefined`.
 *
 * Check out the [documentation](https://grammy.dev/plugins/session.html) on the
 * website to know more about how sessions work in grammY.
 *
 * @param options Optional configuration to pass to the session middleware
 */
export function session<S, C extends Context>(
    options: SessionOptions<S> = {},
): MiddlewareFn<C & SessionFlavor<S>> {
    const getSessionKey = options.getSessionKey ?? defaultGetSessionKey;
    const storage = options.storage ??
        (debug(
            "Storing session data in memory, all data will be lost when the bot restarts.",
        ),
            new MemorySessionStorage());
    return async (ctx, next) => {
        const key = await getSessionKey(ctx);
        let value = key === undefined
            ? undefined
            : (await storage.read(key)) ?? options.initial?.();
        Object.defineProperty(ctx, "session", {
            get() {
                if (key === undefined) {
                    const msg = undef("access", getSessionKey);
                    throw new Error(msg);
                }
                return value;
            },
            set(v) {
                if (key === undefined) {
                    const msg = undef("assign", getSessionKey);
                    throw new Error(msg);
                }
                value = v;
            },
        });
        await next(); // no catch: do not write back if middleware throws
        if (key !== undefined) {
            if (value == null) await storage.delete(key);
            else await storage.write(key, value);
        }
    };
}

/**
 * > This is an advanced function of grammY.
 *
 * Generally speaking, lazy sessions work just like normal sessions—just they
 * are loaded on demand. Except for a few `async`s and `await`s here and there,
 * their usage looks 100 % identical.
 *
 * Instead of directly querying the storage every time an update arrives, lazy
 * sessions quickly do this _once you access_ `ctx.session`. This can
 * significantly reduce the database traffic (especially when your bot is added
 * to group chats), because it skips a read and a wrote operation for all
 * updates that the bot does not react to.
 *
 * ```ts
 * // The options are identical
 * bot.use(lazySession({ storage: ... }))
 *
 * bot.on('message', async ctx => {
 *   // The session object is persisted across updates!
 *   const session = await ctx.session
 *   //                        ^
 *   //                        |
 *   //                       This plain property access (no function call) will trigger the database query!
 * })
 * ```
 *
 * Check out the
 * [documentation](https://grammy.dev/plugins/session.html#lazy-sessions) on the
 * website to know more about how lazy sessions work in grammY.
 *
 * @param options Optional configuration to pass to the session middleware
 */
export function lazySession<S, C extends Context>(
    options: SessionOptions<S> = {},
): MiddlewareFn<C & LazySessionFlavor<S>> {
    const getSessionKey = options.getSessionKey ?? defaultGetSessionKey;
    const storage = options.storage ??
        (debug(
            "Storing session data in memory, all data will be lost when the bot restarts.",
        ),
            new MemorySessionStorage());
    return async (ctx, next) => {
        const key = await getSessionKey(ctx);
        let value: MaybePromise<S | undefined> = undefined;
        let promise: Promise<S | undefined> | undefined = undefined;
        let fetching = false;
        let read = false;
        let wrote = false;

        async function load() {
            if (key === undefined) {
                const msg = undef("access", getSessionKey, { lazy: true });
                throw new Error(msg);
            }
            let v: S | undefined = await storage.read(key);
            if (!fetching) return value;
            if (v === undefined) {
                v = options.initial?.();
                if (v !== undefined) {
                    wrote = true;
                    value = v;
                }
            } else {
                value = v;
            }
            return value;
        }

        Object.defineProperty(ctx, "session", {
            get() {
                if (wrote) return value;
                read = true;
                if (promise === undefined) {
                    fetching = true;
                    promise = load();
                }
                return promise;
            },
            set(v) {
                if (key === undefined) {
                    const msg = undef("assign", getSessionKey, { lazy: true });
                    throw new Error(msg);
                }
                wrote = true;
                fetching = false;
                value = v;
            },
        });
        await next(); // no catch: do not wrote back if middleware throws
        if (key !== undefined) {
            if (read) await promise;
            if (read || wrote) {
                value = await value;
                if (value == null) await storage.delete(key);
                else await storage.write(key, value);
            }
        }
    };
}

/** Stores session data per chat by default */
function defaultGetSessionKey(ctx: Context): string | undefined {
    return ctx.chat?.id.toString();
}

/** Returns a useful error message for when the session key is undefined */
function undef<C extends Context>(
    op: "access" | "assign",
    getSessionKey: SessionOptions<C>["getSessionKey"],
    opts: { lazy?: boolean } = {},
) {
    const lazy = opts.lazy ?? false;
    const reason = getSessionKey === defaultGetSessionKey
        ? "this update does not belong to a chat, so the session key is undefined"
        : "the custom `getSessionKey` function returned undefined for this update";
    return `Cannot ${op} ${lazy ? "lazy " : ""}session data because ${reason}!`;
}

/**
 * The memory session storage is a built-in storage adapter that saves your
 * session data in RAM using a regular JavaScript `Map` object. If you use this
 * storage adapter, all sessions will be lost when your process terminates or
 * restarts. Hence, you should only use it for short-lived data that is not
 * important to persist.
 *
 * This class is used as default if you do not provide a storage adapter, e.g.
 * to your database.
 *
 * This storage adapter features expiring sessions. When instatiating this class
 * yourself, you can pass a time to live in milliseconds that will be used for
 * each session object. If a session for a user expired, the session data will
 * be discarded on its first read, and a fresh session object as returned by the
 * `inital` option (or undefined) will be put into place.
 */
export class MemorySessionStorage<S> implements StorageAdapter<S> {
    /**
     * Internally used `Map` instance that stores the session data
     */
    protected readonly storage = new Map<
        string,
        { session: S; expires?: number }
    >();

    /**
     * Constructs a new memory session storage with the given time to live. Note
     * that this storage adapter will not store your data permanently.
     *
     * @param timeToLive TTL in milliseconds, default is `Infinity`
     */
    constructor(private readonly timeToLive = Infinity) {}

    read(key: string) {
        const value = this.storage.get(key);
        if (value === undefined) return undefined;
        if (value.expires !== undefined && value.expires < Date.now()) {
            this.delete(key);
            return undefined;
        }
        return value.session;
    }

    /**
     * Reads the values for all keys of the session storage, and returns them as
     * an array.
     */
    readAll() {
        return Array
            .from(this.storage.keys())
            .map((key) => this.read(key))
            .filter((value): value is S => value !== undefined);
    }

    write(key: string, value: S) {
        this.storage.set(key, this.addExpiryDate(value));
    }

    private addExpiryDate(value: S) {
        const ttl = this.timeToLive;
        if (ttl !== undefined && ttl < Infinity) {
            const now = Date.now();
            return { session: value, expires: now + ttl };
        } else {
            return { session: value };
        }
    }

    delete(key: string) {
        this.storage.delete(key);
    }
}