src/cursor/abstract_cursor.ts

import { Readable, Transform } from 'stream';

import { type BSONSerializeOptions, type Document, Long, pluckBSONSerializeOptions } from '../bson';
import { type OnDemandDocumentDeserializeOptions } from '../cmap/wire_protocol/on_demand/document';
import { type CursorResponse } from '../cmap/wire_protocol/responses';
import {
  MongoAPIError,
  MongoCursorExhaustedError,
  MongoCursorInUseError,
  MongoInvalidArgumentError,
  MongoRuntimeError,
  MongoTailableCursorError
} from '../error';
import type { MongoClient } from '../mongo_client';
import { type Abortable, TypedEventEmitter } from '../mongo_types';
import { executeOperation } from '../operations/execute_operation';
import { GetMoreOperation } from '../operations/get_more';
import { KillCursorsOperation } from '../operations/kill_cursors';
import { ReadConcern, type ReadConcernLike } from '../read_concern';
import { ReadPreference, type ReadPreferenceLike } from '../read_preference';
import { type AsyncDisposable, configureResourceManagement } from '../resource_management';
import type { Server } from '../sdam/server';
import { ClientSession, maybeClearPinnedConnection } from '../sessions';
import { type CSOTTimeoutContext, type Timeout, TimeoutContext } from '../timeout';
import {
  addAbortListener,
  type Disposable,
  kDispose,
  type MongoDBNamespace,
  noop,
  squashError
} from '../utils';

/**
 * @internal
 * TODO(NODE-2882): A cursor's getMore commands must be run on the same server it was started on
 * and the same session must be used for the lifetime of the cursor. This object serves to get the
 * server and session (along with the response) out of executeOperation back to the AbstractCursor.
 *
 * There may be a better design for communicating these values back to the cursor, currently an operation
 * MUST store the selected server on itself so it can be read after executeOperation has returned.
 */
export interface InitialCursorResponse {
  /** The server selected for the operation */
  server: Server;
  /** The session used for this operation, may be implicitly created */
  session?: ClientSession;
  /** The raw server response for the operation */
  response: CursorResponse;
}

/** @public */
export const CURSOR_FLAGS = [
  'tailable',
  'oplogReplay',
  'noCursorTimeout',
  'awaitData',
  'exhaust',
  'partial'
] as const;

/** @public */
export interface CursorStreamOptions {
  /** A transformation method applied to each document emitted by the stream */
  transform?(this: void, doc: Document): Document;
}

/** @public */
export type CursorFlag = (typeof CURSOR_FLAGS)[number];

function removeActiveCursor(this: AbstractCursor) {
  this.client.s.activeCursors.delete(this);
}

/**
 * @public
 * @experimental
 * Specifies how `timeoutMS` is applied to the cursor. Can be either `'cursorLifeTime'` or `'iteration'`
 * When set to `'iteration'`, the deadline specified by `timeoutMS` applies to each call of
 * `cursor.next()`.
 * When set to `'cursorLifetime'`, the deadline applies to the life of the entire cursor.
 *
 * Depending on the type of cursor being used, this option has different default values.
 * For non-tailable cursors, this value defaults to `'cursorLifetime'`
 * For tailable cursors, this value defaults to `'iteration'` since tailable cursors, by
 * definition can have an arbitrarily long lifetime.
 *
 * @example
 * ```ts
 * const cursor = collection.find({}, {timeoutMS: 100, timeoutMode: 'iteration'});
 * for await (const doc of cursor) {
 *  // process doc
 *  // This will throw a timeout error if any of the iterator's `next()` calls takes more than 100ms, but
 *  // will continue to iterate successfully otherwise, regardless of the number of batches.
 * }
 * ```
 *
 * @example
 * ```ts
 * const cursor = collection.find({}, { timeoutMS: 1000, timeoutMode: 'cursorLifetime' });
 * const docs = await cursor.toArray(); // This entire line will throw a timeout error if all batches are not fetched and returned within 1000ms.
 * ```
 */
export const CursorTimeoutMode = Object.freeze({
  ITERATION: 'iteration',
  LIFETIME: 'cursorLifetime'
} as const);

/**
 * @public
 * @experimental
 */
export type CursorTimeoutMode = (typeof CursorTimeoutMode)[keyof typeof CursorTimeoutMode];

/** @public */
export interface AbstractCursorOptions extends BSONSerializeOptions {
  session?: ClientSession;
  readPreference?: ReadPreferenceLike;
  readConcern?: ReadConcernLike;
  /**
   * Specifies the number of documents to return in each response from MongoDB
   */
  batchSize?: number;
  /**
   * When applicable `maxTimeMS` controls the amount of time the initial command
   * that constructs a cursor should take. (ex. find, aggregate, listCollections)
   */
  maxTimeMS?: number;
  /**
   * When applicable `maxAwaitTimeMS` controls the amount of time subsequent getMores
   * that a cursor uses to fetch more data should take. (ex. cursor.next())
   */
  maxAwaitTimeMS?: number;
  /**
   * Comment to apply to the operation.
   *
   * In server versions pre-4.4, 'comment' must be string.  A server
   * error will be thrown if any other type is provided.
   *
   * In server versions 4.4 and above, 'comment' can be any valid BSON type.
   */
  comment?: unknown;
  /**
   * By default, MongoDB will automatically close a cursor when the
   * client has exhausted all results in the cursor. However, for [capped collections](https://www.mongodb.com/docs/manual/core/capped-collections)
   * you may use a Tailable Cursor that remains open after the client exhausts
   * the results in the initial cursor.
   */
  tailable?: boolean;
  /**
   * If awaitData is set to true, when the cursor reaches the end of the capped collection,
   * MongoDB blocks the query thread for a period of time waiting for new data to arrive.
   * When new data is inserted into the capped collection, the blocked thread is signaled
   * to wake up and return the next batch to the client.
   */
  awaitData?: boolean;
  noCursorTimeout?: boolean;
  /** Specifies the time an operation will run until it throws a timeout error. See {@link AbstractCursorOptions.timeoutMode} for more details on how this option applies to cursors. */
  timeoutMS?: number;
  /**
   * @public
   * @experimental
   * Specifies how `timeoutMS` is applied to the cursor. Can be either `'cursorLifeTime'` or `'iteration'`
   * When set to `'iteration'`, the deadline specified by `timeoutMS` applies to each call of
   * `cursor.next()`.
   * When set to `'cursorLifetime'`, the deadline applies to the life of the entire cursor.
   *
   * Depending on the type of cursor being used, this option has different default values.
   * For non-tailable cursors, this value defaults to `'cursorLifetime'`
   * For tailable cursors, this value defaults to `'iteration'` since tailable cursors, by
   * definition can have an arbitrarily long lifetime.
   *
   * @example
   * ```ts
   * const cursor = collection.find({}, {timeoutMS: 100, timeoutMode: 'iteration'});
   * for await (const doc of cursor) {
   *  // process doc
   *  // This will throw a timeout error if any of the iterator's `next()` calls takes more than 100ms, but
   *  // will continue to iterate successfully otherwise, regardless of the number of batches.
   * }
   * ```
   *
   * @example
   * ```ts
   * const cursor = collection.find({}, { timeoutMS: 1000, timeoutMode: 'cursorLifetime' });
   * const docs = await cursor.toArray(); // This entire line will throw a timeout error if all batches are not fetched and returned within 1000ms.
   * ```
   */
  timeoutMode?: CursorTimeoutMode;

  /**
   * @internal
   *
   * A timeout context to govern the total time the cursor can live.  If provided, the cursor
   * cannot be used in ITERATION mode.
   */
  timeoutContext?: CursorTimeoutContext;
}

/** @internal */
export type InternalAbstractCursorOptions = Omit<AbstractCursorOptions, 'readPreference'> & {
  // resolved
  readPreference: ReadPreference;
  readConcern?: ReadConcern;

  // cursor flags, some are deprecated
  oplogReplay?: boolean;
  exhaust?: boolean;
  partial?: boolean;

  omitMaxTimeMS?: boolean;
};

/** @public */
export type AbstractCursorEvents = {
  [AbstractCursor.CLOSE](): void;
};

/** @public */
export abstract class AbstractCursor<
    TSchema = any,
    CursorEvents extends AbstractCursorEvents = AbstractCursorEvents
  >
  extends TypedEventEmitter<CursorEvents>
  implements AsyncDisposable
{
  /** @internal */
  private cursorId: Long | null;
  /** @internal */
  private cursorSession: ClientSession;
  /** @internal */
  private selectedServer?: Server;
  /** @internal */
  private cursorNamespace: MongoDBNamespace;
  /** @internal */
  private documents: CursorResponse | null = null;
  /** @internal */
  private cursorClient: MongoClient;
  /** @internal */
  private transform?: (doc: TSchema) => any;
  /**
   * @internal
   * This is true whether or not the first command fails. It only indicates whether or not the first
   * command has been run.
   */
  private initialized: boolean;
  /** @internal */
  private isClosed: boolean;
  /** @internal */
  private isKilled: boolean;
  /** @internal */
  protected readonly cursorOptions: InternalAbstractCursorOptions;
  /** @internal */
  protected timeoutContext?: CursorTimeoutContext;

  /** @event */
  static readonly CLOSE = 'close' as const;

  /** @internal */
  protected deserializationOptions: OnDemandDocumentDeserializeOptions;
  protected signal: AbortSignal | undefined;
  private abortListener: Disposable | undefined;

  /** @internal */
  protected constructor(
    client: MongoClient,
    namespace: MongoDBNamespace,
    options: AbstractCursorOptions & Abortable = {}
  ) {
    super();
    this.on('error', noop);

    if (!client.s.isMongoClient) {
      throw new MongoRuntimeError('Cursor must be constructed with MongoClient');
    }
    this.cursorClient = client;
    this.cursorNamespace = namespace;
    this.cursorId = null;
    this.initialized = false;
    this.isClosed = false;
    this.isKilled = false;
    this.cursorOptions = {
      readPreference:
        options.readPreference && options.readPreference instanceof ReadPreference
          ? options.readPreference
          : ReadPreference.primary,
      ...pluckBSONSerializeOptions(options),
      timeoutMS: options?.timeoutContext?.csotEnabled()
        ? options.timeoutContext.timeoutMS
        : options.timeoutMS,
      tailable: options.tailable,
      awaitData: options.awaitData
    };

    if (this.cursorOptions.timeoutMS != null) {
      if (options.timeoutMode == null) {
        if (options.tailable) {
          if (options.awaitData) {
            if (
              options.maxAwaitTimeMS != null &&
              options.maxAwaitTimeMS >= this.cursorOptions.timeoutMS
            )
              throw new MongoInvalidArgumentError(
                'Cannot specify maxAwaitTimeMS >= timeoutMS for a tailable awaitData cursor'
              );
          }

          this.cursorOptions.timeoutMode = CursorTimeoutMode.ITERATION;
        } else {
          this.cursorOptions.timeoutMode = CursorTimeoutMode.LIFETIME;
        }
      } else {
        if (options.tailable && options.timeoutMode === CursorTimeoutMode.LIFETIME) {
          throw new MongoInvalidArgumentError(
            "Cannot set tailable cursor's timeoutMode to LIFETIME"
          );
        }
        this.cursorOptions.timeoutMode = options.timeoutMode;
      }
    } else {
      if (options.timeoutMode != null)
        throw new MongoInvalidArgumentError('Cannot set timeoutMode without setting timeoutMS');
    }

    // Set for initial command
    this.cursorOptions.omitMaxTimeMS =
      this.cursorOptions.timeoutMS != null &&
      ((this.cursorOptions.timeoutMode === CursorTimeoutMode.ITERATION &&
        !this.cursorOptions.tailable) ||
        (this.cursorOptions.tailable && !this.cursorOptions.awaitData));

    const readConcern = ReadConcern.fromOptions(options);
    if (readConcern) {
      this.cursorOptions.readConcern = readConcern;
    }

    if (typeof options.batchSize === 'number') {
      this.cursorOptions.batchSize = options.batchSize;
    }

    // we check for undefined specifically here to allow falsy values
    // eslint-disable-next-line no-restricted-syntax
    if (options.comment !== undefined) {
      this.cursorOptions.comment = options.comment;
    }

    if (typeof options.maxTimeMS === 'number') {
      this.cursorOptions.maxTimeMS = options.maxTimeMS;
    }

    if (typeof options.maxAwaitTimeMS === 'number') {
      this.cursorOptions.maxAwaitTimeMS = options.maxAwaitTimeMS;
    }

    if (options.session instanceof ClientSession) {
      this.cursorSession = options.session;
    } else {
      this.cursorSession = this.cursorClient.startSession({ owner: this, explicit: false });
    }

    this.deserializationOptions = {
      ...this.cursorOptions,
      validation: {
        utf8: options?.enableUtf8Validation === false ? false : true
      }
    };

    this.timeoutContext = options.timeoutContext;
    this.signal = options.signal;
    this.abortListener = addAbortListener(
      this.signal,
      () => void this.close().then(undefined, squashError)
    );
    this.trackCursor();
  }

  /**
   * The cursor has no id until it receives a response from the initial cursor creating command.
   *
   * It is non-zero for as long as the database has an open cursor.
   *
   * The initiating command may receive a zero id if the entire result is in the `firstBatch`.
   */
  get id(): Long | undefined {
    return this.cursorId ?? undefined;
  }

  /** @internal */
  get isDead() {
    return (this.cursorId?.isZero() ?? false) || this.isClosed || this.isKilled;
  }

  /** @internal */
  get client(): MongoClient {
    return this.cursorClient;
  }

  /** @internal */
  get server(): Server | undefined {
    return this.selectedServer;
  }

  get namespace(): MongoDBNamespace {
    return this.cursorNamespace;
  }

  get readPreference(): ReadPreference {
    return this.cursorOptions.readPreference;
  }

  get readConcern(): ReadConcern | undefined {
    return this.cursorOptions.readConcern;
  }

  /** @internal */
  get session(): ClientSession {
    return this.cursorSession;
  }

  set session(clientSession: ClientSession) {
    this.cursorSession = clientSession;
  }

  /**
   * The cursor is closed and all remaining locally buffered documents have been iterated.
   */
  get closed(): boolean {
    return this.isClosed && (this.documents?.length ?? 0) === 0;
  }

  /**
   * A `killCursors` command was attempted on this cursor.
   * This is performed if the cursor id is non zero.
   */
  get killed(): boolean {
    return this.isKilled;
  }

  get loadBalanced(): boolean {
    return !!this.cursorClient.topology?.loadBalanced;
  }

  /**
   * @beta
   * @experimental
   * An alias for {@link AbstractCursor.close|AbstractCursor.close()}.
   */
  declare [Symbol.asyncDispose]: () => Promise<void>;
  /** @internal */
  async asyncDispose() {
    await this.close();
  }

  /** Adds cursor to client's tracking so it will be closed by MongoClient.close() */
  private trackCursor() {
    this.cursorClient.s.activeCursors.add(this);
    if (!this.listeners('close').includes(removeActiveCursor)) {
      this.once('close', removeActiveCursor);
    }
  }

  /** Returns current buffered documents length */
  bufferedCount(): number {
    return this.documents?.length ?? 0;
  }

  /** Returns current buffered documents */
  readBufferedDocuments(number?: number): NonNullable<TSchema>[] {
    const bufferedDocs: NonNullable<TSchema>[] = [];
    const documentsToRead = Math.min(
      number ?? this.documents?.length ?? 0,
      this.documents?.length ?? 0
    );

    for (let count = 0; count < documentsToRead; count++) {
      const document = this.documents?.shift(this.deserializationOptions);
      if (document != null) {
        bufferedDocs.push(document);
      }
    }

    return bufferedDocs;
  }

  async *[Symbol.asyncIterator](): AsyncGenerator<TSchema, void, void> {
    this.signal?.throwIfAborted();

    if (this.closed) {
      return;
    }

    try {
      while (true) {
        if (this.isKilled) {
          return;
        }

        if (this.closed) {
          return;
        }

        if (this.cursorId != null && this.isDead && (this.documents?.length ?? 0) === 0) {
          return;
        }

        const document = await this.next();

        // eslint-disable-next-line no-restricted-syntax
        if (document === null) {
          return;
        }

        yield document;

        this.signal?.throwIfAborted();
      }
    } finally {
      // Only close the cursor if it has not already been closed. This finally clause handles
      // the case when a user would break out of a for await of loop early.
      if (!this.isClosed) {
        try {
          await this.close();
        } catch (error) {
          squashError(error);
        }
      }
    }
  }

  stream(options?: CursorStreamOptions): Readable & AsyncIterable<TSchema> {
    const readable = new ReadableCursorStream(this);
    const abortListener = addAbortListener(this.signal, function () {
      readable.destroy(this.reason);
    });
    readable.once('end', () => {
      abortListener?.[kDispose]();
    });

    if (options?.transform) {
      const transform = options.transform;

      const transformedStream = readable.pipe(
        new Transform({
          objectMode: true,
          highWaterMark: 1,
          transform(chunk, _, callback) {
            try {
              const transformed = transform(chunk);
              callback(undefined, transformed);
            } catch (err) {
              callback(err);
            }
          }
        })
      );

      // Bubble errors to transformed stream, because otherwise no way
      // to handle this error.
      readable.on('error', err => transformedStream.emit('error', err));

      return transformedStream;
    }

    return readable;
  }

  async hasNext(): Promise<boolean> {
    this.signal?.throwIfAborted();

    if (this.cursorId === Long.ZERO) {
      return false;
    }

    if (this.cursorOptions.timeoutMode === CursorTimeoutMode.ITERATION && this.cursorId != null) {
      this.timeoutContext?.refresh();
    }
    try {
      do {
        if ((this.documents?.length ?? 0) !== 0) {
          return true;
        }
        await this.fetchBatch();
      } while (!this.isDead || (this.documents?.length ?? 0) !== 0);
    } finally {
      if (this.cursorOptions.timeoutMode === CursorTimeoutMode.ITERATION) {
        this.timeoutContext?.clear();
      }
    }

    return false;
  }

  /** Get the next available document from the cursor, returns null if no more documents are available. */
  async next(): Promise<TSchema | null> {
    this.signal?.throwIfAborted();

    if (this.cursorId === Long.ZERO) {
      throw new MongoCursorExhaustedError();
    }

    if (this.cursorOptions.timeoutMode === CursorTimeoutMode.ITERATION && this.cursorId != null) {
      this.timeoutContext?.refresh();
    }

    try {
      do {
        const doc = this.documents?.shift(this.deserializationOptions);
        if (doc != null) {
          if (this.transform != null) return await this.transformDocument(doc);
          return doc;
        }
        await this.fetchBatch();
      } while (!this.isDead || (this.documents?.length ?? 0) !== 0);
    } finally {
      if (this.cursorOptions.timeoutMode === CursorTimeoutMode.ITERATION) {
        this.timeoutContext?.clear();
      }
    }

    return null;
  }

  /**
   * Try to get the next available document from the cursor or `null` if an empty batch is returned
   */
  async tryNext(): Promise<TSchema | null> {
    this.signal?.throwIfAborted();

    if (this.cursorId === Long.ZERO) {
      throw new MongoCursorExhaustedError();
    }

    if (this.cursorOptions.timeoutMode === CursorTimeoutMode.ITERATION && this.cursorId != null) {
      this.timeoutContext?.refresh();
    }
    try {
      let doc = this.documents?.shift(this.deserializationOptions);
      if (doc != null) {
        if (this.transform != null) return await this.transformDocument(doc);
        return doc;
      }

      await this.fetchBatch();

      doc = this.documents?.shift(this.deserializationOptions);
      if (doc != null) {
        if (this.transform != null) return await this.transformDocument(doc);
        return doc;
      }
    } finally {
      if (this.cursorOptions.timeoutMode === CursorTimeoutMode.ITERATION) {
        this.timeoutContext?.clear();
      }
    }

    return null;
  }

  /**
   * Iterates over all the documents for this cursor using the iterator, callback pattern.
   *
   * If the iterator returns `false`, iteration will stop.
   *
   * @param iterator - The iteration callback.
   * @deprecated - Will be removed in a future release. Use for await...of instead.
   */
  async forEach(iterator: (doc: TSchema) => boolean | void): Promise<void> {
    this.signal?.throwIfAborted();

    if (typeof iterator !== 'function') {
      throw new MongoInvalidArgumentError('Argument "iterator" must be a function');
    }
    for await (const document of this) {
      const result = iterator(document);
      if (result === false) {
        break;
      }
    }
  }

  /**
   * Frees any client-side resources used by the cursor.
   */
  async close(options?: { timeoutMS?: number }): Promise<void> {
    await this.cleanup(options?.timeoutMS);
  }

  /**
   * Returns an array of documents. The caller is responsible for making sure that there
   * is enough memory to store the results. Note that the array only contains partial
   * results when this cursor had been previously accessed. In that case,
   * cursor.rewind() can be used to reset the cursor.
   */
  async toArray(): Promise<TSchema[]> {
    this.signal?.throwIfAborted();

    const array: TSchema[] = [];
    // at the end of the loop (since readBufferedDocuments is called) the buffer will be empty
    // then, the 'await of' syntax will run a getMore call
    for await (const document of this) {
      array.push(document);
      const docs = this.readBufferedDocuments();
      if (this.transform != null) {
        for (const doc of docs) {
          array.push(await this.transformDocument(doc));
        }
      } else {
        array.push(...docs);
      }
    }
    return array;
  }
  /**
   * Add a cursor flag to the cursor
   *
   * @param flag - The flag to set, must be one of following ['tailable', 'oplogReplay', 'noCursorTimeout', 'awaitData', 'partial' -.
   * @param value - The flag boolean value.
   */
  addCursorFlag(flag: CursorFlag, value: boolean): this {
    this.throwIfInitialized();
    if (!CURSOR_FLAGS.includes(flag)) {
      throw new MongoInvalidArgumentError(`Flag ${flag} is not one of ${CURSOR_FLAGS}`);
    }

    if (typeof value !== 'boolean') {
      throw new MongoInvalidArgumentError(`Flag ${flag} must be a boolean value`);
    }

    this.cursorOptions[flag] = value;
    return this;
  }

  /**
   * Map all documents using the provided function
   * If there is a transform set on the cursor, that will be called first and the result passed to
   * this function's transform.
   *
   * @remarks
   *
   * **Note** Cursors use `null` internally to indicate that there are no more documents in the cursor. Providing a mapping
   * function that maps values to `null` will result in the cursor closing itself before it has finished iterating
   * all documents.  This will **not** result in a memory leak, just surprising behavior.  For example:
   *
   * ```typescript
   * const cursor = collection.find({});
   * cursor.map(() => null);
   *
   * const documents = await cursor.toArray();
   * // documents is always [], regardless of how many documents are in the collection.
   * ```
   *
   * Other falsey values are allowed:
   *
   * ```typescript
   * const cursor = collection.find({});
   * cursor.map(() => '');
   *
   * const documents = await cursor.toArray();
   * // documents is now an array of empty strings
   * ```
   *
   * **Note for Typescript Users:** adding a transform changes the return type of the iteration of this cursor,
   * it **does not** return a new instance of a cursor. This means when calling map,
   * you should always assign the result to a new variable in order to get a correctly typed cursor variable.
   * Take note of the following example:
   *
   * @example
   * ```typescript
   * const cursor: FindCursor<Document> = coll.find();
   * const mappedCursor: FindCursor<number> = cursor.map(doc => Object.keys(doc).length);
   * const keyCounts: number[] = await mappedCursor.toArray(); // cursor.toArray() still returns Document[]
   * ```
   * @param transform - The mapping transformation method.
   */
  map<T = any>(transform: (doc: TSchema) => T): AbstractCursor<T> {
    this.throwIfInitialized();
    const oldTransform = this.transform;
    if (oldTransform) {
      this.transform = doc => {
        return transform(oldTransform(doc));
      };
    } else {
      this.transform = transform;
    }

    return this as unknown as AbstractCursor<T>;
  }

  /**
   * Set the ReadPreference for the cursor.
   *
   * @param readPreference - The new read preference for the cursor.
   */
  withReadPreference(readPreference: ReadPreferenceLike): this {
    this.throwIfInitialized();
    if (readPreference instanceof ReadPreference) {
      this.cursorOptions.readPreference = readPreference;
    } else if (typeof readPreference === 'string') {
      this.cursorOptions.readPreference = ReadPreference.fromString(readPreference);
    } else {
      throw new MongoInvalidArgumentError(`Invalid read preference: ${readPreference}`);
    }

    return this;
  }

  /**
   * Set the ReadPreference for the cursor.
   *
   * @param readPreference - The new read preference for the cursor.
   */
  withReadConcern(readConcern: ReadConcernLike): this {
    this.throwIfInitialized();
    const resolvedReadConcern = ReadConcern.fromOptions({ readConcern });
    if (resolvedReadConcern) {
      this.cursorOptions.readConcern = resolvedReadConcern;
    }

    return this;
  }

  /**
   * Set a maxTimeMS on the cursor query, allowing for hard timeout limits on queries (Only supported on MongoDB 2.6 or higher)
   *
   * @param value - Number of milliseconds to wait before aborting the query.
   */
  maxTimeMS(value: number): this {
    this.throwIfInitialized();
    if (typeof value !== 'number') {
      throw new MongoInvalidArgumentError('Argument for maxTimeMS must be a number');
    }

    this.cursorOptions.maxTimeMS = value;
    return this;
  }

  /**
   * Set the batch size for the cursor.
   *
   * @param value - The number of documents to return per batch. See {@link https://www.mongodb.com/docs/manual/reference/command/find/|find command documentation}.
   */
  batchSize(value: number): this {
    this.throwIfInitialized();
    if (this.cursorOptions.tailable) {
      throw new MongoTailableCursorError('Tailable cursor does not support batchSize');
    }

    if (typeof value !== 'number') {
      throw new MongoInvalidArgumentError('Operation "batchSize" requires an integer');
    }

    this.cursorOptions.batchSize = value;
    return this;
  }

  /**
   * Rewind this cursor to its uninitialized state. Any options that are present on the cursor will
   * remain in effect. Iterating this cursor will cause new queries to be sent to the server, even
   * if the resultant data has already been retrieved by this cursor.
   */
  rewind(): void {
    if (this.timeoutContext && this.timeoutContext.owner !== this) {
      throw new MongoAPIError(`Cannot rewind cursor that does not own its timeout context.`);
    }
    if (!this.initialized) {
      return;
    }

    this.cursorId = null;
    this.documents?.clear();
    this.timeoutContext?.clear();
    this.timeoutContext = undefined;
    this.isClosed = false;
    this.isKilled = false;
    this.initialized = false;
    this.hasEmittedClose = false;
    this.trackCursor();

    // We only want to end this session if we created it, and it hasn't ended yet
    if (this.cursorSession.explicit === false) {
      if (!this.cursorSession.hasEnded) {
        this.cursorSession.endSession().then(undefined, squashError);
      }
      this.cursorSession = this.cursorClient.startSession({ owner: this, explicit: false });
    }
  }

  /**
   * Returns a new uninitialized copy of this cursor, with options matching those that have been set on the current instance
   */
  abstract clone(): AbstractCursor<TSchema>;

  /** @internal */
  protected abstract _initialize(
    session: ClientSession | undefined
  ): Promise<InitialCursorResponse>;

  /** @internal */
  async getMore(batchSize: number): Promise<CursorResponse> {
    if (this.cursorId == null) {
      throw new MongoRuntimeError(
        'Unexpected null cursor id. A cursor creating command should have set this'
      );
    }
    if (this.selectedServer == null) {
      throw new MongoRuntimeError(
        'Unexpected null selectedServer. A cursor creating command should have set this'
      );
    }
    const getMoreOptions = {
      ...this.cursorOptions,
      session: this.cursorSession,
      batchSize
    };

    const getMoreOperation = new GetMoreOperation(
      this.cursorNamespace,
      this.cursorId,
      this.selectedServer,
      getMoreOptions
    );

    return await executeOperation(this.cursorClient, getMoreOperation, this.timeoutContext);
  }

  /**
   * @internal
   *
   * This function is exposed for the unified test runner's createChangeStream
   * operation.  We cannot refactor to use the abstract _initialize method without
   * a significant refactor.
   */
  private async cursorInit(): Promise<void> {
    if (this.cursorOptions.timeoutMS != null) {
      this.timeoutContext ??= new CursorTimeoutContext(
        TimeoutContext.create({
          serverSelectionTimeoutMS: this.client.s.options.serverSelectionTimeoutMS,
          timeoutMS: this.cursorOptions.timeoutMS
        }),
        this
      );
    }
    try {
      const state = await this._initialize(this.cursorSession);
      // Set omitMaxTimeMS to the value needed for subsequent getMore calls
      this.cursorOptions.omitMaxTimeMS = this.cursorOptions.timeoutMS != null;
      const response = state.response;
      this.selectedServer = state.server;
      this.cursorId = response.id;
      this.cursorNamespace = response.ns ?? this.namespace;
      this.documents = response;
      this.initialized = true; // the cursor is now initialized, even if it is dead
    } catch (error) {
      // the cursor is now initialized, even if an error occurred
      this.initialized = true;
      await this.cleanup(undefined, error);
      throw error;
    }

    if (this.isDead) {
      await this.cleanup();
    }

    return;
  }

  /** @internal Attempt to obtain more documents */
  private async fetchBatch(): Promise<void> {
    if (this.isClosed) {
      return;
    }

    if (this.isDead) {
      // if the cursor is dead, we clean it up
      // cleanupCursor should never throw, but if it does it indicates a bug in the driver
      // and we should surface the error
      await this.cleanup();
      return;
    }

    if (this.cursorId == null) {
      await this.cursorInit();
      // If the cursor died or returned documents, return
      if ((this.documents?.length ?? 0) !== 0 || this.isDead) return;
      // Otherwise, run a getMore
    }

    // otherwise need to call getMore
    const batchSize = this.cursorOptions.batchSize || 1000;

    try {
      const response = await this.getMore(batchSize);
      this.cursorId = response.id;
      this.documents = response;
    } catch (error) {
      try {
        await this.cleanup(undefined, error);
      } catch (cleanupError) {
        // `cleanupCursor` should never throw, squash and throw the original error
        squashError(cleanupError);
      }
      throw error;
    }

    if (this.isDead) {
      // If we successfully received a response from a cursor BUT the cursor indicates that it is exhausted,
      // we intentionally clean up the cursor to release its session back into the pool before the cursor
      // is iterated.  This prevents a cursor that is exhausted on the server from holding
      // onto a session indefinitely until the AbstractCursor is iterated.
      //
      // cleanupCursorAsync should never throw, but if it does it indicates a bug in the driver
      // and we should surface the error
      await this.cleanup();
    }
  }

  /** @internal */
  private async cleanup(timeoutMS?: number, error?: Error) {
    this.abortListener?.[kDispose]();
    this.isClosed = true;
    const timeoutContextForKillCursors = (): CursorTimeoutContext | undefined => {
      if (timeoutMS != null) {
        this.timeoutContext?.clear();
        return new CursorTimeoutContext(
          TimeoutContext.create({
            serverSelectionTimeoutMS: this.client.s.options.serverSelectionTimeoutMS,
            timeoutMS
          }),
          this
        );
      } else {
        return this.timeoutContext?.refreshed();
      }
    };
    try {
      if (
        !this.isKilled &&
        this.cursorId &&
        !this.cursorId.isZero() &&
        this.cursorNamespace &&
        this.selectedServer &&
        !this.cursorSession.hasEnded
      ) {
        this.isKilled = true;
        const cursorId = this.cursorId;
        this.cursorId = Long.ZERO;

        await executeOperation(
          this.cursorClient,
          new KillCursorsOperation(cursorId, this.cursorNamespace, this.selectedServer, {
            session: this.cursorSession
          }),
          timeoutContextForKillCursors()
        );
      }
    } catch (error) {
      squashError(error);
    } finally {
      try {
        if (this.cursorSession?.owner === this) {
          await this.cursorSession.endSession({ error });
        }
        if (!this.cursorSession?.inTransaction()) {
          maybeClearPinnedConnection(this.cursorSession, { error });
        }
      } finally {
        this.emitClose();
      }
    }
  }

  /** @internal */
  private hasEmittedClose = false;
  /** @internal */
  private emitClose() {
    try {
      if (!this.hasEmittedClose && ((this.documents?.length ?? 0) === 0 || this.isClosed)) {
        // @ts-expect-error: CursorEvents is generic so Parameters<CursorEvents["close"]> may not be assignable to `[]`. Not sure how to require extenders do not add parameters.
        this.emit('close');
      }
    } finally {
      this.hasEmittedClose = true;
    }
  }

  /** @internal */
  private async transformDocument(document: NonNullable<TSchema>): Promise<NonNullable<TSchema>> {
    if (this.transform == null) return document;

    try {
      const transformedDocument = this.transform(document);
      // eslint-disable-next-line no-restricted-syntax
      if (transformedDocument === null) {
        const TRANSFORM_TO_NULL_ERROR =
          'Cursor returned a `null` document, but the cursor is not exhausted.  Mapping documents to `null` is not supported in the cursor transform.';
        throw new MongoAPIError(TRANSFORM_TO_NULL_ERROR);
      }
      return transformedDocument;
    } catch (transformError) {
      try {
        await this.close();
      } catch (closeError) {
        squashError(closeError);
      }
      throw transformError;
    }
  }

  /** @internal */
  protected throwIfInitialized() {
    if (this.initialized) throw new MongoCursorInUseError();
  }
}

class ReadableCursorStream extends Readable {
  private _cursor: AbstractCursor;
  private _readInProgress = false;

  constructor(cursor: AbstractCursor) {
    super({
      objectMode: true,
      autoDestroy: false,
      highWaterMark: 1
    });
    this._cursor = cursor;
  }

  // eslint-disable-next-line @typescript-eslint/no-unused-vars
  override _read(size: number): void {
    if (!this._readInProgress) {
      this._readInProgress = true;
      this._readNext();
    }
  }

  override _destroy(error: Error | null, callback: (error?: Error | null) => void): void {
    this._cursor.close().then(
      () => callback(error),
      closeError => callback(closeError)
    );
  }

  private _readNext() {
    if (this._cursor.id === Long.ZERO) {
      this.push(null);
      return;
    }

    this._cursor
      .next()
      .then(
        // result from next()
        result => {
          if (result == null) {
            this.push(null);
          } else if (this.destroyed) {
            this._cursor.close().then(undefined, squashError);
          } else {
            if (this.push(result)) {
              return this._readNext();
            }

            this._readInProgress = false;
          }
        },
        // error from next()
        err => {
          // NOTE: This is questionable, but we have a test backing the behavior. It seems the
          //       desired behavior is that a stream ends cleanly when a user explicitly closes
          //       a client during iteration. Alternatively, we could do the "right" thing and
          //       propagate the error message by removing this special case.
          if (err.message.match(/server is closed/)) {
            this._cursor.close().then(undefined, squashError);
            return this.push(null);
          }

          // NOTE: This is also perhaps questionable. The rationale here is that these errors tend
          //       to be "operation was interrupted", where a cursor has been closed but there is an
          //       active getMore in-flight. This used to check if the cursor was killed but once
          //       that changed to happen in cleanup legitimate errors would not destroy the
          //       stream. There are change streams test specifically test these cases.
          if (err.message.match(/operation was interrupted/)) {
            return this.push(null);
          }

          // NOTE: The two above checks on the message of the error will cause a null to be pushed
          //       to the stream, thus closing the stream before the destroy call happens. This means
          //       that either of those error messages on a change stream will not get a proper
          //       'error' event to be emitted (the error passed to destroy). Change stream resumability
          //       relies on that error event to be emitted to create its new cursor and thus was not
          //       working on 4.4 servers because the error emitted on failover was "interrupted at
          //       shutdown" while on 5.0+ it is "The server is in quiesce mode and will shut down".
          //       See NODE-4475.
          return this.destroy(err);
        }
      )
      // if either of the above handlers throw
      .catch(error => {
        this._readInProgress = false;
        this.destroy(error);
      });
  }
}

configureResourceManagement(AbstractCursor.prototype);

/**
 * @internal
 * The cursor timeout context is a wrapper around a timeout context
 * that keeps track of the "owner" of the cursor.  For timeout contexts
 * instantiated inside a cursor, the owner will be the cursor.
 *
 * All timeout behavior is exactly the same as the wrapped timeout context's.
 */
export class CursorTimeoutContext extends TimeoutContext {
  constructor(
    public timeoutContext: TimeoutContext,
    public owner: symbol | AbstractCursor
  ) {
    super();
  }
  override get serverSelectionTimeout(): Timeout | null {
    return this.timeoutContext.serverSelectionTimeout;
  }
  override get connectionCheckoutTimeout(): Timeout | null {
    return this.timeoutContext.connectionCheckoutTimeout;
  }
  override get clearServerSelectionTimeout(): boolean {
    return this.timeoutContext.clearServerSelectionTimeout;
  }
  override get timeoutForSocketWrite(): Timeout | null {
    return this.timeoutContext.timeoutForSocketWrite;
  }
  override get timeoutForSocketRead(): Timeout | null {
    return this.timeoutContext.timeoutForSocketRead;
  }
  override csotEnabled(): this is CSOTTimeoutContext {
    return this.timeoutContext.csotEnabled();
  }
  override refresh(): void {
    if (typeof this.owner !== 'symbol') return this.timeoutContext.refresh();
  }
  override clear(): void {
    if (typeof this.owner !== 'symbol') return this.timeoutContext.clear();
  }
  override get maxTimeMS(): number | null {
    return this.timeoutContext.maxTimeMS;
  }
  get timeoutMS(): number | null {
    return this.timeoutContext.csotEnabled() ? this.timeoutContext.timeoutMS : null;
  }
  override refreshed(): CursorTimeoutContext {
    return new CursorTimeoutContext(this.timeoutContext.refreshed(), this.owner);
  }
  override addMaxTimeMSToCommand(command: Document, options: { omitMaxTimeMS?: boolean }): void {
    this.timeoutContext.addMaxTimeMSToCommand(command, options);
  }
  override getSocketTimeoutMS(): number | undefined {
    return this.timeoutContext.getSocketTimeoutMS();
  }
}