import { setTimeout } from 'timers/promises';

import { Binary, ByteUtils, type Document, Long, type Timestamp } from './bson';
import type { CommandOptions, Connection } from './cmap/connection';
import { ConnectionPoolMetrics } from './cmap/metrics';
import { type MongoDBResponse } from './cmap/wire_protocol/responses';
import { PINNED, UNPINNED } from './constants';
import type { AbstractCursor } from './cursor/abstract_cursor';
import {
  type AnyError,
  isRetryableWriteError,
  MongoAPIError,
  MongoCompatibilityError,
  MONGODB_ERROR_CODES,
  type MongoDriverError,
  MongoError,
  MongoErrorLabel,
  MongoExpiredSessionError,
  MongoInvalidArgumentError,
  MongoOperationTimeoutError,
  MongoRuntimeError,
  MongoServerError,
  MongoTransactionError,
  MongoWriteConcernError
} from './error';
import type { MongoClient, MongoOptions } from './mongo_client';
import { TypedEventEmitter } from './mongo_types';
import { executeOperation } from './operations/execute_operation';
import { RunCommandOperation } from './operations/run_command';
import { ReadConcernLevel } from './read_concern';
import { ReadPreference } from './read_preference';
import { _advanceClusterTime, type ClusterTime, TopologyType } from './sdam/common';
import { TimeoutContext } from './timeout';
import {
  isTransactionCommand,
  Transaction,
  type TransactionOptions,
  TxnState
} from './transactions';
import {
  calculateDurationInMs,
  commandSupportsReadConcern,
  isPromiseLike,
  List,
  MongoDBNamespace,
  noop,
  processTimeMS,
  squashError,
  uuidV4
} from './utils';
import { WriteConcern, type WriteConcernOptions, type WriteConcernSettings } from './write_concern';

/** @public */
export interface ClientSessionOptions {
  /** Whether causal consistency should be enabled on this session */
  causalConsistency?: boolean;
  /** Whether all read operations should be read from the same snapshot for this session (NOTE: not compatible with `causalConsistency=true`) */
  snapshot?: boolean;
  /** The default TransactionOptions to use for transactions started on this session. */
  defaultTransactionOptions?: TransactionOptions;
  /**
   * @public
   * @experimental
   * An overriding timeoutMS value to use for a client-side timeout.
   * If not provided the session uses the timeoutMS specified on the MongoClient.
   */
  defaultTimeoutMS?: number;

  /** @internal */
  owner?: symbol | AbstractCursor;
  /** @internal */
  explicit?: boolean;
  /** @internal */
  initialClusterTime?: ClusterTime;
}

/** @public */
export type WithTransactionCallback<T = any> = (session: ClientSession) => Promise<T>;

/** @public */
export type ClientSessionEvents = {
  ended(session: ClientSession): void;
};

/** @public */
export interface EndSessionOptions {
  /**
   * An optional error which caused the call to end this session
   * @internal
   */
  error?: AnyError;
  force?: boolean;
  forceClear?: boolean;

  /** Specifies the time an operation will run until it throws a timeout error */
  timeoutMS?: number;
}

/**
 * A class representing a client session on the server
 *
 * NOTE: not meant to be instantiated directly.
 * @public
 */
export class ClientSession
  extends TypedEventEmitter<ClientSessionEvents>
  implements AsyncDisposable
{
  /** @internal */
  client: MongoClient;
  /** @internal */
  sessionPool: ServerSessionPool;
  hasEnded: boolean;
  clientOptions: MongoOptions;
  supports: { causalConsistency: boolean };
  clusterTime?: ClusterTime;
  operationTime?: Timestamp;
  explicit: boolean;
  /** @internal */
  owner?: symbol | AbstractCursor;
  defaultTransactionOptions: TransactionOptions;
  /** @internal */
  transaction: Transaction;
  /**
   * @internal
   * Keeps track of whether or not the current transaction has attempted to be committed. Is
   * initially undefined. Gets set to false when startTransaction is called. When commitTransaction is sent to server, if the commitTransaction succeeds, it is then set to undefined, otherwise, set to true
   */
  private commitAttempted?: boolean;
  public readonly snapshotEnabled: boolean;

  /** @internal */
  private _serverSession: ServerSession | null;
  /** @internal */
  public snapshotTime?: Timestamp;
  /** @internal */
  public pinnedConnection?: Connection;
  /** @internal */
  public txnNumberIncrement: number;
  /**
   * @experimental
   * Specifies the time an operation in a given `ClientSession` will run until it throws a timeout error
   */
  timeoutMS?: number;

  /** @internal */
  public timeoutContext: TimeoutContext | null = null;

  /**
   * Create a client session.
   * @internal
   * @param client - The current client
   * @param sessionPool - The server session pool (Internal Class)
   * @param options - Optional settings
   * @param clientOptions - Optional settings provided when creating a MongoClient
   */
  constructor(
    client: MongoClient,
    sessionPool: ServerSessionPool,
    options: ClientSessionOptions,
    clientOptions: MongoOptions
  ) {
    super();
    this.on('error', noop);

    if (client == null) {
      // TODO(NODE-3483)
      throw new MongoRuntimeError('ClientSession requires a MongoClient');
    }

    if (sessionPool == null || !(sessionPool instanceof ServerSessionPool)) {
      // TODO(NODE-3483)
      throw new MongoRuntimeError('ClientSession requires a ServerSessionPool');
    }

    options = options ?? {};

    this.snapshotEnabled = options.snapshot === true;
    if (options.causalConsistency === true && this.snapshotEnabled) {
      throw new MongoInvalidArgumentError(
        'Properties "causalConsistency" and "snapshot" are mutually exclusive'
      );
    }

    this.client = client;
    this.sessionPool = sessionPool;
    this.hasEnded = false;
    this.clientOptions = clientOptions;
    this.timeoutMS = options.defaultTimeoutMS ?? client.s.options?.timeoutMS;

    this.explicit = !!options.explicit;
    this._serverSession = this.explicit ? this.sessionPool.acquire() : null;
    this.txnNumberIncrement = 0;

    const defaultCausalConsistencyValue = this.explicit && options.snapshot !== true;
    this.supports = {
      // if we can enable causal consistency, do so by default
      causalConsistency: options.causalConsistency ?? defaultCausalConsistencyValue
    };

    this.clusterTime = options.initialClusterTime;

    this.operationTime = undefined;
    this.owner = options.owner;
    this.defaultTransactionOptions = { ...options.defaultTransactionOptions };
    this.transaction = new Transaction();
  }

  /** The server id associated with this session */
  get id(): ServerSessionId | undefined {
    return this.serverSession?.id;
  }

  get serverSession(): ServerSession {
    let serverSession = this._serverSession;
    if (serverSession == null) {
      if (this.explicit) {
        throw new MongoRuntimeError('Unexpected null serverSession for an explicit session');
      }
      if (this.hasEnded) {
        throw new MongoRuntimeError('Unexpected null serverSession for an ended implicit session');
      }
      serverSession = this.sessionPool.acquire();
      this._serverSession = serverSession;
    }
    return serverSession;
  }

  get loadBalanced(): boolean {
    return this.client.topology?.description.type === TopologyType.LoadBalanced;
  }

  /** @internal */
  pin(conn: Connection): void {
    if (this.pinnedConnection) {
      throw TypeError('Cannot pin multiple connections to the same session');
    }

    this.pinnedConnection = conn;
    conn.emit(
      PINNED,
      this.inTransaction() ? ConnectionPoolMetrics.TXN : ConnectionPoolMetrics.CURSOR
    );
  }

  /** @internal */
  unpin(options?: { force?: boolean; forceClear?: boolean; error?: AnyError }): void {
    if (this.loadBalanced) {
      return maybeClearPinnedConnection(this, options);
    }

    this.transaction.unpinServer();
  }

  get isPinned(): boolean {
    return this.loadBalanced ? !!this.pinnedConnection : this.transaction.isPinned;
  }

  /**
   * Frees any client-side resources held by the current session.  If a session is in a transaction,
   * the transaction is aborted.
   *
   * Does not end the session on the server.
   *
   * @param options - Optional settings. Currently reserved for future use
   */
  async endSession(options?: EndSessionOptions): Promise<void> {
    try {
      if (this.inTransaction()) {
        await this.abortTransaction({ ...options, throwTimeout: true });
      }
    } catch (error) {
      // spec indicates that we should ignore all errors for `endSessions`
      if (error.name === 'MongoOperationTimeoutError') throw error;
      squashError(error);
    } finally {
      if (!this.hasEnded) {
        const serverSession = this.serverSession;
        if (serverSession != null) {
          // release the server session back to the pool
          this.sessionPool.release(serverSession);
          // Store a clone of the server session for reference (debugging)
          this._serverSession = new ServerSession(serverSession);
        }
        // mark the session as ended, and emit a signal
        this.hasEnded = true;
        this.emit('ended', this);
      }
      maybeClearPinnedConnection(this, { force: true, ...options });
    }
  }
  /**
   * @experimental
   * An alias for {@link ClientSession.endSession|ClientSession.endSession()}.
   */
  async [Symbol.asyncDispose]() {
    await this.endSession({ force: true });
  }

  /**
   * Advances the operationTime for a ClientSession.
   *
   * @param operationTime - the `BSON.Timestamp` of the operation type it is desired to advance to
   */
  advanceOperationTime(operationTime: Timestamp): void {
    if (this.operationTime == null) {
      this.operationTime = operationTime;
      return;
    }

    if (operationTime.greaterThan(this.operationTime)) {
      this.operationTime = operationTime;
    }
  }

  /**
   * Advances the clusterTime for a ClientSession to the provided clusterTime of another ClientSession
   *
   * @param clusterTime - the $clusterTime returned by the server from another session in the form of a document containing the `BSON.Timestamp` clusterTime and signature
   */
  advanceClusterTime(clusterTime: ClusterTime): void {
    if (!clusterTime || typeof clusterTime !== 'object') {
      throw new MongoInvalidArgumentError('input cluster time must be an object');
    }
    if (!clusterTime.clusterTime || clusterTime.clusterTime._bsontype !== 'Timestamp') {
      throw new MongoInvalidArgumentError(
        'input cluster time "clusterTime" property must be a valid BSON Timestamp'
      );
    }
    if (
      !clusterTime.signature ||
      clusterTime.signature.hash?._bsontype !== 'Binary' ||
      (typeof clusterTime.signature.keyId !== 'bigint' &&
        typeof clusterTime.signature.keyId !== 'number' &&
        clusterTime.signature.keyId?._bsontype !== 'Long') // apparently we decode the key to number?
    ) {
      throw new MongoInvalidArgumentError(
        'input cluster time must have a valid "signature" property with BSON Binary hash and BSON Long keyId'
      );
    }

    _advanceClusterTime(this, clusterTime);
  }

  /**
   * Used to determine if this session equals another
   *
   * @param session - The session to compare to
   */
  equals(session: ClientSession): boolean {
    if (!(session instanceof ClientSession)) {
      return false;
    }

    if (this.id == null || session.id == null) {
      return false;
    }

    return ByteUtils.equals(this.id.id.buffer, session.id.id.buffer);
  }

  /**
   * Increment the transaction number on the internal ServerSession
   *
   * @privateRemarks
   * This helper increments a value stored on the client session that will be
   * added to the serverSession's txnNumber upon applying it to a command.
   * This is because the serverSession is lazily acquired after a connection is obtained
   */
  incrementTransactionNumber(): void {
    this.txnNumberIncrement += 1;
  }

  /** @returns whether this session is currently in a transaction or not */
  inTransaction(): boolean {
    return this.transaction.isActive;
  }

  /**
   * Starts a new transaction with the given options.
   *
   * @remarks
   * **IMPORTANT**: Running operations in parallel is not supported during a transaction. The use of `Promise.all`,
   * `Promise.allSettled`, `Promise.race`, etc to parallelize operations inside a transaction is
   * undefined behaviour.
   *
   * @param options - Options for the transaction
   */
  startTransaction(options?: TransactionOptions): void {
    if (this.snapshotEnabled) {
      throw new MongoCompatibilityError('Transactions are not supported in snapshot sessions');
    }

    if (this.inTransaction()) {
      throw new MongoTransactionError('Transaction already in progress');
    }

    if (this.isPinned && this.transaction.isCommitted) {
      this.unpin();
    }

    this.commitAttempted = false;
    // increment txnNumber
    this.incrementTransactionNumber();
    // create transaction state
    this.transaction = new Transaction({
      readConcern:
        options?.readConcern ??
        this.defaultTransactionOptions.readConcern ??
        this.clientOptions?.readConcern,
      writeConcern:
        options?.writeConcern ??
        this.defaultTransactionOptions.writeConcern ??
        this.clientOptions?.writeConcern,
      readPreference:
        options?.readPreference ??
        this.defaultTransactionOptions.readPreference ??
        this.clientOptions?.readPreference,
      maxCommitTimeMS: options?.maxCommitTimeMS ?? this.defaultTransactionOptions.maxCommitTimeMS
    });

    this.transaction.transition(TxnState.STARTING_TRANSACTION);
  }

  /**
   * Commits the currently active transaction in this session.
   *
   * @param options - Optional options, can be used to override `defaultTimeoutMS`.
   */
  async commitTransaction(options?: { timeoutMS?: number }): Promise<void> {
    if (this.transaction.state === TxnState.NO_TRANSACTION) {
      throw new MongoTransactionError('No transaction started');
    }

    if (
      this.transaction.state === TxnState.STARTING_TRANSACTION ||
      this.transaction.state === TxnState.TRANSACTION_COMMITTED_EMPTY
    ) {
      // the transaction was never started, we can safely exit here
      this.transaction.transition(TxnState.TRANSACTION_COMMITTED_EMPTY);
      return;
    }

    if (this.transaction.state === TxnState.TRANSACTION_ABORTED) {
      throw new MongoTransactionError(
        'Cannot call commitTransaction after calling abortTransaction'
      );
    }

    const command: {
      commitTransaction: 1;
      writeConcern?: WriteConcernSettings;
      recoveryToken?: Document;
      maxTimeMS?: number;
    } = { commitTransaction: 1 };

    const timeoutMS =
      typeof options?.timeoutMS === 'number'
        ? options.timeoutMS
        : typeof this.timeoutMS === 'number'
          ? this.timeoutMS
          : null;

    const wc = this.transaction.options.writeConcern ?? this.clientOptions?.writeConcern;
    if (wc != null) {
      if (timeoutMS == null && this.timeoutContext == null) {
        WriteConcern.apply(command, { wtimeoutMS: 10000, w: 'majority', ...wc });
      } else {
        const wcKeys = Object.keys(wc);
        if (wcKeys.length > 2 || (!wcKeys.includes('wtimeoutMS') && !wcKeys.includes('wTimeoutMS')))
          // if the write concern was specified with wTimeoutMS, then we set both wtimeoutMS
          // and wTimeoutMS, guaranteeing at least two keys, so if we have more than two keys,
          // then we can automatically assume that we should add the write concern to the command.
          // If it has 2 or fewer keys, we need to check that those keys aren't the wtimeoutMS
          // or wTimeoutMS options before we add the write concern to the command
          WriteConcern.apply(command, { ...wc, wtimeoutMS: undefined });
      }
    }

    if (this.transaction.state === TxnState.TRANSACTION_COMMITTED || this.commitAttempted) {
      if (timeoutMS == null && this.timeoutContext == null) {
        WriteConcern.apply(command, { wtimeoutMS: 10000, ...wc, w: 'majority' });
      } else {
        WriteConcern.apply(command, { w: 'majority', ...wc, wtimeoutMS: undefined });
      }
    }

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

    if (this.transaction.recoveryToken) {
      command.recoveryToken = this.transaction.recoveryToken;
    }

    const operation = new RunCommandOperation(new MongoDBNamespace('admin'), command, {
      session: this,
      readPreference: ReadPreference.primary,
      bypassPinningCheck: true
    });
    operation.maxAttempts = this.clientOptions.maxAdaptiveRetries + 1;

    const timeoutContext =
      this.timeoutContext ??
      (typeof timeoutMS === 'number'
        ? TimeoutContext.create({
            serverSelectionTimeoutMS: this.clientOptions.serverSelectionTimeoutMS,
            socketTimeoutMS: this.clientOptions.socketTimeoutMS,
            timeoutMS
          })
        : null);

    try {
      await executeOperation(this.client, operation, timeoutContext);
      this.commitAttempted = undefined;
      return;
    } catch (firstCommitError) {
      this.commitAttempted = true;

      const remainingAttempts = this.clientOptions.maxAdaptiveRetries + 1 - operation.attemptsMade;
      if (remainingAttempts <= 0) {
        throw firstCommitError;
      }

      if (firstCommitError instanceof MongoError && isRetryableWriteError(firstCommitError)) {
        // SPEC-1185: apply majority write concern when retrying commitTransaction
        WriteConcern.apply(command, { wtimeoutMS: 10000, ...wc, w: 'majority' });
        // per txns spec, must unpin session in this case
        this.unpin({ force: true });

        try {
          const op = new RunCommandOperation(new MongoDBNamespace('admin'), command, {
            session: this,
            readPreference: ReadPreference.primary,
            bypassPinningCheck: true
          });
          op.maxAttempts = remainingAttempts;
          await executeOperation(this.client, op, timeoutContext);
          return;
        } catch (retryCommitError) {
          // If the retry failed, we process that error instead of the original
          if (shouldAddUnknownTransactionCommitResultLabel(retryCommitError)) {
            retryCommitError.addErrorLabel(MongoErrorLabel.UnknownTransactionCommitResult);
          }

          if (shouldUnpinAfterCommitError(retryCommitError)) {
            this.unpin({ error: retryCommitError });
          }

          throw retryCommitError;
        }
      }

      if (shouldAddUnknownTransactionCommitResultLabel(firstCommitError)) {
        firstCommitError.addErrorLabel(MongoErrorLabel.UnknownTransactionCommitResult);
      }

      if (shouldUnpinAfterCommitError(firstCommitError)) {
        this.unpin({ error: firstCommitError });
      }

      throw firstCommitError;
    } finally {
      this.transaction.transition(TxnState.TRANSACTION_COMMITTED);
    }
  }

  /**
   * Aborts the currently active transaction in this session.
   *
   * @param options - Optional options, can be used to override `defaultTimeoutMS`.
   */
  async abortTransaction(options?: { timeoutMS?: number }): Promise<void>;
  /** @internal */
  async abortTransaction(options?: { timeoutMS?: number; throwTimeout?: true }): Promise<void>;
  async abortTransaction(options?: { timeoutMS?: number; throwTimeout?: true }): Promise<void> {
    if (this.transaction.state === TxnState.NO_TRANSACTION) {
      throw new MongoTransactionError('No transaction started');
    }

    if (this.transaction.state === TxnState.STARTING_TRANSACTION) {
      // the transaction was never started, we can safely exit here
      this.transaction.transition(TxnState.TRANSACTION_ABORTED);
      return;
    }

    if (this.transaction.state === TxnState.TRANSACTION_ABORTED) {
      throw new MongoTransactionError('Cannot call abortTransaction twice');
    }

    if (
      this.transaction.state === TxnState.TRANSACTION_COMMITTED ||
      this.transaction.state === TxnState.TRANSACTION_COMMITTED_EMPTY
    ) {
      throw new MongoTransactionError(
        'Cannot call abortTransaction after calling commitTransaction'
      );
    }

    const command: {
      abortTransaction: 1;
      writeConcern?: WriteConcernOptions;
      recoveryToken?: Document;
    } = { abortTransaction: 1 };

    const timeoutMS =
      typeof options?.timeoutMS === 'number'
        ? options.timeoutMS
        : this.timeoutContext?.csotEnabled()
          ? this.timeoutContext.timeoutMS // refresh timeoutMS for abort operation
          : typeof this.timeoutMS === 'number'
            ? this.timeoutMS
            : null;

    const timeoutContext =
      timeoutMS != null
        ? TimeoutContext.create({
            timeoutMS,
            serverSelectionTimeoutMS: this.clientOptions.serverSelectionTimeoutMS,
            socketTimeoutMS: this.clientOptions.socketTimeoutMS
          })
        : null;

    const wc = this.transaction.options.writeConcern ?? this.clientOptions?.writeConcern;
    if (wc != null && timeoutMS == null) {
      WriteConcern.apply(command, { wtimeoutMS: 10000, w: 'majority', ...wc });
    }

    if (this.transaction.recoveryToken) {
      command.recoveryToken = this.transaction.recoveryToken;
    }

    const operation = new RunCommandOperation(new MongoDBNamespace('admin'), command, {
      session: this,
      readPreference: ReadPreference.primary,
      bypassPinningCheck: true
    });

    try {
      await executeOperation(this.client, operation, timeoutContext);
      this.unpin();
      return;
    } catch (firstAbortError) {
      this.unpin();

      if (firstAbortError.name === 'MongoRuntimeError') throw firstAbortError;
      if (options?.throwTimeout && firstAbortError.name === 'MongoOperationTimeoutError') {
        throw firstAbortError;
      }

      if (firstAbortError instanceof MongoError && isRetryableWriteError(firstAbortError)) {
        try {
          await executeOperation(this.client, operation, timeoutContext);
          return;
        } catch (secondAbortError) {
          if (secondAbortError.name === 'MongoRuntimeError') throw secondAbortError;
          if (options?.throwTimeout && secondAbortError.name === 'MongoOperationTimeoutError') {
            throw secondAbortError;
          }
          // we do not retry the retry
        }
      }

      // The spec indicates that if the operation times out or fails with a non-retryable error, we should ignore all errors on `abortTransaction`
    } finally {
      this.transaction.transition(TxnState.TRANSACTION_ABORTED);
      if (this.loadBalanced) {
        maybeClearPinnedConnection(this, { force: false });
      }
    }
  }

  /**
   * This is here to ensure that ClientSession is never serialized to BSON.
   */
  toBSON(): never {
    throw new MongoRuntimeError('ClientSession cannot be serialized to BSON.');
  }

  /**
   * Starts a transaction and runs a provided function, ensuring the commitTransaction is always attempted when all operations run in the function have completed.
   *
   * **IMPORTANT:** This method requires the function passed in to return a Promise. That promise must be made by `await`-ing all operations in such a way that rejections are propagated to the returned promise.
   *
   * **IMPORTANT:** Running operations in parallel is not supported during a transaction. The use of `Promise.all`,
   * `Promise.allSettled`, `Promise.race`, etc to parallelize operations inside a transaction is
   * undefined behaviour.
   *
   * **IMPORTANT:** When running an operation inside a `withTransaction` callback, if it is not
   * provided the explicit session in its options, it will not be part of the transaction and it will not respect timeoutMS.
   *
   *
   * @remarks
   * - If all operations successfully complete and the `commitTransaction` operation is successful, then the provided function will return the result of the provided function.
   * - If the transaction is unable to complete or an error is thrown from within the provided function, then the provided function will throw an error.
   *   - If the transaction is manually aborted within the provided function it will not throw.
   * - If the driver needs to attempt to retry the operations, the provided function may be called multiple times.
   *
   * Checkout a descriptive example here:
   * @see https://www.mongodb.com/blog/post/quick-start-nodejs--mongodb--how-to-implement-transactions
   *
   * If a command inside withTransaction fails:
   * - It may cause the transaction on the server to be aborted.
   * - This situation is normally handled transparently by the driver.
   * - However, if the application catches such an error and does not rethrow it, the driver will not be able to determine whether the transaction was aborted or not.
   * - The driver will then retry the transaction indefinitely.
   *
   * To avoid this situation, the application must not silently handle errors within the provided function.
   * If the application needs to handle errors within, it must await all operations such that if an operation is rejected it becomes the rejection of the callback function passed into withTransaction.
   *
   * @param fn - callback to run within a transaction
   * @param options - optional settings for the transaction
   * @returns A raw command response or undefined
   */
  async withTransaction<T = any>(
    fn: WithTransactionCallback<T>,
    options?: TransactionOptions & {
      /**
       * Configures a timeoutMS expiry for the entire withTransactionCallback.
       *
       * @remarks
       * - The remaining timeout will not be applied to callback operations that do not use the ClientSession.
       * - Overriding timeoutMS for operations executed using the explicit session inside the provided callback will result in a client-side error.
       */
      timeoutMS?: number;
    }
  ): Promise<T> {
    const MAX_TIMEOUT = 120_000;

    const timeoutMS = options?.timeoutMS ?? this.timeoutMS ?? null;
    this.timeoutContext =
      timeoutMS != null
        ? TimeoutContext.create({
            timeoutMS,
            serverSelectionTimeoutMS: this.clientOptions.serverSelectionTimeoutMS,
            socketTimeoutMS: this.clientOptions.socketTimeoutMS
          })
        : null;

    // 1. Define the following:
    // 1.1 Record the current monotonic time, which will be used to enforce the 120-second / CSOT timeout before later retry attempts.
    // 1.2 Set `transactionAttempt` to `0`.
    // 1.3 Set `TIMEOUT_MS` to be `timeoutMS` if given, otherwise MAX_TIMEOUT (120-seconds).

    // Timeout Error propagation
    // When the previously encountered error needs to be propagated because there is no more time for another attempt,
    // and it is not already a timeout error, then:
    //  - A timeout error MUST be propagated instead. It MUST expose the previously encountered error as specified in
    //    the "Errors" section of the CSOT specification.
    //  - If exposing the previously encountered error from a timeout error is impossible in a driver, then the driver
    //    is exempt from the requirement and MUST propagate the previously encountered error as is. The timeout error
    //    MUST copy all error labels from the previously encountered error.

    // The spec describes timeout checks as "elapsed time < TIMEOUT_MS" (where elapsed = now - start).
    // We precompute `deadline = now + remainingTimeMS` so each check becomes simply `now < deadline`.
    const csotEnabled = !!this.timeoutContext?.csotEnabled();
    const remainingTimeMS = this.timeoutContext?.csotEnabled()
      ? this.timeoutContext.remainingTimeMS
      : MAX_TIMEOUT;
    const deadline = processTimeMS() + remainingTimeMS;

    let committed = false;
    let result: T;

    let lastError: Error | null = null;

    try {
      retryTransaction: for (
        let transactionAttempt = 0, isRetry = false;
        !committed;
        ++transactionAttempt, isRetry = transactionAttempt > 0
      ) {
        // 2. If `transactionAttempt` > 0:
        if (isRetry) {
          // 2.1 Calculate backoffMS to be jitter * min(BACKOFF_INITIAL * 1.5 ** (transactionAttempt - 1), BACKOFF_MAX).
          //  If elapsed time + backoffMS > TIMEOUT_MS, then propagate the previously encountered error to the caller of
          //  withTransaction as per timeout error propagation and return immediately. Otherwise, sleep for backoffMS.
          //  2.1.1 jitter is a random float between [0, 1), optionally including 1, depending on what is most natural
          //    for the given driver language.
          //  2.1.2 transactionAttempt is the variable defined in step 1.
          //  2.1.3 BACKOFF_INITIAL is 5ms
          //  2.1.4 BACKOFF_MAX is 500ms
          const BACKOFF_INITIAL_MS = 5;
          const BACKOFF_MAX_MS = 500;
          const BACKOFF_GROWTH = 1.5;
          const jitter = Math.random();
          const backoffMS =
            jitter *
            Math.min(
              BACKOFF_INITIAL_MS * BACKOFF_GROWTH ** (transactionAttempt - 1),
              BACKOFF_MAX_MS
            );

          if (processTimeMS() + backoffMS >= deadline) {
            throw makeTimeoutError(
              lastError ??
                new MongoRuntimeError(
                  `Transaction retry did not record an error: should never occur. Please file a bug.`
                ),
              csotEnabled
            );
          }

          await setTimeout(backoffMS);
        }

        // 3. Invoke startTransaction on the session and increment transactionAttempt. If TransactionOptions were
        // specified in the call to withTransaction, those MUST be used for startTransaction. Note that
        // ClientSession.defaultTransactionOptions will be used in the absence of any explicit TransactionOptions.
        // 4. If startTransaction reported an error, propagate that error to the caller of withTransaction as is and
        // return immediately.
        this.startTransaction(options);

        try {
          // 5. Invoke the callback. Drivers MUST ensure that the ClientSession can be accessed within the callback
          // (e.g. pass ClientSession as the first parameter, rely on lexical scoping). Drivers MAY pass additional
          // parameters as needed (e.g. user data solicited by withTransaction).
          const promise = fn(this);
          if (!isPromiseLike(promise)) {
            throw new MongoInvalidArgumentError(
              'Function provided to `withTransaction` must return a Promise'
            );
          }

          // 6. Control returns to withTransaction. Determine the current state of the ClientSession and whether the
          // callback reported an error (e.g. thrown exception, error output parameter).
          result = await promise;

          // 8. If the ClientSession is in the "no transaction", "transaction aborted", or "transaction committed"
          // state, assume the callback intentionally aborted or committed the transaction and return immediately.
          if (
            this.transaction.state === TxnState.NO_TRANSACTION ||
            this.transaction.state === TxnState.TRANSACTION_COMMITTED ||
            this.transaction.state === TxnState.TRANSACTION_ABORTED
          ) {
            return result;
          }
        } catch (fnError) {
          // 7. If the callback reported an error
          if (!(fnError instanceof MongoError) || fnError instanceof MongoInvalidArgumentError) {
            // This first preemptive abort regardless of TxnState isn't spec,
            // and it's unclear whether it's serving a practical purpose, but this logic is OLD
            await this.abortTransaction();
            throw fnError;
          }

          lastError = fnError;

          // 7.1 If the ClientSession is in the "starting transaction" or "transaction in progress"
          // state, invoke abortTransaction on the session.
          if (
            this.transaction.state === TxnState.STARTING_TRANSACTION ||
            this.transaction.state === TxnState.TRANSACTION_IN_PROGRESS
          ) {
            await this.abortTransaction();
          }

          // 7.2 If the callback's error includes a "TransientTransactionError" label, jump back to step two.
          if (fnError.hasErrorLabel(MongoErrorLabel.TransientTransactionError)) {
            if (processTimeMS() >= deadline) {
              throw makeTimeoutError(lastError, csotEnabled);
            }
            continue retryTransaction;
          }

          // 7.3 If the callback's error includes a "UnknownTransactionCommitResult" label, the callback must
          // have manually committed a transaction, propagate the callback's error to the caller of withTransaction
          // as is and return immediately.
          // 7.4 Otherwise, propagate the callback's error to the caller of withTransaction as is and return immediately.
          throw fnError;
        }

        retryCommit: while (!committed) {
          try {
            // 9. Invoke commitTransaction on the session.
            await this.commitTransaction();
            committed = true;
          } catch (commitError) {
            // 10. If commitTransaction reported an error:
            lastError = commitError;

            // 10.1 If the commitTransaction error includes a UnknownTransactionCommitResult label and the error is
            // not MaxTimeMSExpired
            if (
              commitError.hasErrorLabel(MongoErrorLabel.UnknownTransactionCommitResult) &&
              !isMaxTimeMSExpiredError(commitError)
            ) {
              // 10.1.1 If the elapsed time of withTransaction exceeded TIMEOUT_MS, propagate the commitTransaction
              // error to the caller of withTransaction as per timeout error propagation and return immediately.
              if (processTimeMS() >= deadline) {
                throw makeTimeoutError(commitError, csotEnabled);
              }
              // 10.1.2 Otherwise, jump back to step nine. We will trust commitTransaction to apply a majority write
              // concern on retry attempts (see: Majority write concern is used when retrying commitTransaction).
              continue retryCommit;
            }

            // 10.2 If the commitTransaction error includes a TransientTransactionError label, jump back to step two.
            if (commitError.hasErrorLabel(MongoErrorLabel.TransientTransactionError)) {
              continue retryTransaction;
            }

            // 10.3 Otherwise, propagate the commitTransaction error to the caller of withTransaction as is and return
            // immediately.
            throw commitError;
          }
        }
      }

      // 11. The transaction was committed successfully. Return immediately.
      // @ts-expect-error Result is always defined if we reach here, the for-loop above convinces TS it is not.
      return result;
    } finally {
      this.timeoutContext = null;
    }
  }
}

function makeTimeoutError(cause: Error, csotEnabled: boolean): Error {
  // Async APIs know how to cancel themselves and might return CSOT error
  if (cause instanceof MongoOperationTimeoutError) {
    return cause;
  }
  if (csotEnabled) {
    const timeoutError = new MongoOperationTimeoutError('Timed out during withTransaction', {
      cause
    });
    if (cause instanceof MongoError) {
      for (const label of cause.errorLabels) {
        timeoutError.addErrorLabel(label);
      }
    }
    return timeoutError;
  }
  return cause;
}

const NON_DETERMINISTIC_WRITE_CONCERN_ERRORS = new Set([
  'CannotSatisfyWriteConcern',
  'UnknownReplWriteConcern',
  'UnsatisfiableWriteConcern'
]);

function shouldUnpinAfterCommitError(commitError: Error) {
  if (commitError instanceof MongoError) {
    if (
      isRetryableWriteError(commitError) ||
      commitError instanceof MongoWriteConcernError ||
      isMaxTimeMSExpiredError(commitError)
    ) {
      if (isUnknownTransactionCommitResult(commitError)) {
        // per txns spec, must unpin session in this case
        return true;
      }
    } else if (commitError.hasErrorLabel(MongoErrorLabel.TransientTransactionError)) {
      return true;
    }
  }
  return false;
}

function shouldAddUnknownTransactionCommitResultLabel(commitError: MongoError) {
  let ok = isRetryableWriteError(commitError);
  ok ||= commitError instanceof MongoWriteConcernError;
  ok ||= isMaxTimeMSExpiredError(commitError);
  ok &&= isUnknownTransactionCommitResult(commitError);
  return ok;
}

function isUnknownTransactionCommitResult(err: MongoError): err is MongoError {
  const isNonDeterministicWriteConcernError =
    err instanceof MongoServerError &&
    err.codeName &&
    NON_DETERMINISTIC_WRITE_CONCERN_ERRORS.has(err.codeName);

  return (
    isMaxTimeMSExpiredError(err) ||
    (!isNonDeterministicWriteConcernError &&
      err.code !== MONGODB_ERROR_CODES.UnsatisfiableWriteConcern &&
      err.code !== MONGODB_ERROR_CODES.UnknownReplWriteConcern)
  );
}

export function maybeClearPinnedConnection(
  session: ClientSession,
  options?: EndSessionOptions
): void {
  // unpin a connection if it has been pinned
  const conn = session.pinnedConnection;
  const error = options?.error;

  if (
    session.inTransaction() &&
    error &&
    error instanceof MongoError &&
    error.hasErrorLabel(MongoErrorLabel.TransientTransactionError)
  ) {
    return;
  }

  const topology = session.client.topology;
  // NOTE: the spec talks about what to do on a network error only, but the tests seem to
  //       to validate that we don't unpin on _all_ errors?
  if (conn && topology != null) {
    const servers = Array.from(topology.s.servers.values());
    const loadBalancer = servers[0];

    if (options?.error == null || options?.force) {
      loadBalancer.pool.checkIn(conn);
      session.pinnedConnection = undefined;
      conn.emit(
        UNPINNED,
        session.transaction.state !== TxnState.NO_TRANSACTION
          ? ConnectionPoolMetrics.TXN
          : ConnectionPoolMetrics.CURSOR
      );

      if (options?.forceClear) {
        loadBalancer.pool.clear({ serviceId: conn.serviceId });
      }
    }
  }
}

function isMaxTimeMSExpiredError(err: MongoError): boolean {
  if (err == null || !(err instanceof MongoServerError)) {
    return false;
  }

  return (
    err.code === MONGODB_ERROR_CODES.MaxTimeMSExpired ||
    err.writeConcernError?.code === MONGODB_ERROR_CODES.MaxTimeMSExpired
  );
}

/** @public */
export type ServerSessionId = { id: Binary };

/**
 * Reflects the existence of a session on the server. Can be reused by the session pool.
 * WARNING: not meant to be instantiated directly. For internal use only.
 * @public
 */
export class ServerSession {
  id: ServerSessionId;
  lastUse: number;
  txnNumber: number;
  isDirty: boolean;

  /** @internal */
  constructor(cloned?: ServerSession | null) {
    if (cloned != null) {
      const idBytes = ByteUtils.allocateUnsafe(16);
      idBytes.set(cloned.id.id.buffer);
      this.id = { id: new Binary(idBytes, cloned.id.id.sub_type) };
      this.lastUse = cloned.lastUse;
      this.txnNumber = cloned.txnNumber;
      this.isDirty = cloned.isDirty;
      return;
    }
    this.id = { id: new Binary(uuidV4(), Binary.SUBTYPE_UUID) };
    this.lastUse = processTimeMS();
    this.txnNumber = 0;
    this.isDirty = false;
  }

  /**
   * Determines if the server session has timed out.
   *
   * @param sessionTimeoutMinutes - The server's "logicalSessionTimeoutMinutes"
   */
  hasTimedOut(sessionTimeoutMinutes: number): boolean {
    // Take the difference of the lastUse timestamp and now, which will result in a value in
    // milliseconds, and then convert milliseconds to minutes to compare to `sessionTimeoutMinutes`
    const idleTimeMinutes = Math.round(
      ((calculateDurationInMs(this.lastUse) % 86400000) % 3600000) / 60000
    );

    return idleTimeMinutes > sessionTimeoutMinutes - 1;
  }
}

/**
 * Maintains a pool of Server Sessions.
 * For internal use only
 * @internal
 */
export class ServerSessionPool {
  client: MongoClient;
  sessions: List<ServerSession>;

  constructor(client: MongoClient) {
    if (client == null) {
      throw new MongoRuntimeError('ServerSessionPool requires a MongoClient');
    }

    this.client = client;
    this.sessions = new List<ServerSession>();
  }

  /**
   * Acquire a Server Session from the pool.
   * Iterates through each session in the pool, removing any stale sessions
   * along the way. The first non-stale session found is removed from the
   * pool and returned. If no non-stale session is found, a new ServerSession is created.
   */
  acquire(): ServerSession {
    const sessionTimeoutMinutes = this.client.topology?.logicalSessionTimeoutMinutes ?? 10;

    let session: ServerSession | null = null;

    // Try to obtain from session pool
    while (this.sessions.length > 0) {
      const potentialSession = this.sessions.shift();
      if (
        potentialSession != null &&
        (!!this.client.topology?.loadBalanced ||
          !potentialSession.hasTimedOut(sessionTimeoutMinutes))
      ) {
        session = potentialSession;
        break;
      }
    }

    // If nothing valid came from the pool make a new one
    if (session == null) {
      session = new ServerSession();
    }

    return session;
  }

  /**
   * Release a session to the session pool
   * Adds the session back to the session pool if the session has not timed out yet.
   * This method also removes any stale sessions from the pool.
   *
   * @param session - The session to release to the pool
   */
  release(session: ServerSession): void {
    const sessionTimeoutMinutes = this.client.topology?.logicalSessionTimeoutMinutes ?? 10;

    if (this.client.topology?.loadBalanced && !sessionTimeoutMinutes) {
      this.sessions.unshift(session);
    }

    if (!sessionTimeoutMinutes) {
      return;
    }

    this.sessions.prune(session => session.hasTimedOut(sessionTimeoutMinutes));

    if (!session.hasTimedOut(sessionTimeoutMinutes)) {
      if (session.isDirty) {
        return;
      }

      // otherwise, readd this session to the session pool
      this.sessions.unshift(session);
    }
  }
}

/**
 * Optionally decorate a command with sessions specific keys
 *
 * @param session - the session tracking transaction state
 * @param command - the command to decorate
 * @param options - Optional settings passed to calling operation
 *
 * @internal
 */
export function applySession(
  session: ClientSession,
  command: Document,
  options: CommandOptions
): MongoDriverError | undefined {
  if (session.hasEnded) {
    return new MongoExpiredSessionError();
  }

  // May acquire serverSession here
  const serverSession = session.serverSession;
  if (serverSession == null) {
    return new MongoRuntimeError('Unable to acquire server session');
  }

  if (options.writeConcern?.w === 0) {
    if (session && session.explicit) {
      // Error if user provided an explicit session to an unacknowledged write (SPEC-1019)
      return new MongoAPIError('Cannot have explicit session with unacknowledged writes');
    }
    return;
  }

  // mark the last use of this session, and apply the `lsid`
  serverSession.lastUse = processTimeMS();
  command.lsid = serverSession.id;

  const inTxnOrTxnCommand = session.inTransaction() || isTransactionCommand(command);
  const isRetryableWrite = !!options.willRetryWrite;

  if (isRetryableWrite || inTxnOrTxnCommand) {
    serverSession.txnNumber += session.txnNumberIncrement;
    session.txnNumberIncrement = 0;
    // TODO(NODE-2674): Preserve int64 sent from MongoDB
    command.txnNumber = Long.fromNumber(serverSession.txnNumber);
  }

  if (!inTxnOrTxnCommand) {
    if (session.transaction.state !== TxnState.NO_TRANSACTION) {
      session.transaction.transition(TxnState.NO_TRANSACTION);
    }

    if (
      session.supports.causalConsistency &&
      session.operationTime &&
      commandSupportsReadConcern(command)
    ) {
      command.readConcern = command.readConcern || {};
      Object.assign(command.readConcern, { afterClusterTime: session.operationTime });
    } else if (session.snapshotEnabled) {
      command.readConcern = command.readConcern || { level: ReadConcernLevel.snapshot };
      if (session.snapshotTime != null) {
        Object.assign(command.readConcern, { atClusterTime: session.snapshotTime });
      }
    }

    return;
  }

  // now attempt to apply transaction-specific sessions data

  // `autocommit` must always be false to differentiate from retryable writes
  command.autocommit = false;

  if (session.transaction.state === TxnState.STARTING_TRANSACTION) {
    command.startTransaction = true;

    const readConcern =
      session.transaction.options.readConcern || session?.clientOptions?.readConcern;
    if (readConcern) {
      command.readConcern = readConcern;
    }

    if (session.supports.causalConsistency && session.operationTime) {
      command.readConcern = command.readConcern || {};
      Object.assign(command.readConcern, { afterClusterTime: session.operationTime });
    }
  }
  return;
}

export function updateSessionFromResponse(session: ClientSession, document: MongoDBResponse): void {
  if (document.$clusterTime) {
    _advanceClusterTime(session, document.$clusterTime);
  }

  if (document.operationTime && session && session.supports.causalConsistency) {
    session.advanceOperationTime(document.operationTime);
  }

  if (document.recoveryToken && session && session.inTransaction()) {
    session.transaction._recoveryToken = document.recoveryToken;
  }

  if (session?.snapshotEnabled && session.snapshotTime == null) {
    // find and aggregate commands return atClusterTime on the cursor
    // distinct includes it in the response body
    const atClusterTime = document.atClusterTime;
    if (atClusterTime) {
      session.snapshotTime = atClusterTime;
    }
  }

  if (session.transaction.state === TxnState.STARTING_TRANSACTION) {
    if (document.ok === 1) {
      session.transaction.transition(TxnState.TRANSACTION_IN_PROGRESS);
    } else {
      const error = new MongoServerError(document.toObject());
      const isRetryableError = error.hasErrorLabel(MongoErrorLabel.RetryableError);

      if (!isRetryableError) {
        session.transaction.transition(TxnState.TRANSACTION_IN_PROGRESS);
      }
    }
  }
}