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import { makePromiseKit } from '@endo/promise-kit';
import type { Reader } from '@endo/stream';
import { stringify } from '@metamask/kernel-utils';
import { is, literal, object } from '@metamask/superstruct';
import type { Infer } from '@metamask/superstruct';
import type { BaseReader, BaseWriter, ValidateInput } from './BaseStream.ts';
import { makeDoneResult } from './utils.ts';
export const DuplexStreamSentinel = {
Syn: '@@Syn',
Ack: '@@Ack',
} as const;
const SynStruct = object({
[DuplexStreamSentinel.Syn]: literal(true),
});
type DuplexStreamSyn = Infer<typeof SynStruct>;
export const isSyn = (value: unknown): value is DuplexStreamSyn =>
is(value, SynStruct);
export const makeSyn = (): DuplexStreamSyn => ({
[DuplexStreamSentinel.Syn]: true,
});
const AckStruct = object({
[DuplexStreamSentinel.Ack]: literal(true),
});
type DuplexStreamAck = Infer<typeof AckStruct>;
export const isAck = (value: unknown): value is DuplexStreamAck =>
is(value, AckStruct);
export const makeAck = (): DuplexStreamAck => ({
[DuplexStreamSentinel.Ack]: true,
});
type DuplexStreamSignal = DuplexStreamSyn | DuplexStreamAck;
export const isDuplexStreamSignal = (
value: unknown,
): value is DuplexStreamSignal => isSyn(value) || isAck(value);
/**
* Make a validator for input to a duplex stream. Constructor helper for concrete
* duplex stream implementations.
*
* Validators passed in by consumers must be augmented such that errors aren't
* thrown for {@link DuplexStreamSignal} values.
*
* @param validateInput - The validator for the stream's input type.
* @returns A validator for the stream's input type, or `undefined` if no
* validation is desired.
*/
export const makeDuplexStreamInputValidator = <Read>(
validateInput?: ValidateInput<Read>,
): ((value: unknown) => value is Read) | undefined =>
validateInput &&
((value: unknown): value is Read =>
isDuplexStreamSignal(value) || validateInput(value));
const SynchronizationStatus = {
Idle: 0,
Pending: 1,
Complete: 2,
Failed: 3,
} as const;
type SynchronizationStatus =
(typeof SynchronizationStatus)[keyof typeof SynchronizationStatus];
const isEnded = (status: SynchronizationStatus): boolean =>
status === SynchronizationStatus.Complete ||
status === SynchronizationStatus.Failed;
/**
* The base of a duplex stream. Essentially a {@link BaseReader} with a `write()` method.
* Backed up by separate {@link BaseReader} and {@link BaseWriter} instances under the hood.
*/
export abstract class BaseDuplexStream<
Read,
ReadStream extends BaseReader<Read>,
Write = Read,
WriteStream extends BaseWriter<Write> = BaseWriter<Write>,
> implements Reader<Read>
{
/**
* The underlying reader for the duplex stream.
*/
readonly #reader: ReadStream;
/**
* The underlying writer for the duplex stream.
*/
readonly #writer: WriteStream;
/**
* The promise for the synchronization of the stream with its remote
* counterpart.
*/
#syncKit: PromiseKit<void> = makePromiseKit<void>();
/**
* Whether the stream is synchronized with its remote counterpart.
*/
#synchronizationStatus: SynchronizationStatus = SynchronizationStatus.Idle;
/**
* Reads the next value from the stream.
*
* @returns The next value from the stream.
*/
next: () => Promise<IteratorResult<Read, undefined>>;
/**
* Writes a value to the stream.
*
* @param value - The next value to write to the stream.
* @returns The result of writing the value.
*/
write: (value: Write) => Promise<IteratorResult<undefined, undefined>>;
/**
* Constructs a new {@link BaseDuplexStream}.
*
* @param reader - The underlying reader for the duplex stream.
* @param writer - The underlying writer for the duplex stream.
*/
constructor(reader: ReadStream, writer: WriteStream) {
// Set a catch handler to avoid unhandled rejection errors. The promise may
// reject before reads or writes occur, in which case there are no handlers.
this.#syncKit.promise.catch(() => undefined);
// Next and write only work if synchronization completes.
this.next = async () => {
if (this.#synchronizationStatus !== SynchronizationStatus.Complete) {
return this.#syncKit.promise.then(async () => reader.next());
}
const result = await reader.next();
// If we receive a SYN message, we re-synchronize.
if (isSyn(result.value)) {
this.#resetSynchronizationStatus();
this.#performSynchronization(result).catch((error) => {
this.#failSynchronization(error);
});
return this.#syncKit.promise.then(async () => reader.next());
}
return result;
};
this.write = async (value: Write) =>
this.#synchronizationStatus === SynchronizationStatus.Complete
? writer.next(value)
: this.#syncKit.promise.then(async () => writer.next(value));
this.#reader = reader;
this.#writer = writer;
harden(this);
}
/**
* Resets the synchronization status to idle and creates a new promise kit.
*/
#resetSynchronizationStatus(): void {
this.#synchronizationStatus = SynchronizationStatus.Idle;
this.#syncKit = makePromiseKit<void>();
this.#syncKit.promise.catch(() => undefined);
}
/**
* Synchronizes the duplex stream with its remote counterpart. Must be awaited
* before values can be read from or written to the stream. Idempotent.
*
* @returns A promise that resolves when the stream is synchronized.
*/
async synchronize(): Promise<void> {
if (this.#synchronizationStatus !== SynchronizationStatus.Idle) {
return this.#syncKit.promise;
}
try {
await this.#performSynchronization();
} catch (error) {
this.#failSynchronization(error as Error);
}
return this.#syncKit.promise;
}
/**
* Performs the synchronization protocol.
*
* **ATTN:** The synchronization protocol requires sending values that do not
* conform to the read and write types of the stream. We do not currently have
* the type system to express this, so we just override TypeScript and do it
* anyway. This is far from ideal, but it works because (1) the streams always
* allow stream signal values at runtime, and (2) the special values cannot
* be observed by users of the stream.
*
* @param previousResult - The result of the previous `next()` call.
* This method will call `next()` on the reader if not provided. Used
* when the other side reinitializes synchronization.
*/
async #performSynchronization(
previousResult?: IteratorResult<Read, undefined>,
): Promise<void> {
this.#synchronizationStatus = SynchronizationStatus.Pending;
let receivedSyn = false;
let sentAck = false;
const sendAck = async (): Promise<void> => {
sentAck = true;
// @ts-expect-error See docstring.
await this.#writer.next(makeAck());
};
// @ts-expect-error See docstring.
await this.#writer.next(makeSyn());
let result = previousResult ?? (await this.#reader.next());
while (this.#synchronizationStatus === SynchronizationStatus.Pending) {
if (isAck(result.value)) {
if (!sentAck) {
await sendAck();
}
this.#completeSynchronization();
} else if (isSyn(result.value)) {
if (receivedSyn) {
this.#failSynchronization(
new Error('Received duplicate SYN message during synchronization'),
);
} else {
receivedSyn = true;
await sendAck();
result = await this.#reader.next();
}
} else {
this.#failSynchronization(
new Error(
`Received unexpected message during synchronization: ${stringify(result)}`,
),
);
}
}
}
/**
* Completes the synchronization by resolving the sync promise. Idempotent.
*/
#completeSynchronization(): void {
if (isEnded(this.#synchronizationStatus)) {
return;
}
this.#synchronizationStatus = SynchronizationStatus.Complete;
this.#syncKit.resolve();
}
/**
* Fails the synchronization by rejecting the sync promise with the given error. Idempotent.
*
* @param error - The error to reject the sync promise with.
*/
#failSynchronization(error: Error): void {
if (isEnded(this.#synchronizationStatus)) {
return;
}
this.#synchronizationStatus = SynchronizationStatus.Failed;
this.#syncKit.reject(error);
}
/**
* Returns the async iterator for this stream.
*
* @returns This stream as an async iterator.
*/
[Symbol.asyncIterator](): typeof this {
return this;
}
/**
* Drains the stream by passing each value to a handler function.
*
* @param handler - The function that will receive each value from the stream.
*/
async drain(handler: (value: Read) => void | Promise<void>): Promise<void> {
for await (const value of this) {
await handler(value);
}
}
/**
* Pipes the stream to another duplex stream.
*
* @param sink - The duplex stream to pipe to.
*/
async pipe<Read2>(sink: DuplexStream<Read2, Read>): Promise<void> {
await this.drain(async (value) => {
await sink.write(value);
});
}
/**
* Closes the stream. Idempotent.
*
* @returns The final result for this stream.
*/
async return(): Promise<IteratorResult<Read, undefined>> {
this.#completeSynchronization();
await Promise.all([this.#writer.return(), this.#reader.return()]);
return makeDoneResult();
}
/**
* Closes the stream with an error. Idempotent.
*
* @param error - The error to close the stream with.
* @returns The final result for this stream.
*/
async throw(error: Error): Promise<IteratorResult<Read, undefined>> {
this.#failSynchronization(error);
// eslint-disable-next-line promise/no-promise-in-callback
await Promise.all([this.#writer.throw(error), this.#reader.throw(error)]);
return makeDoneResult();
}
/**
* Closes the stream. Syntactic sugar for `throw(error)` or `return()`. Idempotent.
*
* @param error - The error to close the stream with.
* @returns The final result for this stream.
*/
async end(error?: Error): Promise<IteratorResult<Read, undefined>> {
return error ? this.throw(error) : this.return();
}
}
harden(BaseDuplexStream);
/**
* A duplex stream. Essentially a {@link Reader} with a `write()` method.
*/
export type DuplexStream<Read, Write = Read> = Pick<
BaseDuplexStream<Read, BaseReader<Read>, Write, BaseWriter<Write>>,
'next' | 'write' | 'drain' | 'pipe' | 'return' | 'throw' | 'end'
> & {
[Symbol.asyncIterator]: () => DuplexStream<Read, Write>;
};
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