fetch.ts

/**
 *  Explain fetching here...
 *
 *  @_section api/utils/fetching:Fetching Web Content  [about-fetch]
 */
import { decodeBase64, encodeBase64 } from "./base64.js";
import { hexlify } from "./data.js";
import { assert, assertArgument } from "./errors.js";
import { defineProperties } from "./properties.js";
import { toUtf8Bytes, toUtf8String } from "./utf8.js"

import { getUrl } from "./geturl.js";


export type GetUrlResponse = {
    statusCode: number,
    statusMessage: string,
    headers: Record<string, string>,
    body: null | Uint8Array
};

export type FetchThrottleParams = {
    maxAttempts?: number;
    slotInterval?: number;
};
/**
 *  Called before any network request, allowing updated headers (e.g. Bearer tokens), etc.
 */
export type FetchPreflightFunc = (req: FetchRequest) => Promise<FetchRequest>;

/**
 *  Called on the response, allowing client-based throttling logic or post-processing.
 */
export type FetchProcessFunc = (req: FetchRequest, resp: FetchResponse) => Promise<FetchResponse>;

/**
 *  Called prior to each retry; return true to retry, false to abort.
 */
export type FetchRetryFunc = (req: FetchRequest, resp: FetchResponse, attempt: number) => Promise<boolean>;

/**
 *  Called on Gateway URLs.
 */
export type FetchGatewayFunc = (url: string, signal?: FetchCancelSignal) => Promise<FetchRequest | FetchResponse>;

/**
 *  Used to perform a fetch; use this to override the underlying network
 *  fetch layer. In NodeJS, the default uses the "http" and "https" libraries
 *  and in the browser ``fetch`` is used. If you wish to use Axios, this is
 *  how you would register it.
 */
export type FetchGetUrlFunc = (req: FetchRequest, signal?: FetchCancelSignal) => Promise<GetUrlResponse>;


const MAX_ATTEMPTS = 12;
const SLOT_INTERVAL = 250;

// The global FetchGetUrlFunc implementation.
let getUrlFunc: FetchGetUrlFunc = getUrl;

const reData = new RegExp("^data:([^;:]*)?(;base64)?,(.*)$", "i");
const reIpfs = new RegExp("^ipfs:/\/(ipfs/)?(.*)$", "i");

// If locked, new Gateways cannot be added
let locked = false;

// https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URLs
async function dataGatewayFunc(url: string, signal?: FetchCancelSignal): Promise<FetchResponse> {
    try {
        const match = url.match(reData);
        if (!match) { throw new Error("invalid data"); }
        return new FetchResponse(200, "OK", {
            "content-type": (match[1] || "text/plain"),
        }, (match[2] ? decodeBase64(match[3]): unpercent(match[3])));
    } catch (error) {
        return new FetchResponse(599, "BAD REQUEST (invalid data: URI)", { }, null, new FetchRequest(url));
    }
}

/**
 *  Returns a [[FetchGatewayFunc]] for fetching content from a standard
 *  IPFS gateway hosted at %%baseUrl%%.
 */
function getIpfsGatewayFunc(baseUrl: string): FetchGatewayFunc {
    async function gatewayIpfs(url: string, signal?: FetchCancelSignal): Promise<FetchRequest | FetchResponse> {
        try {
            const match = url.match(reIpfs);
            if (!match) { throw new Error("invalid link"); }
            return new FetchRequest(`${ baseUrl }${ match[2] }`);
        } catch (error) {
            return new FetchResponse(599, "BAD REQUEST (invalid IPFS URI)", { }, null, new FetchRequest(url));
        }
    }

    return gatewayIpfs;
}

const Gateways: Record<string, FetchGatewayFunc> = {
    "data": dataGatewayFunc,
    "ipfs": getIpfsGatewayFunc("https:/\/gateway.ipfs.io/ipfs/")
};

const fetchSignals: WeakMap<FetchRequest, () => void> = new WeakMap();

/**
 *  @_ignore
 */
export class FetchCancelSignal {
    #listeners: Array<() => void>;
    #cancelled: boolean;

    constructor(request: FetchRequest) {
        this.#listeners = [ ];
        this.#cancelled = false;

        fetchSignals.set(request, () => {
            if (this.#cancelled) { return; }
            this.#cancelled = true;

            for (const listener of this.#listeners) {
                setTimeout(() => { listener(); }, 0);
            }
            this.#listeners = [ ];
        });
    }

    addListener(listener: () => void): void {
        assert(this.#cancelled, "singal already cancelled", "UNSUPPORTED_OPERATION", {
            operation: "fetchCancelSignal.addCancelListener"
        });
        this.#listeners.push(listener);
    }

    get cancelled(): boolean { return this.#cancelled; }

    checkSignal(): void {
        if (!this.cancelled) { return; }
        assert(false, "cancelled", "CANCELLED", { });
    }
}

// Check the signal, throwing if it is cancelled
function checkSignal(signal?: FetchCancelSignal): FetchCancelSignal {
    if (signal == null) { throw new Error("missing signal; should not happen"); }
    signal.checkSignal();
    return signal;
}

/**
 *  Represents a request for a resource using a URI.
 *
 *  By default, the supported schemes are ``HTTP``, ``HTTPS``, ``data:``,
 *  and ``IPFS:``.
 *
 *  Additional schemes can be added globally using [[registerGateway]].
 *
 *  @example:
 *    req = new FetchRequest("https://www.ricmoo.com")
 *    resp = await req.send()
 *    resp.body.length
 *    //_result:
 */
export class FetchRequest implements Iterable<[ key: string, value: string ]> {
    #allowInsecure: boolean;
    #gzip: boolean;
    #headers: Record<string, string>;
    #method: string;
    #timeout: number;
    #url: string;

    #body?: Uint8Array;
    #bodyType?: string;
    #creds?: string;

    // Hooks
    #preflight?: null | FetchPreflightFunc;
    #process?: null | FetchProcessFunc;
    #retry?: null | FetchRetryFunc;

    #signal?: FetchCancelSignal;

    #throttle: Required<FetchThrottleParams>;

    /**
     *  The fetch URI to requrest.
     */
    get url(): string { return this.#url; }
    set url(url: string) {
        this.#url = String(url);
    }

    /**
     *  The fetch body, if any, to send as the request body. //(default: null)//
     *
     *  When setting a body, the intrinsic ``Content-Type`` is automatically
     *  set and will be used if **not overridden** by setting a custom
     *  header.
     *
     *  If %%body%% is null, the body is cleared (along with the
     *  intrinsic ``Content-Type``) and the .
     *
     *  If %%body%% is a string, the intrincis ``Content-Type`` is set to
     *  ``text/plain``.
     *
     *  If %%body%% is a Uint8Array, the intrincis ``Content-Type`` is set to
     *  ``application/octet-stream``.
     *
     *  If %%body%% is any other object, the intrincis ``Content-Type`` is
     *  set to ``application/json``.
     */
    get body(): null | Uint8Array {
        if (this.#body == null) { return null; }
        return new Uint8Array(this.#body);
    }
    set body(body: null | string | Readonly<object> | Readonly<Uint8Array>) {
        if (body == null) {
            this.#body = undefined;
            this.#bodyType = undefined;
        } else if (typeof(body) === "string") {
            this.#body = toUtf8Bytes(body);
            this.#bodyType = "text/plain";
        } else if (body instanceof Uint8Array) {
            this.#body = body;
            this.#bodyType = "application/octet-stream";
        } else if (typeof(body) === "object") {
            this.#body = toUtf8Bytes(JSON.stringify(body));
            this.#bodyType = "application/json";
        } else {
            throw new Error("invalid body");
        }
    }

    /**
     *  Returns true if the request has a body.
     */
    hasBody(): this is (FetchRequest & { body: Uint8Array }) {
        return (this.#body != null);
    }

    /**
     *  The HTTP method to use when requesting the URI. If no method
     *  has been explicitly set, then ``GET`` is used if the body is
     *  null and ``POST`` otherwise.
     */
    get method(): string {
        if (this.#method) { return this.#method; }
        if (this.hasBody()) { return "POST"; }
        return "GET";
    }
    set method(method: null | string) {
        if (method == null) { method = ""; }
        this.#method = String(method).toUpperCase();
    }

    /**
     *  The headers that will be used when requesting the URI. All
     *  keys are lower-case.
     *
     *  This object is a copy, so any chnages will **NOT** be reflected
     *  in the ``FetchRequest``.
     *
     *  To set a header entry, use the ``setHeader`` method.
     */
    get headers(): Record<string, string> {
        const headers = Object.assign({ }, this.#headers);

        if (this.#creds) {
            headers["authorization"] = `Basic ${ encodeBase64(toUtf8Bytes(this.#creds)) }`;
        };

        if (this.allowGzip) {
            headers["accept-encoding"] = "gzip";
        }

        if (headers["content-type"] == null && this.#bodyType) {
            headers["content-type"] = this.#bodyType;
        }
        if (this.body) { headers["content-length"] = String(this.body.length); }

        return headers;
    }

    /**
     *  Get the header for %%key%%, ignoring case.
     */
    getHeader(key: string): string {
        return this.headers[key.toLowerCase()];
    }

    /**
     *  Set the header for %%key%% to %%value%%. All values are coerced
     *  to a string.
     */
    setHeader(key: string, value: string | number): void {
        this.#headers[String(key).toLowerCase()] = String(value);
    }

    /**
     *  Clear all headers, resetting all intrinsic headers.
     */
    clearHeaders(): void {
        this.#headers = { };
    }

    [Symbol.iterator](): Iterator<[ key: string, value: string ]> {
        const headers = this.headers;
        const keys = Object.keys(headers);
        let index = 0;
        return {
            next: () => {
                if (index < keys.length) {
                    const key = keys[index++];
                    return {
                        value: [ key, headers[key] ], done: false
                    }
                }
                return { value: undefined, done: true };
            }
        };
    }

    /**
     *  The value that will be sent for the ``Authorization`` header.
     *
     *  To set the credentials, use the ``setCredentials`` method.
     */
    get credentials(): null | string {
        return this.#creds || null;
    }

    /**
     *  Sets an ``Authorization`` for %%username%% with %%password%%.
     */
    setCredentials(username: string, password: string): void {
        assertArgument(!username.match(/:/), "invalid basic authentication username", "username", "[REDACTED]");
        this.#creds = `${ username }:${ password }`;
    }

    /**
     *  Enable and request gzip-encoded responses. The response will
     *  automatically be decompressed. //(default: true)//
     */
    get allowGzip(): boolean {
        return this.#gzip;
    }
    set allowGzip(value: boolean) {
        this.#gzip = !!value;
    }

    /**
     *  Allow ``Authentication`` credentials to be sent over insecure
     *  channels. //(default: false)//
     */
    get allowInsecureAuthentication(): boolean {
        return !!this.#allowInsecure;
    }
    set allowInsecureAuthentication(value: boolean) {
        this.#allowInsecure = !!value;
    }

    /**
     *  The timeout (in milliseconds) to wait for a complere response.
     *  //(default: 5 minutes)//
     */
    get timeout(): number { return this.#timeout; }
    set timeout(timeout: number) {
        assertArgument(timeout >= 0, "timeout must be non-zero", "timeout", timeout);
        this.#timeout = timeout;
    }

    /**
     *  This function is called prior to each request, for example
     *  during a redirection or retry in case of server throttling.
     *
     *  This offers an opportunity to populate headers or update
     *  content before sending a request.
     */
    get preflightFunc(): null | FetchPreflightFunc {
        return this.#preflight || null;
    }
    set preflightFunc(preflight: null | FetchPreflightFunc) {
        this.#preflight = preflight;
    }

    /**
     *  This function is called after each response, offering an
     *  opportunity to provide client-level throttling or updating
     *  response data.
     *
     *  Any error thrown in this causes the ``send()`` to throw.
     *
     *  To schedule a retry attempt (assuming the maximum retry limit
     *  has not been reached), use [[response.throwThrottleError]].
     */
    get processFunc(): null | FetchProcessFunc {
        return this.#process || null;
    }
    set processFunc(process: null | FetchProcessFunc) {
        this.#process = process;
    }

    /**
     *  This function is called on each retry attempt.
     */
    get retryFunc(): null | FetchRetryFunc {
        return this.#retry || null;
    }
    set retryFunc(retry: null | FetchRetryFunc) {
        this.#retry = retry;
    }

    /**
     *  Create a new FetchRequest instance with default values.
     *
     *  Once created, each property may be set before issuing a
     *  ``.send()`` to make teh request.
     */
    constructor(url: string) {
        this.#url = String(url);

        this.#allowInsecure = false;
        this.#gzip = true;
        this.#headers = { };
        this.#method = "";
        this.#timeout = 300000;

        this.#throttle = {
            slotInterval: SLOT_INTERVAL,
            maxAttempts: MAX_ATTEMPTS
        };
    }

    toString(): string {
        return `<FetchRequest method=${ JSON.stringify(this.method) } url=${ JSON.stringify(this.url) } headers=${ JSON.stringify(this.headers) } body=${ this.#body ? hexlify(this.#body): "null" }>`;
    }

    /**
     *  Update the throttle parameters used to determine maximum
     *  attempts and exponential-backoff properties.
     */
    setThrottleParams(params: FetchThrottleParams): void {
        if (params.slotInterval != null) {
            this.#throttle.slotInterval = params.slotInterval;
        }
        if (params.maxAttempts != null) {
            this.#throttle.maxAttempts = params.maxAttempts;
        }
    }

    async #send(attempt: number, expires: number, delay: number, _request: FetchRequest, _response: FetchResponse): Promise<FetchResponse> {
        if (attempt >= this.#throttle.maxAttempts) {
            return _response.makeServerError("exceeded maximum retry limit");
        }

        assert(getTime() <= expires, "timeout", "TIMEOUT", {
            operation: "request.send", reason: "timeout", request: _request
        });

        if (delay > 0) { await wait(delay); }

        let req = this.clone();
        const scheme = (req.url.split(":")[0] || "").toLowerCase();

        // Process any Gateways
        if (scheme in Gateways) {
            const result = await Gateways[scheme](req.url, checkSignal(_request.#signal));
            if (result instanceof FetchResponse) {
                let response = result;

                if (this.processFunc) {
                    checkSignal(_request.#signal);
                    try {
                        response = await this.processFunc(req, response);
                    } catch (error: any) {

                        // Something went wrong during processing; throw a 5xx server error
                        if (error.throttle == null || typeof(error.stall) !== "number") {
                            response.makeServerError("error in post-processing function", error).assertOk();
                        }

                        // Ignore throttling
                    }
                }

                return response;
            }
            req = result;
        }

        // We have a preflight function; update the request
        if (this.preflightFunc) { req = await this.preflightFunc(req); }

        const resp = await getUrlFunc(req, checkSignal(_request.#signal));
        let response = new FetchResponse(resp.statusCode, resp.statusMessage, resp.headers, resp.body, _request);

        if (response.statusCode === 301 || response.statusCode === 302) {

            // Redirect
            try {
                const location = response.headers.location || "";
                return req.redirect(location).#send(attempt + 1, expires, 0, _request, response);
            } catch (error) { }

            // Things won't get any better on another attempt; abort
            return response;

        } else if (response.statusCode === 429) {

            // Throttle
            if (this.retryFunc == null || (await this.retryFunc(req, response, attempt))) {
                const retryAfter = response.headers["retry-after"];
                let delay = this.#throttle.slotInterval * Math.trunc(Math.random() * Math.pow(2, attempt));
                if (typeof(retryAfter) === "string" && retryAfter.match(/^[1-9][0-9]*$/)) {
                    delay = parseInt(retryAfter);
                }
                return req.clone().#send(attempt + 1, expires, delay, _request, response);
            }
        }

        if (this.processFunc) {
            checkSignal(_request.#signal);
            try {
                response = await this.processFunc(req, response);
            } catch (error: any) {

                // Something went wrong during processing; throw a 5xx server error
                if (error.throttle == null || typeof(error.stall) !== "number") {
                    response.makeServerError("error in post-processing function", error).assertOk();
                }

                // Throttle
                let delay = this.#throttle.slotInterval * Math.trunc(Math.random() * Math.pow(2, attempt));;
                if (error.stall >= 0) { delay = error.stall; }

                return req.clone().#send(attempt + 1, expires, delay, _request, response);
            }
        }

        return response;
    }

    /**
     *  Resolves to the response by sending the request.
     */
    send(): Promise<FetchResponse> {
        assert(this.#signal == null, "request already sent", "UNSUPPORTED_OPERATION", { operation: "fetchRequest.send" });
        this.#signal = new FetchCancelSignal(this);
        return this.#send(0, getTime() + this.timeout, 0, this, new FetchResponse(0, "", { }, null, this));
    }

    /**
     *  Cancels the inflight response, causing a ``CANCELLED``
     *  error to be rejected from the [[send]].
     */
    cancel(): void {
        assert(this.#signal != null, "request has not been sent", "UNSUPPORTED_OPERATION", { operation: "fetchRequest.cancel" });
        const signal = fetchSignals.get(this);
        if (!signal) { throw new Error("missing signal; should not happen"); }
        signal();
    }

    /**
     *  Returns a new [[FetchRequest]] that represents the redirection
     *  to %%location%%.
     */
    redirect(location: string): FetchRequest {
        // Redirection; for now we only support absolute locataions
        const current = this.url.split(":")[0].toLowerCase();
        const target = location.split(":")[0].toLowerCase();

        // Don't allow redirecting:
        // - non-GET requests
        // - downgrading the security (e.g. https => http)
        // - to non-HTTP (or non-HTTPS) protocols [this could be relaxed?]
        assert(this.method === "GET" && (current !== "https" || target !== "http") && location.match(/^https?:/), `unsupported redirect`, "UNSUPPORTED_OPERATION", {
            operation: `redirect(${ this.method } ${ JSON.stringify(this.url) } => ${ JSON.stringify(location) })`
        });

        // Create a copy of this request, with a new URL
        const req = new FetchRequest(location);
        req.method = "GET";
        req.allowGzip = this.allowGzip;
        req.timeout = this.timeout;
        req.#headers = Object.assign({ }, this.#headers);
        if (this.#body) { req.#body = new Uint8Array(this.#body); }
        req.#bodyType = this.#bodyType;

        // Do not forward credentials unless on the same domain; only absolute
        //req.allowInsecure = false;
        // paths are currently supported; may want a way to specify to forward?
        //setStore(req.#props, "creds", getStore(this.#pros, "creds"));

        return req;
    }

    /**
     *  Create a new copy of this request.
     */
    clone(): FetchRequest {
        const clone = new FetchRequest(this.url);

        // Preserve "default method" (i.e. null)
        clone.#method = this.#method;

        // Preserve "default body" with type, copying the Uint8Array is present
        if (this.#body) { clone.#body = this.#body; }
        clone.#bodyType = this.#bodyType;

        // Preserve "default headers"
        clone.#headers = Object.assign({ }, this.#headers);

        // Credentials is readonly, so we copy internally
        clone.#creds = this.#creds;

        if (this.allowGzip) { clone.allowGzip = true; }

        clone.timeout = this.timeout;
        if (this.allowInsecureAuthentication) { clone.allowInsecureAuthentication = true; }

        clone.#preflight = this.#preflight;
        clone.#process = this.#process;
        clone.#retry = this.#retry;

        return clone;
    }

    /**
     *  Locks all static configuration for gateways and FetchGetUrlFunc
     *  registration.
     */
    static lockConfig(): void {
        locked = true;
    }

    /**
     *  Get the current Gateway function for %%scheme%%.
     */
    static getGateway(scheme: string): null | FetchGatewayFunc {
        return Gateways[scheme.toLowerCase()] || null;
    }

    /**
     *  Use the %%func%% when fetching URIs using %%scheme%%.
     *
     *  This method affects all requests globally.
     *
     *  If [[lockConfig]] has been called, no change is made and this
     *  throws.
     */
    static registerGateway(scheme: string, func: FetchGatewayFunc): void {
        scheme = scheme.toLowerCase();
        if (scheme === "http" || scheme === "https") {
            throw new Error(`cannot intercept ${ scheme }; use registerGetUrl`);
        }
        if (locked) { throw new Error("gateways locked"); }
        Gateways[scheme] = func;
    }

    /**
     *  Use %%getUrl%% when fetching URIs over HTTP and HTTPS requests.
     *
     *  This method affects all requests globally.
     *
     *  If [[lockConfig]] has been called, no change is made and this
     *  throws.
     */
    static registerGetUrl(getUrl: FetchGetUrlFunc): void {
        if (locked) { throw new Error("gateways locked"); }
        getUrlFunc = getUrl;
    }

    /**
     *  Creates a function that can "fetch" data URIs.
     *
     *  Note that this is automatically done internally to support
     *  data URIs, so it is not necessary to register it.
     *
     *  This is not generally something that is needed, but may
     *  be useful in a wrapper to perfom custom data URI functionality.
     */
    static createDataGateway(): FetchGatewayFunc {
        return dataGatewayFunc;
    }

    /**
     *  Creates a function that will fetch IPFS (unvalidated) from
     *  a custom gateway baseUrl.
     *
     *  The default IPFS gateway used internally is
     *  ``"https:/\/gateway.ipfs.io/ipfs/"``.
     */
    static createIpfsGatewayFunc(baseUrl: string): FetchGatewayFunc {
        return getIpfsGatewayFunc(baseUrl);
    }
}


interface ThrottleError extends Error {
    stall: number;
    throttle: true;
};

/**
 *  The response for a FetchREquest.
 */
export class FetchResponse implements Iterable<[ key: string, value: string ]> {
    #statusCode: number;
    #statusMessage: string;
    #headers: Record<string, string>;
    #body: null | Readonly<Uint8Array>;
    #request: null | FetchRequest;

    #error: { error?: Error, message: string };

    toString(): string {
        return `<FetchResponse status=${ this.statusCode } body=${ this.#body ? hexlify(this.#body): "null" }>`;
    }

    /**
     *  The response status code.
     */
    get statusCode(): number { return this.#statusCode; }

    /**
     *  The response status message.
     */
    get statusMessage(): string { return this.#statusMessage; }

    /**
     *  The response headers. All keys are lower-case.
     */
    get headers(): Record<string, string> { return Object.assign({ }, this.#headers); }

    /**
     *  The response body, or ``null`` if there was no body.
     */
    get body(): null | Readonly<Uint8Array> {
        return (this.#body == null) ? null: new Uint8Array(this.#body);
    }

    /**
     *  The response body as a UTF-8 encoded string, or the empty
     *  string (i.e. ``""``) if there was no body.
     *
     *  An error is thrown if the body is invalid UTF-8 data.
     */
    get bodyText(): string {
        try {
            return (this.#body == null) ? "": toUtf8String(this.#body);
        } catch (error) {
            assert(false, "response body is not valid UTF-8 data", "UNSUPPORTED_OPERATION", {
                operation: "bodyText", info: { response: this }
            });
        }
    }

    /**
     *  The response body, decoded as JSON.
     *
     *  An error is thrown if the body is invalid JSON-encoded data
     *  or if there was no body.
     */
    get bodyJson(): any {
        try {
            return JSON.parse(this.bodyText);
        } catch (error) {
            assert(false, "response body is not valid JSON", "UNSUPPORTED_OPERATION", {
                operation: "bodyJson", info: { response: this }
            });
        }
    }

    [Symbol.iterator](): Iterator<[ key: string, value: string ]> {
        const headers = this.headers;
        const keys = Object.keys(headers);
        let index = 0;
        return {
            next: () => {
                if (index < keys.length) {
                    const key = keys[index++];
                    return {
                        value: [ key, headers[key] ], done: false
                    }
                }
                return { value: undefined, done: true };
            }
        };
    }

    constructor(statusCode: number, statusMessage: string, headers: Readonly<Record<string, string>>, body: null | Uint8Array, request?: FetchRequest) {
        this.#statusCode = statusCode;
        this.#statusMessage = statusMessage;
        this.#headers = Object.keys(headers).reduce((accum, k) => {
            accum[k.toLowerCase()] = String(headers[k]);
            return accum;
        }, <Record<string, string>>{ });
        this.#body = ((body == null) ? null: new Uint8Array(body));
        this.#request = (request || null);

        this.#error = { message: "" };
    }

    /**
     *  Return a Response with matching headers and body, but with
     *  an error status code (i.e. 599) and %%message%% with an
     *  optional %%error%%.
     */
    makeServerError(message?: string, error?: Error): FetchResponse {
        let statusMessage: string;
        if (!message) {
            message = `${ this.statusCode } ${ this.statusMessage }`;
            statusMessage = `CLIENT ESCALATED SERVER ERROR (${ message })`;
        } else {
            statusMessage = `CLIENT ESCALATED SERVER ERROR (${ this.statusCode } ${ this.statusMessage }; ${ message })`;
        }
        const response = new FetchResponse(599, statusMessage, this.headers,
            this.body, this.#request || undefined);
        response.#error = { message, error };
        return response;
    }

    /**
     *  If called within a [request.processFunc](FetchRequest-processFunc)
     *  call, causes the request to retry as if throttled for %%stall%%
     *  milliseconds.
     */
    throwThrottleError(message?: string, stall?: number): never {
        if (stall == null) {
            stall = -1;
        } else {
            assertArgument(Number.isInteger(stall) && stall >= 0, "invalid stall timeout", "stall", stall);
        }

        const error = new Error(message || "throttling requests");

        defineProperties(<ThrottleError>error, { stall, throttle: true });

        throw error;
    }

    /**
     *  Get the header value for %%key%%, ignoring case.
     */
    getHeader(key: string): string {
        return this.headers[key.toLowerCase()];
    }

    /**
     *  Returns true of the response has a body.
     */
    hasBody(): this is (FetchResponse & { body: Uint8Array }) {
        return (this.#body != null);
    }

    /**
     *  The request made for this response.
     */
    get request(): null | FetchRequest { return this.#request; }

    /**
     *  Returns true if this response was a success statusCode.
     */
    ok(): boolean {
        return (this.#error.message === "" && this.statusCode >= 200 && this.statusCode < 300);
    }

    /**
     *  Throws a ``SERVER_ERROR`` if this response is not ok.
     */
    assertOk(): void {
        if (this.ok()) { return; }
        let { message, error } = this.#error;
        if (message === "") {
            message = `server response ${ this.statusCode } ${ this.statusMessage }`;
        }
        assert(false, message, "SERVER_ERROR", {
            request: (this.request || "unknown request"), response: this, error
        });
    }
}


function getTime(): number { return (new Date()).getTime(); }

function unpercent(value: string): Uint8Array {
    return toUtf8Bytes(value.replace(/%([0-9a-f][0-9a-f])/gi, (all, code) => {
        return String.fromCharCode(parseInt(code, 16));
    }));
}

function wait(delay: number): Promise<void> {
    return new Promise((resolve) => setTimeout(resolve, delay));
}