lib/src/http/lifecycle/Response.ts

import {Request} from './Request'
import {ErrorWithContext, HTTPStatus, BehaviorSubject} from '../../util'
import {ServerResponse} from 'http'
import {HTTPCookieJar} from '../kernel/HTTPCookieJar'
import {Readable} from 'stream'
import {Logging} from '../../service/Logging'

/**
 * Error thrown when the server tries to re-send headers after they have been sent once.
 */
export class HeadersAlreadySentError extends ErrorWithContext {
    constructor(response: Response, headerName?: string) {
        super(`Cannot modify or re-send headers for this request as they have already been sent.`)
        this.context = { headerName }
    }
}

/**
 * Error thrown when the server tries to re-send a response that has already been sent.
 */
export class ResponseAlreadySentError extends ErrorWithContext {
    constructor(public readonly response: Response) {
        super(`Cannot modify or re-send response as it has already ended.`)
    }
}

/**
 * A class representing an HTTP response to a client.
 */
export class Response {
    /** Mapping of headers that should be sent back to the client. */
    private headers: {[key: string]: string | string[]} = {}

    /** True if the headers have been sent. */
    private sentHeaders = false

    /** True if the response has been sent and closed. */
    private responseEnded = false

    /** The HTTP status code that should be sent to the client. */
    private status: HTTPStatus = HTTPStatus.OK

    /**
     * If this is true, then some module in the kernel has flagged the response
     * as being interrupted and handled. Subsequent modules should NOT overwrite
     * the response.
     * @private
     */
    private isBlockingWriteback = false

    /** The body contents that should be written to the response. */
    public body: string | Buffer | Uint8Array | Readable = ''

    /**
     * Behavior subject fired right before the response content is written.
     */
    public readonly sending$: BehaviorSubject<Response> = new BehaviorSubject<Response>()

    /**
     * Behavior subject fired right after the response content is written.
     */
    public readonly sent$: BehaviorSubject<Response> = new BehaviorSubject<Response>()

    constructor(
        /** The associated request object. */
        public readonly request: Request,

        /** The native Node.js ServerResponse. */
        protected readonly serverResponse?: ServerResponse,
    ) { }

    protected get logging(): Logging {
        return this.request.make(Logging)
    }

    /** Get the currently set response status. */
    public getStatus(): HTTPStatus {
        return this.status
    }

    /** Set a new response status. */
    public setStatus(status: HTTPStatus): this {
        if ( this.sentHeaders ) {
            throw new HeadersAlreadySentError(this, 'status')
        }

        this.status = status
        return this
    }

    /** Get the HTTPCookieJar for the client. */
    public get cookies(): HTTPCookieJar {
        return this.request.cookies
    }

    /** Get the value of the response header, if it exists. */
    public getHeader(name: string): string | string[] | undefined {
        return this.headers[name]
    }

    /** Set the value of the response header. */
    public setHeader(name: string, value: string | string[]): this {
        this.logging.verbose(`Will set header on response: ${name}`)
        if ( this.sentHeaders ) {
            throw new HeadersAlreadySentError(this, name)
        }

        this.headers[name] = value
        return this
    }

    /**
     * Remove a header from the response by name.
     * @param name
     */
    public unsetHeader(name: string): this {
        this.logging.verbose(`Will unset header on response: ${name}`)
        if ( this.sentHeaders ) {
            throw new HeadersAlreadySentError(this, name)
        }

        delete this.headers[name]
        return this
    }

    /**
     * Bulk set the specified headers in the response.
     * @param data
     */
    public setHeaders(data: {[name: string]: string | string[]}): this {
        this.logging.verbose(`Will set headers on response: ${Object.keys(data).join(', ')}`)
        if ( this.sentHeaders ) {
            throw new HeadersAlreadySentError(this)
        }

        this.headers = {...this.headers,
            ...data}
        return this
    }

    /**
     * Add the given value as a header, appending it to an existing header if one exists.
     * @param name
     * @param value
     */
    public appendHeader(name: string, value: string | string[]): this {
        this.logging.verbose(`Will append header: ${name}`)

        if ( this.sentHeaders ) {
            throw new HeadersAlreadySentError(this, name)
        }

        if ( !Array.isArray(value) ) {
            value = [value]
        }

        let existing = this.headers[name] ?? []
        if ( !Array.isArray(existing) ) {
            existing = [existing]
        }

        existing = [...existing, ...value]
        if ( existing.length === 1 ) {
            existing = existing[0]
        }

        this.headers[name] = existing
        return this
    }

    /**
     * Write the headers to the client.
     */
    public sendHeaders(): this {
        this.logging.verbose(`Sending headers...`)
        if ( !this.serverResponse ) {
            throw new ErrorWithContext('Unable to send headers: Response has no underlying connection.', {
                suggestion: 'This usually means the Request was created by an alternative server, like WebsocketServer. You should use that server to handle the request.',
            })
        }
        const headers = {} as any

        const setCookieHeaders = this.cookies.getSetCookieHeaders()
        if ( setCookieHeaders.length ) {
            headers['Set-Cookie'] = setCookieHeaders
        }

        for ( const key in this.headers ) {
            if ( !Object.prototype.hasOwnProperty.call(this.headers, key) ) {
                continue
            }

            headers[key] = this.headers[key]
        }

        this.serverResponse.writeHead(this.status, headers)
        this.sentHeaders = true
        return this
    }

    /** Returns true if the headers have been sent. */
    public hasSentHeaders(): boolean {
        return this.sentHeaders
    }

    /** Returns true if a body has been set in the response. */
    public hasBody(): boolean {
        return Boolean(this.body)
    }

    /**
     * Get or set the flag for whether the writeback should be blocked.
     * @param set - if this is specified, the value will be set.
     */
    public blockingWriteback(set?: boolean): boolean {
        if ( typeof set !== 'undefined' ) {
            this.isBlockingWriteback = set
        }

        return this.isBlockingWriteback
    }

    /**
     * Write the headers and specified data to the client.
     * @param data
     */
    public async write(data: string | Buffer | Uint8Array | Readable): Promise<void> {
        this.logging.verbose(`Writing headers & data to response... (destroyed? ${!this.serverResponse || this.serverResponse.destroyed})`)
        return new Promise<void>((res, rej) => {
            if ( !this.serverResponse ) {
                throw new ErrorWithContext('Unable to write response: Response has no underlying connection.', {
                    suggestion: 'This usually means the Request was created by an alternative server, like WebsocketServer. You should use that server to handle the request.',
                })
            }

            if ( this.responseEnded || this.serverResponse.destroyed ) {
                throw new ErrorWithContext('Tried to write to Response after lifecycle ended.')
            }

            if ( !this.sentHeaders ) {
                this.sendHeaders()
            }

            if ( data instanceof Readable ) {
                data.pipe(this.serverResponse)
                    .on('finish', () => {
                        res()
                    })
                    .on('error', error => {
                        rej(error)
                    })
            } else {
                this.serverResponse.write(data, error => {
                    if ( error ) {
                        rej(error)
                    } else {
                        res()
                    }
                })
            }
        })
    }

    /**
     * Send the response to the client, writing the headers and configured body.
     */
    public async send(): Promise<void> {
        await this.sending$.next(this)

        if ( !(this.body instanceof Readable) ) {
            this.setHeader('Content-Length', String(Buffer.from(this.body ?? '').length))
        }

        this.setHeader('Date', (new Date()).toUTCString())
        this.setHeader('Permissions-Policy', 'interest-cohort=()')

        await this.write(this.body ?? '')
        this.end()

        await this.sent$.next(this)
    }

    /**
     * Returns true if the response can still be sent. False if it has been sent
     * or the connection has been destroyed.
     */
    public canSend(): boolean {
        return !(this.responseEnded || !this.serverResponse || this.serverResponse.destroyed)
    }

    /**
     * Mark the response as ended and close the socket.
     */
    public end(): this {
        if ( this.responseEnded ) {
            throw new ResponseAlreadySentError(this)
        }

        this.sentHeaders = true
        this.serverResponse?.end()
        return this
    }

    // location?
}