file_download.js

/**
 * Responsible for handling the actual downloading of a requested file from B2.
 */

import { rewriteErrorResponse } from './error_handling'
import {
    CACHE_AGE_SECONDS,
    PLAIN_TEXT_CONTENT_TYPE,
} from './constants'


/**
 * Fetch and return a file from Backblaze B2 by supplying the download authorization
 * token in the Authorization header.
 * Errors are rewritten to not give away that this is a B2 bucket.
 * Headers starting with "x-bz" are removed before being returned to the user.
 * Adds cache and security-related headers.
 *
 * @param request the request from a client that will be rewritten to fetch from
 *                a Backblaze B2 bucket
 * @param {object} b2 the b2config object
 * @returns {Promise<Response>} the Response to the client that will either be
 *                              the requested file or an error page
 */
async function getB2File(request, b2) {
    let requestedUrl = new URL(request.url)
    console.log(`requestedUrl = ${requestedUrl.toString()}`)
    if(requestedUrl.hostname === DIR_DOMAIN) {
        requestedUrl.hostname = MAIN_DOMAIN
        return Response.redirect(requestedUrl.toString(), 301)
    }
    let url = new URL(b2.data.downloadUrl)
    url.pathname = `/file/${B2BUCKET}/${requestedUrl.pathname}`

    const response = await fetch(url.toString(), {
        cf: {
            cacheTtl: 60,
            cacheEverything: true,
        },
        headers: {
            "Authorization": b2.data.authorizationToken,
        }
    })

    if(response.ok) {
        return modifiedB2Response(request, response)
    }
    else {
        return rewriteErrorResponse(request, response)
    }
}


/**
 * Adds cache headers, converts some Backblaze B2-specific headers to standard
 * headers, and deletes the headers that start with "x-bz"
 *
 * @param request a request for a file on B2
 * @param response the successful response from B2 that will be copied and modified
 * @param convertHeaders if true, convert x-bz headers to standard headers then delete them
 * @returns {Promise<Response>} the modified response
 */
async function modifiedB2Response(request, response, convertHeaders=true) {
    console.log("modifiedB2Response...")
    const newResponse = new Response(response.body, response)

    // cache for a week
    newResponse.headers.set("Cache-Control", `public, immutable, max-age=${CACHE_AGE_SECONDS}`)
    newResponse.headers.set("Expires", new Date(Date.now() + CACHE_AGE_SECONDS * 1000).toUTCString())
    if(convertHeaders) {
        convertB2Headers(request, newResponse)
    }

    return newResponse
}


/**
 * Converts the x-bz-content-sha1 header to an ETag header.
 * Converts the x-bz-upload-timestamp header to a Last-Modified header.
 * By default also deletes all headers that start with "x-bz"
 *
 * @param request the request from the client for a B2 file
 * @param response the response from B2 that will be modified in-place
 * @param deleteHeaders if true, delete the x-bz headers in the response
 */
function convertB2Headers(request, response, deleteHeaders=true) {
    console.log("convertB2Headers...")
    // get a Last-Modified header from x-bz-upload-timestamp
    let bzts = response.headers.get("x-bz-upload-timestamp")
    bzts = parseInt(bzts)
    let d = new Date(bzts)
    let lastModified = d.toUTCString()
    response.headers.set("Last-Modified", lastModified)

    // get an ETag header from x-bz-content-sha1
    let bzsha = response.headers.get("x-bz-content-sha1")
    bzsha = bzsha.replace(/^unverified:/, "")  // in case it was uploaded without a checksum
    bzsha = bzsha.substring(0, 16)  // just get the first 16 characters of it
    bzsha = `"${bzsha}"`  // CloudFlare wants the ETag wrapped in quotes
    response.headers.set("ETag", bzsha)

    if(deleteHeaders) {
        // remove the 'x-bz-' Backblaze headers
        for(const header of response.headers.keys()) {
            if(header.match(/^x-bz/i)) {
                response.headers.delete(header)
            }
        }
    }

    // these file extensions we want to show up as plain text
    let url = new URL(request.url)
    if(/\.(pub|boot|cfg)$/.test(url.pathname)) {
        response.headers.set("Content-Type", PLAIN_TEXT_CONTENT_TYPE)
    }
}


export default getB2File