Service worker files to cache don't include base path

[finnbear]

Describe the bug
The paths passed to the service worker for caching do not include the basepath

Logs
Here is the compiled service worker for my project, despite the fact that I specified a two-directory-deep base path following the instructions in the documentation. As you can see, many of the paths in the service worker start with /client rather than /my/basepath/client.

!function(){"use strict";const e=1610090779060,t=`cache${e}`,c=["/client/client.4a48bc03.js","/client/inject_styles.5607aec6.js","/client/index.a638974d.js","/client/integer.23b05fd4.js","/client/Select.f63838d9.js","/client/Slider.cdb985c1.js","/client/matrix.4cf47436.js","/client/cache.f20914d9.js","/client/float.8d03a59d.js","/client/heap.50efc23e.js"].concat(["/service-worker-index.html","/favicon.png","/global.css","/manifest.json"]),n=new Set(c);self.addEventListener("install",(e=>{e.waitUntil(caches.open(t).then((e=>e.addAll(c))).then((()=>{self.skipWaiting()})))})),self.addEventListener("activate",(e=>{e.waitUntil(caches.keys().then((async e=>{for(const c of e)c!==t&&await caches.delete(c);self.clients.claim()})))})),self.addEventListener("fetch",(t=>{if("GET"!==t.request.method||t.request.headers.has("range"))return;const c=new URL(t.request.url),s=c.protocol.startsWith("http"),a=c.hostname===self.location.hostname&&c.port!==self.location.port,i=c.host===self.location.host&&n.has(c.pathname),l="only-if-cached"===t.request.cache&&!i;!s||a||l||t.respondWith((async()=>i&&await caches.match(t.request)||async function(t){const c=await caches.open(`offline${e}`);try{const e=await fetch(t);return c.put(t,e.clone()),e}catch(e){const n=await c.match(t);if(n)return n;throw e}}(t.request))())}))}();

To Reproduce

1. Create a sapper app
2. Add a basepath, following the documentations instructions to add it to both the server and the argument to sapper export
3. Load the site
4. Look for failed caching requests to incorrect urls

Expected behavior
Service worker should be passed paths that conform to the basepath.

• The output of npx envinfo --system --npmPackages svelte,sapper,rollup,webpack --binaries --browsers
  System:
  OS: Linux 5.4 Ubuntu 20.04.1 LTS (Focal Fossa)
  CPU: (8) x64 Intel(R) Core(TM) i7-7700K CPU @ 4.20GHz
  Memory: 7.26 GB / 31.31 GB
  Container: Yes
  Shell: 5.0.17 - /bin/bash
  Binaries:
  Node: 14.13.0 - ~/.nvm/versions/node/v14.13.0/bin/node
  npm: 6.14.8 - ~/.nvm/versions/node/v14.13.0/bin/npm
  Browsers:
  Firefox: 81.0

• Your browser: Firefox 81

• Your hosting environment (i.e. Local, GCP/AWS/Azure, Vercel/Begin, etc...): Simple http server

• If it is an exported (npm run export) or dynamic application: Exported/static

Severity
My site works fine, but nothing is cached properly by the service worker.

[benmccann]

This was also reported in #1538 and #1441

I'm not quite sure what the best fix is. See the description in #1545 (comment). I'm leaning towards prepending the base path. Though I'm not actively working on Sapper at the moment. But if you'd like to send a PR I'll review it

[finnbear]

Those do look like the same issue. I guess we can close this as duplicate.