Wasm: save data in case of canceled requests

There's currently no way for a service worker to know if a request has been canceled. This includes understanding if the user skipped forward in a video, for example.
To avoid returning the entire file every time, we're limiting Range requests from audio/video tags to 2MB chunks only.

See: w3c/ServiceWorker#1544

ui/src/sw/sw.js

@@ -89,21 +89,6 @@ self.addEventListener('message', async (event) => {
     }
 })
 
-/*
-;(async function main() {
-    // Load the WebAssembly
-    const result = await WebAssembly.instantiateStreaming(fetch('http://localhost:3129/ui/app.wasm'), go.importObject)
-    go.run(result.instance)
-
-    // Get the index
-    const index = Prvt.getIndex('http://localhost:3129', new Uint8Array(DecodeArrayBuffer(RemovePaddingChars(masterKey))))
-    console.log(index)
-    console.log(await index.listFolder('/'))
-    console.log(await index.stat())
-    console.log(await index.getFileByPath('/bensound-energy.mp3'))
-    console.log(await index.getFileById('f7a82545-fe70-4681-a5b2-ab593ebfab37'))
-})()*/
-
 /*
 Test:
 

wasm/decrypt.go

@@ -128,6 +128,20 @@ func DecryptRequest() js.Func {
 			method = decryptRequestPromise(masterKey, fileId)
 		} else {
 			// Range requests
+
+			// If this is a request for a video or audio, then we will limit the returned range to to 2 MB max
+			// This is because we have no way yet to detect if requests are canceled (waiting for https://github.com/w3c/ServiceWorker/issues/1544) and this way we at least waste less data when requests are canceled
+			// We determine if the request is for a video or audio by looking at the destination (see https://developer.mozilla.org/en-US/docs/Web/API/Request/destination)
+			destination := req.Get("destination").String()
+			if destination == "audio" || destination == "track" || destination == "video" {
+				// Limit to 2 MB
+				// Also the case of Length = 0, which means that the request was for the entire file
+				if rng.Length > (2<<20) || rng.Length == 0 {
+					rng.Length = 2 << 20
+				}
+			}
+
+			// Make the request
 			method = decryptRangeRequestPromise(masterKey, fileId, rng)
 		}
 		promiseConstructor := js.Global().Get("Promise")
@@ -391,7 +405,6 @@ func responseUnderlyingSource(body io.ReadCloser, decryptFunc decryptFuncSignatu
 				for {
 					buf := make([]byte, 65536)
 					n, err := pr.Read(buf)
-					fmt.Println("Read", n, err)
 					if err != nil && err != io.EOF {
 						// Stream had an error
 						controller.Call("error", jsError(err.Error()))
@@ -402,7 +415,6 @@ func responseUnderlyingSource(body io.ReadCloser, decryptFunc decryptFuncSignatu
 					}
 					if err == io.EOF {
 						// Stream is done
-						fmt.Println("Stream closed")
 						controller.Call("close")
 						return
 					}