Make bufToStr in polyfill strict, and document difference with native implementation (#5285)

js/ccf-app/src/global.ts

@@ -603,6 +603,9 @@ export interface CCF {
 
   /**
    * Convert an ArrayBuffer into a string.
+   *
+   * Note that this function does not perform any encoding validation, and may produce
+   * an invalid JS string if the input is not valid UTF-8.
    */
   bufToStr(v: ArrayBuffer): string;
 

js/ccf-app/src/polyfill.ts

@@ -489,8 +489,10 @@ class CCFPolyfill implements CCF {
     return typedArrToArrBuf(new TextEncoder().encode(s));
   }
 
+  // Note: this is stricter than CCF's bufToStr, as it will
+  // reject buffers that are not valid UTF-8.
   bufToStr(v: ArrayBuffer): string {
-    return new TextDecoder().decode(v);
+    return new TextDecoder("utf-8", { fatal: true }).decode(v);
   }
 
   jsonCompatibleToBuf<T extends JsonCompatible<T>>(v: T): ArrayBuffer {