Fix TextDecoder for SharedArrayBuffer backed TypedArray (#1940)

js/text_encoding.ts

@@ -356,6 +356,13 @@ export interface TextDecoderOptions {
   ignoreBOM?: false;
 }
 
+type EitherArrayBuffer = SharedArrayBuffer | ArrayBuffer;
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+function isEitherArrayBuffer(x: any): x is EitherArrayBuffer {
+  return x instanceof SharedArrayBuffer || x instanceof ArrayBuffer;
+}
+
 export class TextDecoder {
   private _encoding: string;
 
@@ -400,12 +407,14 @@ export class TextDecoder {
     }
 
     let bytes: Uint8Array;
-    if (typeof input === "object" && input instanceof ArrayBuffer) {
+    if (input instanceof Uint8Array) {
+      bytes = input;
+    } else if (isEitherArrayBuffer(input)) {
       bytes = new Uint8Array(input);
     } else if (
       typeof input === "object" &&
       "buffer" in input &&
-      input.buffer instanceof ArrayBuffer
+      isEitherArrayBuffer(input.buffer)
     ) {
       bytes = new Uint8Array(input.buffer, input.byteOffset, input.byteLength);
     } else {

js/text_encoding_test.ts

@@ -65,3 +65,29 @@ test(function textEncoder2() {
     0xf0, 0x9d, 0x93, 0xbd
   ]);
 });
+
+test(function textDecoderSharedUint8Array() {
+  const ab = new SharedArrayBuffer(6);
+  const dataView = new DataView(ab);
+  const charCodeA = "A".charCodeAt(0);
+  for (let i = 0; i < ab.byteLength; i++) {
+    dataView.setUint8(i, charCodeA + i);
+  }
+  const ui8 = new Uint8Array(ab);
+  const decoder = new TextDecoder();
+  const actual = decoder.decode(ui8);
+  assertEquals(actual, "ABCDEF");
+});
+
+test(function textDecoderSharedInt32Array() {
+  const ab = new SharedArrayBuffer(8);
+  const dataView = new DataView(ab);
+  const charCodeA = "A".charCodeAt(0);
+  for (let i = 0; i < ab.byteLength; i++) {
+    dataView.setUint8(i, charCodeA + i);
+  }
+  const i32 = new Int32Array(ab);
+  const decoder = new TextDecoder();
+  const actual = decoder.decode(i32);
+  assertEquals(actual, "ABCDEFGH");
+});