Fix infinite recursion in web crypto randomUUID polyfill (#858)

On web (browser/Electron), expo-crypto's randomUUID() and getRandomValues()
just forward to globalThis.crypto.* (see expo-crypto/src/ExpoCrypto.web.ts).
The previous polyfill installed `g.crypto.randomUUID = () => ExpoCrypto.randomUUID()`,
which on web reads back through the same `globalThis.crypto.randomUUID` it
just installed, recursing until the stack overflows. The same trap exists
for getRandomValues, but it rarely triggers because the native version is
almost always present.

Capture a bound reference to the native getRandomValues *before* installing
the polyfill, and generate UUID v4 in JS from 16 random bytes (RFC 4122
version + variant bits). The fallback path stays on ExpoCrypto.getRandomValues
(non-recursive on native), and ExpoCrypto.randomUUID is no longer used.

packages/app/src/polyfills/crypto.test.ts

@@ -0,0 +1,40 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const expoCryptoMock = vi.hoisted(() => ({
+  getRandomValues: vi.fn(<T extends ArrayBufferView>(array: T): T => array),
+  randomUUID: vi.fn(() => {
+    throw new Error("ExpoCrypto.randomUUID should not be used for the web fallback");
+  }),
+}));
+
+vi.mock("expo-crypto", () => expoCryptoMock);
+
+describe("polyfillCrypto", () => {
+  beforeEach(() => {
+    vi.resetModules();
+    vi.unstubAllGlobals();
+    expoCryptoMock.getRandomValues.mockClear();
+    expoCryptoMock.randomUUID.mockClear();
+  });
+
+  it("generates randomUUID from getRandomValues when Web Crypto randomUUID is unavailable", async () => {
+    const sourceBytes = Uint8Array.from([
+      0x00, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 0xee,
+      0xff,
+    ]);
+    const getRandomValues = vi.fn(<T extends ArrayBufferView | null>(array: T): T => {
+      if (array && ArrayBuffer.isView(array)) {
+        new Uint8Array(array.buffer, array.byteOffset, array.byteLength).set(sourceBytes);
+      }
+      return array;
+    });
+    vi.stubGlobal("crypto", { getRandomValues });
+
+    const { polyfillCrypto } = await import("./crypto");
+    polyfillCrypto();
+
+    expect(globalThis.crypto.randomUUID()).toBe("00112233-4455-4677-8899-aabbccddeeff");
+    expect(getRandomValues).toHaveBeenCalledTimes(1);
+    expect(expoCryptoMock.randomUUID).not.toHaveBeenCalled();
+  });
+});

packages/app/src/polyfills/crypto.ts

@@ -13,8 +13,26 @@ interface MutableGlobal {
   crypto?: Crypto;
 }
 
+type RandomUUID = `${string}-${string}-${string}-${string}-${string}`;
+type FillRandomValues = <T extends ArrayBufferView | null>(array: T) => T;
+
+function createUuidV4(fillRandomValues: FillRandomValues): RandomUUID {
+  const bytes = fillRandomValues(new Uint8Array(16));
+  bytes[6] = (bytes[6]! & 0x0f) | 0x40;
+  bytes[8] = (bytes[8]! & 0x3f) | 0x80;
+
+  const hex = Array.from(bytes, (byte) => byte.toString(16).padStart(2, "0"));
+  return `${hex.slice(0, 4).join("")}-${hex.slice(4, 6).join("")}-${hex
+    .slice(6, 8)
+    .join("")}-${hex.slice(8, 10).join("")}-${hex.slice(10, 16).join("")}` as RandomUUID;
+}
+
 export function polyfillCrypto(): void {
   const g = globalThis as unknown as MutableGlobal;
+  const nativeGetRandomValues =
+    typeof g.crypto?.getRandomValues === "function"
+      ? g.crypto.getRandomValues.bind(g.crypto)
+      : null;
 
   // Ensure TextEncoder/TextDecoder exist for shared E2EE code (tweetnacl + relay transport).
   // Hermes may not provide them in all configurations.
@@ -47,17 +65,21 @@ export function polyfillCrypto(): void {
     g.crypto = {} as Crypto;
   }
 
+  const fillRandomValues: FillRandomValues = <T extends ArrayBufferView | null>(array: T): T => {
+    if (array === null) return array;
+    if (nativeGetRandomValues) {
+      return nativeGetRandomValues(array as unknown as ArrayBufferView<ArrayBuffer>) as T;
+    }
+    return ExpoCrypto.getRandomValues(
+      array as unknown as Parameters<typeof ExpoCrypto.getRandomValues>[0],
+    ) as unknown as T;
+  };
+
   if (typeof g.crypto.randomUUID !== "function") {
-    g.crypto.randomUUID = () =>
-      ExpoCrypto.randomUUID() as `${string}-${string}-${string}-${string}-${string}`;
+    g.crypto.randomUUID = () => createUuidV4(fillRandomValues);
   }
 
   if (typeof g.crypto.getRandomValues !== "function") {
-    g.crypto.getRandomValues = <T extends ArrayBufferView | null>(array: T): T => {
-      if (array === null) return array;
-      return ExpoCrypto.getRandomValues(
-        array as unknown as Parameters<typeof ExpoCrypto.getRandomValues>[0],
-      ) as unknown as T;
-    };
+    g.crypto.getRandomValues = fillRandomValues;
   }
 }