crypto/rand: use runtime.getRandomData on js/wasm

This mechanism ultimately calls the same JavaScript method, but being
consistent between runtime and crypto/rand lets us reuse test coverage
across them. Also, no allocations.

Cq-Include-Trybots: luci.golang.try:gotip-js-wasm
Change-Id: I035da1ed603fbcdad61945f2b57f3fb527a959fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/608396
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>

src/crypto/rand/rand_js.go

@@ -4,41 +4,24 @@
 
 package rand
 
-import "syscall/js"
-
 // The maximum buffer size for crypto.getRandomValues is 65536 bytes.
 // https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues#exceptions
 const maxGetRandomRead = 64 << 10
 
-// read implements a pseudorandom generator
-// using JavaScript crypto.getRandomValues method.
+//go:wasmimport gojs runtime.getRandomData
+//go:noescape
+func getRandomValues(r []byte)
+
+// read calls the JavaScript Crypto.getRandomValues() method.
 // See https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues.
-var read = batched(getRandom, maxGetRandomRead)
-
-var jsCrypto = js.Global().Get("crypto")
-var uint8Array = js.Global().Get("Uint8Array")
-
-func getRandom(b []byte) error {
-	a := uint8Array.New(len(b))
-	jsCrypto.Call("getRandomValues", a)
-	js.CopyBytesToGo(b, a)
+func read(b []byte) error {
+	for len(b) > 0 {
+		size := len(b)
+		if size > maxGetRandomRead {
+			size = maxGetRandomRead
+		}
+		getRandomValues(b[:size])
+		b = b[size:]
+	}
 	return nil
 }
-
-// batched returns a function that calls f to populate a []byte by chunking it
-// into subslices of, at most, readMax bytes.
-func batched(f func([]byte) error, readMax int) func([]byte) error {
-	return func(out []byte) error {
-		for len(out) > 0 {
-			read := len(out)
-			if read > readMax {
-				read = readMax
-			}
-			if err := f(out[:read]); err != nil {
-				return err
-			}
-			out = out[read:]
-		}
-		return nil
-	}
-}

src/crypto/rand/rand_test.go

@@ -160,9 +160,6 @@ func TestAllocations(t *testing.T) {
 		// Might be fixable with https://go.dev/issue/56378.
 		t.Skip("boringcrypto allocates")
 	}
-	if runtime.GOOS == "js" {
-		t.Skip("syscall/js allocates")
-	}
 	if race.Enabled {
 		t.Skip("urandomRead allocates under -race")
 	}