Forward String::length in RuntimeDecorator and WithRuntimeDecorator

Summary:
`IRuntime::length(const String&)` came in with "Add `String::length` API" (590396bd4 on static_h, D96030262), and that commit didn't touch `decorator.h`. Neither `RuntimeDecorator` nor `WithRuntimeDecorator` overrides it, so on a decorated runtime `length()` falls back to the default `utf16(str).size()` and never reaches the wrapped runtime.

It looks like an oversight. The same stack added `push`, `tryGetMutableBuffer`, `detached` and the TypedArray methods, and each of those commits added decorator overrides, including one for this method's overload `length(const TypedArray&)`. The `createError*` commit (facebook/hermes#2203) and this one didn't.

For Hermes-backed decorators the value is already right, because Hermes' `utf16()` is exact, but every call copies the whole string instead of reading the length. Runtimes with a native `length()` and the default `utf16()` get wrong answers. React Native's `JSCRuntime` is one: built against the system JavaScriptCore and wrapped in a `RuntimeDecorator`, `'a\uD800b'` has length 1 instead of 3 and `'x\uDC00yz'` has 1 instead of 4. With this change both match the undecorated runtime. That setup lives outside this repo, so it isn't a test here.

`WithRuntimeDecorator` needs the override too. Otherwise it would inherit the plain forward and `length()` would skip `Around`, which it currently goes through via `utf16()`.

`TracingRuntime` no longer records an incidental `Utf16Record` when `length()` is called. That record only came from the `utf16()` fallback. I didn't add a record for `length()`: strings are immutable, and `TracingRuntime` already leaves pure queries like `size(Array)` untraced.

X-link: facebook/hermes#2205

Reviewed By: avp

Differential Revision: D123943189

Pulled By: lavenzg

Author: mrousavy

packages/react-native/ReactCommon/jsi/jsi/decorator.h

@@ -243,6 +243,10 @@ class RuntimeDecorator : public Base, private jsi::Instrumentation {
     return plain_.utf16(sym);
   }
 
+  size_t length(const String& str) override {
+    return plain_.length(str);
+  }
+
   void getStringData(
       const jsi::String& str,
       void* ctx,
@@ -822,6 +826,11 @@ class WithRuntimeDecorator : public RuntimeDecorator<Plain, Base> {
     return RD::utf16(sym);
   }
 
+  size_t length(const String& str) override {
+    Around around{with_};
+    return RD::length(str);
+  }
+
   void getStringData(
       const jsi::String& str,
       void* ctx,

packages/react-native/ReactCommon/jsi/jsi/test/testlib.cpp

@@ -138,6 +138,61 @@ TEST_P(JSITest, StringLengthTest) {
   EXPECT_EQ(invalid.length(rt), 2);
 }
 
+TEST_P(JSITest, DecoratorForwardsStringLength) {
+  // Decorators should forward length(const String&) to the decorated runtime
+  // instead of falling back to the default Runtime::length, which copies the
+  // string via utf16() and bypasses the runtime's own implementation.
+  class Inner : public RuntimeDecorator<Runtime, Runtime> {
+   public:
+    explicit Inner(Runtime& rt) : RuntimeDecorator(rt) {}
+
+    size_t length(const String&) override {
+      ++lengthCalls;
+      return 42;
+    }
+
+    std::u16string utf16(const String& str) override {
+      ++utf16Calls;
+      return RuntimeDecorator::utf16(str);
+    }
+
+    int lengthCalls = 0;
+    int utf16Calls = 0;
+  };
+
+  class Outer : public RuntimeDecorator<Runtime, Runtime> {
+   public:
+    explicit Outer(Runtime& rt) : RuntimeDecorator(rt) {}
+  };
+
+  struct Count {
+    void before() {
+      ++beforeCalls;
+    }
+    void after() {
+      ++afterCalls;
+    }
+    int beforeCalls = 0;
+    int afterCalls = 0;
+  };
+
+  Inner inner(rt);
+  String str = String::createFromAscii(inner, "hello");
+
+  Outer outer(inner);
+  EXPECT_EQ(str.length(outer), 42);
+  EXPECT_EQ(inner.lengthCalls, 1);
+  EXPECT_EQ(inner.utf16Calls, 0);
+
+  Count count;
+  WithRuntimeDecorator<Count> with(inner, count);
+  EXPECT_EQ(str.length(with), 42);
+  EXPECT_EQ(inner.lengthCalls, 2);
+  EXPECT_EQ(inner.utf16Calls, 0);
+  EXPECT_EQ(count.beforeCalls, 1);
+  EXPECT_EQ(count.afterCalls, 1);
+}
+
 TEST_P(JSITest, ObjectTest) {
   eval("x = {1:2, '3':4, 5:'six', 'seven':['eight', 'nine']}");
   Object x = rt.global().getPropertyAsObject(rt, "x");