Support for external v8 strings (#641)

src/binding.cc

@@ -832,6 +832,32 @@ int v8__String__WriteUtf8(const v8::String& self, v8::Isolate* isolate,
   return self.WriteUtf8(isolate, buffer, length, nchars_ref, options);
 }
 
+class ExternalStaticOneByteStringResource: public v8::String::ExternalOneByteStringResource {
+  public:
+    ExternalStaticOneByteStringResource(const char *data, int length):
+                                        _data(data), _length(length) {}
+    const char* data() const override { return _data; }
+    size_t length() const override { return _length; }
+
+  private:
+    const char* _data;
+    const int _length;
+};
+
+const v8::String* v8__String__NewExternalOneByteStatic(v8::Isolate *isolate,
+                                                      const char *data, int length) {
+  return maybe_local_to_ptr(
+    v8::String::NewExternalOneByte(
+      isolate, new ExternalStaticOneByteStringResource(data, length)
+    )
+  );
+}
+
+bool v8__String__IsExternal(const v8::String& self) { return self.IsExternal(); }
+bool v8__String__IsExternalOneByte(const v8::String& self) { return self.IsExternalOneByte(); }
+bool v8__String__IsExternalTwoByte(const v8::String& self) { return self.IsExternalTwoByte(); }
+bool v8__String__IsOneByte(const v8::String& self) { return self.IsOneByte(); }
+
 const v8::Symbol* v8__Symbol__New(v8::Isolate* isolate,
                                   const v8::String& description) {
   return local_to_ptr(v8::Symbol::New(isolate, ptr_to_local(&description)));

src/string.rs

@@ -32,6 +32,17 @@ extern "C" {
     nchars_ref: *mut int,
     options: WriteOptions,
   ) -> int;
+
+  fn v8__String__NewExternalOneByteStatic(
+    isolate: *mut Isolate,
+    buffer: *const char,
+    length: int,
+  ) -> *const String;
+
+  fn v8__String__IsExternal(this: *const String) -> bool;
+  fn v8__String__IsExternalOneByte(this: *const String) -> bool;
+  fn v8__String__IsExternalTwoByte(this: *const String) -> bool;
+  fn v8__String__IsOneByte(this: *const String) -> bool;
 }
 
 #[repr(C)]
@@ -134,6 +145,57 @@ impl String {
     Self::new_from_utf8(scope, value.as_ref(), NewStringType::Normal)
   }
 
+  // Creates a v8::String from a `&'static str`,
+  // must be Latin-1 or ASCII, not UTF-8 !
+  pub fn new_external_onebyte_static<'s>(
+    scope: &mut HandleScope<'s, ()>,
+    value: &'static str,
+  ) -> Option<Local<'s, String>> {
+    let buffer: &[u8] = value.as_ref();
+    if buffer.is_empty() {
+      return None;
+    }
+    let buffer_len = buffer.len().try_into().ok()?;
+    unsafe {
+      scope.cast_local(|sd| {
+        v8__String__NewExternalOneByteStatic(
+          sd.get_isolate_ptr(),
+          buffer.as_ptr() as *const char,
+          buffer_len,
+        )
+      })
+    }
+  }
+
+  /// True if string is external
+  pub fn is_external(&self) -> bool {
+    // TODO: re-enable on next v8-release
+    // Right now it fallbacks to Value::IsExternal, which is incorrect
+    // See: https://source.chromium.org/chromium/_/chromium/v8/v8.git/+/1dd8624b524d14076160c1743f7da0b20fbe68e0
+    // unsafe { v8__String__IsExternal(self) }
+
+    // Fallback for now (though functionally identical)
+    self.is_external_onebyte() || self.is_external_twobyte()
+  }
+
+  /// True if string is external & one-byte
+  /// (e.g: created with new_external_onebyte_static)
+  pub fn is_external_onebyte(&self) -> bool {
+    unsafe { v8__String__IsExternalOneByte(self) }
+  }
+
+  /// True if string is external & two-byte
+  /// NOTE: can't yet be created via rusty_v8
+  pub fn is_external_twobyte(&self) -> bool {
+    unsafe { v8__String__IsExternalTwoByte(self) }
+  }
+
+  /// True if string is known to contain only one-byte data
+  /// doesn't read the string so can return false positives
+  pub fn is_onebyte(&self) -> bool {
+    unsafe { v8__String__IsExternalOneByte(self) }
+  }
+
   /// Convenience function not present in the original V8 API.
   pub fn to_rust_string_lossy(
     &self,

tests/test_api.rs

@@ -4959,3 +4959,41 @@ fn code_cache_script() {
   let ret = script.run(scope).unwrap();
   assert_eq!(ret.uint32_value(scope).unwrap(), 2);
 }
+
+#[test]
+fn external_strings() {
+  let _setup_guard = setup();
+  let isolate = &mut v8::Isolate::new(Default::default());
+  let scope = &mut v8::HandleScope::new(isolate);
+  let context = v8::Context::new(scope);
+  let scope = &mut v8::ContextScope::new(scope, context);
+
+  // Parse JSON from an external string
+  let json_static = "{\"a\": 1, \"b\": 2}";
+  let json_external =
+    v8::String::new_external_onebyte_static(scope, json_static).unwrap();
+  let maybe_value = v8::json::parse(scope, json_external);
+  assert!(maybe_value.is_some());
+  // Check length
+  assert!(json_external.length() == 16);
+  // Externality checks
+  assert!(json_external.is_external());
+  assert!(json_external.is_external_onebyte());
+  assert!(json_external.is_onebyte());
+
+  // In & out
+  let hello =
+    v8::String::new_external_onebyte_static(scope, "hello world").unwrap();
+  let rust_str = hello.to_rust_string_lossy(scope);
+  assert_eq!(rust_str, "hello world");
+  // Externality checks
+  assert!(hello.is_external());
+  assert!(hello.is_external_onebyte());
+  assert!(hello.is_onebyte());
+
+  // two-byte "internal" test
+  let gradients = v8::String::new(scope, "∇gradients").unwrap();
+  assert!(!gradients.is_external());
+  assert!(!gradients.is_external_onebyte());
+  assert!(!gradients.is_onebyte());
+}