perf(encoding): avoid copying the input data in `TextDecoder` (#12573)

The implementation of `TextDecoder` had a bug where it was copying the
input data in every case. This change removes that copy in
non-`SharedArrayBuffer` cases.

Since passing a shared buffer source to Rust would fail, this copy of
the input data was making `TextDecoder` work in cases where the input
is shared. In order to avoid a breaking change, the copy is retained in
those cases.

1 files changed, 8 insertions, 1 deletions

diff --git a/ext/web/08_text_encoding.js b/ext/web/08_text_encoding.js
index 28f2848b8..d3f6d45fb 100644
--- a/ext/web/08_text_encoding.js
+++ b/ext/web/08_text_encoding.js
@@ -114,7 +114,14 @@
         } else {
           input = new Uint8Array(input);
         }
-        return core.opSync("op_encoding_decode", new Uint8Array(input), {
+        if (input.buffer instanceof SharedArrayBuffer) {
+          // We clone the data into a non-shared ArrayBuffer so we can pass it
+          // to Rust.
+          // `input` is now a Uint8Array, and calling the TypedArray constructor
+          // with a TypedArray argument copies the data.
+          input = new Uint8Array(input);
+        }
+        return core.opSync("op_encoding_decode", input, {
           rid: this.#rid,
           stream: options.stream,
         });