fix(ext/web): Prevent TextDecoderStream resource leak on stream cancellation (#21074)

This PR uses the new `cancel` method of `TransformStream` to properly
clean up the internal `TextDecoder` used in `TextDecoderStream` if the
stream is cancelled.

Fixes #13142

Co-authored-by: Bartek Iwańczuk <biwanczuk@gmail.com>

3 files changed, 29 insertions, 0 deletions

diff --git a/cli/tests/unit/text_encoding_test.ts b/cli/tests/unit/text_encoding_test.ts
index b76e9a9bf..270fd07a8 100644
--- a/cli/tests/unit/text_encoding_test.ts
+++ b/cli/tests/unit/text_encoding_test.ts
@@ -319,3 +319,20 @@ Deno.test(function binaryEncode() {
     assertEquals(Array.from(bytes), decodeBinary(binaryString));
   }
 });
+
+Deno.test(
+  { permissions: { read: true } },
+  async function textDecoderStreamCleansUpOnCancel() {
+    const filename = "cli/tests/testdata/assets/hello.txt";
+    const file = await Deno.open(filename);
+    const readable = file.readable.pipeThrough(new TextDecoderStream());
+    const chunks = [];
+    for await (const chunk of readable) {
+      chunks.push(chunk);
+      // breaking out of the loop prevents normal shutdown at end of async iterator values and triggers the cancel method of the stream instead
+      break;
+    }
+    assertEquals(chunks.length, 1);
+    assertEquals(chunks[0].length, 12);
+  },
+);
diff --git a/ext/web/06_streams.js b/ext/web/06_streams.js
index c5306ca9c..7ce045e68 100644
--- a/ext/web/06_streams.js
+++ b/ext/web/06_streams.js
@@ -6681,6 +6681,10 @@ webidl.converters.Transformer = webidl
       converter: webidl.converters.Function,
     },
     {
+      key: "cancel",
+      converter: webidl.converters.Function,
+    },
+    {
       key: "readableType",
       converter: webidl.converters.any,
     },
diff --git a/ext/web/08_text_encoding.js b/ext/web/08_text_encoding.js
index 45dbad538..5f8391e12 100644
--- a/ext/web/08_text_encoding.js
+++ b/ext/web/08_text_encoding.js
@@ -301,6 +301,14 @@ class TextDecoderStream {
           return PromiseReject(err);
         }
       },
+      cancel: (_reason) => {
+        try {
+          const _ = this.#decoder.decode();
+          return PromiseResolve();
+        } catch (err) {
+          return PromiseReject(err);
+        }
+      },
     });
     this[webidl.brand] = webidl.brand;
   }