EFF-808 Fix Channel.decodeText UTF-8 chunk boundaries

.changeset/shy-cycles-flow.md

@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Fix Channel.decodeText corrupting UTF-8 characters split across chunk boundaries.

packages/effect/src/Channel.ts

@@ -6257,8 +6257,9 @@ export const splitLines = <Err, Done>(): Channel<
 /**
  * Decodes incoming `Uint8Array` chunks into strings using `TextDecoder`.
  *
- * Each `Uint8Array` inside an emitted array is decoded independently. The
- * optional `encoding` and `options` are passed to `TextDecoder`.
+ * Input chunks are decoded with streaming enabled so multi-byte characters may
+ * span `Uint8Array` boundaries. The optional `encoding` and `options` are
+ * passed to `TextDecoder`.
  *
  * @category String manipulation
  * @since 4.0.0
@@ -6274,7 +6275,8 @@ export const decodeText = <Err, Done>(encoding?: string, options?: TextDecoderOp
   fromTransform((upstream, _scope) =>
     Effect.sync(() => {
       const decoder = new TextDecoder(encoding, options)
-      return Effect.map(upstream, Arr.map((line) => decoder.decode(line)))
+      const streamOptions = { stream: true }
+      return Effect.map(upstream, Arr.map((line) => decoder.decode(line, streamOptions)))
     })
   )
 

packages/effect/test/Channel.test.ts

@@ -225,6 +225,22 @@ describe("Channel", () => {
       }))
   })
 
+  describe("encoding", () => {
+    it.effect("decodeText handles multi-byte characters split across Uint8Array boundaries", () =>
+      Effect.gen(function*() {
+        const bytes = new TextEncoder().encode("a🌍b")
+        const chunks: ReadonlyArray<readonly [Uint8Array, ...ReadonlyArray<Uint8Array>]> = [
+          [bytes.slice(0, 2)],
+          [bytes.slice(2, 4), bytes.slice(4)]
+        ]
+        const result = yield* Channel.fromArray(chunks).pipe(
+          Channel.pipeTo(Channel.decodeText()),
+          Channel.runCollect
+        )
+        assert.strictEqual(result.flat().join(""), "a🌍b")
+      }))
+  })
+
   describe("merging", () => {
     it.effect("merge - interrupts left side if halt strategy is set to 'right'", () =>
       Effect.gen(function*() {