wasi/cli: restore cli({ passthrough }) — a consumer depends on it

wasi/src/cli.ts

@@ -46,6 +46,13 @@ export interface CliOptions {
   cwd?: string;
   /** `get-stdin`'s buffer contents; default empty (matches contract: "stdin (empty)"). */
   stdinBuffer?: Uint8Array;
+  /**
+   * Also `console.log`/`console.error` captured stdout/stderr writes as
+   * they happen. Default false. For embedders whose guest's stderr is its
+   * only diagnostic channel (lann/wosh): a buffer nobody reads is a black
+   * box exactly when something is going wrong.
+   */
+  passthrough?: boolean;
   /** `exit()` throws `ExitError` instead of merely recording. Default false. */
   throwOnExit?: boolean;
 }
@@ -94,15 +101,18 @@ function concat(chunks: Uint8Array[]): Uint8Array {
 export function cli(options: CliOptions = {}): CliResult {
   const stdoutChunks: Uint8Array[] = [];
   const stderrChunks: Uint8Array[] = [];
+  const passthrough = options.passthrough ?? false;
   let exited = false;
   let exitOk: boolean | undefined;
   let exitCode: number | undefined;
 
   const stdout = new OutputStream((chunk) => {
     stdoutChunks.push(chunk);
+    if (passthrough) console.log(new TextDecoder().decode(chunk));
   });
   const stderr = new OutputStream((chunk) => {
     stderrChunks.push(chunk);
+    if (passthrough) console.error(new TextDecoder().decode(chunk));
   });
 
   const captured: CliCaptured = {
@@ -118,11 +128,13 @@ export function cli(options: CliOptions = {}): CliResult {
   /** 0.3 write-via-stream into a capture buffer (the promise IS the future — embedder-api.md §"Streams and futures"). */
   const captureViaStream = (
     chunks: Uint8Array[],
+    mirror: ((text: string) => void) | undefined,
   ) =>
   async (data: CliByteSource): Promise<CliIoResult> => {
     for await (const chunk of data as AsyncIterable<Uint8Array | number[]>) {
       const bytes = chunk instanceof Uint8Array ? chunk : Uint8Array.from(chunk);
       chunks.push(bytes);
+      mirror?.(new TextDecoder().decode(bytes));
     }
     return { kind: "ok" };
   };
@@ -188,10 +200,16 @@ export function cli(options: CliOptions = {}): CliResult {
       ],
     },
     "wasi:cli/stdout@0.3": {
-      writeViaStream: captureViaStream(stdoutChunks),
+      writeViaStream: captureViaStream(
+        stdoutChunks,
+        passthrough ? (t) => console.log(t) : undefined,
+      ),
     },
     "wasi:cli/stderr@0.3": {
-      writeViaStream: captureViaStream(stderrChunks),
+      writeViaStream: captureViaStream(
+        stderrChunks,
+        passthrough ? (t) => console.error(t) : undefined,
+      ),
     },
     "wasi:cli/terminal-input@0.3": { TerminalInput },
     "wasi:cli/terminal-output@0.3": { TerminalOutput },

wasi/tests/cli_test.ts

@@ -100,6 +100,42 @@ Deno.test("cli: no terminal is ever attached (option collapses to undefined)", (
   assertEq(stdinTerm.getTerminalStdin(), undefined);
 });
 
+// `passthrough` mirrors writes to the console as they happen, on both
+// tracks, while still capturing. A consumer (lann/wosh) depends on it:
+// its guest's stderr is the only diagnostic channel it has.
+Deno.test("cli: passthrough mirrors stdout/stderr to console on both tracks; off by default", async () => {
+  const logged: string[] = [];
+  const errored: string[] = [];
+  const origLog = console.log, origError = console.error;
+  console.log = (...a: unknown[]) => logged.push(a.join(" "));
+  console.error = (...a: unknown[]) => errored.push(a.join(" "));
+  try {
+    const quiet = cli();
+    (quiet.imports["wasi:cli/stdout@0.2"] as { getStdout(): { write(c: Uint8Array): void } })
+      .getStdout().write(new TextEncoder().encode("silent"));
+    assertEq(logged.length, 0);
+    assertEq(quiet.captured.stdoutText(), "silent");
+
+    const { imports, captured } = cli({ passthrough: true });
+    (imports["wasi:cli/stdout@0.2"] as { getStdout(): { write(c: Uint8Array): void } })
+      .getStdout().write(new TextEncoder().encode("out2"));
+    (imports["wasi:cli/stderr@0.2"] as { getStderr(): { write(c: Uint8Array): void } })
+      .getStderr().write(new TextEncoder().encode("err2"));
+    await (imports["wasi:cli/stderr@0.3"] as {
+      writeViaStream(data: AsyncIterable<Uint8Array>): Promise<{ kind: string }>;
+    }).writeViaStream((async function* () {
+      yield new TextEncoder().encode("err3");
+    })());
+    assertEq(JSON.stringify(logged), JSON.stringify(["out2"]));
+    assertEq(JSON.stringify(errored), JSON.stringify(["err2", "err3"]));
+    assertEq(captured.stdoutText(), "out2");
+    assertEq(captured.stderrText(), "err2err3");
+  } finally {
+    console.log = origLog;
+    console.error = origError;
+  }
+});
+
 // --- the @0.3 track (capture impl) ---------------------------------------------
 
 Deno.test("cli@0.3: write-via-stream captures; read-via-stream serves the buffer", async () => {