Deliver a line to a single line listener without the listener snapshot (#2531)

packages/compiler/test/ts7/baselines/order-parity/tests/corpus/readline-line-listener-changes.ts.json

@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/readline-line-listener-changes.ts"
+ ],
+ "diags": []
+}

packages/runtime/src/scr_readline.c

@@ -105,6 +105,17 @@ void scr_rl_emit(ScrRl *rl, int ev, ScrStr *line, ScrError *err) {
   RlList *l = &rl->ev[ev];
   size_t n = l->n;
   if (n == 0) return;
+  /* One persistent line listener (the usual 'line' handler): no snapshot
+   * and no once-listener pass. The retain keeps the closure alive if the
+   * listener removes itself. */
+  if (n == 1 && ev == RL_EV_LINE && !l->v[0].once && !l->v[0].iter) {
+    RlEntry e = l->v[0];
+    if (scr_exc_pending()) return;
+    scr_closure_retain(e.cb);
+    ((ScrRlLineFn)e.fn)(e.cb, scr_str_retain(line));
+    scr_closure_release(e.cb);
+    return;
+  }
   /* Most events have a listener or two: their snapshot stays on the stack. */
   RlEntry small[4];
   RlEntry *snap = n <= 4 ? small : malloc(n * sizeof *snap);

tests/corpus/readline-line-listener-changes.ts

@@ -0,0 +1,32 @@
+// 'line' listeners that change while a line is delivered: a listener added
+// by the only line listener starts with the next line, after which both
+// run in order; a once listener next to a persistent one runs for one line;
+// and no listener sees a line twice.
+import { createInterface } from "node:readline";
+import { Readable } from "node:stream";
+
+const text = "a\nb\nc\nd\ne\nf\n";
+
+const first = createInterface({ input: Readable.from([text]) });
+let seen = 0;
+const counting = (line: string): void => {
+  seen++;
+  console.log("counting", line);
+  if (seen === 2) first.on("line", (later: string) => console.log("later", later));
+};
+first.on("line", counting);
+first.on("close", () => {
+  console.log("first closed", seen);
+
+  const second = createInterface({ input: Readable.from([text]) });
+  second.once("line", (line: string) => console.log("once", line));
+  second.on("line", (line: string) => console.log("every", line));
+  second.on("close", () => {
+    console.log("second closed");
+
+    const third = createInterface({ input: Readable.from(["x\ny\n", "z\n"]) });
+    const seenLines: string[] = [];
+    third.on("line", (line: string) => seenLines.push(line));
+    third.on("close", () => console.log("third", seenLines.join(",")));
+  });
+});

tests/harness/corpus-costs/readline-line-listener-changes.ts.json

@@ -0,0 +1 @@
+{ "plain": 4500, "san": 7400 }