Keep small listener snapshots of readline and EventEmitter emits on the stack (#2483)

packages/compiler/test/ts7/baselines/order-parity/tests/corpus/emit-listener-snapshots.ts.json

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

packages/runtime/src/scr_events_emitter.c

@@ -1066,7 +1066,9 @@ static bool scr_ee_emit_core(ScrEmitter *em, ScrStr *name, va_list *ap_template,
   ScrEeBucket *b = em->reg ? scr_ee_bucket_of(em->reg, name) : NULL;
   if (!b || b->n == 0) return false;
   size_t n = b->n;
-  ScrEeEntry **snap = malloc(n * sizeof *snap);
+  /* Most events have a listener or two: their snapshot stays on the stack. */
+  ScrEeEntry *small[4];
+  ScrEeEntry **snap = n <= 4 ? small : malloc(n * sizeof *snap);
   if (!snap) scr_ee_oom();
   for (size_t i = 0; i < n; i++) {
     snap[i] = b->ls[i];
@@ -1130,7 +1132,7 @@ static bool scr_ee_emit_core(ScrEmitter *em, ScrStr *name, va_list *ap_template,
     }
     scr_ee_entry_unref(e);
   }
-  free(snap);
+  if (snap != small) free(snap);
   return true;
 }
 

packages/runtime/src/scr_readline.c

@@ -105,7 +105,9 @@ 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;
-  RlEntry *snap = malloc(n * sizeof *snap);
+  /* 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);
   if (!snap) scr_rl_oom();
   memcpy(snap, l->v, n * sizeof *snap);
   for (size_t i = 0; i < n; i++) {
@@ -139,7 +141,7 @@ void scr_rl_emit(ScrRl *rl, int ev, ScrStr *line, ScrError *err) {
     }
     if (!e.iter) scr_closure_release(e.cb);
   }
-  free(snap);
+  if (snap != small) free(snap);
 }
 
 ScrArr *scr_rl_history_of(ScrRl *rl) {

tests/corpus/emit-listener-snapshots.ts

@@ -0,0 +1,46 @@
+// Listeners run from a snapshot of the list taken when the event is
+// emitted: a listener added during an emit runs from the next emit on, a
+// listener removed during an emit still runs in that emit, and a once
+// listener runs once. Covers readline 'line' events and EventEmitter
+// events with fewer and more listeners than a few.
+import { EventEmitter } from "node:events";
+import { createInterface } from "node:readline";
+import { Readable } from "node:stream";
+
+function emitter(count: number): void {
+  const em = new EventEmitter();
+  const log: string[] = [];
+  const listeners: ((n: number) => void)[] = [];
+  for (let i = 0; i < count; i++) {
+    const listener = (n: number): void => {
+      log.push(`${i}:${n}`);
+      if (n === 1 && i === 0) em.on("tick", (m: number) => log.push(`late:${m}`));
+      if (n === 1 && i === 1 && listeners[count - 1]) em.off("tick", listeners[count - 1]!);
+    };
+    listeners.push(listener);
+    em.on("tick", listener);
+  }
+  em.once("tick", (n: number) => log.push(`once:${n}`));
+  em.emit("tick", 1);
+  em.emit("tick", 2);
+  console.log(count, em.listenerCount("tick"), log.join(" "));
+}
+
+for (const count of [1, 2, 4, 5, 7]) emitter(count);
+
+async function lines(count: number): Promise<void> {
+  const input = Readable.from(["alpha\nbeta\n", "gamma\ndelta\n"]);
+  const rl = createInterface({ input, terminal: false });
+  const log: string[] = [];
+  for (let i = 0; i < count; i++) {
+    rl.on("line", (line: string) => {
+      log.push(`${i}:${line}`);
+      if (line === "beta" && i === 0) rl.on("line", (next: string) => log.push(`late:${next}`));
+    });
+  }
+  rl.once("line", (line: string) => log.push(`once:${line}`));
+  await new Promise<void>((resolve) => rl.on("close", () => resolve()));
+  console.log(count, log.join(" "));
+}
+
+for (const count of [1, 3, 6]) await lines(count);

tests/harness/corpus-costs/emit-listener-snapshots.ts.json

@@ -0,0 +1 @@
+{ "plain": 5463, "san": 3430 }