Node islands: read the stack from the frame address, strip ASan's swapcontext warning, fix a race in 6811

- ni_stack_limit classifies the stack by the frame address. Under ASan's
  use-after-return detection a local lives on a fake stack, so a call into
  Node from a hosted promise reaction got the wrong V8 stack limit and hit
  "Maximum call stack size exceeded" (6802, dev, sanitized).
- The ordering harness drops ASan's once-per-process makecontext/swapcontext
  warning from stderr, as the other harnesses do.
- 6811's island timers fire at 15 and 40 ms, after the program's immediate
  and its 5 and 10 ms timers, instead of racing the first loop turn.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/runtime/src/scr_node_island.c

@@ -336,8 +336,9 @@ static void ni_call_job(void *p) {
  * runtime's guard for it: its mapping's start plus the reserve compiled
  * code keeps). */
 static uintptr_t ni_stack_limit(void) {
-  char here;
-  uintptr_t sp = (uintptr_t)&here;
+  /* The frame address, not a local's: under ASan's use-after-return
+   * detection locals live on a fake stack elsewhere in memory. */
+  uintptr_t sp = (uintptr_t)__builtin_frame_address(0);
   if (ni_stack != NULL && sp > (uintptr_t)ni_stack && sp <= (uintptr_t)ni_stack + NI_STACK)
     return (uintptr_t)(ni_stack + NI_GUARD) + NI_STACK / 8;
   if (sp > ni_main_lo && sp <= ni_main_hi) {

tests/corpus/6811-node-island-uncaught-listener/island.cjs

@@ -1,4 +1,4 @@
 exports.timers = () => {
-  setTimeout(() => console.log("island timer 1"), 1);
-  setTimeout(() => console.log("island timer 2"), 20);
+  setTimeout(() => console.log("island timer 1"), 15);
+  setTimeout(() => console.log("island timer 2"), 40);
 };

tests/harness/node-island-order.test.ts

@@ -53,20 +53,26 @@ const programs = existsSync(programsDir)
       .sort()
   : [];
 
+/** Linux ASan's once-per-process warning at the first context switch (the
+ * runtime's fibers and Node's stack): not program output. */
+const ASAN_SWAP = /^==\d+==WARNING: ASan doesn't fully support makecontext\/swapcontext.*\n/gm;
+
 /** One run's observable outcome: stdout, stderr and the exit status. */
 async function outcome(cmd: string, args: string[]): Promise<string> {
   try {
-    const { stdout, stderr } = await execFileAsync(cmd, args, {
+    const { stdout, stderr: raw } = await execFileAsync(cmd, args, {
       cwd: repoRoot,
       encoding: "utf8",
       timeout: 20_000,
       env: { ...process.env, NODE_NO_WARNINGS: "1" },
     });
+    const stderr = raw.replace(ASAN_SWAP, "");
     return `${stdout}${stderr === "" ? "" : `[stderr]\n${stderr}`}[exit 0]`;
   } catch (err) {
     const e = err as { stdout?: string; stderr?: string; code?: unknown; signal?: unknown };
     if (typeof e.code !== "number") return `[failed ${String(e.signal ?? e.code)}]`;
-    return `${e.stdout ?? ""}${e.stderr ? `[stderr]\n${e.stderr}` : ""}[exit ${e.code}]`;
+    const stderr = (e.stderr ?? "").replace(ASAN_SWAP, "");
+    return `${e.stdout ?? ""}${stderr ? `[stderr]\n${stderr}` : ""}[exit ${e.code}]`;
   }
 }