test(process): publish capture readiness atomically

tests/fixtures/processCaptureCancellation.mjs

@@ -1,23 +1,27 @@
 import { spawn } from "node:child_process";
-import { writeFile } from "node:fs/promises";
+import { rename, writeFile } from "node:fs/promises";
 import { join } from "node:path";
 
 const [mode, root] = process.argv.slice(2);
 
+const publishReadyMarker = async (marker) => {
+  const destination = join(root, "ready.json");
+  const temporary = join(root, "ready.json.tmp");
+  await writeFile(temporary, JSON.stringify(marker));
+  await rename(temporary, destination);
+};
+
 if (mode === "settlement") {
   const child = spawn(process.execPath, [process.argv[1], "workload", root], {
     stdio: ["ignore", "ignore", "ignore", "ipc"],
   });
   child.on("message", async (message) => {
     if (message !== "ready" || child.pid === undefined) return;
-    await writeFile(
-      join(root, "ready.json"),
-      JSON.stringify({
-        rootPid: process.pid,
-        childPid: child.pid,
-        runId: process.env.REA_PROCESS_RUN_ID,
-      }),
-    );
+    await publishReadyMarker({
+      rootPid: process.pid,
+      childPid: child.pid,
+      runId: process.env.REA_PROCESS_RUN_ID,
+    });
     child.disconnect();
     child.unref();
     process.stdout.write("root exiting; owned workload remains\n");
@@ -33,13 +37,10 @@ if (mode === "settlement") {
   setTimeout(() => process.exit(0), 12_000);
   if (mode === "workload") process.send?.("ready");
   else {
-    await writeFile(
-      join(root, "ready.json"),
-      JSON.stringify({
-        rootPid: process.pid,
-        runId: process.env.REA_PROCESS_RUN_ID,
-      }),
-    );
+    await publishReadyMarker({
+      rootPid: process.pid,
+      runId: process.env.REA_PROCESS_RUN_ID,
+    });
     process.stdout.write("owned workload ready\n");
   }
 }