refactor(agents): share queued JSONL file writer

src/agents/queued-file-writer.ts

@@ -0,0 +1,34 @@
+import fs from "node:fs/promises";
+import path from "node:path";
+
+export type QueuedFileWriter = {
+  filePath: string;
+  write: (line: string) => void;
+};
+
+export function getQueuedFileWriter(
+  writers: Map<string, QueuedFileWriter>,
+  filePath: string,
+): QueuedFileWriter {
+  const existing = writers.get(filePath);
+  if (existing) {
+    return existing;
+  }
+
+  const dir = path.dirname(filePath);
+  const ready = fs.mkdir(dir, { recursive: true }).catch(() => undefined);
+  let queue = Promise.resolve();
+
+  const writer: QueuedFileWriter = {
+    filePath,
+    write: (line: string) => {
+      queue = queue
+        .then(() => ready)
+        .then(() => fs.appendFile(filePath, line, "utf8"))
+        .catch(() => undefined);
+    },
+  };
+
+  writers.set(filePath, writer);
+  return writer;
+}