// What the child process call sites share: the exit status, read right after
// `spawn` so the exit event cannot pass unseen while the caller reads the
// streams, and echoing output without outrunning a slow reader.
import type { ChildProcess } from "node:child_process";
import { once } from "node:events";
import { constants } from "node:os";
import type { Writable } from "node:stream";

// Resolves once the process exits, before its streams close, with its exit
// code, or with 128 plus the signal number if a signal killed it, as a shell
// reports. A process that fails to start rejects with the spawn error.
export function exitCode(child: ChildProcess): Promise<number> {
  const code = new Promise<number>((resolve, reject) => {
    child.once("error", reject);
    child.once("exit", (exit, signal) => {
      resolve(exit ?? (signal === null ? 1 : 128 + constants.signals[signal]));
    });
  });
  // Callers await it after draining the streams; a failed start must not be
  // reported as an unhandled rejection before then.
  code.catch(() => {});
  return code;
}

// Writes a chunk, waiting for the stream to drain when its buffer is full.
export async function echo(
  stream: Writable,
  chunk: string | Uint8Array,
): Promise<void> {
  if (!stream.write(chunk)) {
    await once(stream, "drain");
  }
}