/**
 * Atomic, permission-safe write of the agent profile.
 *
 * Extracted from the side-effecting server entrypoint so it can be unit-tested: the
 * property that matters here (the key is never world-readable, even mid-write) is only
 * observable if you can call this directly and interrupt it.
 *
 * The profile holds a live agent key, so 0600 is a security control. Two traps:
 *  - `writeFileSync`'s `mode` applies ONLY when the file is created. A .tmp left behind by
 *    an earlier crash keeps its own permissions, and `renameSync` preserves the source's
 *    mode — so a stale 0644 .tmp yields a 0644 profile. Unlinking it first makes 0600 true
 *    *by construction*: the key is never on disk world-readable, not even in the window
 *    between rename and chmod.
 *  - `chmodSync` used to be the sole enforcement point, and its failure was swallowed by a
 *    bare `catch {}` — the key could sit world-readable with nothing said. Never swallow
 *    it, and verify the result instead of assuming it took.
 */
import { existsSync, writeFileSync, renameSync, chmodSync, unlinkSync, statSync } from "fs";

/** Where diagnostics go. Injectable so tests can assert on them. */
export type Warn = (message: string) => void;

const defaultWarn: Warn = (m) => process.stderr.write(m);

export function safeWriteProfile(path: string, data: unknown, warn: Warn = defaultWarn): void {
  const tmp = path + ".tmp";

  // Residue from an earlier crash would keep its own (possibly 0644) permissions.
  try {
    if (existsSync(tmp)) unlinkSync(tmp);
  } catch (e) {
    warn(`[agentchat] WARNING: stale ${tmp} could not be removed: ${e}\n`);
  }

  // Created fresh → `mode` actually applies, so the key is 0600 from the instant it exists.
  writeFileSync(tmp, JSON.stringify(data, null, 2), { mode: 0o600 });
  renameSync(tmp, path);

  try {
    chmodSync(path, 0o600);
  } catch (e) {
    warn(`[agentchat] WARNING: could not chmod ${path} to 0600: ${e}\n`);
  }

  try {
    const mode = statSync(path).mode & 0o777;
    if (mode !== 0o600) {
      warn(
        `[agentchat] WARNING: ${path} is mode ${mode.toString(8)}, expected 600 — it holds your agent key. Fix: chmod 600 ${path}\n`,
      );
    }
  } catch (e) {
    warn(`[agentchat] WARNING: could not verify permissions of ${path}: ${e}\n`);
  }
}