hive-priv: fix doc comment attribution of nixos-container's flock

hive-priv/src/main.rs

@@ -628,9 +628,14 @@ async fn exec(
 ///
 /// `update` builds the toplevel itself first and passes `--system-path`
 /// instead of `--flake` — see [`nix_build_toplevel`] for why. `create`
-/// keeps the plain `--flake` form: `nixos-container`'s own `buildFlake()`
-/// takes an internal `flock` for `create` (not for `update`), so the
-/// hazard this works around doesn't apply there.
+/// keeps the plain `--flake` form: `nixos-container` takes an exclusive
+/// `flock` for the whole `create` action, and once `update` no longer
+/// calls `buildFlake()` (below), concurrent `create`s are the only
+/// remaining writers of the shared `.tmp` out-link — so they're mutually
+/// excluded. ⚠️ That holds only while nothing else calls `buildFlake()`
+/// unlocked: pre-this-PR, `create`'s own lock bought it nothing against a
+/// concurrent *unlocked* `update` clobbering the same `.tmp` — the lock
+/// alone was never what made `create` safe, this fix is.
 async fn container_flake_action(
     verb: &str,
     name: &str,
@@ -678,9 +683,12 @@ fn toplevel_attr(name: &str) -> String {
 /// path* — `$systemPath` is only ever assigned from the CLI flag or from
 /// its own build result, so with no flag it stays `undef` and
 /// `"$systemPath.tmp"` interpolates to the bare string `.tmp` in
-/// whatever the caller's cwd is. Worse, the `create` action's `flock`
-/// around this isn't taken for `update` at all. hive-priv never sets a
-/// per-call cwd, so two concurrent `update`s (this hive runs
+/// whatever the caller's cwd is. `buildFlake()` itself takes no lock at
+/// all — `create` wraps its *whole action* in an exclusive `flock` before
+/// calling `buildFlake()`, but `update` calls it with no lock, so that
+/// `create`-side lock never protected against a concurrent `update`
+/// clobbering the same `.tmp`. hive-priv never sets a per-call cwd, so two
+/// concurrent `update`s (this hive runs
 /// `services.hyperhive.c0re.buildSlots` > 1) share that one `.tmp`: one's
 /// `readlink(".tmp")` can resolve to the *other's* build output, handing
 /// an agent's container the wrong agent's closure — the "agent container