Opus identified that PR #2227's preservation block had two related bugs in
the parent_session_id handling:
1. During preservation save: code did
_old_parent = s.parent_session_id
s.parent_session_id = None
s.save(touch_updated_at=False, skip_index=True)
s.parent_session_id = _old_parent
The save persisted parent=None to disk. The in-memory restoration didn't
reach the disk copy. Result: a /branch fork session that subsequently
compressed lost its 'Forked from X' badge on the preserved old snapshot.
2. Stamping the continuation: code did
if not s.parent_session_id:
s.parent_session_id = old_sid
The 'if not' guard skipped the stamp when the session already had a
parent_session_id from a prior fork. Result: fork-of-fork compression
broke lineage — the continuation jumped back to the original fork parent
instead of the just-preserved immediate predecessor snapshot.
Fix (matches Opus's recommendation):
- Remove the parent clearing during preservation save (preserve as-is)
- Drop the 'if not' guard; always stamp continuation to old_sid
This makes the lineage chain consistent: new → old → old.parent → ... root.
Traversal from the continuation always walks through the just-preserved
snapshot to get to its parent's parent, never jumping over the snapshot.
Two new regression tests pin both invariants:
- test_parent_session_id_stamped_unconditionally (no 'if not' guard)
- test_old_session_parent_preserved_during_archive_save (no parent=None)
Both pass against the fix. All 8 tests in the file pass.
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