fix: atomic subsystem namespace slot check via FDB transaction

[boddumanohar]

Problem

Parallel clone() and add_lvol_ha() requests with namespaced=True share a subsystem (NQN). All threads read the same namespace count from DB, each decides the slot is free, and each writes an lvol with the same NQN — resulting in more lvols than max_namespace_per_subsys in one subsystem.

This is a classic TOCTOU race: the check (get_next_available_subsystem_on_node) and the act (write_to_db) were not atomic.

Fix

Replace the bare lvol.write_to_db() with a new db_controller.write_lvol_with_ns_check(lvol) that wraps both the count check and the write in a single FDB transaction:

def _write_lvol_with_ns_check_tx(self, tr, node_id, nqn, max_ns, lvol_key, lvol_data):
    live = 0
    for _, v in tr.get_range_startswith(b"object/LVol/"):
        d = json.loads(v)
        if d.get("node_id") == node_id and d.get("nqn") == nqn                 and d.get("status") not in (STATUS_IN_DELETION, STATUS_DELETED):
            live += 1
    if live >= max_ns:
        return False
    tr[lvol_key] = lvol_data
    return True

FDB's OCC means: if two transactions both read the object/LVol/ range and try to commit, the one that sees a stale read loses, gets retried with fresh data, and correctly sees the slot is now taken. No explicit lock, no serialisation of unrelated requests — parallel creates on different subsystems are completely unaffected.

Callers changed

File	Location
snapshot_controller.py	clone() — namespaced clones
lvol_controller.py	add_lvol_ha() — namespaced lvol creates

Both return a retryable error ("Subsystem namespace limit reached concurrently; retry") instead of silently over-allocating when the OCC check fails.

Why not a mutex?

A per-node lock would serialise all clone requests on a node (even ones targeting different subsystems), adding ~15–25 ms of queue wait per request under parallel load. OCC only serialises the rare actual conflict.

Diff size

55 lines across 3 files.

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