MySQL ERROR 1093 forbids referencing the UPDATE target inside a
subquery; the existing UPDATE ... WHERE id = (SELECT id FROM
topology_mutations ...) form blew up on every mutation claim under
the MySQL backend, so no mutation ever progressed past pending.
Wrap the inner SELECT in a derived table (SELECT id FROM (...) AS
_next). MySQL materialises the derived rowset before applying the
UPDATE, sidestepping 1093. SQLite accepts both forms, so the
single-statement atomic claim semantics are preserved on both
backends — racing watchers still serialise correctly.
anti
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