llmcost: per-namespace price overrides never take effect (ResolvePrice orders by namespace DESC without NULLS LAST, so the NULL-namespace global price always wins)

[ever1022]

Summary

In openmeter/llmcost, a per-namespace manual price override (POST /api/v1/llm-cost/overrides → CreateOverride) is silently ignored whenever a global/system price exists for the same (provider, model_id) — which is the normal case, because the background sync job (UpsertGlobalPrice) populates a global price for every known model.

The override row is written and is correctly selected as a candidate at read time, but it loses the final tie-break in ResolvePrice and the global price is returned instead. As a result, manual overrides have no effect on cost computation (e.g. Feature Cost / LLM unit cost).

Root cause

openmeter/llmcost/adapter/price.go, ResolvePrice:

entity, err := a.db.LLMCostPrice.Query().
    Where(pricedb.DeletedAtIsNil()).
    Where(pricedb.Or(
        pricedb.NamespaceEQ(input.Namespace), // override (namespace set)
        pricedb.NamespaceIsNil(),             // global/system price (NULL)
    )).
    Where(pricedb.ProviderEQ(string(input.Provider))).
    Where(pricedb.ModelIDEQ(input.ModelID)).
    Where(pricedb.EffectiveFromLTE(at)).
    Where(pricedb.Or(
        pricedb.EffectiveToIsNil(),
        pricedb.EffectiveToGT(at),
    )).
    Order(
        // Prioritize namespace overrides
        pricedb.ByNamespace(sql.OrderDesc()),   // <-- missing NULLS LAST
        pricedb.ByEffectiveFrom(sql.OrderDesc()),
    ).
    First(ctx)

• Manual overrides are stored with a non-NULL namespace (CreateOverride calls SetNamespace(input.Namespace) with source = manual).
• Global/system prices are stored with namespace = NULL (UpsertGlobalPrice never sets a namespace; the schema field is Optional().Nillable() and is documented as "Nil for global prices, set for namespace overrides").

The ordering intent ("Prioritize namespace overrides") relies on the non-NULL namespace sorting before the NULL namespace. But ByNamespace(sql.OrderDesc()) emits a bare ORDER BY namespace DESC with no NULLS clause, and PostgreSQL's default for DESC is NULLS FIRST. So the NULL-namespace (global) row sorts first and First(ctx) returns it — the override never wins.

ent confirms the emitted SQL has no NULLS clause (entgo.io/ent@v0.14.6, dialect/sql/sql.go, OrderFieldTerm.ToFunc):

if f.Desc { b.WriteString(" DESC") }
if f.NullsFirst { b.WriteString(" NULLS FIRST") } else if f.NullsLast { b.WriteString(" NULLS LAST") }
// neither option set -> bare " DESC", PostgreSQL default = NULLS FIRST

Reproduction (PostgreSQL 14)

CREATE TEMP TABLE t (id text, namespace text, price numeric);
INSERT INTO t VALUES ('global_sync', NULL, 30), ('override', 'default', 20);

-- What ResolvePrice emits today:
SELECT id, namespace, price FROM t ORDER BY namespace DESC LIMIT 1;
--  id          | namespace | price
-- -------------+-----------+-------
--  global_sync |           |    30      <-- override is ignored

-- With the fix:
SELECT id, namespace, price FROM t ORDER BY namespace DESC NULLS LAST LIMIT 1;
--  id       | namespace | price
-- ----------+-----------+-------
--  override | default   |    20      <-- override wins, as intended

Impact

• Any per-namespace LLM price override created via POST /llm-cost/overrides does not affect cost resolution as long as a global price exists for the same (provider, model_id) (the common case, since sync seeds global prices).
• This affects all consumers of ResolvePrice, including LLM unit cost / Feature Cost computation in openmeter/cost/adapter/adapter.go (getLLMPrices passes feat.Namespace correctly; the override is matched as a candidate but loses the sort).
• The override row is persisted and is returned by ListOverrides, so it appears to have been applied — making this a silent, hard-to-notice correctness bug.

Why it wasn't caught

• There is no integration test exercising the real ResolvePrice SQL ordering: openmeter/llmcost/service tests use a mockAdapter, and there is no openmeter/llmcost/adapter/*_test.go. The cost and e2e suites don't create overrides.
• The same NULL-ordering pitfall is handled correctly elsewhere in the codebase — openmeter/subject/adapter/subject.go explicitly adds sql.OrderNullsLast() with the comment "Make sure null display names are last" — confirming the default is NULLS FIRST and must be overridden.

Suggested fix

Make the namespace ordering put NULLs last so non-NULL (override) namespaces sort first:

Order(
    pricedb.ByNamespace(sql.OrderDesc(), sql.OrderNullsLast()),
    pricedb.ByEffectiveFrom(sql.OrderDesc()),
)

A regression test that inserts both a global price and a namespace override for the same (provider, model_id) and asserts ResolvePrice returns the override would lock this down.

Environment

• Package: openmeter/llmcost
• ent: v0.14.6
• PostgreSQL: 14 (default NULLS FIRST on DESC applies to all supported PostgreSQL versions)