fix(credits): make credit deduction an atomic conditional decrement (#71)

lib/database.ts

@@ -807,23 +807,21 @@ export async function getCredits(userId: string) {
 
 export async function deductCredit(userId: string, feature: FeatureKey): Promise<boolean> {
   const admin = await getSupabaseAdmin()
-  const credits = await getCredits(userId)
 
-  if (!credits) return false
+  // Atomic, conditional decrement via the deduct_credit() SQL function.
+  // The DB-side WHERE <col> > 0 guard makes concurrent calls on the last credit
+  // resolve to exactly one success and one failure (no read-then-write race).
+  const { data, error } = await admin.rpc("deduct_credit", {
+    p_user_id: userId,
+    p_feature: feature,
+  })
 
-  const remainingKey = `${feature}_remaining` as keyof Credits
-  const remaining = Number(credits[remainingKey] ?? 0)
-
-  if (remaining <= 0) return false
-
-  const { error } = await admin
-    .from("credits")
-    .update({
-      [remainingKey]: remaining - 1,
-    })
-    .eq("user_id", userId)
+  if (error) {
+    console.error("[deductCredit] RPC error:", error.message)
+    return false
+  }
 
-  return !error
+  return data === true
 }
 
 export async function refillCreditsForTrial(userId: string) {

supabase/migration-atomic-deduct-credit.sql

@@ -0,0 +1,41 @@
+-- Atomic, conditional credit decrement.
+--
+-- Replaces the previous read-then-write in deductCredit(), which let two
+-- concurrent requests for the same user both read the same remaining value
+-- (e.g. 1) and each write 0, permitting two operations on a single credit.
+--
+-- The conditional UPDATE (... WHERE <col> > 0) is evaluated atomically under
+-- row-level locking: when two calls race on the last credit, the first commits
+-- the decrement to 0 and the second re-checks the WHERE against the updated row,
+-- matches nothing, and reports 0 affected rows -> returns FALSE.
+CREATE OR REPLACE FUNCTION public.deduct_credit(p_user_id UUID, p_feature TEXT)
+RETURNS BOOLEAN
+LANGUAGE plpgsql
+AS $$
+DECLARE
+  v_updated INTEGER;
+BEGIN
+  IF p_feature = 'ai_chat' THEN
+    UPDATE public.credits
+    SET ai_chat_remaining = ai_chat_remaining - 1,
+        updated_at        = NOW()
+    WHERE user_id = p_user_id AND ai_chat_remaining > 0;
+  ELSIF p_feature = 'flashcards' THEN
+    UPDATE public.credits
+    SET flashcards_remaining = flashcards_remaining - 1,
+        updated_at           = NOW()
+    WHERE user_id = p_user_id AND flashcards_remaining > 0;
+  ELSIF p_feature = 'study_plan' THEN
+    UPDATE public.credits
+    SET study_plan_remaining = study_plan_remaining - 1,
+        updated_at           = NOW()
+    WHERE user_id = p_user_id AND study_plan_remaining > 0;
+  ELSE
+    -- Unknown feature key: nothing to deduct.
+    RETURN FALSE;
+  END IF;
+
+  GET DIAGNOSTICS v_updated = ROW_COUNT;
+  RETURN v_updated > 0;
+END;
+$$;