Member/CycleGenerator: better delete_cycles errors; UUID-based advisory lock

delete_cycles returns first error for debugging. Advisory lock key
derived from member id (first 8 bytes of UUID hex) to reduce
phash2 collision risk; fallback to phash2 on invalid UUID.

lib/membership/member.ex

@@ -37,6 +37,7 @@ defmodule Mv.Membership.Member do
     data_layer: AshPostgres.DataLayer,
     authorizers: [Ash.Policy.Authorizer]
 
+  import Bitwise
   require Ash.Query
   import Ash.Expr
   alias Ecto.Adapters.SQL, as: EctoSQL
@@ -906,6 +907,25 @@ defmodule Mv.Membership.Member do
     end)
   end
 
+  # Returns a deterministic 64-bit key for pg_advisory_xact_lock from a member id (UUID string).
+  # Reduces collision risk vs phash2 when multiple members are locked.
+  @doc false
+  def advisory_lock_key_for_member_id(member_id) when is_binary(member_id) do
+    hex = String.replace(member_id, "-", "")
+
+    if String.length(hex) >= 16 do
+      first_8_hex = String.slice(hex, 0, 16)
+      bin = Base.decode16!(first_8_hex, case: :lower)
+      decoded = :binary.decode_unsigned(bin, :big)
+      # Postgres bigint is signed 64-bit; keep in non-negative range
+      rem(decoded, 1 <<< 63)
+    else
+      :erlang.phash2(member_id)
+    end
+  rescue
+    ArgumentError -> :erlang.phash2(member_id)
+  end
+
   # Regenerates cycles when membership fee type changes
   # Deletes future unpaid cycles and regenerates them with the new type/amount
   # Uses advisory lock to prevent concurrent modifications
@@ -915,7 +935,7 @@ defmodule Mv.Membership.Member do
   # Uses system actor for cycle regeneration (mandatory side effect)
   def regenerate_cycles_on_type_change(member, _opts \\ []) do
     today = Date.utc_today()
-    lock_key = :erlang.phash2(member.id)
+    lock_key = advisory_lock_key_for_member_id(member.id)
 
     # Use advisory lock to prevent concurrent deletion and regeneration
     # This ensures atomicity when multiple updates happen simultaneously
@@ -1025,18 +1045,17 @@ defmodule Mv.Membership.Member do
     end
   end
 
-  # Deletes cycles and returns :ok if all succeeded, {:error, reason} otherwise
-  # Uses system actor for authorization to ensure deletion always works
+  # Deletes cycles and returns :ok if all succeeded, {:error, reason} otherwise.
+  # Returns the first error for debugging; uses system actor for authorization.
   defp delete_cycles(cycles_to_delete, actor_opts) do
     delete_results =
       Enum.map(cycles_to_delete, fn cycle ->
         Ash.destroy(cycle, actor_opts)
       end)
 
-    if Enum.any?(delete_results, &match?({:error, _}, &1)) do
-      {:error, :deletion_failed}
-    else
-      :ok
+    case Enum.find(delete_results, &match?({:error, _}, &1)) do
+      {:error, reason} -> {:error, reason}
+      nil -> :ok
     end
   end
 

lib/mv/membership_fees/cycle_generator.ex

@@ -112,7 +112,7 @@ defmodule Mv.MembershipFees.CycleGenerator do
   end
 
   defp do_generate_cycles_with_lock(member, today, false, opts) do
-    lock_key = :erlang.phash2(member.id)
+    lock_key = Member.advisory_lock_key_for_member_id(member.id)
 
     Repo.transaction(fn ->
       EctoSQL.query!(Repo, "SELECT pg_advisory_xact_lock($1)", [lock_key])