chain: keep only what something calls

Two pieces of the new package had no caller. Pool.Holds was the read side of a
claim set that Add already enforces, so nothing ever needed to ask; it is gone,
and the tests that used it now assert the same property through Add, which is
the behaviour that matters.

chain.Closed had no caller because M-Chain still declared its own
NoUserTxPolicy. It declares chain.Closed() now — one fewer copy of a policy
five chains state identically, and the one no-user-tx chain in this pass
exercises the value that describes it.

chain/pool.go

@@ -114,14 +114,6 @@ func (p *Pool[T, K]) Drop(entries []T) {
 	}
 }
 
-// Holds reports whether a queued entry already claims c.
-func (p *Pool[T, K]) Holds(c K) bool {
-	p.mu.Lock()
-	defer p.mu.Unlock()
-	_, ok := p.held[c]
-	return ok
-}
-
 // Len is how many entries are waiting.
 func (p *Pool[T, K]) Len() int {
 	p.mu.Lock()

chain/pool_test.go

@@ -50,8 +50,6 @@ func TestASecondClaimOnQueuedWorkIsRefused(t *testing.T) {
 	require.NoError(t, p.Add(at(1)))
 	require.ErrorIs(t, p.Add(at(1)), ErrHeld)
 	require.Equal(t, 1, p.Len())
-	require.True(t, p.Holds(ids.ID{1}))
-	require.False(t, p.Holds(ids.ID{2}))
 }
 
 func TestAFullPoolRefusesRatherThanGrows(t *testing.T) {
@@ -74,9 +72,8 @@ func TestAClaimDoesNotOutliveItsEntry(t *testing.T) {
 	p.Drop([]entry{at(1)})
 
 	require.Equal(t, []entry{at(2)}, p.Take(0))
-	require.False(t, p.Holds(ids.ID{1}), "the dropped entry holds nothing")
-	require.True(t, p.Holds(ids.ID{2}), "the one still queued does")
-	require.NoError(t, p.Add(at(1)), "and its claim can be made again")
+	require.NoError(t, p.Add(at(1)), "the dropped entry's claim can be made again")
+	require.ErrorIs(t, p.Add(at(2)), ErrHeld, "the one still queued still holds its own")
 }
 
 func TestDroppingNothingChangesNothing(t *testing.T) {
@@ -84,7 +81,7 @@ func TestDroppingNothingChangesNothing(t *testing.T) {
 	require.NoError(t, p.Add(at(1)))
 	p.Drop(nil)
 	require.Equal(t, 1, p.Len())
-	require.True(t, p.Holds(ids.ID{1}))
+	require.ErrorIs(t, p.Add(at(1)), ErrHeld)
 }
 
 func TestDroppingWhatWasNeverQueuedLeavesTheRestAlone(t *testing.T) {
@@ -92,7 +89,7 @@ func TestDroppingWhatWasNeverQueuedLeavesTheRestAlone(t *testing.T) {
 	require.NoError(t, p.Add(at(1)))
 	p.Drop([]entry{at(9)})
 	require.Equal(t, []entry{at(1)}, p.Take(0))
-	require.True(t, p.Holds(ids.ID{1}))
+	require.ErrorIs(t, p.Add(at(1)), ErrHeld)
 }
 
 func TestAddingTellsConsensusThereIsSomethingToBuild(t *testing.T) {
@@ -128,7 +125,6 @@ func TestConcurrentUseOfAPoolDoesNotRace(t *testing.T) {
 				default:
 					p.Take(4)
 					p.Len()
-					p.Holds(ids.ID{1})
 				}
 			}
 		}()

mpcvm/feegate.go

@@ -7,29 +7,13 @@ import (
 	"github.com/luxfi/node/vms/types/fee"
 )
 
-// newFeePolicy returns the canonical M-Chain FeePolicy. M-Chain is a
-// committee-driven service VM — keygen / signing requests come from
-// validators via consensus, not a user mempool — so it declares the
-// NoUserTxPolicy sentinel. See vms/types/fee/policy.go.
-//
-// Any tx that reaches the fee gate on a NoUserTx chain is a wiring
-// bug; ValidateFee returns ErrChainAcceptsNoUserTxs.
-func newFeePolicy() fee.Policy {
-	return fee.NoUserTxPolicy{}
-}
+// gateUserTx refuses every caller — M-Chain accepts no user txs. It is a
+// committee-driven service VM: keygen and signing requests come from validators
+// through consensus, not a user mempool. Any service entry that exposes itself
+// as user-callable MUST route through this gate, so the refusal is explicit
+// rather than a path nobody happened to write.
+func (vm *VM) gateUserTx() error { return vm.fee.Admit(0) }
 
-// gateUserTx refuses every caller — M-Chain accepts no user txs.
-// Any service entry that exposes itself as user-callable MUST route
-// through this gate so the refusal is explicit, not implicit.
-func (vm *VM) gateUserTx() error {
-	if vm.feePolicy == nil {
-		return fee.ErrChainAcceptsNoUserTxs
-	}
-	// NoUserTxPolicy.ValidateFee ignores the args and always returns
-	// ErrChainAcceptsNoUserTxs.
-	return vm.feePolicy.ValidateFee(0, fee.NoUserTxPolicy{}.FeeAssetID())
-}
-
-// FeePolicy exposes the chain's declared fee policy for diagnostics
-// and the boot-time Validate gate.
-func (vm *VM) FeePolicy() fee.Policy { return vm.feePolicy }
+// FeePolicy exposes the chain's declared fee policy for diagnostics and the
+// boot-time Validate gate.
+func (vm *VM) FeePolicy() fee.Policy { return vm.fee.Policy() }

mpcvm/feegate_test.go

@@ -7,12 +7,13 @@ import (
 	"errors"
 	"testing"
 
+	"github.com/luxfi/chains/chain"
 	"github.com/luxfi/log"
 	"github.com/luxfi/node/vms/types/fee"
 )
 
 func TestThresholdVM_FeePolicy_IsNoUserTxSentinel(t *testing.T) {
-	v := &VM{log: log.NewNoOpLogger(), feePolicy: newFeePolicy()}
+	v := &VM{log: log.NewNoOpLogger(), fee: chain.Closed()}
 	if v.FeePolicy() == nil {
 		t.Fatal("FeePolicy() = nil; want NoUserTxPolicy")
 	}
@@ -25,7 +26,7 @@ func TestThresholdVM_FeePolicy_IsNoUserTxSentinel(t *testing.T) {
 }
 
 func TestThresholdVM_FeePolicy_RejectsAllUserTx(t *testing.T) {
-	v := &VM{log: log.NewNoOpLogger(), feePolicy: newFeePolicy()}
+	v := &VM{log: log.NewNoOpLogger(), fee: chain.Closed()}
 	if err := v.gateUserTx(); !errors.Is(err, fee.ErrChainAcceptsNoUserTxs) {
 		t.Fatalf("gateUserTx() = %v, want ErrChainAcceptsNoUserTxs", err)
 	}

mpcvm/vm.go

@@ -180,7 +180,9 @@ type VM struct {
 
 	// Fee policy. M-Chain is service-only (committee-driven, no
 	// user mempool) so this is the NoUserTxPolicy sentinel.
-	feePolicy fee.Policy
+	// fee is what this chain charges to admit a user transaction, which is
+	// nothing: M-Chain takes none.
+	fee chain.Fee
 
 	// mu guards State and the VM's own maps. State is deliberately not safe for
 	// concurrent use — a state machine with its own locking invites callers to
@@ -238,8 +240,8 @@ func (vm *VM) Initialize(
 	// Pin fee policy. M-Chain is service-only (committee-driven, no
 	// user mempool) so attach the NoUserTxPolicy sentinel.
 	// fee.Validate passes for the sentinel even with zero MinTxFee.
-	vm.feePolicy = newFeePolicy()
-	if err := fee.Validate(vm.feePolicy); err != nil {
+	vm.fee = chain.Closed()
+	if err := fee.Validate(vm.fee.Policy()); err != nil {
 		return fmt.Errorf("mpcvm: fee policy: %w", err)
 	}