This commit removes the toEngine channel pattern from all VMs and adds an additional API:

```
SubscribeToEvents(ctx context.Context) Message)
```

Which returns when an event (pending transactions or state sync done Message) is sent by the VM.

Unlike the toEngine pattern which only notifies the consumer a single time until a block is built, successive calls to `SubscribeToEvents` will return that transactions are pending as long as the mem-pool is not empty and a block can be built, otherwise the calls block.

The API change is needed to support consensus protocols which facilitate censorship resistance as part of their protocol and therefore need information about whether a block should be built or not on demand, in contrast to the curren toEngine pattern which notifies the consensus when new transactions arrive only once, and then block building may even fail. Having such an API on the VM side (and not implementing the logic in consensus) forces the VM to handle any side effects of block building (such as re-introducing transactions into the mempool).

Signed-off-by: Yacov Manevich <yacov.manevich@avalabs.org>

Author: yacovm

chains/linearizable_vm.go

@@ -33,11 +33,14 @@ type initializeOnLinearizeVM struct {
 	genesisBytes []byte
 	upgradeBytes []byte
 	configBytes  []byte
-	toEngine     chan<- common.Message
 	fxs          []*common.Fx
 	appSender    common.AppSender
 }
 
+func (vm *initializeOnLinearizeVM) SubscribeToEvents(ctx context.Context) common.Message {
+	return vm.vmToInitialize.SubscribeToEvents(ctx)
+}
+
 func (vm *initializeOnLinearizeVM) Linearize(ctx context.Context, stopVertexID ids.ID) error {
 	vm.vmToLinearize.stopVertexID = stopVertexID
 	return vm.vmToInitialize.Initialize(
@@ -47,7 +50,6 @@ func (vm *initializeOnLinearizeVM) Linearize(ctx context.Context, stopVertexID i
 		vm.genesisBytes,
 		vm.upgradeBytes,
 		vm.configBytes,
-		vm.toEngine,
 		vm.fxs,
 		vm.appSender,
 	)
@@ -74,9 +76,8 @@ func (vm *linearizeOnInitializeVM) Initialize(
 	_ []byte,
 	_ []byte,
 	_ []byte,
-	toEngine chan<- common.Message,
 	_ []*common.Fx,
 	_ common.AppSender,
 ) error {
-	return vm.Linearize(ctx, vm.stopVertexID, toEngine)
+	return vm.Linearize(ctx, vm.stopVertexID)
 }

chains/manager.go

@@ -727,10 +727,6 @@ func (m *manager) createAvalancheChain(
 		},
 	)
 
-	// The channel through which a VM may send messages to the consensus engine
-	// VM uses this channel to notify engine that a block is ready to be made
-	msgChan := make(chan common.Message, defaultChannelSize)
-
 	// The only difference between using avalancheMessageSender and
 	// snowmanMessageSender here is where the metrics will be placed. Because we
 	// end up using this sender after the linearization, we pass in
@@ -742,7 +738,6 @@ func (m *manager) createAvalancheChain(
 		genesisData,
 		chainConfig.Upgrade,
 		chainConfig.Config,
-		msgChan,
 		fxs,
 		snowmanMessageSender,
 	)
@@ -823,7 +818,6 @@ func (m *manager) createAvalancheChain(
 		genesisBytes: genesisData,
 		upgradeBytes: chainConfig.Upgrade,
 		configBytes:  chainConfig.Config,
-		toEngine:     msgChan,
 		fxs:          fxs,
 		appSender:    snowmanMessageSender,
 	}
@@ -885,8 +879,8 @@ func (m *manager) createAvalancheChain(
 	// Asynchronously passes messages from the network to the consensus engine
 	h, err := handler.New(
 		ctx,
+		linearizableVM,
 		vdrs,
-		msgChan,
 		m.FrontierPollFrequency,
 		m.ConsensusAppConcurrency,
 		m.ResourceTracker,
@@ -1202,18 +1196,13 @@ func (m *manager) createSnowmanChain(
 		vm = tracedvm.NewBlockVM(vm, "proposervm", m.Tracer)
 	}
 
-	// The channel through which a VM may send messages to the consensus engine
-	// VM uses this channel to notify engine that a block is ready to be made
-	msgChan := make(chan common.Message, defaultChannelSize)
-
 	if err := vm.Initialize(
 		context.TODO(),
 		ctx.Context,
 		vmDB,
 		genesisData,
 		chainConfig.Upgrade,
 		chainConfig.Config,
-		msgChan,
 		fxs,
 		messageSender,
 	); err != nil {
@@ -1277,8 +1266,8 @@ func (m *manager) createSnowmanChain(
 	// Asynchronously passes messages from the network to the consensus engine
 	h, err := handler.New(
 		ctx,
+		vm,
 		vdrs,
-		msgChan,
 		m.FrontierPollFrequency,
 		m.ConsensusAppConcurrency,
 		m.ResourceTracker,

go.mod

@@ -13,9 +13,9 @@ require (
 	github.com/DataDog/zstd v1.5.2
 	github.com/StephenButtolph/canoto v0.15.0
 	github.com/antithesishq/antithesis-sdk-go v0.3.8
-	github.com/ava-labs/coreth v0.15.2-rc.0.0.20250610170140-2fcf45f828a2
+	github.com/ava-labs/coreth v0.15.2-rc.0.0.20250616171632-973a9f30d630
 	github.com/ava-labs/ledger-avalanche/go v0.0.0-20241009183145-e6f90a8a1a60
-	github.com/ava-labs/libevm v0.0.0-20250610142802-2672fbd7cdfc
+	github.com/ava-labs/libevm v1.13.14-0.2.0.release
 	github.com/btcsuite/btcd/btcutil v1.1.3
 	github.com/cockroachdb/pebble v0.0.0-20230928194634-aa077af62593
 	github.com/compose-spec/compose-go v1.20.2

go.sum

@@ -66,12 +66,12 @@ github.com/antithesishq/antithesis-sdk-go v0.3.8/go.mod h1:IUpT2DPAKh6i/YhSbt6Gl
 github.com/armon/consul-api v0.0.0-20180202201655-eb2c6b5be1b6/go.mod h1:grANhF5doyWs3UAsr3K4I6qtAmlQcZDesFNEHPZAzj8=
 github.com/armon/go-socks5 v0.0.0-20160902184237-e75332964ef5 h1:0CwZNZbxp69SHPdPJAN/hZIm0C4OItdklCFmMRWYpio=
 github.com/armon/go-socks5 v0.0.0-20160902184237-e75332964ef5/go.mod h1:wHh0iHkYZB8zMSxRWpUBQtwG5a7fFgvEO+odwuTv2gs=
-github.com/ava-labs/coreth v0.15.2-rc.0.0.20250610170140-2fcf45f828a2 h1:/E1w2S6xvEhkO2+C9sGCV8W80qaeSN5WBFqdKLl12HM=
-github.com/ava-labs/coreth v0.15.2-rc.0.0.20250610170140-2fcf45f828a2/go.mod h1:cqwBag+zzqifDutdPVzZKovfC2d0L8Zxq4YgTGaMCwg=
+github.com/ava-labs/coreth v0.15.2-rc.0.0.20250616171632-973a9f30d630 h1:9QpJ17vK5RGrU4XwN8Xj2iJ2NrGNlPOSINrv0sFxh0U=
+github.com/ava-labs/coreth v0.15.2-rc.0.0.20250616171632-973a9f30d630/go.mod h1:gz03IIszWMvTJZHO55Qb+vJgW7gsrPK/AuT2aygpH3c=
 github.com/ava-labs/ledger-avalanche/go v0.0.0-20241009183145-e6f90a8a1a60 h1:EL66gtXOAwR/4KYBjOV03LTWgkEXvLePribLlJNu4g0=
 github.com/ava-labs/ledger-avalanche/go v0.0.0-20241009183145-e6f90a8a1a60/go.mod h1:/7qKobTfbzBu7eSTVaXMTr56yTYk4j2Px6/8G+idxHo=
-github.com/ava-labs/libevm v0.0.0-20250610142802-2672fbd7cdfc h1:cSXaUY4hdmoJ2FJOgOzn+WiovN/ZB/zkNRgnZhE50OA=
-github.com/ava-labs/libevm v0.0.0-20250610142802-2672fbd7cdfc/go.mod h1:+Iol+sVQ1KyoBsHf3veyrBmHCXr3xXRWq6ZXkgVfNLU=
+github.com/ava-labs/libevm v1.13.14-0.2.0.release h1:uKGCc5/ceeBbfAPRVtBUxbQt50WzB2pEDb8Uy93ePgQ=
+github.com/ava-labs/libevm v1.13.14-0.2.0.release/go.mod h1:+Iol+sVQ1KyoBsHf3veyrBmHCXr3xXRWq6ZXkgVfNLU=
 github.com/ava-labs/simplex v0.0.0-20250611154800-78b82e9820e5 h1:rwPm63i5nJ2XIuNjO2H68gDmMKje0VW7orLZMISPrC8=
 github.com/ava-labs/simplex v0.0.0-20250611154800-78b82e9820e5/go.mod h1:GVzumIo3zR23/qGRN2AdnVkIPHcKMq/D89EGWZfMGQ0=
 github.com/aymerick/raymond v2.0.3-0.20180322193309-b565731e1464+incompatible/go.mod h1:osfaiScAUVup+UC9Nfq76eWqDhXlp+4UYaA8uhTBO6g=

proto/messenger/messenger.proto

@@ -5,7 +5,7 @@ package messenger;
 option go_package = "github.com/ava-labs/avalanchego/proto/pb/messenger";
 
 service Messenger {
-  rpc Notify(NotifyRequest) returns (NotifyResponse);
+  rpc Notify(stream Event) returns (stream EventRequest);
 }
 
 enum Message {
@@ -14,8 +14,13 @@ enum Message {
   MESSAGE_STATE_SYNC_FINISHED = 2;
 }
 
-message NotifyRequest {
-  Message message = 1;
+message EventRequest {
+  oneof event {
+    bool start = 1;
+    bool stop = 2;
+  }
 }
 
-message NotifyResponse {}
+message Event {
+  Message message = 1;
+}