From dac330c8402f0ae9af53bdb8a60dbe643df86508 Mon Sep 17 00:00:00 2001
From: ganeshvanahalli <ganeshdvanahalli@gmail.com>
Date: Mon, 2 Mar 2026 16:53:20 -0600
Subject: [PATCH 1/2] Prevent MEL node startup if have non-MEL entries in
 ConsensusDB

---
 arbnode/node.go                       | 69 +++++++++++++++++++++++----
 changelog/ganeshvanahalli-nit-4571.md |  2 +
 2 files changed, 62 insertions(+), 9 deletions(-)
 create mode 100644 changelog/ganeshvanahalli-nit-4571.md

diff --git a/arbnode/node.go b/arbnode/node.go
index 36e78d4c30..f0de31a460 100644
--- a/arbnode/node.go
+++ b/arbnode/node.go
@@ -26,6 +26,7 @@ import (
 
 	"github.com/offchainlabs/nitro/arbnode/dataposter"
 	"github.com/offchainlabs/nitro/arbnode/dataposter/storage"
+	"github.com/offchainlabs/nitro/arbnode/db/read"
 	"github.com/offchainlabs/nitro/arbnode/db/schema"
 	"github.com/offchainlabs/nitro/arbnode/mel"
 	melrunner "github.com/offchainlabs/nitro/arbnode/mel/runner"
@@ -811,35 +812,85 @@ func getInboxTrackerAndReader(
 	return inboxTracker, inboxReader, nil
 }
 
+func validateAndInitializeDBForMEL(
+	ctx context.Context,
+	l1client *ethclient.Client,
+	deployInfo *chaininfo.RollupAddresses,
+	consensusDB ethdb.Database,
+) error {
+	melDB := melrunner.NewDatabase(consensusDB)
+	_, err := melDB.GetHeadMelState()
+	if err != nil {
+		if !rawdb.IsDbErrNotFound(err) {
+			return err
+		}
+		// SequencerBatchCountKey shouldn't exist
+		hasSequencerBatchCountKey, err := consensusDB.Has(schema.SequencerBatchCountKey)
+		if err != nil {
+			return err
+		}
+		if hasSequencerBatchCountKey {
+			return errors.New("MEL being initialized when DB already has stale keys from inbox reader")
+		}
+		// DelayedMessageCountKey shouldn't exist
+		hasDelayedMessageCountKey, err := consensusDB.Has(schema.DelayedMessageCountKey)
+		if err != nil {
+			return err
+		}
+		if hasDelayedMessageCountKey {
+			return errors.New("MEL being initialized when DB already has stale keys from inbox reader")
+		}
+		// MessageCountKey should be zero (since TxStreamer initializes it to zero if it doesn't exist)
+		msgCount, err := read.Value[uint64](consensusDB, schema.MessageCountKey)
+		if err != nil {
+			return err
+		}
+		if msgCount != 0 {
+			return errors.New("MEL being initialized when DB already has stale msgs")
+		}
+		// Create Initial MEL state
+		initialState, err := createInitialMELState(ctx, deployInfo, l1client)
+		if err != nil {
+			return err
+		}
+		if err = melDB.SaveState(initialState); err != nil {
+			return fmt.Errorf("failed to save initial mel state: %w", err)
+		}
+	}
+	return nil
+}
+
 func getMessageExtractor(
 	ctx context.Context,
 	config *Config,
 	l2Config *params.ChainConfig,
 	l1client *ethclient.Client,
 	deployInfo *chaininfo.RollupAddresses,
-	arbDb ethdb.Database,
+	consensusDB ethdb.Database,
 	txStreamer *TransactionStreamer,
 	dapRegistry *daprovider.DAProviderRegistry,
 ) (*melrunner.MessageExtractor, error) {
 	if !config.MessageExtraction.Enable {
-		return nil, nil
-	}
-	melDB := melrunner.NewDatabase(arbDb)
-	if _, err := melDB.GetHeadMelState(); err != nil {
-		initialState, err := createInitialMELState(ctx, deployInfo, l1client)
+		// HeadMelStateBlockNumKey shouldn't exist, because if it does that means node was started earlier
+		// with MEL and now trying to run with inbox reader and tracker. Error to prevent DB corruption
+		hasHeadMelStateBlockNumKey, err := consensusDB.Has(schema.HeadMelStateBlockNumKey)
 		if err != nil {
 			return nil, err
 		}
-		if err = melDB.SaveState(initialState); err != nil {
-			return nil, fmt.Errorf("failed to save initial mel state: %w", err)
+		if hasHeadMelStateBlockNumKey {
+			return nil, errors.New("node alredy has MEL related database entries and is trying to start inbox reader and tracker, not allowed")
 		}
+		return nil, nil
+	}
+	if err := validateAndInitializeDBForMEL(ctx, l1client, deployInfo, consensusDB); err != nil {
+		return nil, err
 	}
 	msgExtractor, err := melrunner.NewMessageExtractor(
 		config.MessageExtraction,
 		l1client,
 		l2Config,
 		deployInfo,
-		melDB,
+		melrunner.NewDatabase(consensusDB),
 		txStreamer,
 		dapRegistry,
 		nil,
diff --git a/changelog/ganeshvanahalli-nit-4571.md b/changelog/ganeshvanahalli-nit-4571.md
new file mode 100644
index 0000000000..ce2bdbc2a7
--- /dev/null
+++ b/changelog/ganeshvanahalli-nit-4571.md
@@ -0,0 +1,2 @@
+### Added
+ - Prevent MEL node startup if have non-MEL entries in consensus database
\ No newline at end of file

From ebadcef96e4aba5e598aca8adc438b7a2df1de19 Mon Sep 17 00:00:00 2001
From: ganeshvanahalli <ganeshdvanahalli@gmail.com>
Date: Tue, 3 Mar 2026 11:17:32 -0600
Subject: [PATCH 2/2] update in-line documentation

---
 arbnode/node.go | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arbnode/node.go b/arbnode/node.go
index f0de31a460..6ead4b6bc0 100644
--- a/arbnode/node.go
+++ b/arbnode/node.go
@@ -871,8 +871,9 @@ func getMessageExtractor(
 	dapRegistry *daprovider.DAProviderRegistry,
 ) (*melrunner.MessageExtractor, error) {
 	if !config.MessageExtraction.Enable {
-		// HeadMelStateBlockNumKey shouldn't exist, because if it does that means node was started earlier
-		// with MEL and now trying to run with inbox reader and tracker. Error to prevent DB corruption
+		// Prevent database corruption. If HeadMelStateBlockNumKey exists,
+		// it indicates this node was previously run with Message Extraction (MEL) enabled.
+		// Switching back to the standard inbox reader/tracker is not allowed.
 		hasHeadMelStateBlockNumKey, err := consensusDB.Has(schema.HeadMelStateBlockNumKey)
 		if err != nil {
 			return nil, err