Fix: Tiebreaker for chains with equal weight

pkg/protocol/chain.go

@@ -54,6 +54,9 @@ type Chain struct {
 	// Engine contains the engine instance that is used to process blocks for this chain.
 	Engine reactive.Variable[*engine.Engine]
 
+	// IsSolid contains a flag that indicates whether this chain is solid (has a connection to the root).
+	IsSolid reactive.Event
+
 	// IsEvicted contains a flag that indicates whether this chain was evicted.
 	IsEvicted reactive.Event
 
@@ -86,6 +89,7 @@ func newChain(chains *Chains) *Chain {
 		StartEngine:              reactive.NewVariable[bool](),
 		Engine:                   reactive.NewVariable[*engine.Engine](),
 		IsEvicted:                reactive.NewEvent(),
+		IsSolid:                  reactive.NewEvent(),
 		shouldEvict:              reactive.NewEvent(),
 
 		chains:      chains,
@@ -230,6 +234,8 @@ func (c *Chain) initDerivedProperties() (shutdown func()) {
 							parentChain.deriveChildChains(c),
 
 							c.deriveShouldEvict(forkingPoint, parentChain),
+
+							c.deriveIsSolid(forkingPoint, parentChain),
 						)
 					}),
 				)
@@ -264,6 +270,15 @@ func (c *Chain) deriveShouldEvict(forkingPoint *Commitment, parentChain *Chain)
 	return
 }
 
+// deriveIsSolid defines how a chain determines whether it is solid (has a connection to the root).
+func (c *Chain) deriveIsSolid(forkingPoint *Commitment, parentChain *Chain) (shutdown func()) {
+	if parentChain != nil {
+		return c.IsSolid.InheritFrom(parentChain.IsSolid)
+	}
+
+	return c.IsSolid.InheritFrom(forkingPoint.IsRoot)
+}
+
 // deriveWarpSyncMode defines how a chain determines whether it is in warp sync mode or not.
 func (c *Chain) deriveWarpSyncMode(engine *engine.Engine) func() {
 	return c.WarpSyncMode.DeriveValueFrom(reactive.NewDerivedVariable4(func(warpSyncMode bool, engineInitialized bool, latestSyncedSlot iotago.SlotIndex, latestSeenSlot iotago.SlotIndex, outOfSyncThreshold iotago.SlotIndex) bool {

pkg/protocol/chains.go

@@ -351,19 +351,21 @@ func (c *Chains) initChainSwitching() (shutdown func()) {
 
 func (c *Chains) trackHeaviestCandidates(chain *Chain) (teardown func()) {
 	return chain.LatestCommitment.OnUpdate(func(_ *Commitment, latestCommitment *Commitment) {
-		targetSlot := latestCommitment.ID().Index()
+		chain.IsSolid.OnTrigger(func() {
+			targetSlot := latestCommitment.ID().Index()
 
-		if evictionEvent := c.protocol.EvictionEvent(targetSlot); !evictionEvent.WasTriggered() {
-			c.HeaviestClaimedCandidate.registerCommitment(targetSlot, latestCommitment, evictionEvent)
+			if evictionEvent := c.protocol.EvictionEvent(targetSlot); !evictionEvent.WasTriggered() {
+				c.HeaviestClaimedCandidate.registerCommitment(targetSlot, latestCommitment, evictionEvent)
 
-			latestCommitment.IsAttested.OnTrigger(func() {
-				c.HeaviestAttestedCandidate.registerCommitment(targetSlot, latestCommitment, evictionEvent)
-			})
+				latestCommitment.IsAttested.OnTrigger(func() {
+					c.HeaviestAttestedCandidate.registerCommitment(targetSlot, latestCommitment, evictionEvent)
+				})
 
-			latestCommitment.IsVerified.OnTrigger(func() {
-				c.HeaviestVerifiedCandidate.registerCommitment(targetSlot, latestCommitment, evictionEvent)
-			})
-		}
+				latestCommitment.IsVerified.OnTrigger(func() {
+					c.HeaviestVerifiedCandidate.registerCommitment(targetSlot, latestCommitment, evictionEvent)
+				})
+			}
+		})
 	})
 }