light: run detector for sequentially validating light client (#5538)

Closes #5445

light/client.go

@@ -123,7 +123,7 @@ type Client struct {
 	providerMutex tmsync.Mutex
 	// Primary provider of new headers.
 	primary provider.Provider
-	// See Witnesses option
+	// Providers used to "witness" new headers.
 	witnesses []provider.Provider
 
 	// Where trusted light blocks are stored.
@@ -218,7 +218,7 @@ func NewClientFromTrustedStore(
 	}
 
 	// Validate the number of witnesses.
-	if len(c.witnesses) < 1 && c.verificationMode == skipping {
+	if len(c.witnesses) < 1 {
 		return nil, errNoWitnesses{}
 	}
 
@@ -363,10 +363,8 @@ func (c *Client) initializeWithTrustOptions(ctx context.Context, options TrustOp
 	}
 
 	// 3) Cross-verify with witnesses to ensure everybody has the same state.
-	if len(c.witnesses) > 0 {
-		if err := c.compareFirstHeaderWithWitnesses(ctx, l.SignedHeader); err != nil {
-			return err
-		}
+	if err := c.compareFirstHeaderWithWitnesses(ctx, l.SignedHeader); err != nil {
+		return err
 	}
 
 	// 4) Persist both of them and continue.
@@ -443,7 +441,7 @@ func (c *Client) Update(ctx context.Context, now time.Time) (*types.LightBlock,
 }
 
 // VerifyLightBlockAtHeight fetches the light block at the given height
-// and calls verifyLightBlock. It returns the block immediately if it exists in
+// and verifies it. It returns the block immediately if it exists in
 // the trustedStore (no verification is needed).
 //
 // height must be > 0.
@@ -600,6 +598,7 @@ func (c *Client) verifySequential(
 		verifiedBlock = trustedBlock
 		interimBlock  *types.LightBlock
 		err           error
+		trace         = []*types.LightBlock{trustedBlock}
 	)
 
 	for height := trustedBlock.Height + 1; height <= newLightBlock.Height; height++ {
@@ -669,9 +668,17 @@ func (c *Client) verifySequential(
 
 		// 3) Update verifiedBlock
 		verifiedBlock = interimBlock
+
+		// 4) Add verifiedBlock to trace
+		trace = append(trace, verifiedBlock)
 	}
 
-	return nil
+	// Compare header with the witnesses to ensure it's not a fork.
+	// More witnesses we have, more chance to notice one.
+	//
+	// CORRECTNESS ASSUMPTION: there's at least 1 correct full node
+	// (primary or one of the witnesses).
+	return c.detectDivergence(ctx, trace, now)
 }
 
 // see VerifyHeader
@@ -995,6 +1002,10 @@ func (c *Client) compareFirstHeaderWithWitnesses(ctx context.Context, h *types.S
 	compareCtx, cancel := context.WithCancel(ctx)
 	defer cancel()
 
+	if len(c.witnesses) < 1 {
+		return errNoWitnesses{}
+	}
+
 	errc := make(chan error, len(c.witnesses))
 	for i, witness := range c.witnesses {
 		go c.compareNewHeaderWithWitness(compareCtx, errc, h, witness, i)