light: improve provider handling (#6053)

Introduces heuristics that track the amount of no responses or unavailable blocks a provider has for more robust provider handling by the light client. Use concurrent calls to all witnesses when a new primary is needed.
2026-01-04 04:04:00 +00:00 · 2021-03-01 12:04:02 +01:00
parent e2384a00ce
commit 37d36cd5bc
9 changed files with 334 additions and 178 deletions
--- a/light/client.go
+++ b/light/client.go
@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"sort"
+	"sync"
 	"time"

 	"github.com/tendermint/tendermint/libs/log"
@@ -126,8 +127,10 @@ type Client struct {
 	pruningSize uint16
 	// See ConfirmationFunction option
 	confirmationFn func(action string) bool
-
-	quit chan struct{}
+	// The light client keeps track of how many times it has requested a light
+	// block from it's providers. When this exceeds the amount of witnesses the
+	// light client will just return the last error sent by the providers
+	// repeatRequests uint16

 	logger log.Logger
 }
@@ -161,14 +164,14 @@ func NewClient(
 	}

 	if c.latestTrustedBlock != nil {
-		c.logger.Info("Checking trusted light block using options")
+		c.logger.Info("checking trusted light block using options")
 		if err := c.checkTrustedHeaderUsingOptions(ctx, trustOptions); err != nil {
 			return nil, err
 		}
 	}

 	if c.latestTrustedBlock == nil || c.latestTrustedBlock.Height < trustOptions.Height {
-		c.logger.Info("Downloading trusted light block using options")
+		c.logger.Info("downloading trusted light block using options")
 		if err := c.initializeWithTrustOptions(ctx, trustOptions); err != nil {
 			return nil, err
 		}
@@ -199,7 +202,6 @@ func NewClientFromTrustedStore(
 		trustedStore:     trustedStore,
 		pruningSize:      defaultPruningSize,
 		confirmationFn:   func(action string) bool { return true },
-		quit:             make(chan struct{}),
 		logger:           log.NewNopLogger(),
 	}

@@ -237,7 +239,7 @@ func (c *Client) restoreTrustedLightBlock() error {
 			return fmt.Errorf("can't get last trusted light block: %w", err)
 		}
 		c.latestTrustedBlock = trustedBlock
-		c.logger.Info("Restored trusted light block", "height", lastHeight)
+		c.logger.Info("restored trusted light block", "height", lastHeight)
 	}

 	return nil
@@ -273,7 +275,7 @@ func (c *Client) checkTrustedHeaderUsingOptions(ctx context.Context, options Tru
 	case options.Height == c.latestTrustedBlock.Height:
 		primaryHash = options.Hash
 	case options.Height < c.latestTrustedBlock.Height:
-		c.logger.Info("Client initialized with old header (trusted is more recent)",
+		c.logger.Info("client initialized with old header (trusted is more recent)",
 			"old", options.Height,
 			"trustedHeight", c.latestTrustedBlock.Height,
 			"trustedHash", c.latestTrustedBlock.Hash())
@@ -299,11 +301,11 @@ func (c *Client) checkTrustedHeaderUsingOptions(ctx context.Context, options Tru
 	}

 	if !bytes.Equal(primaryHash, c.latestTrustedBlock.Hash()) {
-		c.logger.Info("Prev. trusted header's hash (h1) doesn't match hash from primary provider (h2)",
+		c.logger.Info("previous trusted header's hash (h1) doesn't match hash from primary provider (h2)",
 			"h1", c.latestTrustedBlock.Hash(), "h2", primaryHash)

 		action := fmt.Sprintf(
-			"Prev. trusted header's hash %X doesn't match hash %X from primary provider. Remove all the stored light blocks?",
+			"Previous trusted header's hash %X doesn't match hash %X from primary provider. Remove all the stored light blocks?",
 			c.latestTrustedBlock.Hash(), primaryHash)
 		if c.confirmationFn(action) {
 			err := c.Cleanup()
@@ -415,7 +417,7 @@ func (c *Client) Update(ctx context.Context, now time.Time) (*types.LightBlock,
 		if err != nil {
 			return nil, err
 		}
-		c.logger.Info("Advanced to new state", "height", latestBlock.Height, "hash", latestBlock.Hash())
+		c.logger.Info("advanced to new state", "height", latestBlock.Height, "hash", latestBlock.Hash())
 		return latestBlock, nil
 	}

@@ -440,7 +442,7 @@ func (c *Client) VerifyLightBlockAtHeight(ctx context.Context, height int64, now
 	// Check if the light block is already verified.
 	h, err := c.TrustedLightBlock(height)
 	if err == nil {
-		c.logger.Info("Header has already been verified", "height", height, "hash", h.Hash())
+		c.logger.Debug("header has already been verified", "height", height, "hash", h.Hash())
 		// Return already trusted light block
 		return h, nil
 	}
@@ -497,7 +499,7 @@ func (c *Client) VerifyHeader(ctx context.Context, newHeader *types.Header, now
 		if !bytes.Equal(l.Hash(), newHeader.Hash()) {
 			return fmt.Errorf("existing trusted header %X does not match newHeader %X", l.Hash(), newHeader.Hash())
 		}
-		c.logger.Info("Header has already been verified",
+		c.logger.Debug("header has already been verified",
 			"height", newHeader.Height, "hash", newHeader.Hash())
 		return nil
 	}
@@ -516,7 +518,7 @@ func (c *Client) VerifyHeader(ctx context.Context, newHeader *types.Header, now
 }

 func (c *Client) verifyLightBlock(ctx context.Context, newLightBlock *types.LightBlock, now time.Time) error {
-	c.logger.Info("VerifyHeader", "height", newLightBlock.Height, "hash", newLightBlock.Hash())
+	c.logger.Info("verify light block", "height", newLightBlock.Height, "hash", newLightBlock.Hash())

 	var (
 		verifyFunc func(ctx context.Context, trusted *types.LightBlock, new *types.LightBlock, now time.Time) error
@@ -561,7 +563,7 @@ func (c *Client) verifyLightBlock(ctx context.Context, newLightBlock *types.Ligh
 		err = verifyFunc(ctx, closestBlock, newLightBlock, now)
 	}
 	if err != nil {
-		c.logger.Error("Can't verify", "err", err)
+		c.logger.Error("failed to verify", "err", err)
 		return err
 	}

@@ -595,7 +597,7 @@ func (c *Client) verifySequential(
 		}

 		// 2) Verify them
-		c.logger.Debug("Verify adjacent newLightBlock against verifiedBlock",
+		c.logger.Debug("verify adjacent newLightBlock against verifiedBlock",
 			"trustedHeight", verifiedBlock.Height,
 			"trustedHash", verifiedBlock.Hash(),
 			"newHeight", interimBlock.Height,
@@ -610,32 +612,21 @@ func (c *Client) verifySequential(
 			case ErrInvalidHeader:
 				// If the target header is invalid, return immediately.
 				if err.To == newLightBlock.Height {
-					c.logger.Debug("Target header is invalid", "err", err)
+					c.logger.Debug("target header is invalid", "err", err)
 					return err
 				}

-				// If some intermediate header is invalid, replace the primary and try
-				// again.
-				c.logger.Error("primary sent invalid header -> replacing", "err", err, "primary", c.primary)
-				replaceErr := c.replacePrimaryProvider()
-				if replaceErr != nil {
-					c.logger.Error("Can't replace primary", "err", replaceErr)
-					// return original error
-					return err
-				}
+				// If some intermediate header is invalid, remove the primary and try again.
+				c.logger.Error("primary sent invalid header -> removing", "err", err, "primary", c.primary)

-				replacementBlock, fErr := c.lightBlockFromPrimary(ctx, newLightBlock.Height)
-				if fErr != nil {
-					c.logger.Error("Can't fetch light block from primary", "err", fErr)
-					// return original error
+				replacementBlock, removeErr := c.findNewPrimary(ctx, newLightBlock.Height, true)
+				if removeErr != nil {
+					c.logger.Debug("failed to replace primary. Returning original error", "err", removeErr)
 					return err
 				}

 				if !bytes.Equal(replacementBlock.Hash(), newLightBlock.Hash()) {
-					c.logger.Error("Replacement provider has a different light block",
-						"newHash", newLightBlock.Hash(),
-						"replHash", replacementBlock.Hash())
-					// return original error
+					c.logger.Debug("replaced primary but new primary has a different block to the initial one")
 					return err
 				}

@@ -686,7 +677,7 @@ func (c *Client) verifySkipping(
 	)

 	for {
-		c.logger.Debug("Verify non-adjacent newHeader against verifiedBlock",
+		c.logger.Debug("verify non-adjacent newHeader against verifiedBlock",
 			"trustedHeight", verifiedBlock.Height,
 			"trustedHash", verifiedBlock.Hash(),
 			"newHeight", blockCache[depth].Height,
@@ -716,10 +707,20 @@ func (c *Client) verifySkipping(
 				pivotHeight := verifiedBlock.Height + (blockCache[depth].Height-verifiedBlock.
 					Height)*verifySkippingNumerator/verifySkippingDenominator
 				interimBlock, providerErr := source.LightBlock(ctx, pivotHeight)
-				if providerErr != nil {
+				switch providerErr {
+				case nil:
+					blockCache = append(blockCache, interimBlock)
+
+				// if the error is benign, the client does not need to replace the primary
+				case provider.ErrLightBlockNotFound, provider.ErrNoResponse:
+					return nil, err
+
+				// all other errors such as ErrBadLightBlock or ErrUnreliableProvider are seen as malevolent and the
+				// provider is removed
+				default:
 					return nil, ErrVerificationFailed{From: verifiedBlock.Height, To: pivotHeight, Reason: providerErr}
 				}
-				blockCache = append(blockCache, interimBlock)
+
 			}
 			depth++

@@ -744,32 +745,20 @@ func (c *Client) verifySkippingAgainstPrimary(
 		// If the target header is invalid, return immediately.
 		invalidHeaderHeight := err.(ErrVerificationFailed).To
 		if invalidHeaderHeight == newLightBlock.Height {
-			c.logger.Debug("Target header is invalid", "err", err)
+			c.logger.Debug("target header is invalid", "err", err)
 			return err
 		}

-		// If some intermediate header is invalid, replace the primary and try
-		// again.
+		// If some intermediate header is invalid, remove the primary and try again.
 		c.logger.Error("primary sent invalid header -> replacing", "err", err, "primary", c.primary)
-		replaceErr := c.replacePrimaryProvider()
-		if replaceErr != nil {
-			c.logger.Error("Can't replace primary", "err", replaceErr)
-			// return original error
-			return err
-		}
-
-		replacementBlock, fErr := c.lightBlockFromPrimary(ctx, newLightBlock.Height)
-		if fErr != nil {
-			c.logger.Error("Can't fetch light block from primary", "err", fErr)
-			// return original error
+		replacementBlock, removeErr := c.findNewPrimary(ctx, newLightBlock.Height, true)
+		if removeErr != nil {
+			c.logger.Error("failed to replace primary. Returning original error", "err", removeErr)
 			return err
 		}

 		if !bytes.Equal(replacementBlock.Hash(), newLightBlock.Hash()) {
-			c.logger.Error("Replacement provider has a different light block",
-				"newHash", newLightBlock.Hash(),
-				"replHash", replacementBlock.Hash())
-			// return original error
+			c.logger.Debug("replaced primary but new primary has a different block to the initial one. Returning original error")
 			return err
 		}

@@ -835,7 +824,7 @@ func (c *Client) Witnesses() []provider.Provider {
 // Cleanup removes all the data (headers and validator sets) stored. Note: the
 // client must be stopped at this point.
 func (c *Client) Cleanup() error {
-	c.logger.Info("Removing all light blocks")
+	c.logger.Info("removing all light blocks")
 	c.latestTrustedBlock = nil
 	return c.trustedStore.Prune(0)
 }
@@ -908,21 +897,31 @@ func (c *Client) backwards(
 			return fmt.Errorf("failed to obtain the header at height #%d: %w", verifiedHeader.Height-1, err)
 		}
 		interimHeader = interimBlock.Header
-		c.logger.Debug("Verify newHeader against verifiedHeader",
+		c.logger.Debug("verify newHeader against verifiedHeader",
 			"trustedHeight", verifiedHeader.Height,
 			"trustedHash", verifiedHeader.Hash(),
 			"newHeight", interimHeader.Height,
 			"newHash", interimHeader.Hash())
 		if err := VerifyBackwards(interimHeader, verifiedHeader); err != nil {
-			c.logger.Error("primary sent invalid header -> replacing", "err", err, "primary", c.primary)
-			if replaceErr := c.replacePrimaryProvider(); replaceErr != nil {
-				c.logger.Error("Can't replace primary", "err", replaceErr)
-				// return original error
-				return fmt.Errorf("verify backwards from %d to %d failed: %w",
-					verifiedHeader.Height, interimHeader.Height, err)
+			// verification has failed
+			c.logger.Error("backwards verification failed, replacing primary...", "err", err, "primary", c.primary)
+
+			// the client tries to see if it can get a witness to continue with the request
+			newPrimarysBlock, replaceErr := c.findNewPrimary(ctx, newHeader.Height, true)
+			if replaceErr != nil {
+				c.logger.Debug("failed to replace primary. Returning original error", "err", replaceErr)
+				return err
 			}
-			// we need to verify the header at the same height again
-			continue
+
+			// before continuing we must check that they have the same target header to validate
+			if !bytes.Equal(newPrimarysBlock.Hash(), newHeader.Hash()) {
+				c.logger.Debug("replaced primary but new primary has a different block to the initial one")
+				// return the original error
+				return err
+			}
+
+			// try again with the new primary
+			return c.backwards(ctx, verifiedHeader, newPrimarysBlock.Header)
 		}
 		verifiedHeader = interimHeader
 	}
@@ -930,55 +929,145 @@ func (c *Client) backwards(
 	return nil
 }

-// NOTE: requires a providerMutex locked.
-func (c *Client) removeWitness(idx int) {
-	switch len(c.witnesses) {
-	case 0:
-		panic(fmt.Sprintf("wanted to remove %d element from empty witnesses slice", idx))
-	case 1:
-		c.witnesses = make([]provider.Provider, 0)
-	default:
-		c.witnesses[idx] = c.witnesses[len(c.witnesses)-1]
-		c.witnesses = c.witnesses[:len(c.witnesses)-1]
-	}
-}
-
-// replaceProvider takes the first alternative provider and promotes it as the
-// primary provider.
-func (c *Client) replacePrimaryProvider() error {
-	c.providerMutex.Lock()
-	defer c.providerMutex.Unlock()
-
-	if len(c.witnesses) <= 1 {
-		return ErrNoWitnesses
-	}
-	c.primary = c.witnesses[0]
-	c.witnesses = c.witnesses[1:]
-	c.logger.Info("Replacing primary with the first witness", "new_primary", c.primary)
-
-	return nil
-}
-
 // lightBlockFromPrimary retrieves the lightBlock from the primary provider
-// at the specified height. Handles dropout by the primary provider by swapping
-// with an alternative provider.
+// at the specified height. This method also handles provider behavior as follows:
+//
+// 1. If the provider does not respond or does not have the block, it tries again
+//    with a different provider
+// 2. If all providers return the same error, the light client forwards the error to
+//    where the initial request came from
+// 3. If the provider provides an invalid light block, is deemed unreliable or returns
+//    any other error, the primary is permanently dropped and is replaced by a witness.
 func (c *Client) lightBlockFromPrimary(ctx context.Context, height int64) (*types.LightBlock, error) {
 	c.providerMutex.Lock()
 	l, err := c.primary.LightBlock(ctx, height)
 	c.providerMutex.Unlock()
-	if err != nil {
-		c.logger.Debug("Error on light block request from primary", "error", err, "primary", c.primary)
-		replaceErr := c.replacePrimaryProvider()
-		if replaceErr != nil {
-			return nil, fmt.Errorf("%v. Tried to replace primary but: %w", err.Error(), replaceErr)
-		}
-		// replace primary and request a light block again
-		return c.lightBlockFromPrimary(ctx, height)
+
+	switch err {
+	case nil:
+		// Everything went smoothly. We reset the lightBlockRequests and return the light block
+		return l, nil
+
+	case provider.ErrNoResponse, provider.ErrLightBlockNotFound:
+		// we find a new witness to replace the primary
+		c.logger.Debug("error from light block request from primary, replacing...", "error", err, "primary", c.primary)
+		return c.findNewPrimary(ctx, height, false)
+
+	default:
+		// The light client has most likely received either provider.ErrUnreliableProvider or provider.ErrBadLightBlock
+		// These errors mean that the light client should drop the primary and try with another provider instead
+		c.logger.Error("error from light block request from primary, removing...", "error", err, "primary", c.primary)
+		return c.findNewPrimary(ctx, height, true)
 	}
-	return l, err
 }

-// compareFirstHeaderWithWitnesses compares h with all witnesses. If any
+// NOTE: requires a providerMutex lock
+func (c *Client) removeWitnesses(indexes []int) error {
+	// check that we will still have witnesses remaining
+	if len(c.witnesses) <= len(indexes) {
+		return ErrNoWitnesses
+	}
+
+	// we need to make sure that we remove witnesses by index in the reverse
+	// order so as to not affect the indexes themselves
+	sort.Ints(indexes)
+	for i := len(indexes) - 1; i >= 0; i-- {
+		c.witnesses[indexes[i]] = c.witnesses[len(c.witnesses)-1]
+		c.witnesses = c.witnesses[:len(c.witnesses)-1]
+	}
+
+	return nil
+}
+
+type witnessResponse struct {
+	lb           *types.LightBlock
+	witnessIndex int
+	err          error
+}
+
+// findNewPrimary concurrently sends a light block request, promoting the first witness to return
+// a valid light block as the new primary. The remove option indicates whether the primary should be
+// entire removed or just appended to the back of the witnesses list. This method also handles witness
+// errors. If no witness is available, it returns the last error of the witness.
+func (c *Client) findNewPrimary(ctx context.Context, height int64, remove bool) (*types.LightBlock, error) {
+	c.providerMutex.Lock()
+	defer c.providerMutex.Unlock()
+
+	if len(c.witnesses) <= 1 {
+		return nil, ErrNoWitnesses
+	}
+
+	var (
+		witnessResponsesC = make(chan witnessResponse, len(c.witnesses))
+		witnessesToRemove []int
+		lastError         error
+		wg                sync.WaitGroup
+	)
+
+	// send out a light block request to all witnesses
+	subctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+	for index := range c.witnesses {
+		wg.Add(1)
+		go func(witnessIndex int, witnessResponsesC chan witnessResponse) {
+			defer wg.Done()
+
+			lb, err := c.witnesses[witnessIndex].LightBlock(subctx, height)
+			witnessResponsesC <- witnessResponse{lb, witnessIndex, err}
+		}(index, witnessResponsesC)
+	}
+
+	// process all the responses as they come in
+	for i := 0; i < cap(witnessResponsesC); i++ {
+		response := <-witnessResponsesC
+		switch response.err {
+		// success! We have found a new primary
+		case nil:
+			cancel() // cancel all remaining requests to other witnesses
+
+			wg.Wait() // wait for all goroutines to finish
+
+			// if we are not intending on removing the primary then append the old primary to the end of the witness slice
+			if !remove {
+				c.witnesses = append(c.witnesses, c.primary)
+			}
+
+			// promote respondent as the new primary
+			c.logger.Debug("found new primary", "primary", c.witnesses[response.witnessIndex])
+			c.primary = c.witnesses[response.witnessIndex]
+
+			// add promoted witness to the list of witnesses to be removed
+			witnessesToRemove = append(witnessesToRemove, response.witnessIndex)
+
+			// remove witnesses marked as bad (the client must do this before we alter the witness slice and change the indexes
+			// of witnesses). Removal is done in descending order
+			if err := c.removeWitnesses(witnessesToRemove); err != nil {
+				return nil, err
+			}
+
+			// return the light block that new primary responded with
+			return response.lb, nil
+
+		// process benign errors by logging them only
+		case provider.ErrNoResponse, provider.ErrLightBlockNotFound:
+			lastError = response.err
+			c.logger.Debug("error on light block request from witness",
+				"error", response.err, "primary", c.witnesses[response.witnessIndex])
+			continue
+
+		// process malevolent errors like ErrUnreliableProvider and ErrBadLightBlock by removing the witness
+		default:
+			lastError = response.err
+			c.logger.Error("error on light block request from witness, removing...",
+				"error", response.err, "primary", c.witnesses[response.witnessIndex])
+			witnessesToRemove = append(witnessesToRemove, response.witnessIndex)
+		}
+	}
+
+	return nil, lastError
+}
+
+// compareFirstHeaderWithWitnesses concurrently compares h with all witnesses. If any
 // witness reports a different header than h, the function returns an error.
 func (c *Client) compareFirstHeaderWithWitnesses(ctx context.Context, h *types.SignedHeader) error {
 	compareCtx, cancel := context.WithCancel(ctx)
@@ -1006,26 +1095,20 @@ func (c *Client) compareFirstHeaderWithWitnesses(ctx context.Context, h *types.S
 		case nil:
 			continue
 		case errConflictingHeaders:
-			c.logger.Error(fmt.Sprintf(`Witness #%d has a different header. Please check primary is correct
-and remove witness. Otherwise, use the different primary`, e.WitnessIndex), "witness", c.witnesses[e.WitnessIndex])
+			c.logger.Error(fmt.Sprintf(`witness #%d has a different header. Please check primary is correct
+and remove witness. Otherwise, use a different primary`, e.WitnessIndex), "witness", c.witnesses[e.WitnessIndex])
 			return err
 		case errBadWitness:
 			// If witness sent us an invalid header, then remove it. If it didn't
 			// respond or couldn't find the block, then we ignore it and move on to
 			// the next witness.
 			if _, ok := e.Reason.(provider.ErrBadLightBlock); ok {
-				c.logger.Info("Witness sent us invalid header / vals -> removing it", "witness", c.witnesses[e.WitnessIndex])
+				c.logger.Info("Witness sent an invalid light block, removing...", "witness", c.witnesses[e.WitnessIndex])
 				witnessesToRemove = append(witnessesToRemove, e.WitnessIndex)
 			}
 		}
 	}

-	// we need to make sure that we remove witnesses by index in the reverse
-	// order so as to not affect the indexes themselves
-	sort.Ints(witnessesToRemove)
-	for i := len(witnessesToRemove) - 1; i >= 0; i-- {
-		c.removeWitness(witnessesToRemove[i])
-	}
-
-	return nil
+	// remove all witnesses that misbehaved
+	return c.removeWitnesses(witnessesToRemove)
 }
--- a/light/client_test.go
+++ b/light/client_test.go
@@ -740,7 +740,7 @@ func TestClient_Concurrency(t *testing.T) {
 			defer wg.Done()

 			// NOTE: Cleanup, Stop, VerifyLightBlockAtHeight and Verify are not supposed
-			// to be concurrenly safe.
+			// to be concurrently safe.

 			assert.Equal(t, chainID, c.ChainID())

@@ -774,8 +774,12 @@ func TestClientReplacesPrimaryWithWitnessIfPrimaryIsUnavailable(t *testing.T) {
 	_, err = c.Update(ctx, bTime.Add(2*time.Hour))
 	require.NoError(t, err)

+	// the primary should no longer be the deadNode
 	assert.NotEqual(t, c.Primary(), deadNode)
-	assert.Equal(t, 1, len(c.Witnesses()))
+
+	// we should still have the dead node as a witness because it
+	// hasn't repeatedly been unresponsive yet
+	assert.Equal(t, 2, len(c.Witnesses()))
 }

 func TestClient_BackwardsVerification(t *testing.T) {
--- a/light/detector.go
+++ b/light/detector.go
@@ -5,7 +5,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"sort"
 	"time"

 	"github.com/tendermint/tendermint/light/provider"
@@ -35,7 +34,7 @@ func (c *Client) detectDivergence(ctx context.Context, primaryTrace []*types.Lig
 		lastVerifiedHeader = primaryTrace[len(primaryTrace)-1].SignedHeader
 		witnessesToRemove  = make([]int, 0)
 	)
-	c.logger.Debug("Running detector against trace", "endBlockHeight", lastVerifiedHeader.Height,
+	c.logger.Debug("running detector against trace", "endBlockHeight", lastVerifiedHeader.Height,
 		"endBlockHash", lastVerifiedHeader.Hash, "length", len(primaryTrace))

 	c.providerMutex.Lock()
@@ -75,7 +74,7 @@ func (c *Client) detectDivergence(ctx context.Context, primaryTrace []*types.Lig
 				now,
 			)
 			if err != nil {
-				c.logger.Info("Error validating witness's divergent header", "witness", supportingWitness, "err", err)
+				c.logger.Info("error validating witness's divergent header", "witness", supportingWitness, "err", err)
 				witnessesToRemove = append(witnessesToRemove, e.WitnessIndex)
 				continue
 			}
@@ -83,7 +82,7 @@ func (c *Client) detectDivergence(ctx context.Context, primaryTrace []*types.Lig
 			// We are suspecting that the primary is faulty, hence we hold the witness as the source of truth
 			// and generate evidence against the primary that we can send to the witness
 			primaryEv := newLightClientAttackEvidence(primaryBlock, witnessTrace[len(witnessTrace)-1], witnessTrace[0])
-			c.logger.Error("Attempted attack detected. Sending evidence againt primary by witness", "ev", primaryEv,
+			c.logger.Error("ATTEMPTED ATTACK DETECTED. Sending evidence againt primary by witness", "ev", primaryEv,
 				"primary", c.primary, "witness", supportingWitness)
 			c.sendEvidence(ctx, primaryEv, supportingWitness)

@@ -117,22 +116,17 @@ func (c *Client) detectDivergence(ctx context.Context, primaryTrace []*types.Lig
 			return ErrLightClientAttack

 		case errBadWitness:
-			c.logger.Info("Witness returned an error during header comparison", "witness", c.witnesses[e.WitnessIndex],
-				"err", err)
-			// if witness sent us an invalid header, then remove it. If it didn't respond or couldn't find the block, then we
-			// ignore it and move on to the next witness
-			if _, ok := e.Reason.(provider.ErrBadLightBlock); ok {
-				c.logger.Info("Witness sent us invalid header / vals -> removing it", "witness", c.witnesses[e.WitnessIndex])
-				witnessesToRemove = append(witnessesToRemove, e.WitnessIndex)
-			}
+			c.logger.Info("witness returned an error during header comparison, removing...",
+				"witness", c.witnesses[e.WitnessIndex], "err", err)
+			witnessesToRemove = append(witnessesToRemove, e.WitnessIndex)
+		default:
+			c.logger.Debug("error in light block request to witness", "err", err)
 		}
 	}

-	// we need to make sure that we remove witnesses by index in the reverse
-	// order so as to not affect the indexes themselves
-	sort.Ints(witnessesToRemove)
-	for i := len(witnessesToRemove) - 1; i >= 0; i-- {
-		c.removeWitness(witnessesToRemove[i])
+	// remove witnesses that have misbehaved
+	if err := c.removeWitnesses(witnessesToRemove); err != nil {
+		return err
 	}

 	// 1. If we had at least one witness that returned the same header then we
@@ -156,7 +150,17 @@ func (c *Client) compareNewHeaderWithWitness(ctx context.Context, errc chan erro
 	witness provider.Provider, witnessIndex int) {

 	lightBlock, err := witness.LightBlock(ctx, h.Height)
-	if err != nil {
+	switch err {
+	case nil:
+		break
+
+	case provider.ErrNoResponse, provider.ErrLightBlockNotFound:
+		errc <- err
+		return
+
+	default:
+		// all other errors (i.e. invalid block or unreliable provider) we mark the witness as bad
+		// and remove it
 		errc <- errBadWitness{Reason: err, WitnessIndex: witnessIndex}
 		return
 	}
@@ -165,7 +169,7 @@ func (c *Client) compareNewHeaderWithWitness(ctx context.Context, errc chan erro
 		errc <- errConflictingHeaders{Block: lightBlock, WitnessIndex: witnessIndex}
 	}

-	c.logger.Debug("Matching header received by witness", "height", h.Height, "witness", witnessIndex)
+	c.logger.Debug("matching header received by witness", "height", h.Height, "witness", witnessIndex)
 	errc <- nil
 }

@@ -173,7 +177,7 @@ func (c *Client) compareNewHeaderWithWitness(ctx context.Context, errc chan erro
 func (c *Client) sendEvidence(ctx context.Context, ev *types.LightClientAttackEvidence, receiver provider.Provider) {
 	err := receiver.ReportEvidence(ctx, ev)
 	if err != nil {
-		c.logger.Error("Failed to report evidence to provider", "ev", ev, "provider", receiver)
+		c.logger.Error("failed to report evidence to provider", "ev", ev, "provider", receiver)
 	}
 }

--- a/light/detector_test.go
+++ b/light/detector_test.go
@@ -253,5 +253,5 @@ func TestClientDivergentTraces3(t *testing.T) {

 	_, err = c.VerifyLightBlockAtHeight(ctx, 10, bTime.Add(1*time.Hour))
 	assert.Error(t, err)
-	assert.Equal(t, 0, len(c.Witnesses()))
+	assert.Equal(t, 1, len(c.Witnesses()))
 }
--- a/light/provider/errors.go
+++ b/light/provider/errors.go
@@ -7,15 +7,15 @@ import (

 var (
 	// ErrLightBlockNotFound is returned when a provider can't find the
-	// requested header.
+	// requested header. The light client will not remove the provider
 	ErrLightBlockNotFound = errors.New("light block not found")
 	// ErrNoResponse is returned if the provider doesn't respond to the
-	// request in a gieven time
+	// request in a given time. The light client will not remove the provider
 	ErrNoResponse = errors.New("client failed to respond")
 )

 // ErrBadLightBlock is returned when a provider returns an invalid
-// light block.
+// light block. The light client will remove the provider.
 type ErrBadLightBlock struct {
 	Reason error
 }
@@ -23,3 +23,14 @@ type ErrBadLightBlock struct {
 func (e ErrBadLightBlock) Error() string {
 	return fmt.Sprintf("client provided bad signed header: %s", e.Reason.Error())
 }
+
+// ErrUnreliableProvider is a generic error that indicates that the provider isn't
+// behaving in a reliable manner to the light client. The light client will
+// remove the provider
+type ErrUnreliableProvider struct {
+	Reason string
+}
+
+func (e ErrUnreliableProvider) Error() string {
+	return fmt.Sprintf("client deemed unreliable: %s", e.Reason)
+}
--- a/light/provider/http/http.go
+++ b/light/provider/http/http.go
@@ -18,8 +18,10 @@ import (
 )

 var defaultOptions = Options{
-	MaxRetryAttempts: 5,
-	Timeout:          3 * time.Second,
+	MaxRetryAttempts:    5,
+	Timeout:             3 * time.Second,
+	NoBlockThreshold:    5,
+	NoResponseThreshold: 5,
 }

 // http provider uses an RPC client to obtain the necessary information.
@@ -27,14 +29,37 @@ type http struct {
 	chainID string
 	client  rpcclient.RemoteClient

-	maxRetryAttempts int
+	// httt provider heuristics
+
+	// The provider tracks the amount of times that the
+	// client doesn't respond. If this exceeds the threshold
+	// then the provider will return an unreliable provider error
+	noResponseThreshold uint16
+	noResponseCount     uint16
+
+	// The provider tracks the amount of time the client
+	// doesn't have a block. If this exceeds the threshold
+	// then the provider will return an unreliable provider error
+	noBlockThreshold uint16
+	noBlockCount     uint16
+
+	// In a single request, the provider attempts multiple times
+	// with exponential backoff to reach the client. If this
+	// exceeds the maxRetry attempts, this result in a ErrNoResponse
+	maxRetryAttempts uint16
 }

 type Options struct {
-	// -1 means no limit
-	MaxRetryAttempts int
+	// 0 means no retries
+	MaxRetryAttempts uint16
 	// 0 means no timeout.
 	Timeout time.Duration
+	// The amount of requests that a client doesn't have the block
+	// for before the provider deems the client unreliable
+	NoBlockThreshold uint16
+	// The amount of requests that a client doesn't respond to
+	// before the provider deems the client unreliable
+	NoResponseThreshold uint16
 }

 // New creates a HTTP provider, which is using the rpchttp.HTTP client under
@@ -67,9 +92,11 @@ func NewWithClient(chainID string, client rpcclient.RemoteClient) provider.Provi
 // NewWithClient allows you to provide a custom client.
 func NewWithClientAndOptions(chainID string, client rpcclient.RemoteClient, options Options) provider.Provider {
 	return &http{
-		client:           client,
-		chainID:          chainID,
-		maxRetryAttempts: options.MaxRetryAttempts,
+		client:              client,
+		chainID:             chainID,
+		maxRetryAttempts:    options.MaxRetryAttempts,
+		noResponseThreshold: options.NoResponseThreshold,
+		noBlockThreshold:    options.NoBlockThreshold,
 	}
 }

@@ -130,7 +157,8 @@ func (p *http) validatorSet(ctx context.Context, height *int64) (*types.Validato
 	for len(vals) != total && page <= maxPages {
 		// create another for loop to control retries. If p.maxRetryAttempts
 		// is negative we will keep repeating.
-		for attempt := 0; attempt != p.maxRetryAttempts+1; attempt++ {
+		attempt := uint16(0)
+		for {
 			res, err := p.client.Validators(ctx, height, &page, &perPage)
 			switch e := err.(type) {
 			case nil: // success!! Now we validate the response
@@ -149,8 +177,13 @@ func (p *http) validatorSet(ctx context.Context, height *int64) (*types.Validato

 			case *url.Error:
 				if e.Timeout() {
+					// if we have exceeded retry attempts then return a no response error
+					if attempt == p.maxRetryAttempts {
+						return nil, p.noResponse()
+					}
+					attempt++
 					// request timed out: we wait and try again with exponential backoff
-					time.Sleep(backoffTimeout(uint16(attempt)))
+					time.Sleep(backoffTimeout(attempt))
 					continue
 				}
 				return nil, provider.ErrBadLightBlock{Reason: e}
@@ -188,7 +221,7 @@ func (p *http) validatorSet(ctx context.Context, height *int64) (*types.Validato
 func (p *http) signedHeader(ctx context.Context, height *int64) (*types.SignedHeader, error) {
 	// create a for loop to control retries. If p.maxRetryAttempts
 	// is negative we will keep repeating.
-	for attempt := 0; attempt != p.maxRetryAttempts+1; attempt++ {
+	for attempt := uint16(0); attempt != p.maxRetryAttempts+1; attempt++ {
 		commit, err := p.client.Commit(ctx, height)
 		switch e := err.(type) {
 		case nil: // success!!
@@ -197,24 +230,20 @@ func (p *http) signedHeader(ctx context.Context, height *int64) (*types.SignedHe
 		case *url.Error:
 			if e.Timeout() {
 				// we wait and try again with exponential backoff
-				time.Sleep(backoffTimeout(uint16(attempt)))
+				time.Sleep(backoffTimeout(attempt))
 				continue
 			}
 			return nil, provider.ErrBadLightBlock{Reason: e}

 		case *rpctypes.RPCError:
-			// Check if we got something other than internal error. This shouldn't happen unless the RPC module
-			// or light client has been tampered with. If we do get this error, stop the connection with the
-			// peer and return an error
-			if e.Code != -32603 {
-				return nil, provider.ErrBadLightBlock{Reason: errors.New(e.Data)}
+			// check if the error indicates that the peer doesn't have the block
+			if strings.Contains(e.Data, ctypes.ErrHeightNotAvailable.Error()) ||
+				strings.Contains(e.Data, ctypes.ErrHeightExceedsChainHead.Error()) {
+				return nil, p.noBlock()
 			}

-			// check if the error indicates that the peer doesn't have the block
-			if strings.Contains(err.Error(), ctypes.ErrHeightNotAvailable.Error()) ||
-				strings.Contains(err.Error(), ctypes.ErrHeightExceedsChainHead.Error()) {
-				return nil, provider.ErrLightBlockNotFound
-			}
+			// for every other error, the provider returns a bad block
+			return nil, provider.ErrBadLightBlock{Reason: errors.New(e.Data)}

 		default:
 			// If we don't know the error then by default we return a bad light block error and
@@ -222,7 +251,27 @@ func (p *http) signedHeader(ctx context.Context, height *int64) (*types.SignedHe
 			return nil, provider.ErrBadLightBlock{Reason: e}
 		}
 	}
-	return nil, provider.ErrNoResponse
+	return nil, p.noResponse()
+}
+
+func (p *http) noResponse() error {
+	p.noResponseCount++
+	if p.noResponseCount > p.noResponseThreshold {
+		return provider.ErrUnreliableProvider{
+			Reason: fmt.Sprintf("failed to respond after %d attempts", p.noResponseCount),
+		}
+	}
+	return provider.ErrNoResponse
+}
+
+func (p *http) noBlock() error {
+	p.noBlockCount++
+	if p.noBlockCount > p.noBlockThreshold {
+		return provider.ErrUnreliableProvider{
+			Reason: fmt.Sprintf("failed to provide a block after %d attempts", p.noBlockCount),
+		}
+	}
+	return provider.ErrLightBlockNotFound
 }

 func validateHeight(height int64) (*int64, error) {
--- a/light/provider/http/http_test.go
+++ b/light/provider/http/http_test.go
@@ -66,25 +66,33 @@ func TestProvider(t *testing.T) {
 	require.NoError(t, err)

 	// let's get the highest block
-	sh, err := p.LightBlock(context.Background(), 0)
+	lb, err := p.LightBlock(context.Background(), 0)
 	require.NoError(t, err)
-	assert.True(t, sh.Height < 1000)
+	assert.True(t, lb.Height < 1000)

 	// let's check this is valid somehow
-	assert.Nil(t, sh.ValidateBasic(chainID))
+	assert.Nil(t, lb.ValidateBasic(chainID))

 	// historical queries now work :)
-	lower := sh.Height - 3
-	sh, err = p.LightBlock(context.Background(), lower)
+	lower := lb.Height - 3
+	lb, err = p.LightBlock(context.Background(), lower)
 	require.NoError(t, err)
-	assert.Equal(t, lower, sh.Height)
+	assert.Equal(t, lower, lb.Height)

 	// // fetching missing heights (both future and pruned) should return appropriate errors
-	_, err = p.LightBlock(context.Background(), 1000)
+	lb, err = p.LightBlock(context.Background(), 1000)
 	require.Error(t, err)
+	require.Nil(t, lb)
 	assert.Equal(t, provider.ErrLightBlockNotFound, err)

 	_, err = p.LightBlock(context.Background(), 1)
 	require.Error(t, err)
 	assert.Equal(t, provider.ErrLightBlockNotFound, err)
+
+	// if the provider is unable to provide four more blocks then we should return
+	// an unreliable peer error
+	for i := 0; i < 4; i++ {
+		_, err = p.LightBlock(context.Background(), 1)
+	}
+	assert.IsType(t, provider.ErrUnreliableProvider{}, err)
 }
--- a/light/provider/mock/deadmock.go
+++ b/light/provider/mock/deadmock.go
@@ -2,15 +2,12 @@ package mock

 import (
 	"context"
-	"errors"
 	"fmt"

 	"github.com/tendermint/tendermint/light/provider"
 	"github.com/tendermint/tendermint/types"
 )

-var errNoResp = errors.New("no response from provider")
-
 type deadMock struct {
 	id string
 }
@@ -25,9 +22,9 @@ func (p *deadMock) String() string {
 }

 func (p *deadMock) LightBlock(_ context.Context, height int64) (*types.LightBlock, error) {
-	return nil, errNoResp
+	return nil, provider.ErrNoResponse
 }

 func (p *deadMock) ReportEvidence(_ context.Context, ev types.Evidence) error {
-	return errNoResp
+	return provider.ErrNoResponse
 }
--- a/light/verifier.go
+++ b/light/verifier.go
@@ -130,7 +130,7 @@ func VerifyAdjacent(

 	// Check the validator hashes are the same
 	if !bytes.Equal(untrustedHeader.ValidatorsHash, trustedHeader.NextValidatorsHash) {
-		err := fmt.Errorf("expected old header next validators (%X) to match those from new header (%X)",
+		err := fmt.Errorf("expected old header's next validators (%X) to match those from new header (%X)",
 			trustedHeader.NextValidatorsHash,
 			untrustedHeader.ValidatorsHash,
 		)