path: root/eth/downloader/downloader_test.go

package downloader

import (
    "encoding/binary"
    "fmt"
    "math/big"
    "testing"
    "time"

    "github.com/ethereum/go-ethereum/common"
    "github.com/ethereum/go-ethereum/core"
    "github.com/ethereum/go-ethereum/core/types"
    "github.com/ethereum/go-ethereum/event"
)

var (
    knownHash   = common.Hash{1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1}
    unknownHash = common.Hash{2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2}
    bannedHash  = common.Hash{3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3}

    genesis = createBlock(1, common.Hash{}, knownHash)
)

// idCounter is used by the createHashes method the generate deterministic but unique hashes
var idCounter = int64(2) // #1 is the genesis block

// createHashes generates a batch of hashes rooted at a specific point in the chain.
func createHashes(amount int, root common.Hash) (hashes []common.Hash) {
    hashes = make([]common.Hash, amount+1)
    hashes[len(hashes)-1] = root

    for i := 0; i < len(hashes)-1; i++ {
        binary.BigEndian.PutUint64(hashes[i][:8], uint64(idCounter))
        idCounter++
    }
    return
}

// createBlock assembles a new block at the given chain height.
func createBlock(i int, parent, hash common.Hash) *types.Block {
    header := &types.Header{Number: big.NewInt(int64(i))}
    block := types.NewBlockWithHeader(header)
    block.HeaderHash = hash
    block.ParentHeaderHash = parent
    return block
}

// copyBlock makes a deep copy of a block suitable for local modifications.
func copyBlock(block *types.Block) *types.Block {
    return createBlock(int(block.Number().Int64()), block.ParentHeaderHash, block.HeaderHash)
}

func createBlocksFromHashes(hashes []common.Hash) map[common.Hash]*types.Block {
    blocks := make(map[common.Hash]*types.Block)
    for i := 0; i < len(hashes); i++ {
        parent := knownHash
        if i < len(hashes)-1 {
            parent = hashes[i+1]
        }
        blocks[hashes[i]] = createBlock(len(hashes)-i, parent, hashes[i])
    }
    return blocks
}

type downloadTester struct {
    downloader *Downloader

    ownHashes  []common.Hash                           // Hash chain belonging to the tester
    ownBlocks  map[common.Hash]*types.Block            // Blocks belonging to the tester
    peerHashes map[string][]common.Hash                // Hash chain belonging to different test peers
    peerBlocks map[string]map[common.Hash]*types.Block // Blocks belonging to different test peers

    maxHashFetch int // Overrides the maximum number of retrieved hashes
}

func newTester() *downloadTester {
    tester := &downloadTester{
        ownHashes:  []common.Hash{knownHash},
        ownBlocks:  map[common.Hash]*types.Block{knownHash: genesis},
        peerHashes: make(map[string][]common.Hash),
        peerBlocks: make(map[string]map[common.Hash]*types.Block),
    }
    var mux event.TypeMux
    downloader := New(&mux, tester.hasBlock, tester.getBlock, tester.dropPeer)
    tester.downloader = downloader

    return tester
}

// sync starts synchronizing with a remote peer, blocking until it completes.
func (dl *downloadTester) sync(id string) error {
    return dl.downloader.synchronise(id, dl.peerHashes[id][0])
}

// syncTake is starts synchronising with a remote peer, but concurrently it also
// starts fetching blocks that the downloader retrieved. IT blocks until both go
// routines terminate.
func (dl *downloadTester) syncTake(id string) ([]*Block, error) {
    // Start a block collector to take blocks as they become available
    done := make(chan struct{})
    took := []*Block{}
    go func() {
        for running := true; running; {
            select {
            case <-done:
                running = false
            default:
                time.Sleep(time.Millisecond)
            }
            // Take a batch of blocks and accumulate
            blocks := dl.downloader.TakeBlocks()
            for _, block := range blocks {
                dl.ownHashes = append(dl.ownHashes, block.RawBlock.Hash())
                dl.ownBlocks[block.RawBlock.Hash()] = block.RawBlock
            }
            took = append(took, blocks...)
        }
        done <- struct{}{}
    }()
    // Start the downloading, sync the taker and return
    err := dl.sync(id)

    done <- struct{}{}
    <-done

    return took, err
}

// hasBlock checks if a block is present in the testers canonical chain.
func (dl *downloadTester) hasBlock(hash common.Hash) bool {
    return dl.getBlock(hash) != nil
}

// getBlock retrieves a block from the testers canonical chain.
func (dl *downloadTester) getBlock(hash common.Hash) *types.Block {
    return dl.ownBlocks[hash]
}

// newPeer registers a new block download source into the downloader.
func (dl *downloadTester) newPeer(id string, hashes []common.Hash, blocks map[common.Hash]*types.Block) error {
    err := dl.downloader.RegisterPeer(id, hashes[0], dl.peerGetHashesFn(id), dl.peerGetBlocksFn(id))
    if err == nil {
        // Assign the owned hashes and blocks to the peer (deep copy)
        dl.peerHashes[id] = make([]common.Hash, len(hashes))
        copy(dl.peerHashes[id], hashes)

        dl.peerBlocks[id] = make(map[common.Hash]*types.Block)
        for hash, block := range blocks {
            dl.peerBlocks[id][hash] = copyBlock(block)
        }
    }
    return err
}

// dropPeer simulates a hard peer removal from the connection pool.
func (dl *downloadTester) dropPeer(id string) {
    delete(dl.peerHashes, id)
    delete(dl.peerBlocks, id)

    dl.downloader.UnregisterPeer(id)
}

// peerGetBlocksFn constructs a getHashes function associated with a particular
// peer in the download tester. The returned function can be used to retrieve
// batches of hashes from the particularly requested peer.
func (dl *downloadTester) peerGetHashesFn(id string) func(head common.Hash) error {
    return func(head common.Hash) error {
        limit := MaxHashFetch
        if dl.maxHashFetch > 0 {
            limit = dl.maxHashFetch
        }
        // Gather the next batch of hashes
        hashes := dl.peerHashes[id]
        result := make([]common.Hash, 0, limit)
        for i, hash := range hashes {
            if hash == head {
                i++
                for len(result) < cap(result) && i < len(hashes) {
                    result = append(result, hashes[i])
                    i++
                }
                break
            }
        }
        // Delay delivery a bit to allow attacks to unfold
        go func() {
            time.Sleep(time.Millisecond)
            dl.downloader.DeliverHashes(id, result)
        }()
        return nil
    }
}

// peerGetBlocksFn constructs a getBlocks function associated with a particular
// peer in the download tester. The returned function can be used to retrieve
// batches of blocks from the particularly requested peer.
func (dl *downloadTester) peerGetBlocksFn(id string) func([]common.Hash) error {
    return func(hashes []common.Hash) error {
        blocks := dl.peerBlocks[id]
        result := make([]*types.Block, 0, len(hashes))
        for _, hash := range hashes {
            if block, ok := blocks[hash]; ok {
                result = append(result, block)
            }
        }
        go dl.downloader.DeliverBlocks(id, result)

        return nil
    }
}

// Tests that simple synchronization, without throttling from a good peer works.
func TestSynchronisation(t *testing.T) {
    // Create a small enough block chain to download and the tester
    targetBlocks := blockCacheLimit - 15
    hashes := createHashes(targetBlocks, knownHash)
    blocks := createBlocksFromHashes(hashes)

    tester := newTester()
    tester.newPeer("peer", hashes, blocks)

    // Synchronise with the peer and make sure all blocks were retrieved
    if err := tester.sync("peer"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
    if queued := len(tester.downloader.queue.blockPool); queued != targetBlocks {
        t.Fatalf("synchronised block mismatch: have %v, want %v", queued, targetBlocks)
    }
}

// Tests that the synchronized blocks can be correctly retrieved.
func TestBlockTaking(t *testing.T) {
    // Create a small enough block chain to download and the tester
    targetBlocks := blockCacheLimit - 15
    hashes := createHashes(targetBlocks, knownHash)
    blocks := createBlocksFromHashes(hashes)

    tester := newTester()
    tester.newPeer("peer", hashes, blocks)

    // Synchronise with the peer and test block retrieval
    if err := tester.sync("peer"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
    if took := tester.downloader.TakeBlocks(); len(took) != targetBlocks {
        t.Fatalf("took block mismatch: have %v, want %v", len(took), targetBlocks)
    }
}

// Tests that an inactive downloader will not accept incoming hashes and blocks.
func TestInactiveDownloader(t *testing.T) {
    tester := newTester()

    // Check that neither hashes nor blocks are accepted
    if err := tester.downloader.DeliverHashes("bad peer", []common.Hash{}); err != errNoSyncActive {
        t.Errorf("error mismatch: have %v, want %v", err, errNoSyncActive)
    }
    if err := tester.downloader.DeliverBlocks("bad peer", []*types.Block{}); err != errNoSyncActive {
        t.Errorf("error mismatch: have %v, want %v", err, errNoSyncActive)
    }
}

// Tests that a canceled download wipes all previously accumulated state.
func TestCancel(t *testing.T) {
    // Create a small enough block chain to download and the tester
    targetBlocks := blockCacheLimit - 15
    hashes := createHashes(targetBlocks, knownHash)
    blocks := createBlocksFromHashes(hashes)

    tester := newTester()
    tester.newPeer("peer", hashes, blocks)

    // Synchronise with the peer, but cancel afterwards
    if err := tester.sync("peer"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
    if !tester.downloader.Cancel() {
        t.Fatalf("cancel operation failed")
    }
    // Make sure the queue reports empty and no blocks can be taken
    hashCount, blockCount := tester.downloader.queue.Size()
    if hashCount > 0 || blockCount > 0 {
        t.Errorf("block or hash count mismatch: %d hashes, %d blocks, want 0", hashCount, blockCount)
    }
    if took := tester.downloader.TakeBlocks(); len(took) != 0 {
        t.Errorf("taken blocks mismatch: have %d, want %d", len(took), 0)
    }
}

// Tests that if a large batch of blocks are being downloaded, it is throttled
// until the cached blocks are retrieved.
func TestThrottling(t *testing.T) {
    // Create a long block chain to download and the tester
    targetBlocks := 8 * blockCacheLimit
    hashes := createHashes(targetBlocks, knownHash)
    blocks := createBlocksFromHashes(hashes)

    tester := newTester()
    tester.newPeer("peer", hashes, blocks)

    // Start a synchronisation concurrently
    errc := make(chan error)
    go func() {
        errc <- tester.sync("peer")
    }()
    // Iteratively take some blocks, always checking the retrieval count
    for total := 0; total < targetBlocks; {
        // Wait a bit for sync to complete
        for start := time.Now(); time.Since(start) < 3*time.Second; {
            time.Sleep(25 * time.Millisecond)
            if len(tester.downloader.queue.blockPool) == blockCacheLimit {
                break
            }
        }
        // Fetch the next batch of blocks
        took := tester.downloader.TakeBlocks()
        if len(took) != blockCacheLimit {
            t.Fatalf("block count mismatch: have %v, want %v", len(took), blockCacheLimit)
        }
        total += len(took)
        if total > targetBlocks {
            t.Fatalf("target block count mismatch: have %v, want %v", total, targetBlocks)
        }
    }
    if err := <-errc; err != nil {
        t.Fatalf("block synchronization failed: %v", err)
    }
}

// Tests that if a peer returns an invalid chain with a block pointing to a non-
// existing parent, it is correctly detected and handled.
func TestNonExistingParentAttack(t *testing.T) {
    tester := newTester()

    // Forge a single-link chain with a forged header
    hashes := createHashes(1, knownHash)
    blocks := createBlocksFromHashes(hashes)
    tester.newPeer("valid", hashes, blocks)

    hashes = createHashes(1, knownHash)
    blocks = createBlocksFromHashes(hashes)
    blocks[hashes[0]].ParentHeaderHash = unknownHash
    tester.newPeer("attack", hashes, blocks)

    // Try and sync with the malicious node and check that it fails
    if err := tester.sync("attack"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
    bs := tester.downloader.TakeBlocks()
    if len(bs) != 1 {
        t.Fatalf("retrieved block mismatch: have %v, want %v", len(bs), 1)
    }
    if tester.hasBlock(bs[0].RawBlock.ParentHash()) {
        t.Fatalf("tester knows about the unknown hash")
    }
    tester.downloader.Cancel()

    // Try to synchronize with the valid chain and make sure it succeeds
    if err := tester.sync("valid"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
    bs = tester.downloader.TakeBlocks()
    if len(bs) != 1 {
        t.Fatalf("retrieved block mismatch: have %v, want %v", len(bs), 1)
    }
    if !tester.hasBlock(bs[0].RawBlock.ParentHash()) {
        t.Fatalf("tester doesn't know about the origin hash")
    }
}

// Tests that if a malicious peers keeps sending us repeating hashes, we don't
// loop indefinitely.
func TestRepeatingHashAttack(t *testing.T) { // TODO: Is this thing valid??
    tester := newTester()

    // Create a valid chain, but drop the last link
    hashes := createHashes(blockCacheLimit, knownHash)
    blocks := createBlocksFromHashes(hashes)
    tester.newPeer("valid", hashes, blocks)
    tester.newPeer("attack", hashes[:len(hashes)-1], blocks)

    // Try and sync with the malicious node
    errc := make(chan error)
    go func() {
        errc <- tester.sync("attack")
    }()
    // Make sure that syncing returns and does so with a failure
    select {
    case <-time.After(time.Second):
        t.Fatalf("synchronisation blocked")
    case err := <-errc:
        if err == nil {
            t.Fatalf("synchronisation succeeded")
        }
    }
    // Ensure that a valid chain can still pass sync
    if err := tester.sync("valid"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
}

// Tests that if a malicious peers returns a non-existent block hash, it should
// eventually time out and the sync reattempted.
func TestNonExistingBlockAttack(t *testing.T) {
    tester := newTester()

    // Create a valid chain, but forge the last link
    hashes := createHashes(blockCacheLimit, knownHash)
    blocks := createBlocksFromHashes(hashes)
    tester.newPeer("valid", hashes, blocks)

    hashes[len(hashes)/2] = unknownHash
    tester.newPeer("attack", hashes, blocks)

    // Try and sync with the malicious node and check that it fails
    if err := tester.sync("attack"); err != errPeersUnavailable {
        t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errPeersUnavailable)
    }
    // Ensure that a valid chain can still pass sync
    if err := tester.sync("valid"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
}

// Tests that if a malicious peer is returning hashes in a weird order, that the
// sync throttler doesn't choke on them waiting for the valid blocks.
func TestInvalidHashOrderAttack(t *testing.T) {
    tester := newTester()

    // Create a valid long chain, but reverse some hashes within
    hashes := createHashes(4*blockCacheLimit, knownHash)
    blocks := createBlocksFromHashes(hashes)
    tester.newPeer("valid", hashes, blocks)

    chunk1 := make([]common.Hash, blockCacheLimit)
    chunk2 := make([]common.Hash, blockCacheLimit)
    copy(chunk1, hashes[blockCacheLimit:2*blockCacheLimit])
    copy(chunk2, hashes[2*blockCacheLimit:3*blockCacheLimit])

    copy(hashes[2*blockCacheLimit:], chunk1)
    copy(hashes[blockCacheLimit:], chunk2)
    tester.newPeer("attack", hashes, blocks)

    // Try and sync with the malicious node and check that it fails
    if _, err := tester.syncTake("attack"); err != errInvalidChain {
        t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errInvalidChain)
    }
    // Ensure that a valid chain can still pass sync
    if _, err := tester.syncTake("valid"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
}

// Tests that if a malicious peer makes up a random hash chain and tries to push
// indefinitely, it actually gets caught with it.
func TestMadeupHashChainAttack(t *testing.T) {
    tester := newTester()
    blockSoftTTL = 100 * time.Millisecond
    crossCheckCycle = 25 * time.Millisecond

    // Create a long chain of hashes without backing blocks
    hashes := createHashes(4*blockCacheLimit, knownHash)
    blocks := createBlocksFromHashes(hashes)

    tester.newPeer("valid", hashes, blocks)
    tester.newPeer("attack", createHashes(1024*blockCacheLimit, knownHash), nil)

    // Try and sync with the malicious node and check that it fails
    if _, err := tester.syncTake("attack"); err != errCrossCheckFailed {
        t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errCrossCheckFailed)
    }
    // Ensure that a valid chain can still pass sync
    if _, err := tester.syncTake("valid"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
}

// Tests that if a malicious peer makes up a random hash chain, and tries to push
// indefinitely, one hash at a time, it actually gets caught with it. The reason
// this is separate from the classical made up chain attack is that sending hashes
// one by one prevents reliable block/parent verification.
func TestMadeupHashChainDrippingAttack(t *testing.T) {
    // Create a random chain of hashes to drip
    hashes := createHashes(16*blockCacheLimit, knownHash)
    tester := newTester()

    // Try and sync with the attacker, one hash at a time
    tester.maxHashFetch = 1
    tester.newPeer("attack", hashes, nil)
    if _, err := tester.syncTake("attack"); err != errStallingPeer {
        t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errStallingPeer)
    }
}

// Tests that if a malicious peer makes up a random block chain, and tried to
// push indefinitely, it actually gets caught with it.
func TestMadeupBlockChainAttack(t *testing.T) {
    defaultBlockTTL := blockSoftTTL
    defaultCrossCheckCycle := crossCheckCycle

    blockSoftTTL = 100 * time.Millisecond
    crossCheckCycle = 25 * time.Millisecond

    // Create a long chain of blocks and simulate an invalid chain by dropping every second
    hashes := createHashes(16*blockCacheLimit, knownHash)
    blocks := createBlocksFromHashes(hashes)

    gapped := make([]common.Hash, len(hashes)/2)
    for i := 0; i < len(gapped); i++ {
        gapped[i] = hashes[2*i]
    }
    // Try and sync with the malicious node and check that it fails
    tester := newTester()
    tester.newPeer("attack", gapped, blocks)
    if _, err := tester.syncTake("attack"); err != errCrossCheckFailed {
        t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errCrossCheckFailed)
    }
    // Ensure that a valid chain can still pass sync
    blockSoftTTL = defaultBlockTTL
    crossCheckCycle = defaultCrossCheckCycle

    tester.newPeer("valid", hashes, blocks)
    if _, err := tester.syncTake("valid"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
}

// Advanced form of the above forged blockchain attack, where not only does the
// attacker make up a valid hashes for random blocks, but also forges the block
// parents to point to existing hashes.
func TestMadeupParentBlockChainAttack(t *testing.T) {
    tester := newTester()

    defaultBlockTTL := blockSoftTTL
    defaultCrossCheckCycle := crossCheckCycle

    blockSoftTTL = 100 * time.Millisecond
    crossCheckCycle = 25 * time.Millisecond

    // Create a long chain of blocks and simulate an invalid chain by dropping every second
    hashes := createHashes(16*blockCacheLimit, knownHash)
    blocks := createBlocksFromHashes(hashes)
    tester.newPeer("valid", hashes, blocks)

    for _, block := range blocks {
        block.ParentHeaderHash = knownHash // Simulate pointing to already known hash
    }
    tester.newPeer("attack", hashes, blocks)

    // Try and sync with the malicious node and check that it fails
    if _, err := tester.syncTake("attack"); err != errCrossCheckFailed {
        t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errCrossCheckFailed)
    }
    // Ensure that a valid chain can still pass sync
    blockSoftTTL = defaultBlockTTL
    crossCheckCycle = defaultCrossCheckCycle

    if _, err := tester.syncTake("valid"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
}

// Tests that if one/multiple malicious peers try to feed a banned blockchain to
// the downloader, it will not keep refetching the same chain indefinitely, but
// gradually block pieces of it, until it's head is also blocked.
func TestBannedChainStarvationAttack(t *testing.T) {
    // Create the tester and ban the selected hash
    tester := newTester()
    tester.downloader.banned.Add(bannedHash)

    // Construct a valid chain, for it and ban the fork
    hashes := createHashes(8*blockCacheLimit, knownHash)
    blocks := createBlocksFromHashes(hashes)
    tester.newPeer("valid", hashes, blocks)

    fork := len(hashes)/2 - 23
    hashes = append(createHashes(4*blockCacheLimit, bannedHash), hashes[fork:]...)
    blocks = createBlocksFromHashes(hashes)
    tester.newPeer("attack", hashes, blocks)

    // Iteratively try to sync, and verify that the banned hash list grows until
    // the head of the invalid chain is blocked too.
    for banned := tester.downloader.banned.Size(); ; {
        // Try to sync with the attacker, check hash chain failure
        if _, err := tester.syncTake("attack"); err != errInvalidChain {
            if tester.downloader.banned.Has(hashes[0]) && err == errBannedHead {
                break
            }
            t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errInvalidChain)
        }
        // Check that the ban list grew with at least 1 new item, or all banned
        bans := tester.downloader.banned.Size()
        if bans < banned+1 {
            t.Fatalf("ban count mismatch: have %v, want %v+", bans, banned+1)
        }
        banned = bans
    }
    // Check that after banning an entire chain, bad peers get dropped
    if err := tester.newPeer("new attacker", hashes, blocks); err != errBannedHead {
        t.Fatalf("peer registration mismatch: have %v, want %v", err, errBannedHead)
    }
    if peer := tester.downloader.peers.Peer("new attacker"); peer != nil {
        t.Fatalf("banned attacker registered: %v", peer)
    }
    // Ensure that a valid chain can still pass sync
    if _, err := tester.syncTake("valid"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
}

// Tests that if a peer sends excessively many/large invalid chains that are
// gradually banned, it will have an upper limit on the consumed memory and also
// the origin bad hashes will not be evacuated.
func TestBannedChainMemoryExhaustionAttack(t *testing.T) {
    // Create the tester and ban the selected hash
    tester := newTester()
    tester.downloader.banned.Add(bannedHash)

    // Reduce the test size a bit
    defaultMaxBlockFetch := MaxBlockFetch
    defaultMaxBannedHashes := maxBannedHashes

    MaxBlockFetch = 4
    maxBannedHashes = 256

    // Construct a banned chain with more chunks than the ban limit
    hashes := createHashes(8*blockCacheLimit, knownHash)
    blocks := createBlocksFromHashes(hashes)
    tester.newPeer("valid", hashes, blocks)

    fork := len(hashes)/2 - 23
    hashes = append(createHashes(maxBannedHashes*MaxBlockFetch, bannedHash), hashes[fork:]...)
    blocks = createBlocksFromHashes(hashes)
    tester.newPeer("attack", hashes, blocks)

    // Iteratively try to sync, and verify that the banned hash list grows until
    // the head of the invalid chain is blocked too.
    for {
        // Try to sync with the attacker, check hash chain failure
        if _, err := tester.syncTake("attack"); err != errInvalidChain {
            t.Fatalf("synchronisation error mismatch: have %v, want %v", err, errInvalidChain)
        }
        // Short circuit if the entire chain was banned
        if tester.downloader.banned.Has(hashes[0]) {
            break
        }
        // Otherwise ensure we never exceed the memory allowance and the hard coded bans are untouched
        if bans := tester.downloader.banned.Size(); bans > maxBannedHashes {
            t.Fatalf("ban cap exceeded: have %v, want max %v", bans, maxBannedHashes)
        }
        for hash, _ := range core.BadHashes {
            if !tester.downloader.banned.Has(hash) {
                t.Fatalf("hard coded ban evacuated: %x", hash)
            }
        }
    }
    // Ensure that a valid chain can still pass sync
    MaxBlockFetch = defaultMaxBlockFetch
    maxBannedHashes = defaultMaxBannedHashes

    if _, err := tester.syncTake("valid"); err != nil {
        t.Fatalf("failed to synchronise blocks: %v", err)
    }
}

// Tests that misbehaving peers are disconnected, whilst behaving ones are not.
func TestAttackerDropping(t *testing.T) {
    // Define the disconnection requirement for individual errors
    tests := []struct {
        result error
        drop   bool
    }{
        {nil, false},                 // Sync succeeded, all is well
        {errBusy, false},             // Sync is already in progress, no problem
        {errUnknownPeer, false},      // Peer is unknown, was already dropped, don't double drop
        {errBadPeer, true},           // Peer was deemed bad for some reason, drop it
        {errStallingPeer, true},      // Peer was detected to be stalling, drop it
        {errBannedHead, true},        // Peer's head hash is a known bad hash, drop it
        {errNoPeers, false},          // No peers to download from, soft race, no issue
        {errPendingQueue, false},     // There are blocks still cached, wait to exhaust, no issue
        {errTimeout, true},           // No hashes received in due time, drop the peer
        {errEmptyHashSet, true},      // No hashes were returned as a response, drop as it's a dead end
        {errPeersUnavailable, true},  // Nobody had the advertised blocks, drop the advertiser
        {errInvalidChain, true},      // Hash chain was detected as invalid, definitely drop
        {errCrossCheckFailed, true},  // Hash-origin failed to pass a block cross check, drop
        {errCancelHashFetch, false},  // Synchronisation was canceled, origin may be innocent, don't drop
        {errCancelBlockFetch, false}, // Synchronisation was canceled, origin may be innocent, don't drop
    }
    // Run the tests and check disconnection status
    tester := newTester()
    for i, tt := range tests {
        // Register a new peer and ensure it's presence
        id := fmt.Sprintf("test %d", i)
        if err := tester.newPeer(id, []common.Hash{knownHash}, nil); err != nil {
            t.Fatalf("test %d: failed to register new peer: %v", i, err)
        }
        if _, ok := tester.peerHashes[id]; !ok {
            t.Fatalf("test %d: registered peer not found", i)
        }
        // Simulate a synchronisation and check the required result
        tester.downloader.synchroniseMock = func(string, common.Hash) error { return tt.result }

        tester.downloader.Synchronise(id, knownHash)
        if _, ok := tester.peerHashes[id]; !ok != tt.drop {
            t.Errorf("test %d: peer drop mismatch for %v: have %v, want %v", i, tt.result, !ok, tt.drop)
        }
    }
}