eth/downloader: create repro testcase for beacon header loss

eth/downloader/skeleton.go

@@ -520,7 +520,7 @@ func (s *skeleton) initSync(head *types.Header) {
 				}
 				break
 			}
-			// If the last subchain can be extended, we're lucky. Otherwise create
+			// If the last subchain can be extended, we're lucky. Otherwise, create
 			// a new subchain sync task.
 			var extended bool
 			if n := len(s.progress.Subchains); n > 0 {

eth/downloader/skeleton_test.go

@@ -37,7 +37,7 @@ import (
 type hookedBackfiller struct {
 	// suspendHook is an optional hook to be called when the filler is requested
 	// to be suspended.
-	suspendHook func()
+	suspendHook func() *types.Header
 
 	// resumeHook is an optional hook to be called when the filler is requested
 	// to be resumed.
@@ -56,7 +56,7 @@ func newHookedBackfiller() backfiller {
 // on initial startup.
 func (hf *hookedBackfiller) suspend() *types.Header {
 	if hf.suspendHook != nil {
-		hf.suspendHook()
+		return hf.suspendHook()
 	}
 	return nil // we don't really care about header cleanups for now
 }
@@ -525,9 +525,21 @@ func TestSkeletonSyncRetrievals(t *testing.T) {
 			Number:     big.NewInt(int64(i)),
 		})
 	}
+	// Some tests require a forking side chain to trigger cornercases.
+	var sidechain []*types.Header
+	for i := 0; i < len(chain)/2; i++ { // Fork at block #5000
+		sidechain = append(sidechain, chain[i])
+	}
+	for i := len(chain) / 2; i < len(chain); i++ {
+		sidechain = append(sidechain, &types.Header{
+			ParentHash: sidechain[i-1].Hash(),
+			Number:     big.NewInt(int64(i)),
+			Extra:      []byte("B"), // force a different hash
+		})
+	}
 	tests := []struct {
-		headers  []*types.Header // Database content (beside the genesis)
-		oldstate []*subchain     // Old sync state with various interrupted subchains
+		fill          bool // Whether to run a real backfiller in this test case
+		unpredictable bool // Whether to ignore drops/serves due to uncertain packet assignments
 
 		head     *types.Header       // New head header to announce to reorg to
 		peers    []*skeletonTestPeer // Initial peer set to start the sync with
@@ -760,11 +772,41 @@ func TestSkeletonSyncRetrievals(t *testing.T) {
 			endstate: []*subchain{{Head: 2*requestHeaders + 2, Tail: 1}},
 			endserve: 4 * requestHeaders,
 		},
+		// This test reproduces a bug caught by (@rjl493456442) where a skeleton
+		// header goes missing, causing the sync to get stuck and/or panic.
+		//
+		// The setup requires a previously successfully synced chain up to a block
+		// height N. That results is a single skeleton header (block N) and a single
+		// subchain (head N, Tail N) being stored on disk.
+		//
+		// The following step requires a new sync cycle to a new side chain of a
+		// height higher than N, and an ancestor lower than N (e.g. N-2, N+2).
+		// In this scenario, when processing a batch of headers, a link point of
+		// N-2 will be found, meaning that N-1 and N have been overwritten.
+		//
+		// The link event triggers an early exit, noticing that the previous sub-
+		// chain is a leftover and deletes it (with it's skeleton header N). But
+		// since skeleton header N has been overwritten to the new side chain, we
+		// end up losing it and creating a gap.
+		{
+			fill:          true,
+			unpredictable: true, // We have good and bad peer too, bad may be dropped, test too short for certainty
+
+			head:     chain[len(chain)/2+1], // Sync up until the sidechain common ancestor + 2
+			peers:    []*skeletonTestPeer{newSkeletonTestPeer("test-peer-oldchain", chain)},
+			midstate: []*subchain{{Head: uint64(len(chain)/2 + 1), Tail: 1}},
+
+			newHead:  sidechain[len(sidechain)/2+3], // Sync up until the sidechain common ancestor + 4
+			newPeer:  newSkeletonTestPeer("test-peer-newchain", sidechain),
+			endstate: []*subchain{{Head: uint64(len(sidechain)/2 + 3), Tail: uint64(len(chain) / 2)}},
+		},
 	}
 	for i, tt := range tests {
 		// Create a fresh database and initialize it with the starting state
 		db := rawdb.NewMemoryDatabase()
-		rawdb.WriteHeader(db, chain[0])
+
+		rawdb.WriteBlock(db, types.NewBlockWithHeader(chain[0]))
+		rawdb.WriteReceipts(db, chain[0].Hash(), chain[0].Number.Uint64(), types.Receipts{})
 
 		// Create a peer set to feed headers through
 		peerset := newPeerSet()
@@ -780,8 +822,43 @@ func TestSkeletonSyncRetrievals(t *testing.T) {
 			peerset.Unregister(peer)
 			dropped[peer]++
 		}
+		// Create a backfiller if we need to run more advanced tests
+		filler := newHookedBackfiller()
+		if tt.fill {
+			var filled *types.Header
+
+			filler = &hookedBackfiller{
+				resumeHook: func() {
+					var progress skeletonProgress
+					json.Unmarshal(rawdb.ReadSkeletonSyncStatus(db), &progress)
+
+					for progress.Subchains[0].Tail < progress.Subchains[0].Head {
+						header := rawdb.ReadSkeletonHeader(db, progress.Subchains[0].Tail)
+
+						rawdb.WriteBlock(db, types.NewBlockWithHeader(header))
+						rawdb.WriteReceipts(db, header.Hash(), header.Number.Uint64(), types.Receipts{})
+
+						rawdb.DeleteSkeletonHeader(db, header.Number.Uint64())
+
+						progress.Subchains[0].Tail++
+						progress.Subchains[0].Next = header.Hash()
+					}
+					filled = rawdb.ReadSkeletonHeader(db, progress.Subchains[0].Tail)
+
+					rawdb.WriteBlock(db, types.NewBlockWithHeader(filled))
+					rawdb.WriteReceipts(db, filled.Hash(), filled.Number.Uint64(), types.Receipts{})
+				},
+
+				suspendHook: func() *types.Header {
+					prev := filled
+					filled = nil
+
+					return prev
+				},
+			}
+		}
 		// Create a skeleton sync and run a cycle
-		skeleton := newSkeleton(db, peerset, drop, newHookedBackfiller())
+		skeleton := newSkeleton(db, peerset, drop, filler)
 		skeleton.Sync(tt.head, true)
 
 		var progress skeletonProgress
@@ -815,19 +892,21 @@ func TestSkeletonSyncRetrievals(t *testing.T) {
 			t.Error(err)
 			continue
 		}
-		var served uint64
-		for _, peer := range tt.peers {
-			served += atomic.LoadUint64(&peer.served)
-		}
-		if served != tt.midserve {
-			t.Errorf("test %d, mid state: served headers mismatch: have %d, want %d", i, served, tt.midserve)
-		}
-		var drops uint64
-		for _, peer := range tt.peers {
-			drops += atomic.LoadUint64(&peer.dropped)
-		}
-		if drops != tt.middrop {
-			t.Errorf("test %d, mid state: dropped peers mismatch: have %d, want %d", i, drops, tt.middrop)
+		if !tt.unpredictable {
+			var served uint64
+			for _, peer := range tt.peers {
+				served += atomic.LoadUint64(&peer.served)
+			}
+			if served != tt.midserve {
+				t.Errorf("test %d, mid state: served headers mismatch: have %d, want %d", i, served, tt.midserve)
+			}
+			var drops uint64
+			for _, peer := range tt.peers {
+				drops += atomic.LoadUint64(&peer.dropped)
+			}
+			if drops != tt.middrop {
+				t.Errorf("test %d, mid state: dropped peers mismatch: have %d, want %d", i, drops, tt.middrop)
+			}
 		}
 		// Apply the post-init events if there's any
 		if tt.newHead != nil {
@@ -868,25 +947,27 @@ func TestSkeletonSyncRetrievals(t *testing.T) {
 			continue
 		}
 		// Check that the peers served no more headers than we actually needed
-		served = 0
-		for _, peer := range tt.peers {
-			served += atomic.LoadUint64(&peer.served)
-		}
-		if tt.newPeer != nil {
-			served += atomic.LoadUint64(&tt.newPeer.served)
-		}
-		if served != tt.endserve {
-			t.Errorf("test %d, end state: served headers mismatch: have %d, want %d", i, served, tt.endserve)
-		}
-		drops = 0
-		for _, peer := range tt.peers {
-			drops += atomic.LoadUint64(&peer.dropped)
-		}
-		if tt.newPeer != nil {
-			drops += atomic.LoadUint64(&tt.newPeer.dropped)
-		}
-		if drops != tt.middrop {
-			t.Errorf("test %d, end state: dropped peers mismatch: have %d, want %d", i, drops, tt.middrop)
+		if !tt.unpredictable {
+			served := uint64(0)
+			for _, peer := range tt.peers {
+				served += atomic.LoadUint64(&peer.served)
+			}
+			if tt.newPeer != nil {
+				served += atomic.LoadUint64(&tt.newPeer.served)
+			}
+			if served != tt.endserve {
+				t.Errorf("test %d, end state: served headers mismatch: have %d, want %d", i, served, tt.endserve)
+			}
+			drops := uint64(0)
+			for _, peer := range tt.peers {
+				drops += atomic.LoadUint64(&peer.dropped)
+			}
+			if tt.newPeer != nil {
+				drops += atomic.LoadUint64(&tt.newPeer.dropped)
+			}
+			if drops != tt.enddrop {
+				t.Errorf("test %d, end state: dropped peers mismatch: have %d, want %d", i, drops, tt.middrop)
+			}
 		}
 		// Clean up any leftover skeleton sync resources
 		skeleton.Terminate()