eventstore: fuzz testing found us a bug!

eventstore/boltdb/helpers.go

@@ -113,14 +113,12 @@ type iterators []*iterator
 // i.e. [1, 700, 25, 312, 44, 28] with k=3 becomes something like [700, 312, 44, 1, 25, 28]
 // in this case it's hardcoded to use the 'last' field of the iterator
 // copied from https://github.com/chrislee87/go-quickselect
-// this is modified to also return the highest 'last' (because it's not guaranteed it will be the first item)
-func (its iterators) quickselect(k int) uint32 {
+func (its iterators) quickselect(k int) {
 	if len(its) == 0 || k >= len(its) {
-		return 0
+		return
 	}
 
 	left, right := 0, len(its)-1
-
 	for {
 		// insertion sort for small ranges
 		if right-left <= 20 {
@@ -129,7 +127,7 @@ func (its iterators) quickselect(k int) uint32 {
 					its[j], its[j-1] = its[j-1], its[j]
 				}
 			}
-			return its[0].last
+			return
 		}
 
 		// median-of-three to choose pivot
@@ -165,14 +163,7 @@ func (its iterators) quickselect(k int) uint32 {
 		pivotIndex = rr
 
 		if k == pivotIndex {
-			// now that stuff is selected we get the highest "last"
-			highest := its[0].last
-			for i := 1; i < k; i++ {
-				if its[i].last > highest {
-					highest = its[i].last
-				}
-			}
-			return highest
+			return
 		}
 
 		if k < pivotIndex {
@@ -183,6 +174,17 @@ func (its iterators) quickselect(k int) uint32 {
 	}
 }
 
+// return the highest 'last' value among the first k items in its
+func (its iterators) threshold(k int) uint32 {
+	highest := its[0].last
+	for i := 1; i < k; i++ {
+		if its[i].last > highest {
+			highest = its[i].last
+		}
+	}
+	return highest
+}
+
 type key struct {
 	bucket  []byte
 	fullkey []byte

eventstore/boltdb/query.go

@@ -110,7 +110,9 @@ func (b *BoltBackend) query(txn *bbolt.Tx, filter nostr.Filter, limit int, yield
 
 		// after pulling from all iterators once we now find out what iterators are
 		// the ones we should keep pulling from next (i.e. which one's last emitted timestamp is the highest)
-		threshold := iterators.quickselect(min(numberOfIteratorsToPullOnEachRound, len(iterators)))
+		k := min(numberOfIteratorsToPullOnEachRound, len(iterators))
+		iterators.quickselect(k)
+		threshold := iterators.threshold(k)
 
 		// so we can emit all the events higher than the threshold
 		for i := range iterators {

eventstore/boltdb/testdata/fuzz/FuzzQuery/bca5f7631b59619d

@@ -0,0 +1,8 @@
+go test fuzz v1
+uint(200)
+uint(15)
+uint(11)
+uint(49)
+uint(2)
+uint(91)
+uint(1)

eventstore/boltdb/utils_test.go

@@ -1,67 +0,0 @@
-package boltdb
-
-import (
-	"testing"
-
-	"github.com/stretchr/testify/require"
-)
-
-func TestQuickselect(t *testing.T) {
-	{
-		its := iterators{
-			{last: 781},
-			{last: 900},
-			{last: 1},
-			{last: 81},
-			{last: 325},
-			{last: 781},
-			{last: 562},
-			{last: 81},
-			{last: 444},
-		}
-
-		its.quickselect(3)
-		require.ElementsMatch(t,
-			[]uint32{its[0].last, its[1].last, its[2].last},
-			[]uint32{900, 781, 781},
-		)
-	}
-
-	{
-		its := iterators{
-			{last: 781},
-			{last: 781},
-			{last: 900},
-			{last: 1},
-			{last: 87},
-			{last: 315},
-			{last: 789},
-			{last: 500},
-			{last: 812},
-			{last: 306},
-			{last: 612},
-			{last: 444},
-			{last: 59},
-			{last: 441},
-			{last: 901},
-			{last: 901},
-			{last: 2},
-			{last: 81},
-			{last: 325},
-			{last: 781},
-			{last: 562},
-			{last: 81},
-			{last: 326},
-			{last: 662},
-			{last: 444},
-			{last: 81},
-			{last: 444},
-		}
-
-		its.quickselect(6)
-		require.ElementsMatch(t,
-			[]uint32{its[0].last, its[1].last, its[2].last, its[3].last, its[4].last, its[5].last},
-			[]uint32{901, 900, 901, 781, 812, 789},
-		)
-	}
-}