sstable: fix nil pointer in twoLevelIterator.NextPrefix

sstable/reader_iter_two_lvl.go

@@ -794,7 +794,6 @@ func (i *twoLevelIterator[D, PD]) NextPrefix(succKey []byte) *base.InternalKV {
 
 	// Did not find prefix in the existing second-level index block. This is the
 	// slow-path where we seek the iterator.
-	var ikv *base.InternalKV
 	if !i.topLevelIndex.SeekGE(succKey) {
 		PD(&i.secondLevel.data).Invalidate()
 		i.secondLevel.index.Invalidate()
@@ -812,7 +811,7 @@ func (i *twoLevelIterator[D, PD]) NextPrefix(succKey []byte) *base.InternalKV {
 		// span multiple index blocks. If upper is exclusive we use >= below,
 		// else we use >.
 		if i.secondLevel.upper != nil {
-			cmp := i.secondLevel.cmp(ikv.K.UserKey, i.secondLevel.upper)
+			cmp := i.secondLevel.cmp(i.topLevelIndex.Separator(), i.secondLevel.upper)
 			if (!i.secondLevel.endKeyInclusive && cmp >= 0) || cmp > 0 {
 				i.secondLevel.exhaustedBounds = +1
 			}

sstable/testdata/block_properties

@@ -696,3 +696,50 @@ filter point-filter=(suffix-point-keys-only,10,20)
 ----
 points: false
 ranges: true (no filters provided)
+
+build collectors=(suffix-point-keys-only) index-block-size=48 block-size=1
+a@10.SET.1:foo
+a@9.SET.2:bar
+a@8.SET.2:baz
+a@7.SET.2:bax
+aa@9.SET.3:mop
+aa@8.SET.3:box
+f@5.SET.3:mop
+----
+point:    [a@10#1,SET-f@5#3,SET]
+seqnums:  [1-3]
+
+block-props
+----
+a@9#2,SET:
+  0: [9, 11)
+  a@10#1,SET:
+    0: [10, 11)
+  a@9#2,SET:
+    0: [9, 10)
+a@7#2,SET:
+  0: [7, 9)
+  a@8#2,SET:
+    0: [8, 9)
+  a@7#2,SET:
+    0: [7, 8)
+b#inf,SEPARATOR:
+  0: [8, 10)
+  aa@9#3,SET:
+    0: [9, 10)
+  b#inf,SEPARATOR:
+    0: [8, 9)
+g#inf,SEPARATOR:
+  0: [5, 6)
+  g#inf,SEPARATOR:
+    0: [5, 6)
+
+# Regression test for a nil-pointer when we find a second-level index block
+# irrelevant and enforce an upper bound.
+
+iter point-key-filter=(suffix-point-keys-only,5,8) upper=c
+seek-ge a
+next-prefix
+----
+<a@7:2>
+.