kv/kvserver/spanset: fix read of invalid iterator's UnsafeKey

Previously, the spanset.MVCCIterator implementation would retrieve the
contained iterator's UsafeKey() on an invalid iterator in Next, NextKey and
Prev. This retrieval was harmless because it always checked the validity of the
iterator before using it, but recent assertions in test builds prevent this
usage.

Release note: None

pkg/kv/kvserver/spanset/batch.go

@@ -102,21 +102,37 @@ func (i *MVCCIterator) SeekLT(key storage.MVCCKey) {
 // Next is part of the storage.MVCCIterator interface.
 func (i *MVCCIterator) Next() {
 	i.i.Next()
-	i.checkAllowed(roachpb.Span{Key: i.UnsafeKey().Key}, false)
+	i.checkAllowedCurrPosForward(false)
 }
 
 // Prev is part of the storage.MVCCIterator interface.
 func (i *MVCCIterator) Prev() {
 	i.i.Prev()
-	i.checkAllowed(roachpb.Span{Key: i.UnsafeKey().Key}, false)
+	i.checkAllowedCurrPosForward(false)
 }
 
 // NextKey is part of the storage.MVCCIterator interface.
 func (i *MVCCIterator) NextKey() {
 	i.i.NextKey()
-	i.checkAllowed(roachpb.Span{Key: i.UnsafeKey().Key}, false)
+	i.checkAllowedCurrPosForward(false)
 }
 
+// checkAllowedCurrPosForward checks the span starting at the current iterator
+// position, if the current iterator position is valid.
+func (i *MVCCIterator) checkAllowedCurrPosForward(errIfDisallowed bool) {
+	i.invalid = false
+	i.err = nil
+	if ok, _ := i.i.Valid(); !ok {
+		// If the iterator is invalid after the operation, there's nothing to
+		// check. We allow uses of iterators to exceed the declared span bounds
+		// as long as the iterator itself is configured with proper boundaries.
+		return
+	}
+	i.checkAllowedValidPos(roachpb.Span{Key: i.UnsafeKey().Key}, errIfDisallowed)
+}
+
+// checkAllowedCurrPosForward checks the provided span if the current iterator
+// position is valid.
 func (i *MVCCIterator) checkAllowed(span roachpb.Span, errIfDisallowed bool) {
 	i.invalid = false
 	i.err = nil
@@ -126,6 +142,10 @@ func (i *MVCCIterator) checkAllowed(span roachpb.Span, errIfDisallowed bool) {
 		// as long as the iterator itself is configured with proper boundaries.
 		return
 	}
+	i.checkAllowedValidPos(span, errIfDisallowed)
+}
+
+func (i *MVCCIterator) checkAllowedValidPos(span roachpb.Span, errIfDisallowed bool) {
 	var err error
 	if i.spansOnly {
 		err = i.spans.CheckAllowed(SpanReadOnly, span)