fix: assert errors when relative positioning functions are called

For functions that require positioning, assert errors instead of attempting
to load index again. The caller must comply with the contract.

Author: xxmplus

sstable/reader_iter_single_lvl.go

@@ -439,9 +439,8 @@ func (i *singleLevelIterator[I, PI, P, PD]) SetContext(ctx context.Context) {
 // unpositioned. If unsuccessful, it sets i.err to any error encountered, which
 // may be nil if we have simply exhausted the entire table.
 func (i *singleLevelIterator[I, PI, P, PD]) loadDataBlock(dir int8) loadBlockResult {
-	if !i.ensureIndexLoaded() {
-		// Ensure the data block iterator is invalidated
-		PD(&i.data).Invalidate()
+	if !i.indexLoaded {
+		i.err = errors.AssertionFailedf("index block is not loaded")
 		return loadBlockFailed
 	}
 	if !PI(&i.index).Valid() {
@@ -608,7 +607,7 @@ func (i *singleLevelIterator[I, PI, D, PD]) trySeekLTUsingPrevWithinBlock(
 	key []byte,
 ) (kv *base.InternalKV, done bool) {
 	kv = PD(&i.data).KV()
-	for j := 0; j < numStepsBeforeSeek; j++ {
+	for range numStepsBeforeSeek {
 		curKeyCmp := i.cmp(kv.K.UserKey, key)
 		if curKeyCmp < 0 {
 			if i.blockLower != nil && i.cmp(kv.K.UserKey, i.blockLower) < 0 {
@@ -1284,7 +1283,8 @@ func (i *singleLevelIterator[I, PI, D, PD]) NextPrefix(succKey []byte) *base.Int
 	// Did not find prefix in the existing data block. This is the slow-path
 	// where we effectively seek the iterator.
 	// The key is likely to be in the next data block, so try one step.
-	if !i.ensureIndexLoaded() {
+	if !i.indexLoaded {
+		i.err = errors.AssertionFailedf("index block is not loaded")
 		return nil
 	}
 	if !PI(&i.index).Next() {
@@ -1358,7 +1358,8 @@ func (i *singleLevelIterator[I, PI, D, PD]) Prev() *base.InternalKV {
 
 func (i *singleLevelIterator[I, PI, D, PD]) skipForward() *base.InternalKV {
 	for {
-		if !i.ensureIndexLoaded() {
+		if !i.indexLoaded {
+			i.err = errors.AssertionFailedf("index block is not loaded")
 			return nil
 		}
 		if !PI(&i.index).Next() {
@@ -1439,7 +1440,8 @@ func (i *singleLevelIterator[I, PI, D, PD]) skipForward() *base.InternalKV {
 
 func (i *singleLevelIterator[I, PI, D, PD]) skipBackward() *base.InternalKV {
 	for {
-		if !i.ensureIndexLoaded() {
+		if !i.indexLoaded {
+			i.err = errors.AssertionFailedf("index block is not loaded")
 			return nil
 		}
 		if !PI(&i.index).Prev() {

sstable/reader_iter_test.go

@@ -444,9 +444,12 @@ func TestLazyLoadingStressOperations(t *testing.T) {
 	require.NoError(t, err)
 	defer iter.Close()
 
+	// Ensure iterator is positioned before operations
+	iter.First()
+
 	// Perform many repeated operations
 	const numOperations = 5000
-	for i := 0; i < numOperations; i++ {
+	for i := range numOperations {
 		switch i % 5 {
 		case 0:
 			iter.First()
@@ -523,7 +526,10 @@ func performConcurrentOps(reader *Reader, keys [][]byte, numOps int, goroutineID
 	}
 	defer iter.Close()
 
-	for i := 0; i < numOps; i++ {
+	// Ensure iterator is positioned before any operations
+	iter.First()
+
+	for i := range numOps {
 		// Vary operations based on goroutine ID and iteration
 		switch (goroutineID + i) % 4 {
 		case 0:

sstable/reader_iter_two_lvl.go

@@ -854,7 +854,8 @@ func (i *twoLevelIterator[I, PI, D, PD]) NextPrefix(succKey []byte) *base.Intern
 	// Did not find prefix in the existing second-level index block. This is the
 	// slow-path where we seek the iterator.
 
-	if !i.ensureTopLevelIndexLoaded() {
+	if !i.topLevelIndexLoaded {
+		i.secondLevel.err = errors.AssertionFailedf("top level index is not loaded")
 		return nil
 	}
 
@@ -905,7 +906,8 @@ func (i *twoLevelIterator[I, PI, D, PD]) skipForward() *base.InternalKV {
 			return nil
 		}
 
-		if !i.ensureTopLevelIndexLoaded() {
+		if !i.topLevelIndexLoaded {
+			i.secondLevel.err = errors.AssertionFailedf("top level index is not loaded")
 			return nil
 		}
 
@@ -987,7 +989,8 @@ func (i *twoLevelIterator[I, PI, D, PD]) skipBackward() *base.InternalKV {
 			return nil
 		}
 
-		if !i.ensureTopLevelIndexLoaded() {
+		if !i.topLevelIndexLoaded {
+			i.secondLevel.err = errors.AssertionFailedf("top level index is not loaded")
 			return nil
 		}
 

sstable/reader_test.go

@@ -1139,6 +1139,8 @@ func (rw *readerWorkload) handleInvalid(callType readCallType, iter Iterator) *b
 	switch callType {
 	case SeekGE, Next, Last:
 		if len(rw.seekKeyAfterInvalid) == 0 {
+			// Ensure iterator is positioned before calling Prev()
+			iter.Last()
 			return iter.Prev()
 		}
 		return iter.SeekLT(rw.seekKeyAfterInvalid, base.SeekLTFlagsNone)
@@ -1149,6 +1151,8 @@ func (rw *readerWorkload) handleInvalid(callType readCallType, iter Iterator) *b
 		return iter.SeekLT(rw.seekKeyAfterInvalid, base.SeekLTFlagsNone)
 	case SeekLT, Prev, First:
 		if len(rw.seekKeyAfterInvalid) == 0 {
+			// Ensure iterator is positioned before calling Next()
+			iter.First()
 			return iter.Next()
 		}
 		return iter.SeekGE(rw.seekKeyAfterInvalid, base.SeekGEFlagsNone)
@@ -1386,6 +1390,10 @@ func TestRandomizedPrefixSuffixRewriter(t *testing.T) {
 			iter, cleanup := createIter(fileName, syntheticSuffix, syntheticPrefix)
 			defer cleanup()
 
+			// Position both iterators to ensure index blocks are loaded
+			eIter.First()
+			iter.First()
+
 			w := createReadWorkload(t, rng, callCount, ks, maxTs+2, syntheticPrefix)
 			workloadChecker := checker{
 				t: t,