row: check for uninitialized KVFetcher

Previously, when GetRangesInfo() and GetBytesRead() was called on
row.Fetcher before the underlying KVFetcher is initialized, it
would panic due to NPE. Now this is fixed. The problem was introduced
by the recent refactor of moving CFetcher from sql/row into sql/colexec.

Release justification: Release blocker.

Release note: None

pkg/sql/row/fetcher.go

@@ -1423,7 +1423,7 @@ func (rf *Fetcher) PartialKey(nCols int) (roachpb.Key, error) {
 // GetRangesInfo returns information about the ranges where the rows came from.
 // The RangeInfo's are deduped and not ordered.
 func (rf *Fetcher) GetRangesInfo() []roachpb.RangeInfo {
-	f := rf.kvFetcher.kvBatchFetcher
+	f := rf.kvFetcher
 	if f == nil {
 		// Not yet initialized.
 		return nil
@@ -1433,7 +1433,12 @@ func (rf *Fetcher) GetRangesInfo() []roachpb.RangeInfo {
 
 // GetBytesRead returns total number of bytes read by the underlying KVFetcher.
 func (rf *Fetcher) GetBytesRead() int64 {
-	return rf.kvFetcher.bytesRead
+	f := rf.kvFetcher
+	if f == nil {
+		// Not yet initialized.
+		return 0
+	}
+	return f.bytesRead
 }
 
 // Only unique secondary indexes have extra columns to decode (namely the

pkg/sql/row/fetcher_test.go

@@ -27,6 +27,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/util"
 	"github.com/cockroachdb/cockroach/pkg/util/encoding"
 	"github.com/cockroachdb/cockroach/pkg/util/leaktest"
+	"github.com/stretchr/testify/assert"
 )
 
 type initFetcherArgs struct {
@@ -1052,3 +1053,12 @@ func TestRowFetcherReset(t *testing.T) {
 func idLookupKey(tableID TableID, indexID sqlbase.IndexID) uint64 {
 	return (uint64(tableID) << 32) | uint64(indexID)
 }
+
+func TestFetcherUninitialized(t *testing.T) {
+	// Regression test for #39013: make sure it's okay to call GetRangesInfo and
+	// GetBytesReader even before the fetcher was fully initialized.
+	var fetcher Fetcher
+
+	assert.Nil(t, fetcher.GetRangesInfo())
+	assert.Zero(t, fetcher.GetBytesRead())
+}