fix edge case for watermark when index is zero

Fixes #1962

We have assumed that index won't be zero for a WaterMark but in
badger's unmanaged mode we start transactions with readTs = 0.
This affects oracle.readMark that could have values starting at 0.

watermark_edge_test.go

@@ -0,0 +1,119 @@
+/*
+ * Copyright 2023 Dgraph Labs, Inc. and Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package badger
+
+import (
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"math/big"
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestWaterMarkEdgeCase(t *testing.T) {
+	const N = 1_000
+	runBadgerTest(t, nil, func(t *testing.T, db *DB) {
+		eg := make(chan error, N)
+		defer close(eg)
+
+		var wg sync.WaitGroup
+		wg.Add(N)
+		for i := 0; i < N; i++ {
+			go func(j int) {
+				defer wg.Done()
+				if err := doWork(db, j); errors.Is(err, ErrConflict) {
+					eg <- nil
+				} else {
+					eg <- fmt.Errorf("expected conflict not found, err: %v, i = %v", err, j)
+				}
+			}(i)
+		}
+		wg.Wait()
+
+		for i := 0; i < N; i++ {
+			if err := <-eg; err != nil {
+				t.Fatal(err)
+			}
+		}
+	})
+}
+
+func doWork(db *DB, i int) error {
+	delay()
+
+	key1 := fmt.Sprintf("v:%d:%s", i, generateRandomBytes())
+	key2 := fmt.Sprintf("v:%d:%s", i, generateRandomBytes())
+
+	tx1 := db.NewTransaction(true)
+	defer tx1.Discard()
+	tx2 := db.NewTransaction(true)
+	defer tx2.Discard()
+
+	getValue(tx2, key1)
+	getValue(tx2, key2)
+	getValue(tx1, key1)
+	getValue(tx2, key1)
+	setValue(tx2, key1, "value1")
+	setValue(tx2, key2, "value2")
+
+	if err := tx2.Commit(); err != nil {
+		return fmt.Errorf("tx2 failed: %w (key1 = %s, key2 = %s)", err, key1, key2)
+	}
+
+	setValue(tx1, key1, "value1-second")
+	getValue(tx1, key1)
+	setValue(tx1, key1, "value1-third")
+
+	delay()
+	if err := tx1.Commit(); err != nil {
+		return fmt.Errorf("tx1 failed: %w (key1 = %s, key2 = %s)", err, key1, key2)
+	}
+
+	return nil
+}
+
+func generateRandomBytes() []byte {
+	b := make([]byte, 20)
+	if _, err := rand.Read(b); err != nil {
+		panic(err)
+	}
+	return b
+}
+
+func getValue(txn *Txn, key string) {
+	if _, err := txn.Get([]byte(key)); err != nil {
+		if !errors.Is(err, ErrKeyNotFound) {
+			panic(err)
+		}
+	}
+}
+
+func setValue(txn *Txn, key, value string) {
+	if err := txn.Set([]byte(key), []byte(value)); err != nil {
+		panic(err)
+	}
+}
+
+func delay() {
+	jitter, err := rand.Int(rand.Reader, big.NewInt(100))
+	if err != nil {
+		panic(err)
+	}
+	<-time.After(time.Duration(jitter.Int64()) * time.Millisecond)
+}

y/watermark.go

@@ -228,7 +228,8 @@ func (w *WaterMark) process(closer *z.Closer) {
 					}
 				}
 			} else {
-				if mark.index > 0 {
+				// it is possible that mark.index is zero. We need to handle that case as well.
+				if mark.index > 0 || (mark.index == 0 && len(mark.indices) == 0) {
 					processOne(mark.index, mark.done)
 				}
 				for _, index := range mark.indices {