Fix scan range write skew

txn.go

@@ -62,7 +62,9 @@ type oracle struct {
 type committedTxn struct {
 	ts uint64
 	// ConflictKeys Keeps track of the entries written at timestamp ts.
-	conflictKeys map[uint64]struct{}
+	// The first one is the hash of the write key. The second one is the
+	// the len of the reads when insert this write
+	conflictKeys map[uint64]int
 }
 
 func newOracle(opt Options) *oracle {
@@ -151,11 +153,33 @@ func (o *oracle) hasConflict(txn *Txn) bool {
 			continue
 		}
 
+		// check if there is any direct conflict
 		for _, ro := range txn.reads {
 			if _, has := committedTxn.conflictKeys[ro]; has {
 				return true
 			}
 		}
+
+		// check if there is any indirect conflict (logical conflict)
+		for _, i := range txn.conflictKeys {
+			if i == -1 {
+				continue
+			}
+
+			happensBeforeReads := txn.reads[:i]
+
+			for _, j := range committedTxn.conflictKeys {
+				otherHappensBeforeReads := txn.reads[:j]
+
+				for fp := range happensBeforeReads {
+					for otherFp := range otherHappensBeforeReads {
+						if fp == otherFp {
+							return true
+						}
+					}
+				}
+			}
+		}
 	}
 
 	return false
@@ -256,7 +280,8 @@ type Txn struct {
 
 	reads []uint64 // contains fingerprints of keys read.
 	// contains fingerprints of keys written. This is used for conflict detection.
-	conflictKeys map[uint64]struct{}
+	// The value int is the len of the reads when insert this write
+	conflictKeys map[uint64]int
 	readsLock    sync.Mutex // guards the reads slice. See addReadKey.
 
 	pendingWrites   map[string]*Entry // cache stores any writes done by txn.
@@ -393,8 +418,8 @@ func (txn *Txn) modify(e *Entry) error {
 	// The txn.conflictKeys is used for conflict detection. If conflict detection
 	// is disabled, we don't need to store key hashes in this map.
 	if txn.db.opt.DetectConflicts {
-		fp := z.MemHash(e.Key) // Avoid dealing with byte arrays.
-		txn.conflictKeys[fp] = struct{}{}
+		fp := z.MemHash(e.Key)                    // Avoid dealing with byte arrays.
+		txn.conflictKeys[fp] = len(txn.reads) - 1 // store this write happens after how many reads
 	}
 	// If a duplicate entry was inserted in managed mode, move it to the duplicate writes slice.
 	// Add the entry to duplicateWrites only if both the entries have different versions. For
@@ -783,7 +808,7 @@ func (db *DB) newTransaction(update, isManaged bool) *Txn {
 	}
 	if update {
 		if db.opt.DetectConflicts {
-			txn.conflictKeys = make(map[uint64]struct{})
+			txn.conflictKeys = make(map[uint64]int)
 		}
 		txn.pendingWrites = make(map[string]*Entry)
 	}

txn_test.go

@@ -328,6 +328,64 @@ func TestTxnWriteSkew(t *testing.T) {
 	})
 }
 
+// https://wiki.postgresql.org/wiki/SSI#Intersecting_Data
+func TestTxnWriteSkew2(t *testing.T) {
+	runBadgerTest(t, nil, func(t *testing.T, db *DB) {
+		// Setup
+		db.opt.managedTxns = false
+		txn := db.NewTransaction(true)
+		defer txn.Discard()
+		txn.SetEntry(NewEntry([]byte("a1"), []byte("10")))
+		txn.SetEntry(NewEntry([]byte("a2"), []byte("20")))
+		txn.SetEntry(NewEntry([]byte("b1"), []byte("100")))
+		txn.SetEntry(NewEntry([]byte("b2"), []byte("200")))
+		require.NoError(t, txn.Commit())
+
+		txn1 := db.NewTransaction(true)
+		defer txn1.Discard()
+
+		itr := txn1.NewIterator(DefaultIteratorOptions)
+		sum := 0
+		{
+			for itr.Rewind(); itr.Valid(); itr.Next() {
+				if itr.Item().Key()[0] == 'a' {
+					a, _ := itr.Item().ValueCopy(nil)
+					val, _ := strconv.ParseUint(string(a), 10, 64)
+					sum += int(val)
+				}
+			}
+			itr.Close()
+		}
+
+		txn1.SetEntry(NewEntry([]byte("b3"), []byte("30")))
+
+		txn2 := db.NewTransaction(true)
+		defer txn2.Discard()
+		{
+			itr = txn2.NewIterator(DefaultIteratorOptions)
+			sum = 0
+			for itr.Rewind(); itr.Valid(); itr.Next() {
+				if itr.Item().Key()[0] == 'b' {
+					a, _ := itr.Item().ValueCopy(nil)
+					val, _ := strconv.ParseUint(string(a), 10, 64)
+					sum += int(val)
+				}
+			}
+			itr.Close()
+		}
+
+		txn2.SetEntry(NewEntry([]byte("a3"), []byte("300")))
+		require.NoError(t, txn2.Commit())
+
+		// Each transaction has modified what the other transaction would have read.
+		// If both were allowed to commit, this would break serializable behavior,
+		// because if they were run one at a time, one of the transactions would have seen the INSERT the other committed.
+		// We wait for a successful COMMIT of one of the transactions before we roll anything back,
+		// though, to ensure progress and prevent thrashing.
+		require.Error(t, txn1.Commit())
+	})
+}
+
 // a3, a2, b4 (del), b3, c2, c1
 // Read at ts=4 -> a3, c2
 // Read at ts=4(Uncommitted) -> a3, b4