vtgate/engine: Fix race condition in join logic (#14435)

Signed-off-by: Dirkjan Bussink <[email protected]>

go/vt/vtgate/engine/join.go

@@ -20,6 +20,7 @@ import (
 	"context"
 	"fmt"
 	"strings"
+	"sync"
 	"sync/atomic"
 
 	"vitess.io/vitess/go/sqltypes"
@@ -115,22 +116,31 @@ func bindvarForType(t querypb.Type) *querypb.BindVariable {
 
 // TryStreamExecute performs a streaming exec.
 func (jn *Join) TryStreamExecute(ctx context.Context, vcursor VCursor, bindVars map[string]*querypb.BindVariable, wantfields bool, callback func(*sqltypes.Result) error) error {
-	var fieldNeeded atomic.Bool
-	fieldNeeded.Store(wantfields)
-	err := vcursor.StreamExecutePrimitive(ctx, jn.Left, bindVars, fieldNeeded.Load(), func(lresult *sqltypes.Result) error {
+	var mu sync.Mutex
+	// We need to use this atomic since we're also reading this
+	// value outside of it being locked with the mu lock.
+	// This is still racy, but worst case it means that we may
+	// retrieve the right hand side fields twice instead of once.
+	var fieldsSent atomic.Bool
+	fieldsSent.Store(!wantfields)
+	err := vcursor.StreamExecutePrimitive(ctx, jn.Left, bindVars, wantfields, func(lresult *sqltypes.Result) error {
 		joinVars := make(map[string]*querypb.BindVariable)
 		for _, lrow := range lresult.Rows {
 			for k, col := range jn.Vars {
 				joinVars[k] = sqltypes.ValueBindVariable(lrow[col])
 			}
 			var rowSent atomic.Bool
-			err := vcursor.StreamExecutePrimitive(ctx, jn.Right, combineVars(bindVars, joinVars), fieldNeeded.Load(), func(rresult *sqltypes.Result) error {
+			err := vcursor.StreamExecutePrimitive(ctx, jn.Right, combineVars(bindVars, joinVars), !fieldsSent.Load(), func(rresult *sqltypes.Result) error {
+				// This needs to be locking since it's not safe to just use
+				// fieldsSent. This is because we can't have a race between
+				// checking fieldsSent and then actually calling the callback
+				// and in parallel another goroutine doing the same. That
+				// can lead to out of order execution of the callback. So the callback
+				// itself and the check need to be covered by the same lock.
+				mu.Lock()
+				defer mu.Unlock()
 				result := &sqltypes.Result{}
-				if fieldNeeded.Load() {
-					// This code is currently unreachable because the first result
-					// will always be just the field info, which will cause the outer
-					// wantfields code path to be executed. But this may change in the future.
-					fieldNeeded.Store(false)
+				if fieldsSent.CompareAndSwap(false, true) {
 					result.Fields = joinFields(lresult.Fields, rresult.Fields, jn.Cols)
 				}
 				for _, rrow := range rresult.Rows {
@@ -154,8 +164,15 @@ func (jn *Join) TryStreamExecute(ctx context.Context, vcursor VCursor, bindVars
 				return callback(result)
 			}
 		}
-		if fieldNeeded.Load() {
-			fieldNeeded.Store(false)
+		// This needs to be locking since it's not safe to just use
+		// fieldsSent. This is because we can't have a race between
+		// checking fieldsSent and then actually calling the callback
+		// and in parallel another goroutine doing the same. That
+		// can lead to out of order execution of the callback. So the callback
+		// itself and the check need to be covered by the same lock.
+		mu.Lock()
+		defer mu.Unlock()
+		if fieldsSent.CompareAndSwap(false, true) {
 			for k := range jn.Vars {
 				joinVars[k] = sqltypes.NullBindVariable
 			}

go/vt/vtgate/engine/scalar_aggregation.go

@@ -112,7 +112,7 @@ func (sa *ScalarAggregate) TryStreamExecute(ctx context.Context, vcursor VCursor
 	var mu sync.Mutex
 	var agg aggregationState
 	var fields []*querypb.Field
-	var fieldsSent bool
+	fieldsSent := !wantfields
 
 	err := vcursor.StreamExecutePrimitive(ctx, sa.Input, bindVars, wantfields, func(result *sqltypes.Result) error {
 		// as the underlying primitive call is not sync
@@ -121,7 +121,7 @@ func (sa *ScalarAggregate) TryStreamExecute(ctx context.Context, vcursor VCursor
 		mu.Lock()
 		defer mu.Unlock()
 
-		if agg == nil {
+		if agg == nil && len(result.Fields) != 0 {
 			var err error
 			agg, fields, err = newAggregation(result.Fields, sa.Aggregates)
 			if err != nil {