projection: Return correct collation information

When we have a project, we correctly convert the value to the connection
collation using:

```go
c.Value(vcursor.ConnCollation())
```

But we then don't update / change the collation on the fields
information. This needs to match the actual data, otherwise the client
might corrupt the data.

This ensures we add the same rules for the resulting collation.

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

go/vt/vtgate/engine/projection.go

@@ -21,6 +21,7 @@ import (
 	"sync"
 
 	"vitess.io/vitess/go/mysql"
+	"vitess.io/vitess/go/mysql/collations"
 	"vitess.io/vitess/go/sqltypes"
 	querypb "vitess.io/vitess/go/vt/proto/query"
 	"vitess.io/vitess/go/vt/sqlparser"
@@ -75,7 +76,7 @@ func (p *Projection) TryExecute(ctx context.Context, vcursor VCursor, bindVars m
 		resultRows = append(resultRows, resultRow)
 	}
 	if wantfields {
-		result.Fields, err = p.evalFields(env, result.Fields)
+		result.Fields, err = p.evalFields(env, result.Fields, vcursor.ConnCollation())
 		if err != nil {
 			return nil, err
 		}
@@ -96,7 +97,7 @@ func (p *Projection) TryStreamExecute(ctx context.Context, vcursor VCursor, bind
 		defer mu.Unlock()
 		if wantfields {
 			once.Do(func() {
-				fields, err = p.evalFields(env, qr.Fields)
+				fields, err = p.evalFields(env, qr.Fields, vcursor.ConnCollation())
 				if err != nil {
 					return
 				}
@@ -135,14 +136,14 @@ func (p *Projection) GetFields(ctx context.Context, vcursor VCursor, bindVars ma
 		return nil, err
 	}
 	env := evalengine.NewExpressionEnv(ctx, bindVars, vcursor)
-	qr.Fields, err = p.evalFields(env, qr.Fields)
+	qr.Fields, err = p.evalFields(env, qr.Fields, vcursor.ConnCollation())
 	if err != nil {
 		return nil, err
 	}
 	return qr, nil
 }
 
-func (p *Projection) evalFields(env *evalengine.ExpressionEnv, infields []*querypb.Field) ([]*querypb.Field, error) {
+func (p *Projection) evalFields(env *evalengine.ExpressionEnv, infields []*querypb.Field, coll collations.ID) ([]*querypb.Field, error) {
 	// TODO: once the evalengine becomes smart enough, we should be able to remove the
 	// dependency on these fields altogether
 	env.Fields = infields
@@ -157,10 +158,15 @@ func (p *Projection) evalFields(env *evalengine.ExpressionEnv, infields []*query
 		if !sqltypes.IsNull(typ.Type()) && !typ.Nullable() {
 			fl |= uint32(querypb.MySqlFlag_NOT_NULL_FLAG)
 		}
+		typCol := typ.Collation()
+		if sqltypes.IsTextOrBinary(typ.Type()) && typCol != collations.CollationBinaryID {
+			typCol = coll
+		}
+
 		fields = append(fields, &querypb.Field{
 			Name:         col,
 			Type:         typ.Type(),
-			Charset:      uint32(typ.Collation()),
+			Charset:      uint32(typCol),
 			ColumnLength: uint32(typ.Size()),
 			Decimals:     uint32(typ.Scale()),
 			Flags:        fl,

go/vt/vtgate/engine/projection_test.go

@@ -228,3 +228,41 @@ func TestFields(t *testing.T) {
 		})
 	}
 }
+
+func TestFieldConversion(t *testing.T) {
+	var testCases = []struct {
+		name      string
+		expr      string
+		typ       querypb.Type
+		collation collations.ID
+	}{
+		{
+			name:      `convert different charset`,
+			expr:      `_latin1 0xFF`,
+			typ:       sqltypes.VarChar,
+			collation: collations.MySQL8().DefaultConnectionCharset(),
+		},
+	}
+
+	for _, testCase := range testCases {
+		t.Run(testCase.name, func(t *testing.T) {
+			arg, err := sqlparser.NewTestParser().ParseExpr(testCase.expr)
+			require.NoError(t, err)
+			bindExpr, err := evalengine.Translate(arg, &evalengine.Config{
+				Environment: vtenv.NewTestEnv(),
+				Collation:   collations.MySQL8().DefaultConnectionCharset(),
+			})
+			require.NoError(t, err)
+			proj := &Projection{
+				Cols:       []string{"col"},
+				Exprs:      []evalengine.Expr{bindExpr},
+				Input:      &SingleRow{},
+				noTxNeeded: noTxNeeded{},
+			}
+			qr, err := proj.TryExecute(context.Background(), &noopVCursor{}, nil, true)
+			require.NoError(t, err)
+			assert.Equal(t, testCase.typ, qr.Fields[0].Type)
+			assert.Equal(t, testCase.collation, collations.ID(qr.Fields[0].Charset))
+		})
+	}
+}