proto: fix handling of required fields after multiple violations (#679)

The previous logic only treated a required not set violation as non-fatal
for the first instance. Afterwards, the logic incorrectly switched back
to being fatal.

proto/all_test.go

@@ -2324,6 +2324,28 @@ func TestInvalidUTF8(t *testing.T) {
 	}
 }
 
+func TestRequired(t *testing.T) {
+	// The F_BoolRequired field appears after all of the required fields.
+	// It should still be handled even after multiple required field violations.
+	m := &GoTest{F_BoolRequired: Bool(true)}
+	got, err := Marshal(m)
+	if _, ok := err.(*RequiredNotSetError); !ok {
+		t.Errorf("Marshal() = %v, want RequiredNotSetError error", err)
+	}
+	if want := []byte{0x50, 0x01}; !bytes.Equal(got, want) {
+		t.Errorf("Marshal() = %x, want %x", got, want)
+	}
+
+	m = new(GoTest)
+	err = Unmarshal(got, m)
+	if _, ok := err.(*RequiredNotSetError); !ok {
+		t.Errorf("Marshal() = %v, want RequiredNotSetError error", err)
+	}
+	if !m.GetF_BoolRequired() {
+		t.Error("m.F_BoolRequired = false, want true")
+	}
+}
+
 // Benchmarks
 
 func testMsg() *GoTest {

proto/table_marshal.go

@@ -252,11 +252,13 @@ func (u *marshalInfo) marshal(b []byte, ptr pointer, deterministic bool) ([]byte
 		}
 	}
 	for _, f := range u.fields {
-		if f.required && errLater == nil {
+		if f.required {
 			if ptr.offset(f.field).getPointer().isNil() {
 				// Required field is not set.
 				// We record the error but keep going, to give a complete marshaling.
-				errLater = &RequiredNotSetError{f.name}
+				if errLater == nil {
+					errLater = &RequiredNotSetError{f.name}
+				}
 				continue
 			}
 		}
@@ -2592,7 +2594,7 @@ func (u *marshalInfo) appendMessageSet(b []byte, ext *XXX_InternalExtensions, de
 		p := toAddrPointer(&v, ei.isptr)
 		b, err = ei.marshaler(b, p, 3<<3|WireBytes, deterministic)
 		b = append(b, 1<<3|WireEndGroup)
-		if nerr.Merge(err) {
+		if !nerr.Merge(err) {
 			return b, err
 		}
 	}

proto/table_unmarshal.go

@@ -175,10 +175,12 @@ func (u *unmarshalInfo) unmarshal(m pointer, b []byte) error {
 				reqMask |= f.reqMask
 				continue
 			}
-			if r, ok := err.(*RequiredNotSetError); ok && errLater == nil {
+			if r, ok := err.(*RequiredNotSetError); ok {
 				// Remember this error, but keep parsing. We need to produce
 				// a full parse even if a required field is missing.
-				errLater = r
+				if errLater == nil {
+					errLater = r
+				}
 				reqMask |= f.reqMask
 				continue
 			}