encoding/xml: allow overriding by empty namespace

The namespace defined by xmlns="value" can be overridden in every included tag
by the empty namespace xmlns="" without a prefix.

Method to calculate indent of XML handles depth of tag and its associated namespace is
still active even when no indent is required.

An XMLName field in a struct means that namespace must be enforced even if empty.
This occurs only on an inner tag as an override of any non-empty namespace of its outer tag.
An attribute is added to have the required namespace display.

Fixes #7113

Change-Id: I57f2308e98c66f04108ab136d350bdc3a6091e98
Reviewed-on: https://go-review.googlesource.com/c/go/+/108796
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/encoding/xml/marshal.go

@@ -543,6 +543,11 @@ func (p *printer) marshalValue(val reflect.Value, finfo *fieldInfo, startTemplat
 		}
 	}
 
+	// If a name was found, namespace is overridden with an empty space
+	if tinfo.xmlname != nil && start.Name.Space == "" &&
+		len(p.tags) != 0 && p.tags[len(p.tags)-1].Space != "" {
+		start.Attr = append(start.Attr, Attr{Name{"", xmlnsPrefix}, ""})
+	}
 	if err := p.writeStart(&start); err != nil {
 		return err
 	}

src/encoding/xml/xml_test.go

@@ -1059,6 +1059,56 @@ func TestIssue12417(t *testing.T) {
 	}
 }
 
+func TestIssue7113(t *testing.T) {
+	type C struct {
+		XMLName Name `xml:""` // Sets empty namespace
+	}
+
+	type A struct {
+		XMLName Name `xml:""`
+		C       C    `xml:""`
+	}
+
+	var a A
+	structSpace := "b"
+	xmlTest := `<A xmlns="` + structSpace + `"><C xmlns=""></C></A>`
+	t.Log(xmlTest)
+	err := Unmarshal([]byte(xmlTest), &a)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if a.XMLName.Space != structSpace {
+		t.Errorf("overidding with empty namespace: unmarshalling, got %s, want %s\n", a.XMLName.Space, structSpace)
+	}
+	if len(a.C.XMLName.Space) != 0 {
+		t.Fatalf("overidding with empty namespace: unmarshalling, got %s, want empty\n", a.C.XMLName.Space)
+	}
+
+	var b []byte
+	b, err = Marshal(&a)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(a.C.XMLName.Space) != 0 {
+		t.Errorf("overidding with empty namespace: marshaling, got %s in C tag which should be empty\n", a.C.XMLName.Space)
+	}
+	if string(b) != xmlTest {
+		t.Fatalf("overidding with empty namespace: marshalling, got %s, want %s\n", b, xmlTest)
+	}
+	var c A
+	err = Unmarshal(b, &c)
+	if err != nil {
+		t.Fatalf("second Unmarshal failed: %s", err)
+	}
+	if c.XMLName.Space != "b" {
+		t.Errorf("overidding with empty namespace: after marshaling & unmarshaling, XML name space: got %s, want %s\n", a.XMLName.Space, structSpace)
+	}
+	if len(c.C.XMLName.Space) != 0 {
+		t.Errorf("overidding with empty namespace: after marshaling & unmarshaling, got %s, want empty\n", a.C.XMLName.Space)
+	}
+}
+
 func TestIssue20396(t *testing.T) {
 
 	var attrError = UnmarshalError("XML syntax error on line 1: expected attribute name in element")