[release-branch.go1.8] encoding/xml: disable checking of attribute syntax, like Go 1.7

Consider this struct, which expects an attribute A and a child C both ints:

    type X struct {
        XMLName xml.Name `xml:"X"`
        A       int      `xml:",attr"`
        C       int
    }

Go 1.2 through Go 1.7 were consistent: attributes unchecked,
children strictly checked:

    $ go1.7 run /tmp/x.go
    <X></X>              ok
    <X A=""></X>         ok
    <X A="bad"></X>      ok
    <X></X>              ok
    <X><C></C></X>       ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C/></X>          ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C>bad</C></X>    ERROR strconv.ParseInt: parsing "bad": invalid syntax
    $

Go 1.8 made attributes strictly checked, matching children:

    $ go1.8 run /tmp/x.go
    <X></X>              ok
    <X A=""></X>         ERROR strconv.ParseInt: parsing "": invalid syntax
    <X A="bad"></X>      ERROR strconv.ParseInt: parsing "bad": invalid syntax
    <X></X>              ok
    <X><C></C></X>       ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C/></X>          ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C>bad</C></X>    ERROR strconv.ParseInt: parsing "bad": invalid syntax
    $

but this broke XML code that had empty attributes (#19333).

In Go 1.9 we plan to start allowing empty children (#13417).
The fix for that will also make empty attributes work again:

    $ go run /tmp/x.go  # Go 1.9 development
    <X></X>              ok
    <X A=""></X>         ok
    <X A="bad"></X>      ERROR strconv.ParseInt: parsing "bad": invalid syntax
    <X></X>              ok
    <X><C></C></X>       ok
    <X><C/></X>          ok
    <X><C>bad</C></X>    ERROR strconv.ParseInt: parsing "bad": invalid syntax
    $

For Go 1.8.1, we want to restore the empty attribute behavior
to match Go 1.7 but not yet change the child behavior as planned for Go 1.9,
since that change hasn't been through release testing.

Instead, restore the more lax Go 1.7 behavior, so that XML files
with empty attributes will not be broken until Go 1.9:

    $ go run /tmp/x.go  # after this CL
    <X></X>              ok
    <X A=""></X>         ok
    <X A="bad"></X>      ok
    <X></X>              ok
    <X><C></C></X>       ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C/></X>          ERROR strconv.ParseInt: parsing "": invalid syntax
    <X><C>bad</C></X>    ERROR strconv.ParseInt: parsing "bad": invalid syntax
    $

Fixes #19333.

Change-Id: I3d38ebd2509f5b6ea3fd4856327f887f9a1a8085
Reviewed-on: https://go-review.googlesource.com/39607
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Sarah Adams <shadams@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

3 files changed, 41 insertions, 3 deletions

diff --git a/src/encoding/xml/marshal_test.go b/src/encoding/xml/marshal_test.go
index 5ec7ececa4..0126146d33 100644
--- a/src/encoding/xml/marshal_test.go
+++ b/src/encoding/xml/marshal_test.go
@@ -2428,7 +2428,10 @@ func TestIssue16158(t *testing.T) {
 	err := Unmarshal([]byte(data), &struct {
 		B byte `xml:"b,attr,omitempty"`
 	}{})
-	if err == nil {
-		t.Errorf("Unmarshal: expected error, got nil")
+
+	// For Go 1.8.1 we've restored the old "no errors reported" behavior.
+	// We'll try again in Go 1.9 to report errors.
+	if err != nil {
+		t.Errorf("Unmarshal: expected nil, got error")
 	}
 }
diff --git a/src/encoding/xml/read.go b/src/encoding/xml/read.go
index 5a89d5f504..799b57e9d1 100644
--- a/src/encoding/xml/read.go
+++ b/src/encoding/xml/read.go
@@ -285,7 +285,8 @@ func (p *Decoder) unmarshalAttr(val reflect.Value, attr Attr) error {
 		return nil
 	}
 
-	return copyValue(val, []byte(attr.Value))
+	copyValue(val, []byte(attr.Value))
+	return nil
 }
 
 var (
diff --git a/src/encoding/xml/xml_test.go b/src/encoding/xml/xml_test.go
index dad6ed98c1..f43a5e7eeb 100644
--- a/src/encoding/xml/xml_test.go
+++ b/src/encoding/xml/xml_test.go
@@ -797,3 +797,37 @@ func TestIssue12417(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue19333(t *testing.T) {
+	type X struct {
+		XMLName Name `xml:"X"`
+		A       int  `xml:",attr"`
+		C       int
+	}
+
+	var tests = []struct {
+		input string
+		ok    bool
+	}{
+		{`<X></X>`, true},
+		{`<X A=""></X>`, true},
+		{`<X A="bad"></X>`, true},
+		{`<X></X>`, true},
+		{`<X><C></C></X>`, false},
+		{`<X><C/></X>`, false},
+		{`<X><C>bad</C></X>`, false},
+	}
+
+	for _, tt := range tests {
+		err := Unmarshal([]byte(tt.input), new(X))
+		if tt.ok {
+			if err != nil {
+				t.Errorf("%s: unexpected error: %v", tt.input, err)
+			}
+		} else {
+			if err == nil {
+				t.Errorf("%s: unexpected success", tt.input)
+			}
+		}
+	}
+}