improve RPCMsg xml encoding testing (#36)

Add unittest for RPCMsg encoding and ensure that operation is supplied
when encoding `rpc` request messages (i.e `ReqMSG`) via custom
MarshalXML method.

Future work to make sure structs passed in are named.

**Test Plan:**
```
$ go test . -v -run=MarshalRPCMsg
=== RUN   TestMarshalRPCMsg
=== RUN   TestMarshalRPCMsg/nil
    msg_test.go:55: out: 
=== RUN   TestMarshalRPCMsg/string
    msg_test.go:55: out: <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="1"><foo><bar/></foo></rpc>
=== RUN   TestMarshalRPCMsg/byteslice
    msg_test.go:55: out: <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="1"><baz><qux/></baz></rpc>
=== RUN   TestMarshalRPCMsg/validate
    msg_test.go:55: out: <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="1"><validate><source><running/></source></validate></rpc>
=== RUN   TestMarshalRPCMsg/customStruct
    msg_test.go:55: out: <rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="1"><command xmlns="http://xml.juniper.net/junos/22.4R0/junos">show bgp neighbors</command></rpc>
--- PASS: TestMarshalRPCMsg (0.00s)
    --- PASS: TestMarshalRPCMsg/nil (0.00s)
    --- PASS: TestMarshalRPCMsg/string (0.00s)
    --- PASS: TestMarshalRPCMsg/byteslice (0.00s)
    --- PASS: TestMarshalRPCMsg/validate (0.00s)
    --- PASS: TestMarshalRPCMsg/customStruct (0.00s)
PASS
ok      github.com/nemith/netconf       (cached)
```

.vscode/settings.json

@@ -1,3 +1,10 @@
 {
-  "cSpell.words": ["netconf"]
+  "cSpell.words": [
+    "byteslice",
+    "innerxml",
+    "inttest",
+    "Junos",
+    "nemith",
+    "netconf"
+  ]
 }

inttest/ssh_test.go

@@ -5,8 +5,10 @@ package inttest
 
 import (
 	"context"
+	"encoding/xml"
 	"net"
 	"os"
+	"strings"
 	"testing"
 
 	"github.com/nemith/netconf"
@@ -15,7 +17,19 @@ import (
 	"golang.org/x/crypto/ssh"
 )
 
+func onlyFlavor(t *testing.T, flavors ...string) {
+	t.Helper()
+	for _, flavor := range flavors {
+		if os.Getenv("NETCONF_DUT_FLAVOR") == flavor {
+			return
+		}
+	}
+	t.Skipf("test only for flavors '%s'.  Skipping", strings.Join(flavors, ","))
+}
+
 func sshAuth(t *testing.T) ssh.AuthMethod {
+	t.Helper()
+
 	switch {
 	case os.Getenv("NETCONF_DUT_SSHPASS") != "":
 		return ssh.Password(os.Getenv("NETCONF_DUT_SSHPASS"))
@@ -117,3 +131,19 @@ func TestBadGetConfig(t *testing.T) {
 	assert.ErrorAs(t, err, &rpcErrors)
 	assert.Len(t, rpcErrors, 1)
 }
+
+func TestJunosCommand(t *testing.T) {
+	onlyFlavor(t, "junos")
+	session := setupSSH(t)
+
+	cmd := struct {
+		XMLName xml.Name `xml:"command"`
+		Command string   `xml:",innerxml"`
+	}{
+		Command: "show version",
+	}
+
+	ctx := context.Background()
+	err := session.Call(ctx, &cmd, nil)
+	assert.NoError(t, err)
+}

msg.go

@@ -21,6 +21,19 @@ type RPCMsg struct {
 	Operation interface{} `xml:",innerxml"`
 }
 
+func (msg *RPCMsg) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
+	if msg.Operation == nil {
+		return fmt.Errorf("operation cannot be nil")
+	}
+
+	// TODO: validate operation is named?
+
+	// alias the type to not cause recursion calling e.Encode
+	type rpcMsg RPCMsg
+	inner := rpcMsg(*msg)
+	return e.Encode(&inner)
+}
+
 // RPCReplyMsg maps the xml value of <rpc-reply> in RFC6241
 type RPCReplyMsg struct {
 	XMLName   xml.Name `xml:"urn:ietf:params:xml:ns:netconf:base:1.0 rpc-reply"`

msg_test.go

@@ -0,0 +1,75 @@
+package netconf
+
+import (
+	"encoding/xml"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestMarshalRPCMsg(t *testing.T) {
+	tt := []struct {
+		name      string
+		operation interface{}
+		err       bool
+		want      []byte
+	}{
+		{
+			name:      "nil",
+			operation: nil,
+			err:       true,
+		},
+		{
+			name:      "string",
+			operation: "<foo><bar/></foo>",
+			want:      []byte(`<rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="1"><foo><bar/></foo></rpc>`),
+		},
+		{
+			name:      "byteslice",
+			operation: []byte("<baz><qux/></baz>"),
+			want:      []byte(`<rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="1"><baz><qux/></baz></rpc>`),
+		},
+		{
+			name:      "validate",
+			operation: validateReq{Source: Running},
+			want:      []byte(`<rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="1"><validate><source><running/></source></validate></rpc>`),
+		},
+		{
+			name: "namedStruct",
+			operation: struct {
+				XMLName xml.Name `xml:"http://xml.juniper.net/junos/22.4R0/junos command"`
+				Command string   `xml:",innerxml"`
+			}{
+				Command: "show bgp neighbors",
+			},
+			want: []byte(`<rpc xmlns="urn:ietf:params:xml:ns:netconf:base:1.0" message-id="1"><command xmlns="http://xml.juniper.net/junos/22.4R0/junos">show bgp neighbors</command></rpc>`),
+		},
+		/*
+			{
+				name: "unnamedStruct",
+				operation: struct {
+					Command string `xml:"command"`
+				}{
+					Command: "show version",
+				},
+			},*/
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.name, func(t *testing.T) {
+			out, err := xml.Marshal(&RPCMsg{
+				MessageID: 1,
+				Operation: tc.operation,
+			})
+			t.Logf("out: %s", out)
+
+			if tc.err {
+				assert.Error(t, err)
+				return
+			}
+
+			assert.NoError(t, err)
+			assert.Equal(t, out, tc.want)
+		})
+	}
+}