Fix marshaling of StringVariable to match unmarshaling (#19)

When I added "pebble plan" to fetch the plan, I didn't add a custom
MarshalYAML function on plan.StringVariable, so it marshaled like so:

environment:
    - name: FOO
      value: bar

This changes it to matching the unmarshaling, like so:

environment:
    - FOO: bar

Corresponding Python Operator Framework issue:
canonical/operator#486

internal/plan/plan.go

@@ -70,10 +70,16 @@ const (
 )
 
 type StringVariable struct {
-	Name  string `yaml:"name"`
-	Value string `yaml:"value"`
+	Name  string
+	Value string
 }
 
+// Ensure we're implementing both the custom unmarshaler and marshaller. Note
+// that the unmarshaller is on *StringVariable, but the marshaller needs to be
+// on the non-pointer type.
+var _ yaml.Unmarshaler = &StringVariable{}
+var _ yaml.Marshaler = StringVariable{}
+
 func (sv *StringVariable) UnmarshalYAML(node *yaml.Node) error {
 	if node.ShortTag() != "!!map" || len(node.Content) != 2 {
 		return fmt.Errorf("environment must be a list of single-item maps (- name: value)")
@@ -91,6 +97,10 @@ func (sv *StringVariable) UnmarshalYAML(node *yaml.Node) error {
 	return nil
 }
 
+func (sv StringVariable) MarshalYAML() (interface{}, error) {
+	return map[string]string{sv.Name: sv.Value}, nil
+}
+
 // CombineLayers combines the given layers into a Plan, with the later layers
 // layers overriding earlier ones.
 func CombineLayers(layers ...*Layer) (*Layer, error) {

internal/plan/plan_test.go

@@ -23,17 +23,18 @@ import (
 	"path/filepath"
 	"strings"
 
-	"github.com/canonical/pebble/internal/plan"
-
 	. "gopkg.in/check.v1"
+	"gopkg.in/yaml.v3"
+
+	"github.com/canonical/pebble/internal/plan"
 )
 
 // TODOs:
 // - command-chain
 // - error on invalid keys
 // - constraints on service names
 
-// The YAML on tests below passes throught this function to deindent and
+// The YAML on tests below passes through this function to deindent and
 // replace tabs by spaces, so we can keep the code here sane.
 func reindent(in string) []byte {
 	var buf bytes.Buffer
@@ -478,3 +479,37 @@ func (s *S) TestReadDirDupNames(c *C) {
 		}
 	}
 }
+
+func (s *S) TestMarshalLayer(c *C) {
+	layerBytes := reindent(`
+		summary: Simple layer
+		description: A simple layer.
+		services:
+			srv1:
+				summary: Service summary
+				default: start
+				override: replace
+				command: cmd arg1 "arg2 arg3"
+				after:
+					- srv2
+				before:
+					- srv3
+				requires:
+					- srv2
+					- srv3
+				environment:
+					- var1: val1
+					- var0: val0
+					- var2: val2
+			srv2:
+				override: replace
+				command: srv2cmd
+			srv3:
+				override: replace
+				command: srv3cmd`)
+	layer, err := plan.ParseLayer(1, "layer1", layerBytes)
+	c.Assert(err, IsNil)
+	out, err := yaml.Marshal(layer)
+	c.Assert(err, IsNil)
+	c.Assert(string(out), Equals, string(layerBytes))
+}