Canonicalise empty plugin configs to nil

plugin.go

@@ -35,13 +35,27 @@ func (p *Plugin) MarshalJSON() ([]byte, error) {
 	return json.Marshal(o)
 }
 
-// MarshalYAML returns the plugin in either "one-item map" form. Plugin sources
+// MarshalYAML returns the plugin in "one-item map" form. Plugin sources
 // are marshalled into "full" form.  Plugins originally specified as a single
 // string (no config, only source) are canonicalised into "one-item map" with
-// config nil.
+// config nil. Configs that are zero-length maps are canonicalised to nil.
 func (p *Plugin) MarshalYAML() (any, error) {
+	cfg := p.Config
+	switch x := cfg.(type) {
+	case map[string]any:
+		if len(x) == 0 {
+			cfg = nil
+		}
+
+	case []any:
+		// Should be invalid, but a different part of the process should be
+		// responsible for checking and complaining.
+		if len(x) == 0 {
+			cfg = nil
+		}
+	}
 	return map[string]any{
-		p.FullSource(): p.Config,
+		p.FullSource(): cfg,
 	}, nil
 }
 

plugin_test.go

@@ -1,6 +1,7 @@
 package pipeline
 
 import (
+	"encoding/json"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -87,6 +88,53 @@ func TestPluginFullSource(t *testing.T) {
 	}
 }
 
+func TestPluginMarshalJSON_Canonicalisation(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		p    *Plugin
+	}{
+		{
+			name: "nil interface",
+			p:    &Plugin{Source: "docker#v1.2.3", Config: nil},
+		},
+		{
+			name: "nil map",
+			p:    &Plugin{Source: "docker#v1.2.3", Config: map[string]any(nil)},
+		},
+		{
+			name: "empty map",
+			p:    &Plugin{Source: "docker#v1.2.3", Config: map[string]any{}},
+		},
+		{
+			name: "nil slice??",
+			p:    &Plugin{Source: "docker#v1.2.3", Config: []any(nil)},
+		},
+		{
+			name: "empty slice??",
+			p:    &Plugin{Source: "docker#v1.2.3", Config: []any{}},
+		},
+	}
+
+	const want = `{"github.com/buildkite-plugins/docker-buildkite-plugin#v1.2.3":null}`
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := json.Marshal(test.p)
+			if err != nil {
+				t.Errorf("json.Marshal(%+v) error = %v", test.p, err)
+			}
+
+			if diff := cmp.Diff(string(got), want); diff != "" {
+				t.Errorf("JSON marshalled plugin diff (-got +want):\n%s", diff)
+			}
+		})
+	}
+}
+
 func TestPluginMatrixInterpolate(t *testing.T) {
 	t.Parallel()
 
@@ -151,5 +199,4 @@ func TestPluginMatrixInterpolate(t *testing.T) {
 			}
 		})
 	}
-
 }