fix: skip empty text/cloud-config parts in multipart response

pkg/cloud_config/validate.go

@@ -0,0 +1,28 @@
+// Package cloud_config provides helpers for validating cloud-config content
+// before it is served to nodes.
+package cloud_config
+
+import "strings"
+
+// IsEmptyCloudConfig reports whether content is a text/cloud-config part
+// that contains no module directives — i.e. it is only the '#cloud-config'
+// header line plus optional blank lines/comments.
+//
+// Such empty parts cause cloud-init on the node to log warnings and can
+// trigger TypeErrors in module handlers that expect a non-None config
+// value (see issue #100).
+func IsEmptyCloudConfig(contentType, content string) bool {
+	if !strings.HasPrefix(strings.TrimSpace(contentType), "text/cloud-config") {
+		return false
+	}
+	for _, line := range strings.Split(content, "\n") {
+		trimmed := strings.TrimSpace(line)
+		// Skip blank lines and the mandatory #cloud-config marker.
+		if trimmed == "" || trimmed == "#cloud-config" || strings.HasPrefix(trimmed, "#") {
+			continue
+		}
+		// Found at least one non-comment, non-blank line — not empty.
+		return false
+	}
+	return true
+}

pkg/cloud_config/validate_test.go

@@ -0,0 +1,52 @@
+package cloud_config
+
+import "testing"
+
+func TestIsEmptyCloudConfig(t *testing.T) {
+	tests := []struct {
+		name        string
+		contentType string
+		content     string
+		want        bool
+	}{
+		{
+			name:        "empty part - header only",
+			contentType: "text/cloud-config",
+			content:     "#cloud-config\n",
+			want:        true,
+		},
+		{
+			name:        "empty part - header with blanks",
+			contentType: "text/cloud-config",
+			content:     "#cloud-config\n\n   \n",
+			want:        true,
+		},
+		{
+			name:        "non-empty part - has write_files",
+			contentType: "text/cloud-config",
+			content:     "#cloud-config\nwrite_files:\n  - path: /tmp/foo\n",
+			want:        false,
+		},
+		{
+			name:        "wrong content type",
+			contentType: "text/x-shellscript",
+			content:     "#cloud-config\n",
+			want:        false,
+		},
+		{
+			name:        "content type with charset param",
+			contentType: "text/cloud-config; charset=utf-8",
+			content:     "#cloud-config\n",
+			want:        true,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := IsEmptyCloudConfig(tc.contentType, tc.content)
+			if got != tc.want {
+				t.Errorf("IsEmptyCloudConfig(%q, ...) = %v, want %v", tc.contentType, got, tc.want)
+			}
+		})
+	}
+}