fix reparsing flags

d2plugin/serve.go

@@ -21,6 +21,7 @@ import (
 // Also see execPlugin in exec.go for the d2 binary plugin protocol.
 func Serve(p Plugin) xmain.RunFunc {
 	return func(ctx context.Context, ms *xmain.State) (err error) {
+		if !ms.Opts.Flags.Parsed() {
 			fs, err := p.Flags(ctx)
 			if err != nil {
 				return err
@@ -36,6 +37,7 @@ func Serve(p Plugin) xmain.RunFunc {
 				// At some point we want to write a friendly help.
 				return info(ctx, p, ms)
 			}
+		}
 
 		if len(ms.Opts.Flags.Args()) < 1 {
 			return xmain.UsageErrorf("expected first argument to be subcmd name")

e2etests-cli/main_test.go

@@ -58,6 +58,14 @@ func TestCLI_E2E(t *testing.T) {
 				assert.Testdata(t, ".svg", svg)
 			},
 		},
+		{
+			name: "flags-panic",
+			run: func(t *testing.T, ctx context.Context, dir string, env *xos.Env) {
+				writeFile(t, dir, "hello-world.d2", `x -> y`)
+				err := runTestMain(t, ctx, dir, env, "layout", "dagre", "--dagre-nodesep", "50", "hello-world.d2")
+				assert.ErrorString(t, err, `failed to wait xmain test: e2etests-cli/d2: failed to unmarshal input to graph: `)
+			},
+		},
 		{
 			name: "empty-layer",
 			run: func(t *testing.T, ctx context.Context, dir string, env *xos.Env) {