cmd: add synthetic command validate-interfaces

The command is injected when running xcaddy with one of Caddy's commands, though `validate-interfaces` does not exist in Caddy itself. This is useful for [guest modules](https://caddyserver.com/docs/extending-caddy) developers to validate their implementation meets the expectation of the host module.

builder.go

@@ -45,6 +45,15 @@ type Builder struct {
 	Debug        bool          `json:"debug,omitempty"`
 	BuildFlags   string        `json:"build_flags,omitempty"`
 	ModFlags     string        `json:"mod_flags,omitempty"`
+
+	// Experimental: Inject extra imports into the build. These
+	// imports are to add Go modules that may be needed in the
+	// Prequels.
+	ExtraImports []string
+	// Experimental: Inject extra code before running Caddy's `main`.
+	// If any of the prequels requires an import, it's you responsibility
+	// to add the import to the ExtraImports slice.
+	Prequels []string
 }
 
 // Build builds Caddy at the configured version with the

cmd/main.go

@@ -223,6 +223,8 @@ func runDev(ctx context.Context, args []string) error {
 		SkipBuild:    skipBuild,
 		SkipCleanup:  skipCleanup,
 		Debug:        buildDebugOutput,
+		ExtraImports: []string{`caddy	"github.com/caddyserver/caddy/v2"`, `"fmt"`, `"github.com/spf13/cobra"`},
+		Prequels:     []string{interfaceValidationCommand},
 	}
 	err = builder.Build(ctx, binOutput)
 	if err != nil {
@@ -439,3 +441,22 @@ func goModule() *debug.Module {
 	}
 	return mod
 }
+
+const interfaceValidationCommand = `
+	caddycmd.RegisterCommand(caddycmd.Command{
+		Name:  "validate-interfaces",
+		Short: "Validate that all modules conform to their namespaces",
+		CobraFunc: func(cmd *cobra.Command) {
+			cmd.RunE = caddycmd.WrapCommandFuncForCobra(func(f caddycmd.Flags) (int, error) {
+				for _, v := range caddy.Modules() {
+					mod, _ := caddy.GetModule(v)
+					if ok, err := caddy.ConformsToNamespace(mod.New(), mod.ID.Namespace()); !ok {
+						return caddy.ExitCodeFailedStartup, fmt.Errorf("module %s does not conform to its namespace: %v", mod.ID, err)
+					}
+				}
+				fmt.Println("All modules conform to their namespaces.")
+				return caddy.ExitCodeSuccess, nil
+			})
+		},
+	})
+`

environment.go

@@ -52,6 +52,8 @@ func (b Builder) newEnvironment(ctx context.Context) (*environment, error) {
 	// create the context for the main module template
 	tplCtx := goModTemplateContext{
 		CaddyModule:  caddyModulePath,
+		ExtraImports: b.ExtraImports,
+		Prequels:     b.Prequels,
 	}
 	for _, p := range b.Plugins {
 		tplCtx.Plugins = append(tplCtx.Plugins, p.PackagePath)
@@ -86,7 +88,7 @@ func (b Builder) newEnvironment(ctx context.Context) (*environment, error) {
 	// write the main module file to temporary folder
 	mainPath := filepath.Join(tempFolder, "main.go")
 	log.Printf("[INFO] Writing main module: %s\n%s", mainPath, buf.Bytes())
-	err = os.WriteFile(mainPath, buf.Bytes(), 0644)
+	err = os.WriteFile(mainPath, buf.Bytes(), 0o644)
 	if err != nil {
 		return nil, err
 	}
@@ -319,6 +321,8 @@ func (env environment) execGoGet(ctx context.Context, modulePath, moduleVersion,
 type goModTemplateContext struct {
 	CaddyModule  string
 	Plugins      []string
+	ExtraImports []string
+	Prequels     []string
 }
 
 const mainModuleTemplate = `package main
@@ -331,9 +335,15 @@ import (
 	{{- range .Plugins}}
 	_ "{{.}}"
 	{{- end}}
+	{{- range .ExtraImports}}
+	{{.}}
+	{{- end}}
 )
 
 func main() {
+	{{- range .Prequels}}
+	{{.}}
+	{{- end}}
 	caddycmd.Main()
 }
 `