internal/gocompat: slight cleanup

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

internal/gocompat/Makefile

@@ -1,6 +1,8 @@
 .PHONY: verify
 verify: generate
+	echo "module github.com/docker/cli" > ../../go.mod
 	GO111MODULE=on go test -v
+	rm ../../go.mod
 
 .PHONY: generate
 generate: clean

internal/gocompat/modulegenerator.go

@@ -1,13 +1,16 @@
 //go:build ignore
+// +build ignore
 
 package main
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"log"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"strings"
 	"text/template"
 
@@ -18,7 +21,8 @@ func main() {
 	if err := generateApp(); err != nil {
 		log.Fatal(err)
 	}
-	if err := generateModule(); err != nil {
+	rootDir := "../../"
+	if err := generateModule(rootDir); err != nil {
 		log.Fatal(err)
 	}
 }
@@ -51,42 +55,33 @@ func generateApp() error {
 	return os.WriteFile("main_test.go", buf.Bytes(), 0o644)
 }
 
-func generateModule() error {
-	content, err := os.ReadFile("../../go.mod")
-	if err != nil {
-		if !os.IsNotExist(err) {
-			return err
-		}
-		content = []byte("module github.com/docker/cli\n")
-		if err := os.WriteFile("../../go.mod", content, 0o644); err != nil {
-			return err
-		}
-		// Let's be nice, and remove the go.mod if we created it.
-		// FIXME(thaJeztah): we need to clean up the go.mod after running the test, but need to know if we created it (or if it was an existing go.mod)
-		// defer os.Remove("../../go.mod")
-	} else {
-		log.Println("WARN: go.mod exists in the repository root!")
-		log.Println("WARN: Using your go.mod instead of our generated version -- this may misbehave!")
-	}
-	mod, err := modfile.Parse("../../go.mod", content, nil)
-	if err != nil {
-		return err
+func generateModule(rootDir string) error {
+	modFile := filepath.Join(rootDir, "go.mod")
+	_, err := os.Stat(modFile)
+	if err == nil {
+		return errors.New("go.mod exists in the repository root")
 	}
-	if mod.Go != nil && mod.Go.Version != "" {
-		return fmt.Errorf("main go.mod must not contain a go version")
+
+	// create an empty go.mod without go version.
+	//
+	// this go.mod must exist when running the test.
+	err = os.WriteFile(modFile, []byte("module github.com/docker/cli\n"), 0o644)
+	if err != nil {
+		return fmt.Errorf("failed to write go.mod: %w", err)
 	}
-	content, err = os.ReadFile("../../vendor.mod")
+
+	content, err := os.ReadFile(filepath.Join(rootDir, "vendor.mod"))
 	if err != nil {
 		return err
 	}
-	mod, err = modfile.Parse("../../vendor.mod", content, nil)
+	mod, err := modfile.Parse(filepath.Join(rootDir, "vendor.mod"), content, nil)
 	if err != nil {
 		return err
 	}
 	if err := mod.AddModuleStmt("example.com/gocompat"); err != nil {
 		return err
 	}
-	if err := mod.AddReplace("github.com/docker/cli", "", "../../", ""); err != nil {
+	if err := mod.AddReplace("github.com/docker/cli", "", rootDir, ""); err != nil {
 		return err
 	}
 	if err := mod.AddGoStmt("1.24"); err != nil {