improve zip path handling

zip.go

@@ -2,15 +2,22 @@ package main
 
 import (
 	"flag"
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"golang.org/x/mod/modfile"
 	"golang.org/x/mod/module"
 	"golang.org/x/mod/zip"
 )
 
+func modbasename(path string) string {
+	parts := strings.Split(path, "/")
+	return parts[len(parts)-1]
+}
+
 func zipcmd(args []string) {
 	var (
 		version    string
@@ -19,13 +26,18 @@ func zipcmd(args []string) {
 
 	flags := flag.NewFlagSet("zip", flag.ExitOnError)
 	flags.StringVar(&version, "version", "", "package version")
-	flags.StringVar(&outputPath, "o", "a.zip", "output file path")
+	flags.StringVar(&outputPath, "o", "", "output file path")
 	flags.Parse(args)
+
 	if version == "" {
 		bail(1, "target release version is required")
 	}
 
 	pkgdir := flags.Arg(0)
+	if pkgdir == "" {
+		pkgdir = "."
+	}
+
 	modfilePath := filepath.Join(pkgdir, "go.mod")
 	b, err := ioutil.ReadFile(modfilePath)
 	if err != nil {
@@ -37,14 +49,33 @@ func zipcmd(args []string) {
 	if err != nil {
 		bail(1, "unable to parse modfile: %v", err)
 	}
+
 	modpath := f.Module.Mod.Path
 	log_info.Print("parsed modfile")
 	log_info.Printf("module path in modfile: %s", modpath)
 	log_info.Printf("module major version in modfile: %s", f.Module.Mod.Version)
 	log_info.Printf("target release version: %s", version)
+
+	// major version compatibility check
 	if err := module.Check(modpath, version); err != nil {
 		shutdown(err)
 	}
+
+	// default to basename@version.zip
+	if outputPath == "" {
+		outputPath = fmt.Sprintf("%s@%s.zip", modbasename(modpath), version)
+	}
+
+	log_info.Printf("destination: %s", outputPath)
+	switch _, err := os.Stat(outputPath); {
+	case err == nil:
+		bail(1, "a file at %q already exists", outputPath)
+	case os.IsNotExist(err):
+		break
+	default:
+		bail(1, "unable to check for file at %q: %v", outputPath, err)
+	}
+
 	zf, err := os.OpenFile(outputPath, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0644)
 	if err != nil {
 		bail(1, "output file not opened: %v", err)