Refactor code which deals with Windows' `.exe` suffix

Signed-off-by: Ian Campbell <ijc@docker.com>

cli-plugins/manager/manager.go

@@ -5,7 +5,6 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
-	"runtime"
 	"strings"
 
 	"github.com/docker/cli/cli/command"
@@ -61,13 +60,10 @@ func addPluginCandidatesFromDir(res map[string][]string, d string) error {
 			continue
 		}
 		name = strings.TrimPrefix(name, NamePrefix)
-		if runtime.GOOS == "windows" {
-			exe := ".exe"
-			if !strings.HasSuffix(name, exe) {
+		var err error
+		if name, err = trimExeSuffix(name); err != nil {
 			continue
 		}
-			name = strings.TrimSuffix(name, exe)
-		}
 		res[name] = append(res[name], filepath.Join(d, dentry.Name()))
 	}
 	return nil
@@ -131,10 +127,7 @@ func PluginRunCommand(dockerCli command.Cli, name string, rootcmd *cobra.Command
 		// fallback to their "invalid" command path.
 		return nil, errPluginNotFound(name)
 	}
-	exename := NamePrefix + name
-	if runtime.GOOS == "windows" {
-		exename = exename + ".exe"
-	}
+	exename := addExeSuffix(NamePrefix + name)
 	for _, d := range getPluginDirs(dockerCli) {
 		path := filepath.Join(d, exename)
 

cli-plugins/manager/plugin.go

@@ -4,7 +4,6 @@ import (
 	"encoding/json"
 	"path/filepath"
 	"regexp"
-	"runtime"
 	"strings"
 
 	"github.com/pkg/errors"
@@ -46,15 +45,12 @@ func newPlugin(c Candidate, rootcmd *cobra.Command) (Plugin, error) {
 	if fullname == "." {
 		return Plugin{}, errors.Errorf("unable to determine basename of plugin candidate %q", path)
 	}
-	if runtime.GOOS == "windows" {
-		exe := ".exe"
-		if !strings.HasSuffix(fullname, exe) {
-			return Plugin{}, errors.Errorf("plugin candidate %q lacks required %q suffix", path, exe)
-		}
-		fullname = strings.TrimSuffix(fullname, exe)
+	var err error
+	if fullname, err = trimExeSuffix(fullname); err != nil {
+		return Plugin{}, errors.Wrapf(err, "plugin candidate %q", path)
 	}
 	if !strings.HasPrefix(fullname, NamePrefix) {
-		return Plugin{}, errors.Errorf("plugin candidate %q does not have %q prefix", path, NamePrefix)
+		return Plugin{}, errors.Errorf("plugin candidate %q: does not have %q prefix", path, NamePrefix)
 	}
 
 	p := Plugin{

cli-plugins/manager/suffix_unix.go

@@ -0,0 +1,10 @@
+// +build !windows
+
+package manager
+
+func trimExeSuffix(s string) (string, error) {
+	return s, nil
+}
+func addExeSuffix(s string) string {
+	return s
+}

cli-plugins/manager/suffix_windows.go

@@ -0,0 +1,19 @@
+package manager
+
+import (
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+func trimExeSuffix(s string) (string, error) {
+	exe := ".exe"
+	if !strings.HasSuffix(s, exe) {
+		return "", errors.Errorf("lacks required %q suffix", exe)
+	}
+	return strings.TrimSuffix(s, exe), nil
+}
+
+func addExeSuffix(s string) string {
+	return s + ".exe"
+}