invoke: Enable plugin file names with extensions

A CNI network configuration file contains the plugin's executable file name.
Some platforms like Windows require a file name extension for executables.
This causes unnecessary burden on admins as they now have to maintain two
versions of each type of netconfig file, which differ only by the ".exe"
extension. A much simpler design is for invoke package to also look for
well-known extensions on platforms that require it. Existing tests are
improved and new tests are added to cover the new behavior.

Fixes #360

pkg/invoke/find_test.go

@@ -17,7 +17,9 @@ package invoke_test
 import (
 	"fmt"
 	"io/ioutil"
+	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/containernetworking/cni/pkg/invoke"
 	. "github.com/onsi/ginkgo"
@@ -26,10 +28,12 @@ import (
 
 var _ = Describe("FindInPath", func() {
 	var (
-		multiplePaths  []string
-		pluginName     string
-		pluginDir      string
-		anotherTempDir string
+		multiplePaths         []string
+		pluginName            string
+		plugin2NameWithExt    string
+		plugin2NameWithoutExt string
+		pluginDir             string
+		anotherTempDir        string
 	)
 
 	BeforeEach(func() {
@@ -37,11 +41,24 @@ var _ = Describe("FindInPath", func() {
 		Expect(err).NotTo(HaveOccurred())
 
 		plugin, err := ioutil.TempFile(tempDir, "a-cni-plugin")
+		Expect(err).NotTo(HaveOccurred())
+
+		plugin2Name := "a-plugin-with-extension" + invoke.ExecutableFileExtensions[0]
+		plugin2, err := os.Create(filepath.Join(tempDir, plugin2Name))
+		Expect(err).NotTo(HaveOccurred())
 
 		anotherTempDir, err = ioutil.TempDir("", "nothing-here")
+		Expect(err).NotTo(HaveOccurred())
 
 		multiplePaths = []string{anotherTempDir, tempDir}
 		pluginDir, pluginName = filepath.Split(plugin.Name())
+		_, plugin2NameWithExt = filepath.Split(plugin2.Name())
+		plugin2NameWithoutExt = strings.Split(plugin2NameWithExt, ".")[0]
+	})
+
+	AfterEach(func() {
+		os.RemoveAll(pluginDir)
+		os.RemoveAll(anotherTempDir)
 	})
 
 	Context("when multiple paths are provided", func() {
@@ -52,6 +69,14 @@ var _ = Describe("FindInPath", func() {
 		})
 	})
 
+	Context("when a plugin name without its file name extension is provided", func() {
+		It("returns the path to the plugin, including its extension", func() {
+			pluginPath, err := invoke.FindInPath(plugin2NameWithoutExt, multiplePaths)
+			Expect(err).NotTo(HaveOccurred())
+			Expect(pluginPath).To(Equal(filepath.Join(pluginDir, plugin2NameWithExt)))
+		})
+	})
+
 	Context("when an error occurs", func() {
 		Context("when no paths are provided", func() {
 			It("returns an error noting no paths were provided", func() {