Warn when installing local ext with no executable

pkg/cmd/extension/manager.go

@@ -28,6 +28,7 @@ import (
 
 // ErrInitialCommitFailed indicates the initial commit when making a new extension failed.
 var ErrInitialCommitFailed = errors.New("initial commit failed")
+var ErrExtensionExecutableNotFound = errors.New("an extension has been installed but there is no executable")
 
 const darwinAmd64 = "darwin-amd64"
 
@@ -194,10 +195,24 @@ func (m *Manager) populateLatestVersions(exts []*Extension) {
 func (m *Manager) InstallLocal(dir string) error {
 	name := filepath.Base(dir)
 	targetLink := filepath.Join(m.installDir(), name)
+	cs := m.io.ColorScheme()
+
 	if err := os.MkdirAll(filepath.Dir(targetLink), 0755); err != nil {
 		return err
 	}
-	return makeSymlink(dir, targetLink)
+	if err := makeSymlink(dir, targetLink); err != nil {
+		return err
+	}
+
+	// Check if an executable of the same name exists in the target directory.
+	// An error here doesn't indicate a failed extension installation, but
+	// it does indicate that the user will not be able to run the extension until
+	// the executable file is built or created manually somehow.
+	if _, err := os.Stat(filepath.Join(dir, name)); err != nil {
+		errMsg := fmt.Errorf("%v %w: expected executable file named \"%s\" in %s, perhaps you need to build it?", cs.WarningIcon(), ErrExtensionExecutableNotFound, name, dir)
+		return errMsg
+	}
+	return nil
 }
 
 type binManifest struct {

pkg/cmd/extension/manager_test.go

@@ -719,6 +719,62 @@ func TestManager_UpgradeExtension_GitExtension_Pinned(t *testing.T) {
 	gcOne.AssertExpectations(t)
 }
 
+func TestManager_Install_local(t *testing.T) {
+	extManagerDir := t.TempDir()
+	ios, _, stdout, stderr := iostreams.Test()
+	m := newTestManager(extManagerDir, nil, nil, ios)
+	fakeExtensionName := "local-ext"
+
+	// Create a temporary directory to simulate the local extension repo
+	extensionLocalPath := filepath.Join(extManagerDir, fakeExtensionName)
+	require.NoError(t, os.MkdirAll(extensionLocalPath, 0755))
+
+	// Create a fake executable in the local extension directory
+	fakeExtensionExecutablePath := filepath.Join(extensionLocalPath, fakeExtensionName)
+	require.NoError(t, stubExtension(fakeExtensionExecutablePath))
+
+	err := m.InstallLocal(extensionLocalPath)
+	require.NoError(t, err)
+
+	// This is the path to a file:
+	// on windows this is a file whose contents is a string describing the path to the local extension dir.
+	// on other platforms this file is a real symlink to the local extension dir.
+	extensionLinkFile := filepath.Join(extManagerDir, "extensions", fakeExtensionName)
+
+	if runtime.GOOS == "windows" {
+		// We don't create symlinks on Windows, so check if we made a file
+		// with the correct contents
+		b, err := os.ReadFile(extensionLinkFile)
+		require.NoError(t, err)
+		assert.Equal(t, extensionLocalPath, string(b))
+	} else {
+		// Verify the created symlink points to the correct directory
+		linkTarget, err := os.Readlink(extensionLinkFile)
+		require.NoError(t, err)
+		assert.Equal(t, extensionLocalPath, linkTarget)
+	}
+	assert.Equal(t, "", stdout.String())
+	assert.Equal(t, "", stderr.String())
+}
+
+func TestManager_Install_local_no_executable_found(t *testing.T) {
+	tempDir := t.TempDir()
+	ios, _, stdout, _ := iostreams.Test()
+	m := newTestManager(tempDir, nil, nil, ios)
+	fakeExtensionName := "local-ext"
+
+	// Create a temporary directory to simulate the local extension repo
+	localDir := filepath.Join(tempDir, fakeExtensionName)
+	require.NoError(t, os.MkdirAll(localDir, 0755))
+
+	// Intentionally not creating an executable in the local extension repo
+	// to simulate an attempt to install a local extension without an executable
+
+	err := m.InstallLocal(localDir)
+	require.ErrorIs(t, err, ErrExtensionExecutableNotFound)
+	assert.Equal(t, "", stdout.String())
+}
+
 func TestManager_Install_git(t *testing.T) {
 	tempDir := t.TempDir()