Fix possible panic during plugin installation (#15689)

app/plugin_install.go

@@ -376,6 +376,8 @@ func (a *App) installExtractedPlugin(manifest *model.Manifest, fromPluginDir str
 		updatedManifest, _, err := pluginsEnvironment.Activate(manifest.Id)
 		if err != nil {
 			return nil, model.NewAppError("installExtractedPlugin", "app.plugin.restart.app_error", nil, err.Error(), http.StatusInternalServerError)
+		} else if updatedManifest == nil {
+			return nil, model.NewAppError("installExtractedPlugin", "app.plugin.restart.app_error", nil, "failed to activate plugin: plugin already active", http.StatusInternalServerError)
 		}
 		manifest = updatedManifest
 	}

app/plugin_install_test.go

@@ -8,10 +8,14 @@ import (
 	"bytes"
 	"compress/gzip"
 	"io"
+	"os"
+	"path/filepath"
 	"sort"
 	"testing"
 
 	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v5/utils/fileutils"
+
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -259,3 +263,35 @@ func TestInstallPluginLocally(t *testing.T) {
 		})
 	})
 }
+
+func TestInstallPluginAlreadyActive(t *testing.T) {
+	th := Setup(t)
+	defer th.TearDown()
+
+	path, _ := fileutils.FindDir("tests")
+	reader, err := os.Open(filepath.Join(path, "testplugin.tar.gz"))
+	require.NoError(t, err)
+
+	actualManifest, appError := th.App.InstallPlugin(reader, true)
+	require.NotNil(t, actualManifest)
+	require.Nil(t, appError)
+	appError = th.App.EnablePlugin(actualManifest.Id)
+	require.Nil(t, appError)
+
+	pluginsEnvironment := th.App.GetPluginsEnvironment()
+	require.NotNil(t, pluginsEnvironment)
+	bundleInfos, err := pluginsEnvironment.Available()
+	require.Nil(t, err)
+	require.NotEmpty(t, bundleInfos)
+	for _, bundleInfo := range bundleInfos {
+		if bundleInfo.Manifest.Id == actualManifest.Id {
+			err := os.RemoveAll(bundleInfo.Path)
+			require.NoError(t, err)
+		}
+	}
+
+	actualManifest, appError = th.App.InstallPlugin(reader, true)
+	require.NotNil(t, appError)
+	require.Nil(t, actualManifest)
+	require.Equal(t, "app.plugin.restart.app_error", appError.Id)
+}