[MM-52924] Implement ConfigurationWillBeSaved plugin hook (#23567)

* Implement ConfigurationWillBeSaved plugin hook

* Add comment

* Update comment

* Fix potential nil dereference if plugin environment is unset

* Address PR review

server/channels/app/plugin_api_test.go

@@ -5,6 +5,7 @@ package app
 
 import (
 	"bytes"
+	_ "embed"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -2195,3 +2196,106 @@ func TestPluginUploadsAPI(t *testing.T) {
 	require.NotNil(t, manifest)
 	require.True(t, activated)
 }
+
+//go:embed plugin_api_tests/manual.test_configuration_will_be_saved_hook/main.tmpl
+var configurationWillBeSavedHookTemplate string
+
+func TestConfigurationWillBeSavedHook(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+
+	getPluginCode := func(hookCode string) string {
+		return fmt.Sprintf(configurationWillBeSavedHookTemplate, hookCode)
+	}
+
+	runPlugin := func(t *testing.T, code string) {
+		pluginDir, err := os.MkdirTemp("", "")
+		require.NoError(t, err)
+		webappPluginDir, err := os.MkdirTemp("", "")
+		require.NoError(t, err)
+		defer os.RemoveAll(pluginDir)
+		defer os.RemoveAll(webappPluginDir)
+
+		newPluginAPI := func(manifest *model.Manifest) plugin.API {
+			return th.App.NewPluginAPI(th.Context, manifest)
+		}
+		env, err := plugin.NewEnvironment(newPluginAPI, NewDriverImpl(th.App.Srv()), pluginDir, webappPluginDir, th.App.Log(), nil)
+		require.NoError(t, err)
+
+		th.App.ch.SetPluginsEnvironment(env)
+
+		pluginID := "testplugin"
+		pluginManifest := `{"id": "testplugin", "server": {"executable": "backend.exe"}}`
+		backend := filepath.Join(pluginDir, pluginID, "backend.exe")
+		utils.CompileGo(t, code, backend)
+
+		os.WriteFile(filepath.Join(pluginDir, pluginID, "plugin.json"), []byte(pluginManifest), 0600)
+		manifest, activated, reterr := env.Activate(pluginID)
+		require.NoError(t, reterr)
+		require.NotNil(t, manifest)
+		require.True(t, activated)
+	}
+
+	t.Run("error", func(t *testing.T) {
+		hookCode := `
+    return nil, fmt.Errorf("plugin hook failed")
+    `
+
+		runPlugin(t, getPluginCode(hookCode))
+
+		cfg := th.App.Config()
+		_, _, appErr := th.App.SaveConfig(cfg, false)
+		require.NotNil(t, appErr)
+		require.Equal(t, "saveConfig: An error occurred running the plugin hook on configuration save., plugin hook failed", appErr.Error())
+
+		require.Equal(t, cfg, th.App.Config())
+	})
+
+	t.Run("AppError", func(t *testing.T) {
+		hookCode := `
+    return nil, model.NewAppError("saveConfig", "custom_error", nil, "", 400)
+    `
+
+		runPlugin(t, getPluginCode(hookCode))
+
+		cfg := th.App.Config()
+		_, _, appErr := th.App.SaveConfig(cfg, false)
+		require.NotNil(t, appErr)
+		require.Equal(t, "custom_error", appErr.Id)
+
+		require.Equal(t, cfg, th.App.Config())
+	})
+
+	t.Run("no error, no config change", func(t *testing.T) {
+		hookCode := `
+    return nil, nil
+    `
+
+		runPlugin(t, getPluginCode(hookCode))
+
+		cfg := th.App.Config()
+		_, newCfg, appErr := th.App.SaveConfig(cfg, false)
+		require.Nil(t, appErr)
+		require.Equal(t, cfg, newCfg)
+	})
+
+	t.Run("config change", func(t *testing.T) {
+		hookCode := `
+    cfg := newCfg.Clone()
+		cfg.PluginSettings.Plugins["custom_plugin"] = map[string]any{
+		  "custom_key": "custom_val",
+		}
+    return cfg, nil
+    `
+
+		runPlugin(t, getPluginCode(hookCode))
+
+		cfg := th.App.Config()
+		_, newCfg, appErr := th.App.SaveConfig(cfg, false)
+		require.Nil(t, appErr)
+		require.NotEqual(t, cfg, newCfg)
+		require.Equal(t, map[string]any{
+			"custom_key": "custom_val",
+		}, newCfg.PluginSettings.Plugins["custom_plugin"])
+	})
+}