MM-10516: Added support for PostActions in ephemeral posts (#10258)

* Added support for PostActions in ephemeral posts

The general approach is that we take all the metadata that DoPostAction
needs to process client DoPostActionRequests, and store it in a
serialized, encrypted Cookie field, in the PostAction struct.

The client then must send it back, and it is then used to process
PostActions as a fallback top the metadata in the database.

This PR adds a new config setting, `ServiceSettings.ActionCookieSecret`.
In a cluster environment it must be the same for all instances.

- Added type PostActionCookie, and a Cookie string to PostAction.
- Added App.AddActionCookiesToPost.
- Use App.AddActionCookiesToPost in api4.createEphemeralPost,
  App.SendEphemeralPost, App.UpdateEphemeralPost.
- Added App.DoPostActionWithCookie to process incoming requests with
  cookies. For backward compatibility, it prefers the metadata in the
  database; falls back to cookie.
- Added plugin.API.UpdateEphemeralPost and plugin.API.DeleteEphemeralPost.
- Added App.encryptActionCookie/App.decryptActionCookie.

* Style

* Fixed an unfortunate typo, tested with matterpoll

* minor PR feedback

* Fixed uninitialized Context

* Fixed another test failure

* Fixed permission check

* Added api test for DoPostActionWithCookie

* Replaced config.ActionCookieSecret with Server.PostActionCookieSecret

Modeled after AsymetricSigningKey

* style

* Set DeleteAt in DeleteEphemeralPost

* PR feedback

* Removed deadwood comment

* Added EXPERIMENTAL comment to the 2 APIs in question

api4/integration_action.go

@@ -23,26 +23,46 @@ func doPostAction(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if !c.App.SessionHasPermissionToChannelByPost(c.App.Session, c.Params.PostId, model.PERMISSION_READ_CHANNEL) {
-		c.SetPermissionError(model.PERMISSION_READ_CHANNEL)
-		return
-	}
-
 	actionRequest := model.DoPostActionRequestFromJson(r.Body)
 	if actionRequest == nil {
 		actionRequest = &model.DoPostActionRequest{}
 	}
 
-	var err *model.AppError
+	var cookie *model.PostActionCookie
+	if actionRequest.Cookie != "" {
+		cookie = &model.PostActionCookie{}
+		cookieStr, err := model.DecryptPostActionCookie(actionRequest.Cookie, c.App.PostActionCookieSecret())
+		if err != nil {
+			c.Err = model.NewAppError("DoPostAction", "api.post.do_action.action_integration.app_error", nil, "err="+err.Error(), http.StatusBadRequest)
+			return
+		}
+		err = json.Unmarshal([]byte(cookieStr), &cookie)
+		if err != nil {
+			c.Err = model.NewAppError("DoPostAction", "api.post.do_action.action_integration.app_error", nil, "err="+err.Error(), http.StatusBadRequest)
+			return
+		}
+		if !c.App.SessionHasPermissionToChannel(c.App.Session, cookie.ChannelId, model.PERMISSION_READ_CHANNEL) {
+			c.SetPermissionError(model.PERMISSION_READ_CHANNEL)
+			return
+		}
+	} else {
+		if !c.App.SessionHasPermissionToChannelByPost(c.App.Session, c.Params.PostId, model.PERMISSION_READ_CHANNEL) {
+			c.SetPermissionError(model.PERMISSION_READ_CHANNEL)
+			return
+		}
+	}
+
+	var appErr *model.AppError
 	resp := &model.PostActionAPIResponse{Status: "OK"}
 
-	if resp.TriggerId, err = c.App.DoPostAction(c.Params.PostId, c.Params.ActionId, c.App.Session.UserId, actionRequest.SelectedOption); err != nil {
-		c.Err = err
+	resp.TriggerId, appErr = c.App.DoPostActionWithCookie(c.Params.PostId, c.Params.ActionId, c.App.Session.UserId,
+		actionRequest.SelectedOption, cookie)
+	if appErr != nil {
+		c.Err = appErr
 		return
 	}
 
 	b, _ := json.Marshal(resp)
-
 	w.Write(b)
 }