[MM-21378] Add mutex to model.Post to guard against race conditions on Post.Props (#13884)

* Add mutex to model.Post to guard against race conditions on Post.Props

* Rename mutex

* Add GetProp() method to Post

* Fix more tests

* Fix flaky test

Benchmarks:

BenchmarkPostPropsGet_indirect
BenchmarkPostPropsGet_indirect-2     	85026746	        13.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_indirect-4     	90273747	        13.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_indirect-8     	88324293	        13.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_indirect-16    	91427720	        13.1 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_direct
BenchmarkPostPropsGet_direct-2       	1000000000	         0.242 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_direct-4       	1000000000	         0.241 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_direct-8       	1000000000	         0.240 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_direct-16      	1000000000	         0.241 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsAdd_indirect
BenchmarkPostPropsAdd_indirect-2     	 5602224	       203 ns/op	     336 B/op	       2 allocs/op
BenchmarkPostPropsAdd_indirect-4     	 5959496	       206 ns/op	     336 B/op	       2 allocs/op
BenchmarkPostPropsAdd_indirect-8     	 5833999	       205 ns/op	     336 B/op	       2 allocs/op
BenchmarkPostPropsAdd_indirect-16    	 5802493	       225 ns/op	     336 B/op	       2 allocs/op
BenchmarkPostPropsAdd_direct
BenchmarkPostPropsAdd_direct-2       	100000000	        11.3 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsAdd_direct-4       	100000000	        11.3 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsAdd_direct-8       	100000000	        11.6 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsAdd_direct-16      	99840794	        11.4 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsDel_indirect
BenchmarkPostPropsDel_indirect-2     	18824002	        61.9 ns/op	      48 B/op	       1 allocs/op
BenchmarkPostPropsDel_indirect-4     	19470736	        63.8 ns/op	      48 B/op	       1 allocs/op
BenchmarkPostPropsDel_indirect-8     	17640460	        65.3 ns/op	      48 B/op	       1 allocs/op
BenchmarkPostPropsDel_indirect-16    	18692962	        65.4 ns/op	      48 B/op	       1 allocs/op
BenchmarkPostPropsDel_direct
BenchmarkPostPropsDel_direct-2       	516257440	         2.34 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsDel_direct-4       	514865216	         2.43 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsDel_direct-8       	511330477	         2.37 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsDel_direct-16      	499504010	         2.38 ns/op	       0 B/op	       0 allocs/op

app/notification_push.go

@@ -425,7 +425,7 @@ func DoesNotifyPropsAllowPushNotification(user *model.User, channelNotifyProps m
 	}
 
 	if (userNotify == model.USER_NOTIFY_ALL || channelNotify == model.CHANNEL_NOTIFY_ALL) &&
-		(post.UserId != user.Id || post.Props["from_webhook"] == "true") {
+		(post.UserId != user.Id || post.GetProp("from_webhook") == "true") {
 		return true
 	}
 
@@ -521,16 +521,16 @@ func (a *App) buildFullPushNotificationMessage(contentsConfig string, post *mode
 	}
 
 	msg.SenderName = senderName
-	if ou, ok := post.Props["override_username"].(string); ok && *cfg.ServiceSettings.EnablePostUsernameOverride {
+	if ou, ok := post.GetProp("override_username").(string); ok && *cfg.ServiceSettings.EnablePostUsernameOverride {
 		msg.OverrideUsername = ou
 		msg.SenderName = ou
 	}
 
-	if oi, ok := post.Props["override_icon_url"].(string); ok && *cfg.ServiceSettings.EnablePostIconOverride {
+	if oi, ok := post.GetProp("override_icon_url").(string); ok && *cfg.ServiceSettings.EnablePostIconOverride {
 		msg.OverrideIconUrl = oi
 	}
 
-	if fw, ok := post.Props["from_webhook"].(string); ok {
+	if fw, ok := post.GetProp("from_webhook").(string); ok {
 		msg.FromWebhook = fw
 	}