[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.go

@@ -69,7 +69,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 			mentions.addMention(otherUserId, DMMention)
 		}
 
-		if post.Props["from_webhook"] == "true" {
+		if post.GetProp("from_webhook") == "true" {
 			mentions.addMention(post.UserId, DMMention)
 		}
 	} else {
@@ -81,7 +81,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 		// Add an implicit mention when a user is added to a channel
 		// even if the user has set 'username mentions' to false in account settings.
 		if post.Type == model.POST_ADD_TO_CHANNEL {
-			addedUserId, ok := post.Props[model.POST_PROPS_ADDED_USER_ID].(string)
+			addedUserId, ok := post.GetProp(model.POST_PROPS_ADDED_USER_ID).(string)
 			if ok {
 				mentions.addMention(addedUserId, KeywordMention)
 			}
@@ -103,7 +103,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 		}
 
 		// prevent the user from mentioning themselves
-		if post.Props["from_webhook"] != "true" {
+		if post.GetProp("from_webhook") != "true" {
 			mentions.removeMention(post.UserId)
 		}
 
@@ -118,7 +118,7 @@ func (a *App) SendNotifications(post *model.Post, team *model.Team, channel *mod
 		for _, profile := range profileMap {
 			if (profile.NotifyProps[model.PUSH_NOTIFY_PROP] == model.USER_NOTIFY_ALL ||
 				channelMemberNotifyPropsMap[profile.Id][model.PUSH_NOTIFY_PROP] == model.CHANNEL_NOTIFY_ALL) &&
-				(post.UserId != profile.Id || post.Props["from_webhook"] == "true") &&
+				(post.UserId != profile.Id || post.GetProp("from_webhook") == "true") &&
 				!post.IsSystemMessage() {
 				allActivityPushUserIds = append(allActivityPushUserIds, profile.Id)
 			}
@@ -733,7 +733,7 @@ func (n *PostNotification) GetSenderName(userNameFormat string, overridesAllowed
 	}
 
 	if overridesAllowed && n.Channel.Type != model.CHANNEL_DIRECT {
-		if value, ok := n.Post.Props["override_username"]; ok && n.Post.Props["from_webhook"] == "true" {
+		if value, ok := n.Post.GetProps()["override_username"]; ok && n.Post.GetProp("from_webhook") == "true" {
 			return value.(string)
 		}
 	}