[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/webhook.go

@@ -168,20 +168,21 @@ func SplitWebhookPost(post *model.Post, maxPostSize int) ([]*model.Post, *model.
 	splits := make([]*model.Post, 0)
 	remainingText := post.Message
 
-	base := *post
+	base := post.Clone()
 	base.Message = ""
-	base.Props = make(map[string]interface{})
-	for k, v := range post.Props {
+	base.SetProps(make(map[string]interface{}))
+	for k, v := range post.GetProps() {
 		if k != "attachments" {
-			base.Props[k] = v
+			base.AddProp(k, v)
 		}
 	}
-	if utf8.RuneCountInString(model.StringInterfaceToJson(base.Props)) > model.POST_PROPS_MAX_USER_RUNES {
+
+	if utf8.RuneCountInString(model.StringInterfaceToJson(base.GetProps())) > model.POST_PROPS_MAX_USER_RUNES {
 		return nil, model.NewAppError("SplitWebhookPost", "web.incoming_webhook.split_props_length.app_error", map[string]interface{}{"Max": model.POST_PROPS_MAX_USER_RUNES}, "", http.StatusBadRequest)
 	}
 
 	for utf8.RuneCountInString(remainingText) > maxPostSize {
-		split := base
+		split := base.Clone()
 		x := 0
 		for index := range remainingText {
 			x++
@@ -191,20 +192,20 @@ func SplitWebhookPost(post *model.Post, maxPostSize int) ([]*model.Post, *model.
 				break
 			}
 		}
-		splits = append(splits, &split)
+		splits = append(splits, split)
 	}
 
-	split := base
+	split := base.Clone()
 	split.Message = remainingText
-	splits = append(splits, &split)
+	splits = append(splits, split)
 
-	attachments, _ := post.Props["attachments"].([]*model.SlackAttachment)
+	attachments, _ := post.GetProp("attachments").([]*model.SlackAttachment)
 	for _, attachment := range attachments {
 		newAttachment := *attachment
 		for {
 			lastSplit := splits[len(splits)-1]
 			newProps := make(map[string]interface{})
-			for k, v := range lastSplit.Props {
+			for k, v := range lastSplit.GetProps() {
 				newProps[k] = v
 			}
 			origAttachments, _ := newProps["attachments"].([]*model.SlackAttachment)
@@ -213,13 +214,13 @@ func SplitWebhookPost(post *model.Post, maxPostSize int) ([]*model.Post, *model.
 			runeCount := utf8.RuneCountInString(newPropsString)
 
 			if runeCount <= model.POST_PROPS_MAX_USER_RUNES {
-				lastSplit.Props = newProps
+				lastSplit.SetProps(newProps)
 				break
 			}
 
 			if len(origAttachments) > 0 {
 				newSplit := base
-				splits = append(splits, &newSplit)
+				splits = append(splits, newSplit)
 				continue
 			}
 
@@ -236,7 +237,7 @@ func SplitWebhookPost(post *model.Post, maxPostSize int) ([]*model.Post, *model.
 					break
 				}
 			}
-			lastSplit.Props = newProps
+			lastSplit.SetProps(newProps)
 			break
 		}
 	}