[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_test.go

@@ -302,9 +302,9 @@ func TestCreateWebhookPost(t *testing.T) {
 	}, model.POST_SLACK_ATTACHMENT, "")
 	require.Nil(t, err)
 
-	assert.Contains(t, post.Props, "from_webhook", "missing from_webhook prop")
-	assert.Contains(t, post.Props, "attachments", "missing attachments prop")
-	assert.Contains(t, post.Props, "webhook_display_name", "missing webhook_display_name prop")
+	assert.Contains(t, post.GetProps(), "from_webhook", "missing from_webhook prop")
+	assert.Contains(t, post.GetProps(), "attachments", "missing attachments prop")
+	assert.Contains(t, post.GetProps(), "webhook_display_name", "missing webhook_display_name prop")
 
 	_, err = th.App.CreateWebhookPost(hook.UserId, th.BasicChannel, "foo", "user", "http://iconurl", "", nil, model.POST_SYSTEM_GENERIC, "")
 	require.NotNil(t, err, "Should have failed - bad post type")
@@ -449,7 +449,7 @@ func TestSplitWebhookPost(t *testing.T) {
 			for i, split := range splits {
 				if i < len(tc.Expected) {
 					assert.Equal(t, tc.Expected[i].Message, split.Message)
-					assert.Equal(t, tc.Expected[i].Props["attachments"], split.Props["attachments"])
+					assert.Equal(t, tc.Expected[i].GetProp("attachments"), split.GetProp("attachments"))
 				}
 			}
 		})
@@ -610,17 +610,17 @@ func TestTriggerOutGoingWebhookWithUsernameAndIconURL(t *testing.T) {
 			select {
 			case webhookPost := <-createdPost:
 				assert.Equal(t, webhookPost.Message, "sample response text from test server")
-				assert.Equal(t, webhookPost.Props["from_webhook"], "true")
+				assert.Equal(t, webhookPost.GetProp("from_webhook"), "true")
 				if testCase.ExpectedIconUrl != "" {
-					assert.Equal(t, webhookPost.Props["override_icon_url"], testCase.ExpectedIconUrl)
+					assert.Equal(t, webhookPost.GetProp("override_icon_url"), testCase.ExpectedIconUrl)
 				} else {
-					assert.Nil(t, webhookPost.Props["override_icon_url"])
+					assert.Nil(t, webhookPost.GetProp("override_icon_url"))
 				}
 
 				if testCase.ExpectedUsername != "" {
-					assert.Equal(t, webhookPost.Props["override_username"], testCase.ExpectedUsername)
+					assert.Equal(t, webhookPost.GetProp("override_username"), testCase.ExpectedUsername)
 				} else {
-					assert.Nil(t, webhookPost.Props["override_username"])
+					assert.Nil(t, webhookPost.GetProp("override_username"))
 				}
 			case <-time.After(5 * time.Second):
 				require.Fail(t, "Timeout, webhook response not created as post")