MM-37417: Push notification authz fix. (#18009)

* MM-37417: Push notification authz fix.

* MM-37417: Tests new app method.

app/post_test.go

@@ -2301,3 +2301,29 @@ func TestAutofollowOnPostingAfterUnfollow(t *testing.T) {
 	require.Nil(t, err)
 	require.True(t, m.Following)
 }
+
+func TestGetPostIfAuthorized(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+
+	privateChannel := th.CreatePrivateChannel(th.BasicTeam)
+	post, err := th.App.CreatePost(th.Context, &model.Post{UserId: th.BasicUser.Id, ChannelId: privateChannel.Id, Message: "Hello"}, privateChannel, false, false)
+	require.Nil(t, err)
+	require.NotNil(t, post)
+
+	session1, err := th.App.CreateSession(&model.Session{UserId: th.BasicUser.Id, Props: model.StringMap{}})
+	require.Nil(t, err)
+	require.NotNil(t, session1)
+
+	session2, err := th.App.CreateSession(&model.Session{UserId: th.BasicUser2.Id, Props: model.StringMap{}})
+	require.Nil(t, err)
+	require.NotNil(t, session2)
+
+	// User is not authorized to get post
+	_, err = th.App.GetPostIfAuthorized(post.Id, session2)
+	require.NotNil(t, err)
+
+	// User is authorized to get post
+	_, err = th.App.GetPostIfAuthorized(post.Id, session1)
+	require.Nil(t, err)
+}