todo-api

Stage 2

move clone and locking to functions on tasklist

• refactor code into a nicer structure.
• add readable and writeable functions to my struct implementation so I can move the clone and sync into the structure

impl  TaskList {
	pub fn new() -> TaskList {
		TaskList {
			tasks : Arc::new(RwLock::new(TaskHashMap::new())),
		}
	}

	pub fn readable(self) -> RwLockReadGuard<TaskHashMap>{
		let tl =  self.tasks.clone();
		let mut my_tasks = tl.read().expect("RWLOCK poisoned");	
		my_tasks
	}

	pub fn writable(self) -> RwLockWriteGuard<TaskHashMap>{
		let tl =  self.tasks.clone();
		let mut my_tasks = tl.write().expect("RWLOCK poisoned");	
		my_tasks
	}
}

• turns out that this triggered lifetime issues - that is where I am at currently
• I need to specify a lifetime fo rthe hashmap which will guide the lifetimes for the readable/writable functions.
• Created minimal sample in Rust playground that produces the same errors
  • https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=882b652a8dd423e40a55df79b6447342
• The interesting thing is, once I clear up the reported error I get the real issue
  • https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=882b652a8dd423e40a55df79b6447342

	pub fn readable<'rwlock>(self) -> RwLockReadGuard<'rwlock,TaskHashMap>{
		let tl =  self.tasks.clone();
		let my_tasks = tl.read().expect("RWLOCK poisoned");	
		my_tasks
	}

	pub fn writable<'rwlock>(self) -> RwLockWriteGuard<'rwlock,TaskHashMap>{
		let tl =  self.tasks.clone();
		let my_tasks = tl.write().expect("RWLOCK poisoned");	
		my_tasks
	}

• The real issue is that the RwLockWriteGuard is actually based on the clone which is destroyed on the fn end.
• This is something I didn’t see at first because I was distracted by the Rust but now the error is very clear

   Compiling playground v0.0.1 (/playground)
error[E0515]: cannot return value referencing local variable `tl`
  --> src/lib.rs:28:3
   |
27 |         let my_tasks = tl.read().expect("RWLOCK poisoned");    
   |                        -- `tl` is borrowed here
28 |         my_tasks
   |         ^^^^^^^^ returns a value referencing data owned by the current function

• When I look at the actual issue it is obvious, and I think that the lifetimes diversion actually did help uncover it
• I believe that in C++ I would have only found it at runtime and it might have been a bit tricky to find
• So this refactor is probably not going to work as I need to invoke the ARC clone (to get a shared object) in each thread.
• renamed a few things and set it back to working code.

Seems like a similar issue seen by this user in the error and that what I wanted to do wouldn’t be possible : https://users.rust-lang.org/t/returning-a-value-from-inside-a-cloned-arc-t/6903/3

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