4mz4 / rust_reven

Technical test for esReven Hamza BOUKHRISS

Mission

You are trying to store multiple callbacks in MyStruct and call them with a concrete type in the do_something method. Specializing MyStruct (which is parametrized with the argument type of the closure) introduced a lifetime issue that has to be fixed.

Goal:

• Please detail what is the problem in a specific text file
• Provide a solution that you would propose to fix it
• Add the patched code and documentation
• you are only allowed to modify the file src/changeme.rs
• Provide us a git patch with your work!

Hamza BOUKHRISS 02/05/2024

Result

Problem Explanation

From the error message

> error: lifetime may not live long enough   --> src/changeme.rs:36:13  
> | 24 | impl<'a> MyTrait<MyCallbackData<'a>> for
> MyStruct<MyCallbackData<'a>> {    |      -- lifetime `'a` defined here
> ... 29 |     fn do_something(&self) {    |                     - let's
> call the lifetime of this reference `'1` ... 36 |            
> (cb.callback)(&cb_data);    |             ^^^^^^^^^^^^^^^^^^^^^^^
> argument requires that `'1` must outlive `'a`
> 
> error: could not compile `interview` (bin "interview") due to 1
> previous error

in Rust Langugage lifetimes ensure that references are valid for as long as they are used. The error you're seeing means that the compiler can't guarantee that a reference used inside a method will stay valid as long as another reference it's compared to .

Solution Proposal

Arc (Atomic Reference Counting) , can improve the Rust program's handling of shared state, particularly in concurrent situations , when multiple parts of the code need to access the same data .

Original Problem	Solution with Arc	Benefits
The MyCallbackData struct previously held a reference (&'a [u8]) to a slice of data (data) within MyStruct.	Wrap Data with Arc: The get_arc_data function now creates an Arc from the internal data of MyStruct. This enables multiple references to the same data and avoids relying on specific function parameter lifetimes.	Lifetime Independence: Callbacks are no longer restricted by the lifetimes of function parameters in MyStruct.
This reference was limited by the lifetime ('a) of the function parameter used to access MyStruct.	Modify CallbackData: The MyCallbackData struct now holds an Arc<[u8]> instead of a direct reference. This allows callbacks to access the shared data encapsulated within the Arc.	Shared Data Access: Callbacks can access the same data concurrently through the Arc reference
Callbacks couldn't access the data independently due to these lifetime restrictions.	Clone Arc for Callbacks: Inside the do_something function, a clone of the Arc (Arc::clone(&self.get_arc_data())) is created for each callback. This ensures each callback gets a valid reference to the data without affecting the original Arc's lifetime.	Improved Flexibility: MyStruct can be used in more dynamic scenarios with concurrent callbacks.

Code Modification :

Code	Old Structure	New Structure
pub data: Arc<[u8]>, //data: &'a [u8],	The structure previously used a lifetime 'a to tie the lifetime of data (a slice of bytes) to that of MyCallbackData.	Now uses Arc<[u8]> to manage the data, removing the need for a specified lifetime and allowing the data to be shared safely across multiple instances with automatic reference counting.
impl CallbackData for MyCallbackData {} //impl<'a> CallbackData for MyCallbackData<'a> {}	The implementation was tied to a specific lifetime 'a.	The lifetime specification is removed as Arc handles the lifetime management, simplifying the trait implementation.
impl MyTrait<MyCallbackData> for MyStruct<MyCallbackData> { fn set_callback(&mut self, cb: MyCallback<MyCallbackData>) {	The method accepted callbacks specific to a lifetime 'a.	With the removal of the explicit lifetime, the method now accepts callbacks that operate on any instance of MyCallbackData, regardless of its lifetime context. This broadens the usability of set_callback.
data: Arc::clone(&self.get_arc_data()), //data: &self.data,	Directly used a reference to self.data, which required careful lifetime management to ensure no dangling references.	Uses Arc::clone to safely share a reference to the data, guaranteeing that the data remains valid as long as there's at least one active reference, and avoiding issues related to data lifetime.
impl<T: CallbackData> MyStruct<T> { fn get_arc_data(&self) -> Arc<[u8]> { Arc::new(self.data)}}		This method wraps the static data array in an Arc at runtime, ensuring that any operation that accesses the data does so through a reference-counted pointer, which promotes safe concurrency and data sharing.

N.B : the modification only impacts src/changeme.rs

to Apply This Modification you can use this command :

git apply 0001-Fix-lifetime-issue-in-callback-structure.patch