Changes:

- implemented `Drop` for thread pool
 - thread pool uses a struct to store a function of specific signature

benches/multithreading.rs

@@ -7,7 +7,7 @@
 use std::sync::Arc;
 
 use criterion::{black_box, criterion_group, criterion_main, BenchmarkId, Criterion, Throughput};
-use imsearch::multithreading::ThreadPool;
+use imsearch::multithreading::{Task, ThreadPool};
 
 /// Amount of elements per vector used to calculate the dot product
 const VEC_ELEM_COUNT: usize = 1_000_000;
@@ -45,15 +45,15 @@ fn dot_parallel(a: Arc<Vec<f64>>, b: Arc<Vec<f64>>, threads: usize) {
     for i in 0..threads {
         // offset of the first element for the thread local vec
         let chunk = i * steps;
-        // create a new strong reference to the vector
-        let aa = a.clone();
-        let bb = b.clone();
         // launch a new thread
-        pool.enqueue(move || {
+        pool.enqueue(Task::new(
-            let a = &aa[chunk..(chunk + steps)];
+            (chunk, steps, a.clone(), b.clone()),
-            let b = &bb[chunk..(chunk + steps)];
+            |(block, inc, a, b)| {
-            dot(a, b)
+                let a = &a[block..(block + inc)];
-        });
+                let b = &b[block..(block + inc)];
+                dot(a, b)
+            },
+        ));
     }
 
     pool.join_all();
@@ -115,15 +115,15 @@ fn pool_overusage(a: Arc<Vec<f64>>, b: Arc<Vec<f64>>, threads: usize) {
     for i in 0..threads {
         // offset of the first element for the thread local vec
         let chunk = i * steps;
-        // create a new strong reference to the vector
-        let aa = a.clone();
-        let bb = b.clone();
         // launch a new thread
-        pool.enqueue(move || {
+        pool.enqueue(Task::new(
-            let a = &aa[chunk..(chunk + steps)];
+            (chunk, steps, a.clone(), b.clone()),
-            let b = &bb[chunk..(chunk + steps)];
+            |(block, inc, a, b)| {
-            dot(a, b)
+                let a = &a[block..(block + inc)];
-        });
+                let b = &b[block..(block + inc)];
+                dot(a, b)
+            },
+        ));
     }
 
     pool.join_all();

src/multithreading/mod.rs

@@ -8,10 +8,12 @@
 //! # Example
 //! ```rust
 //! # use imsearch::multithreading::ThreadPool;
+//! # use imsearch::multithreading::Task;
 //! let mut pool = ThreadPool::with_limit(2);
 //!
 //! for i in 0..10 {
-//!     pool.enqueue(move || i);
+//!     pool.enqueue(Task::new(i, |i| i));
+//! //                            ^^^^^^ closure or static function
 //! }
 //!
 //! pool.join_all();
@@ -19,6 +21,7 @@
 //! ```
 
 use std::{
+    any::Any,
     collections::VecDeque,
     num::NonZeroUsize,
     sync::{
@@ -46,19 +49,31 @@ pub enum Priority {
     Low,
 }
 
-/// Jobs are functions which are executed by the thread pool. They can be stalled when no threads are
+/// Traits a return value has to implement when being given back by a function or closure.
-/// free to execute them directly. They are meant to be executed only once and be done.
+pub trait Sendable: Any + Send + 'static + std::panic::UnwindSafe {}
-pub trait Job<T>: Send + 'static + FnOnce() -> T
+
+impl<T> Sendable for T where T: Any + Send + 'static + std::panic::UnwindSafe {}
+
+/// A task that will be executed at some point in the future by the thread pool
+/// At the heart of this struct is the function to be executed. This may be a closure.
+#[derive(Debug, Copy, Clone)]
+pub struct Task<I, T>
 where
-    T: Send,
+    I: Sendable,
+    T: Sendable,
 {
+    job: fn(I) -> T,
+    param: I,
 }
 
-impl<U, T> Job<T> for U
+impl<I, T> Task<I, T>
 where
-    U: Send + 'static + FnOnce() -> T,
+    I: Sendable,
-    T: Send + 'static,
+    T: Sendable,
 {
+    pub fn new(param: I, job: fn(I) -> T) -> Self {
+        Self { job, param }
+    }
 }
 
 /// Thread pool which can be used to execute functions or closures in parallel.
@@ -71,23 +86,33 @@ where
 /// # Example
 /// ```rust
 /// # use imsearch::multithreading::ThreadPool;
+/// # use imsearch::multithreading::Task;
 /// let mut pool = ThreadPool::with_limit(2);
 ///
 /// for i in 0..10 {
-///     pool.enqueue(move || i);
+///     pool.enqueue(Task::new(i, |i| i));
 /// }
 ///
 /// pool.join_all();
 /// assert_eq!(pool.get_results().iter().sum::<i32>(), 45);
 /// ```
+/// # Drop
+/// This struct implements the `Drop` trait. Upon being dropped the pool will wait for all threads
+/// to finsish. This may take up an arbitrary amount of time.
+/// # Panics in the thread
+/// When a function or closure panics, the executing thread will detect the unwind performed by `panic` causing the
+/// thread to print a message on stderr. The thread itself captures panics and won't terminate execution but continue with
+/// the next task in the queue.
+/// Its not recommend to use this pool with custom panic hooks or special functions which abort the process.
+/// Also panicking code from external program written in C++ or others in undefinied behavior according to [`std::panic::catch_unwind`]
 #[derive(Debug)]
-pub struct ThreadPool<T, F>
+pub struct ThreadPool<I, T>
 where
-    T: Send,
+    I: Sendable,
-    F: Job<T>,
+    T: Sendable,
 {
     /// queue for storing the jobs to be executed
-    queue: Arc<Mutex<VecDeque<F>>>,
+    queue: Arc<Mutex<VecDeque<Task<I, T>>>>,
     /// handles for all threads currently running and processing jobs
     handles: Vec<JoinHandle<()>>,
     /// reciver end for channel based communication between threads
@@ -98,10 +123,10 @@ where
     limit: NonZeroUsize,
 }
 
-impl<T, F> Default for ThreadPool<T, F>
+impl<I, T> Default for ThreadPool<I, T>
 where
-    T: Send + 'static,
+    I: Sendable,
-    F: Job<T>,
+    T: Sendable,
 {
     fn default() -> Self {
         let (sender, receiver) = channel::<T>();
@@ -121,10 +146,10 @@ where
     }
 }
 
-impl<T, F> ThreadPool<T, F>
+impl<I, T> ThreadPool<I, T>
 where
-    T: Send + 'static,
+    I: Sendable,
-    F: Job<T>,
+    T: Sendable,
 {
     /// Creates a new thread pool with default thread count determined by either
     /// [`std::thread::available_parallelism`] or [`DEFAULT_THREAD_POOL_SIZE`] in case it fails.
@@ -139,9 +164,13 @@ where
     /// # Panic
     /// This function will fails if `max_threads` is zero.
     pub fn with_limit(max_threads: usize) -> Self {
+        let (sender, receiver) = channel::<T>();
         Self {
             limit: NonZeroUsize::new(max_threads).expect("Thread limit must be non-zero"),
-            ..Default::default()
+            queue: Arc::new(Mutex::new(VecDeque::new())),
+            handles: Vec::new(),
+            sender,
+            receiver,
         }
     }
 
@@ -151,7 +180,7 @@ where
     /// This function will create a new thread if the maximum number of threads in not reached.
     /// In case the maximum number of threads is already used, the job is stalled and will get executed
     /// when a thread is ready and its at the start of the queue.
-    pub fn enqueue_priorize(&mut self, func: F, priority: Priority) {
+    pub fn enqueue_priorize(&mut self, func: Task<I, T>, priority: Priority) {
         // put job into queue
         let mut queue = self.queue.lock().unwrap();
 
@@ -169,8 +198,14 @@ where
             let queue = self.queue.clone();
 
             self.handles.push(thread::spawn(move || {
-                while let Some(job) = queue.lock().unwrap().pop_front() {
+                while let Some(task) = queue.lock().unwrap().pop_front() {
-                    tx.send(job()).expect("cannot send result");
+                    // basically try catch
+                    if let Err(e) = std::panic::catch_unwind(|| {
+                        tx.send((task.job)(task.param))
+                            .expect("unable to send result over channel");
+                    }) {
+                        eprintln!("thread paniced: {:?}", e);
+                    }
                 }
             }));
         }
@@ -183,7 +218,7 @@ where
     /// This function will create a new thread if the maximum number of threads in not reached.
     /// In case the maximum number of threads is already used, the job is stalled and will get executed
     /// when a thread is ready and its at the start of the queue.
-    pub fn enqueue(&mut self, func: F) {
+    pub fn enqueue(&mut self, func: Task<I, T>) {
         self.enqueue_priorize(func, Priority::Low);
     }
 
@@ -196,15 +231,15 @@ where
         }
     }
 
-    /// Returns all results that have been returned by the threads until now
+    /// Sendables all results that have been Sendableed by the threads until now
     /// and haven't been consumed yet.
-    /// All results retrieved from this call won't be returned on a second call.
+    /// All results retrieved from this call won't be Sendableed on a second call.
     /// This function is non blocking.
     pub fn try_get_results(&mut self) -> Vec<T> {
         self.receiver.try_iter().collect()
     }
 
-    /// Returns all results that have been returned by the threads until now
+    /// Sendables all results that have been returned by the threads until now
     /// and haven't been consumed yet. The function will also wait for all threads to finish executing (empty the queue).
     /// All results retrieved from this call won't be returned on a second call.
     /// This function will block the caller thread.
@@ -214,8 +249,20 @@ where
     }
 }
 
+impl<I, T> Drop for ThreadPool<I, T>
+where
+    I: Sendable,
+    T: Sendable,
+{
+    fn drop(&mut self) {
+        self.join_all();
+    }
+}
+
 #[cfg(test)]
 mod test {
+    use std::panic::UnwindSafe;
+
     use super::*;
 
     #[test]
@@ -223,7 +270,7 @@ mod test {
         let mut pool = ThreadPool::default();
 
         for i in 0..10 {
-            pool.enqueue_priorize(move || i, Priority::High);
+            pool.enqueue_priorize(Task::new(i, |i| i), Priority::High);
         }
 
         pool.join_all();
@@ -236,7 +283,7 @@ mod test {
         let mut pool = ThreadPool::with_limit(2);
 
         for i in 0..10 {
-            pool.enqueue(move || i);
+            pool.enqueue(Task::new(i, |i| i));
         }
 
         assert_eq!(pool.handles.len(), 2);
@@ -247,19 +294,62 @@ mod test {
         assert_eq!(pool.get_results().iter().sum::<i32>(), 45);
     }
 
+    trait Object: Send + UnwindSafe {
+        fn get(&mut self) -> i32;
+    }
+
+    #[derive(Default)]
+    struct Test1 {
+        _int: i32,
+    }
+
+    impl Object for Test1 {
+        fn get(&mut self) -> i32 {
+            0
+        }
+    }
+
+    #[derive(Default)]
+    struct Test2 {
+        _c: char,
+    }
+
+    impl Object for Test2 {
+        fn get(&mut self) -> i32 {
+            0
+        }
+    }
+
+    #[derive(Default)]
+    struct Test3 {
+        _s: String,
+    }
+
+    impl Object for Test3 {
+        fn get(&mut self) -> i32 {
+            0
+        }
+    }
+
     #[test]
-    fn test_multiple() {
+    fn test_1() {
         let mut pool = ThreadPool::with_limit(2);
 
-        for i in 0..10 {
+        let feats: Vec<Box<dyn Object>> = vec![
-            pool.enqueue(move || i);
+            Box::new(Test1::default()),
-        }
+            Box::new(Test2::default()),
+            Box::new(Test3::default()),
+        ];
 
-        assert_eq!(pool.handles.len(), 2);
+        for feat in feats {
-        assert_eq!(pool.limit.get(), 2);
+            pool.enqueue(Task::new(feat, |mut i| {
+                let _ = i.get();
+                i
+            }));
+        }
 
         pool.join_all();
 
-        assert_eq!(pool.get_results().iter().sum::<i32>(), 45);
+        let _feats: Vec<Box<dyn Object>> = pool.get_results();
     }
 }