Added try_publish to writer

Author: khopland

src/write.rs

@@ -293,6 +293,36 @@ where
         }
     }
 
+    /// Publishes if all writers have moved to other side without spinning.
+    ///
+    /// This method needs is difftent then publish to only check the readers once and publish
+    /// if all readers have moved. Returns true if it was sucessfully
+
+    pub fn try_publish(&mut self) -> bool {
+        let epochs = Arc::clone(&self.epochs);
+        let mut epochs = epochs.lock().unwrap();
+
+        // we're over-estimating here, but slab doesn't expose its max index
+        self.last_epochs.resize(epochs.capacity(), 0);
+        for (ri, epoch) in epochs.iter() {
+            if self.last_epochs[ri] % 2 == 0 {
+                continue;
+            }
+
+            let now = epoch.load(Ordering::Acquire);
+            if now != self.last_epochs[ri] {
+                // reader must have seen the last swap, since they have done at least one
+                // operation since we last looked at their epoch, which _must_ mean that they
+                // are no longer using the old pointer value.
+                continue;
+            } else {
+                return false;
+            }
+        }
+        self.do_publish(&mut epochs);
+        return true;
+    }
+
     /// Publish all operations append to the log to reads.
     ///
     /// This method needs to wait for all readers to move to the "other" copy of the data so that
@@ -312,6 +342,14 @@ where
 
         self.wait(&mut epochs);
 
+        self.do_publish(&mut epochs)
+    }
+
+    /// Actual doing the publishing
+    fn do_publish(
+        &mut self,
+        epochs: &mut MutexGuard<'_, slab::Slab<Arc<AtomicUsize>>>,
+    ) -> &mut Self {
         if !self.first {
             // all the readers have left!
             // safety: we haven't freed the Box, and no readers are accessing the w_handle
@@ -558,8 +596,8 @@ struct CheckWriteHandleSend;
 
 #[cfg(test)]
 mod tests {
-    use crate::sync::{AtomicUsize, Mutex, Ordering};
-    use crate::Absorb;
+    use crate::sync::{Arc, AtomicUsize, Mutex, Ordering};
+    use crate::{read, Absorb};
     use slab::Slab;
     include!("./utilities.rs");
 
@@ -713,4 +751,28 @@ mod tests {
         w.publish();
         assert_eq!(w.refreshes, 4);
     }
+
+    #[test]
+    fn try_publish() {
+        let (mut w, _r) = crate::new::<i32, _>();
+
+        // Case 1: A reader has not advanced (odd and unchanged) -> returns false
+        let mut epochs_slab = Slab::new();
+        let idx = epochs_slab.insert(Arc::new(AtomicUsize::new(1))); // odd epoch, "in read"
+        // Ensure last_epochs sees this reader as odd and unchanged
+        w.last_epochs = vec![0; epochs_slab.capacity()];
+        w.last_epochs[idx] = 1;
+        w.epochs = Arc::new(Mutex::new(epochs_slab));
+        assert_eq!(w.try_publish(), false);
+
+        // Case 2: All readers have advanced since last swap -> returns true and publishes
+        let mut epochs_slab_ok = Slab::new();
+        let idx_ok = epochs_slab_ok.insert(Arc::new(AtomicUsize::new(2))); // advanced
+        w.last_epochs = vec![0; epochs_slab_ok.capacity()];
+        w.last_epochs[idx_ok] = 1; // previously odd
+        w.epochs = Arc::new(Mutex::new(epochs_slab_ok));
+        let before = w.refreshes;
+        assert_eq!(w.try_publish(), true);
+        assert_eq!(w.refreshes, before + 1);
+    }
 }