[geary/wip/improve-claiming-folder-session: 6/9] Add a nonblocking queue peek() method

[Michael Gratton <mjog src gnome org>]

commit a6e0fe9255dcb43939775ecff584357c3aa4b17f
Author: Michael Gratton <mike vee net>
Date:   Tue Nov 13 15:54:44 2018 +1100

    Add a nonblocking queue peek() method

 src/engine/nonblocking/nonblocking-queue.vala | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)
---
diff --git a/src/engine/nonblocking/nonblocking-queue.vala b/src/engine/nonblocking/nonblocking-queue.vala
index c7924318..cd6d920b 100644
--- a/src/engine/nonblocking/nonblocking-queue.vala
+++ b/src/engine/nonblocking/nonblocking-queue.vala
@@ -115,7 +115,7 @@ public class Geary.Nonblocking.Queue<G> : BaseObject {
     }
 
     /**
-     * Retrieves the next item from the queue, blocking until available.
+     * Removes and returns the next queued item, blocking until available.
      *
      * If the queue is paused, this will continue to wait until
      * unpaused and an item is ready. If `cancellable` is non-null,
@@ -130,6 +130,22 @@ public class Geary.Nonblocking.Queue<G> : BaseObject {
         }
     }
 
+    /**
+     * Returns the next queued item without removal, blocking until available.
+     *
+     * If the queue is paused, this will continue to wait until
+     * unpaused and an item is ready. If `cancellable` is non-null,
+     * when used will cancel this call.
+     */
+    public async G peek(Cancellable? cancellable = null) throws Error {
+        for (;;) {
+            if (queue.size > 0 && !is_paused)
+                return queue.peek();
+
+            yield spinlock.wait_async(cancellable);
+        }
+    }
+
     /**
      * Removes all items in queue, returning the number of removed items.
      */