[shotwell/wip/742047-mutex] Possible fix

[Jim Nelson <jnelson src gnome org>]

commit 3482a3c5112c8cada8ef9ad8ce6d7e0f17ee6967
Author: Jim Nelson <jim yorba org>
Date:   Mon Dec 29 14:53:54 2014 -0800

    Possible fix
    
    As with Geary, unlock the mutex and yield a bit of time to allow
    background thread a window to notify.

 src/threads/Semaphore.vala |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)
---
diff --git a/src/threads/Semaphore.vala b/src/threads/Semaphore.vala
index dfb0a2f..c5a6b1b 100644
--- a/src/threads/Semaphore.vala
+++ b/src/threads/Semaphore.vala
@@ -66,8 +66,14 @@ public abstract class AbstractSemaphore {
     public void wait() {
         mutex.lock();
         
-        while (do_wait() == WaitAction.SLEEP)
+        while (do_wait() == WaitAction.SLEEP) {
+            // release the mutex and yield to give other threads a chance to notify
+            mutex.unlock();
+            Thread.yield();
+            mutex.lock();
+            
             monitor.wait(mutex);
+        }
         
         mutex.unlock();
     }