Unlock if poll triggers an exception

CRuby semantics for an interrupt during lock appear to ensure that
the lock is released before propagating any interrupt exception.
This poll was added without such behavior, leading to locks being
left locked even though a thread interrupt happened. This in turn
breaks some semantics of Mutex#synchronize, since the lock call
does not happen within the try/finally that would release it, so
any exception raised will exit synchronize with the lock still
locked.

The fix here wraps the additional poll with try/catch to ensure
poll-triggered exceptions release the lock before propagating.

Fixes jruby#8585

core/src/main/java/org/jruby/ext/thread/Mutex.java

@@ -39,6 +39,7 @@
 import org.jruby.anno.JRubyClass;
 import org.jruby.anno.JRubyMethod;
 import org.jruby.runtime.Block;
+import org.jruby.runtime.Helpers;
 import org.jruby.runtime.ObjectAllocator;
 import org.jruby.runtime.ThreadContext;
 import org.jruby.runtime.builtin.IRubyObject;
@@ -114,7 +115,15 @@ public IRubyObject lock(ThreadContext context) {
         }
 
         // always check for thread interrupts after acquiring lock
-        thread.pollThreadEvents(context);
+        try {
+            thread.pollThreadEvents(context);
+        } catch (Throwable t) {
+            // Thread poll triggered an exception event, release locked locks before propagating
+            if (lock.isHeldByCurrentThread()) {
+                thread.unlock(lock);
+            }
+            Helpers.throwException(t);
+        }
 
         return this;
     }