Narrow deadlock check for fibers and fix message

* Only raise deadlock error when two different fibers from the same
  thread attempt to lock (jruby#9218).
* Store the real parent thread as the locking thread for deadlock
  checks (fixes second case added in spec).

Fixes jruby#9218

Author: headius

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

@@ -106,7 +106,8 @@ public IRubyObject lock(ThreadContext context) {
 
         checkRelocking(context);
 
-        if (this.lockingThread == parentThread) {
+        RubyThread lockingThread = this.lockingThread;
+        if (lockingThread == parentThread && context.getFiber() != lockingThread.getContext().getFiber()) {
             throw context.runtime.newThreadError("deadlock; lock already owned by another fiber belonging to the same thread");
         }
 
@@ -123,7 +124,7 @@ public IRubyObject lock(ThreadContext context) {
             }
         }
 
-        this.lockingThread = thread;
+        this.lockingThread = parentThread;
 
         // always check for thread interrupts after acquiring lock
         try {
@@ -204,7 +205,7 @@ public IRubyObject owned_p(ThreadContext context) {
 
     private void checkRelocking(ThreadContext context) {
         if (lock.isHeldByCurrentThread()) {
-            throw context.runtime.newThreadError("Mutex relocking by same thread");
+            throw context.runtime.newThreadError("deadlock; recursive locking");
         }
     }