Release the jwack token when doing a synchronous lock wait.

Although we were deadlock-free before, under some circumstances we'd end up
holding a perfectly good token while in sync wait; that would reduce our
parallelism for no good reason.  So give back our tokens before waiting for
anybody else.

builder.py

@@ -273,10 +273,18 @@ def main(targets, shouldbuildfunc):
             fid,t = locked.pop(0)
             lock = state.Lock(fid)
             lock.trylock()
-            if not lock.owned:
+            while not lock.owned:
                 if vars.DEBUG_LOCKS:
                     warn('%s (WAITING)\n' % _nice(t))
+                # this sequence looks a little silly, but the idea is to
+                # give up our personal token while we wait for the lock to
+                # be released; but we should never run get_token() while
+                # holding a lock, or we could cause deadlocks.
+                jwack.release_mine()
                 lock.waitlock()
+                lock.unlock()
+                jwack.get_token(t)
+                lock.trylock()
             assert(lock.owned)
             if vars.DEBUG_LOCKS:
                 log('%s (...unlocked!)\n' % _nice(t))

jwack.py

@@ -24,6 +24,13 @@ def _release(n):
         _mytokens = 1
 
 
+def release_mine():
+    global _mytokens
+    assert(_mytokens >= 1)
+    os.write(_fds[1], 't')
+    _mytokens -= 1
+
+
 def _timeout(sig, frame):
     pass
 
@@ -161,9 +168,12 @@ def running():
 def wait_all():
     _debug("wait_all\n")
     while running():
+        while _mytokens >= 1:
+            release_mine()
         _debug("wait_all: wait()\n")
         wait(want_token=0)
     _debug("wait_all: empty list\n")
+    get_token('self')  # get my token back
     if _toplevel:
         bb = ''
         while 1: