Add cyclic dependence detection.

If a depends on b which depends on a, redo would just freeze.  Now it
aborts with a somewhat helpful error message.

[Updated by apenwarr for coding style and to add a test.]

builder.py

@@ -206,6 +206,7 @@ class BuildJob:
         os.environ['REDO_PWD'] = state.relpath(newp, vars.STARTDIR)
         os.environ['REDO_TARGET'] = self.basename + self.ext
         os.environ['REDO_DEPTH'] = vars.DEPTH + '  '
+        vars.add_lock(str(self.lock.fid))
         if dn:
             os.chdir(dn)
         os.dup2(self.f.fileno(), 1)
@@ -378,7 +379,13 @@ def main(targets, shouldbuildfunc):
                 # be released; but we should never run get_token() while
                 # holding a lock, or we could cause deadlocks.
                 jwack.release_mine()
-                lock.waitlock()
+                try:
+                    lock.waitlock()
+                except state.CyclicDependencyError:
+                    err('cyclic dependency while building %s\n' % _nice(t))
+                    jwack.get_token(t)
+                    retcode[0] = 208
+                    return retcode[0]
                 lock.unlock()
                 jwack.get_token(t)
                 lock.trylock()