builder.py: create temp log file in the same directory as the final one.

We're going to rename() it from the temp name to the final name, which
doesn't work across filesystems, so the safest option is to keep it in
the same directory.

Reported-by: spacefrogg@meterriblecrew.net

redo/builder.py

@@ -240,9 +240,15 @@ class _BuildJob(object):
         # reading a previous instance created during this session.  It
         # should always see either the old or new instance.
         if env.v.LOG:
-            lfd, lfname = tempfile.mkstemp(prefix='redo.', suffix='.log.tmp')
+            lfend = state.logname(self.sf.id)
+            # Make sure the temp file is in the same directory as lfend,
+            # so we can be sure of our ability to rename it atomically later.
+            lfd, lfname = tempfile.mkstemp(
+                prefix='redo.',
+                suffix='.log.tmp',
+                dir=os.path.dirname(lfend))
             os.fdopen(lfd, 'w')
-            os.rename(lfname, state.logname(self.sf.id))
+            os.rename(lfname, lfend)
         dof = state.File(name=os.path.join(dodir, dofile))
         dof.set_static()
         dof.save()