blackbox: avoid creating multiple file handles for a single log

There are multiple ui objects in Mercurial that can relate to a repository,
before this change, each one would have its own file pointer, which
results in unfortunate logging behavior.

Also, any log rotation results would be bad because only the
active blackboxui object's file pointer would be refreshed.

Note that this does not prevent two long running hg commands for the same
repository from causing problems.

hgext/blackbox.py

@@ -49,6 +49,21 b' command = cmdutil.command(cmdtable)'
 testedwith = 'internal'
 lastblackbox = None
 
+filehandles = {}
+
+def _openlog(vfs):
+    path = vfs.join('blackbox.log')
+    if path in filehandles:
+        return filehandles[path]
+    filehandles[path] = fp = vfs('blackbox.log', 'a')
+    return fp
+
+def _closelog(vfs):
+    path = vfs.join('blackbox.log')
+    fp = filehandles[path]
+    del filehandles[path]
+    fp.close()
+
 def wrapui(ui):
     class blackboxui(ui.__class__):
         @util.propertycache
@@ -71,20 +86,20 b' def wrapui(ui):'
                         self.debug("warning: cannot rename '%s' to '%s': %s\n" %
                                    (newpath, oldpath, err.strerror))
 
-            fp = self._bbvfs('blackbox.log', 'a')
+            fp = _openlog(self._bbvfs)
             maxsize = self.configbytes('blackbox', 'maxsize', 1048576)
             if maxsize > 0:
                 st = self._bbvfs.fstat(fp)
                 if st.st_size >= maxsize:
                     path = fp.name
-                    fp.close()
+                    _closelog(self._bbvfs)
                     maxfiles = self.configint('blackbox', 'maxfiles', 7)
                     for i in xrange(maxfiles - 1, 1, -1):
                         rotate(oldpath='%s.%d' % (path, i - 1),
                                newpath='%s.%d' % (path, i))
                     rotate(oldpath=path,
                            newpath=maxfiles > 0 and path + '.1')
-                    fp = self._bbvfs('blackbox.log', 'a')
+                    fp = _openlog(self._bbvfs)
             return fp
 
         def log(self, event, *msg, **opts):