blackbox: do not prevent 'chg init' from working

Previously, blackbox always appends to blackbox.log and creates the
directory for that file on demand. That could be an issue if:

  1. chg starts from `$REPO` directory, so `ui._bbrepo` is set.
  2. `rm -rf $REPO`.
  3. `chg init $REPO`, blackbox writes something and `init` will fail
     because `$REPO` directory is non-empty.

This patch fixes that by verifying whether vfs exists before re-using it.

Differential Revision: https://phab.mercurial-scm.org/D768

hgext/blackbox.py

@@ -77,9 +77,13 b' def wrapui(ui):'
     class blackboxui(ui.__class__):
         @property
         def _bbvfs(self):
+            vfs = None
             repo = getattr(self, '_bbrepo', None)
             if repo:
-                return repo.vfs
+                vfs = repo.vfs
+                if not vfs.isdir('.'):
+                    vfs = None
+            return vfs
 
         @util.propertycache
         def track(self):
@@ -136,6 +140,10 b' def wrapui(ui):'
 
             if not ui:
                 return
+            vfs = ui._bbvfs
+            if not vfs:
+                return
+
             repo = getattr(ui, '_bbrepo', None)
             if not lastui or repo:
                 lastui = ui

tests/test-blackbox.t

@@ -293,6 +293,13 b' when using chg, blackbox.log should get '
 
   $ cd ..
 
+With chg, blackbox should not create the log file if the repo is gone
+
+  $ hg init repo1
+  $ hg --config extensions.a=! -R repo1 log
+  $ rm -rf $TESTTMP/repo1
+  $ hg --config extensions.a=! init repo1
+
 #endif
 
 blackbox should work if repo.ui.log is not called (issue5518)