transaction: allow generating file outside of store

We allow a vfs argument to `addfilegenerator`. This allows generating files outside
of the store directory like bookmarks. However, this is not really working since
we do not have the infrastructure to backup and restore files outside of store.
By chance, the bookmark file is already backed up by another mechanism so we can
restrict this new feature to bookmarks (which is our only interest here) and
proceed.

mercurial/transaction.py

@@ -144,7 +144,7 b' class transaction(object):'
         self.file.flush()
 
     @active
-    def addbackup(self, file, hardlink=True):
+    def addbackup(self, file, hardlink=True, vfs=None):
         """Adds a backup of the file to the transaction
 
         Calling addbackup() creates a hardlink backup of the specified file
@@ -158,8 +158,10 b' class transaction(object):'
         if file in self.map or file in self.backupmap:
             return
         backupfile = "%s.backup.%s" % (self.journal, file)
-        if self.opener.exists(file):
-            filepath = self.opener.join(file)
+        if vfs is None:
+            vfs = self.opener
+        if vfs.exists(file):
+            filepath = vfs.join(file)
             backuppath = self.opener.join(backupfile)
             util.copyfiles(filepath, backuppath, hardlink=hardlink)
         else:
@@ -176,7 +178,7 b' class transaction(object):'
         self.backupsfile.flush()
 
     @active
-    def addfilegenerator(self, genid, filenames, genfunc, order=0):
+    def addfilegenerator(self, genid, filenames, genfunc, order=0, vfs=None):
         """add a function to generates some files at transaction commit
 
         The `genfunc` argument is a function capable of generating proper
@@ -195,7 +197,10 b' class transaction(object):'
         The `order` argument may be used to control the order in which multiple
         generator will be executed.
         """
-        self._filegenerators[genid] = (order, filenames, genfunc)
+        # For now, we are unable to do proper backup and restore of custom vfs
+        # but for bookmarks that are handled outside this mechanism.
+        assert vfs is None or filenames == ('bookmarks',)
+        self._filegenerators[genid] = (order, filenames, genfunc, vfs)
 
     @active
     def find(self, file):
@@ -239,16 +244,19 b' class transaction(object):'
     def close(self):
         '''commit the transaction'''
         # write files registered for generation
-        for order, filenames, genfunc in sorted(self._filegenerators.values()):
+        for entry in sorted(self._filegenerators.values()):
+            order, filenames, genfunc, vfs = entry
+            if vfs is None:
+                vfs = self.opener
             files = []
             try:
                 for name in filenames:
                     # Some files are already backed up when creating the
                     # localrepo. Until this is properly fixed we disable the
                     # backup for them.
-                    if name not in ('phaseroots',):
+                    if name not in ('phaseroots', 'bookmarks'):
                         self.addbackup(name)
-                    files.append(self.opener(name, 'w', atomictemp=True))
+                    files.append(vfs(name, 'w', atomictemp=True))
                 genfunc(*files)
             finally:
                 for f in files: