--- a/mercurial/cmdutil.py	Tue Oct 06 02:23:21 2015 -0700
+++ b/mercurial/cmdutil.py	Tue Oct 06 09:51:24 2015 -0700
@@ -831,6 +831,22 @@
     if runfn:
         return runfn()
 
+## facility to let extension process additional data into an import patch
+# list of identifier to be executed in order
+extrapreimport = []  # run before commit
+# mapping from identifier to actual import function
+#
+# 'preimport' are run before the commit is made and are provided the following
+# arguments:
+# - repo: the localrepository instance,
+# - patchdata: data extracted from patch header (cf m.patch.patchheadermap),
+# - extra: the future extra dictionnary of the changeset, please mutate it,
+# - opts: the import options.
+# XXX ideally, we would just pass an ctx ready to be computed, that would allow
+# mutation of in memory commit and more. Feel free to rework the code to get
+# there.
+extrapreimportmap = {}
+
 def tryimportone(ui, repo, hunk, parents, opts, msgs, updatefunc):
     """Utility function used by commands.import to import a single patch
 
@@ -949,12 +965,15 @@
                 else:
                     editor = getcommiteditor(editform=editform, **opts)
                 allowemptyback = repo.ui.backupconfig('ui', 'allowemptycommit')
+                extra = {}
+                for idfunc in extrapreimport:
+                    extrapreimportmap[idfunc](repo, extractdata, extra, opts)
                 try:
                     if partial:
                         repo.ui.setconfig('ui', 'allowemptycommit', True)
                     n = repo.commit(message, opts.get('user') or user,
                                     opts.get('date') or date, match=m,
-                                    editor=editor)
+                                    editor=editor, extra=extra)
                 finally:
                     repo.ui.restoreconfig(allowemptyback)
             dsguard.close()

--- a/tests/test-import.t	Tue Oct 06 02:23:21 2015 -0700
+++ b/tests/test-import.t	Tue Oct 06 09:51:24 2015 -0700
@@ -1505,3 +1505,43 @@
   $ hg status -c .
   C a
   C b
+
+Importing some extra header
+===========================
+
+  $ cat > $TESTTMP/parseextra.py <<EOF
+  > import mercurial.patch
+  > import mercurial.cmdutil
+  > 
+  > def processfoo(repo, data, extra, opts):
+  >     if 'foo' in data:
+  >         extra['foo'] = data['foo']
+  > 
+  > mercurial.patch.patchheadermap.append(('Foo', 'foo'))
+  > mercurial.cmdutil.extrapreimport.append('foo')
+  > mercurial.cmdutil.extrapreimportmap['foo'] = processfoo
+  > EOF
+  $ printf "[extensions]\nparseextra=$TESTTMP/parseextra.py" >> $HGRCPATH
+  $ hg up -C tip
+  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ cat > $TESTTMP/foo.patch <<EOF
+  > # HG changeset patch
+  > # User Rataxes
+  > # Date 0 0
+  > #      Thu Jan 01 00:00:00 1970 +0000
+  > # Foo bar
+  > height
+  > 
+  > --- a/a	Thu Jan 01 00:00:00 1970 +0000
+  > +++ b/a	Wed Oct 07 09:17:44 2015 +0000
+  > @@ -5,3 +5,4 @@
+  >  five
+  >  six
+  >  seven
+  > +heigt
+  > EOF
+  $ hg import $TESTTMP/foo.patch
+  applying $TESTTMP/foo.patch
+  $ hg log --debug -r . | grep extra
+  extra:       branch=default
+  extra:       foo=bar