--- a/hgext/lfs/__init__.py	Sun Nov 26 14:59:39 2017 -0500
+++ b/hgext/lfs/__init__.py	Wed Nov 22 22:38:50 2017 -0500
@@ -128,6 +128,9 @@
     wrapfilelog(filelog.filelog)
 
     wrapfunction = extensions.wrapfunction
+
+    wrapfunction(scmutil, 'wrapconvertsink', wrapper.convertsink)
+
     wrapfunction(changegroup,
                  'supportedoutgoingversions',
                  wrapper.supportedoutgoingversions)

--- a/hgext/lfs/wrapper.py	Sun Nov 26 14:59:39 2017 -0500
+++ b/hgext/lfs/wrapper.py	Wed Nov 22 22:38:50 2017 -0500
@@ -165,6 +165,31 @@
 def filectxislfs(self):
     return _islfs(self.filelog(), self.filenode())
 
+def convertsink(orig, sink):
+    sink = orig(sink)
+    if sink.repotype == 'hg':
+        class lfssink(sink.__class__):
+            def putcommit(self, files, copies, parents, commit, source, revmap,
+                          full, cleanp2):
+                pc = super(lfssink, self).putcommit
+                node = pc(files, copies, parents, commit, source, revmap, full,
+                          cleanp2)
+
+                if 'lfs' not in self.repo.requirements:
+                    ctx = self.repo[node]
+
+                    # The file list may contain removed files, so check for
+                    # membership before assuming it is in the context.
+                    if any(f in ctx and ctx[f].islfs() for f, n in files):
+                        self.repo.requirements.add('lfs')
+                        self.repo._writerequirements()
+
+                return node
+
+        sink.__class__ = lfssink
+
+    return sink
+
 def vfsinit(orig, self, othervfs):
     orig(self, othervfs)
     # copy lfs related options

--- a/tests/test-lfs-largefiles.t	Sun Nov 26 14:59:39 2017 -0500
+++ b/tests/test-lfs-largefiles.t	Wed Nov 22 22:38:50 2017 -0500
@@ -290,14 +290,16 @@
   0 remove large_by_size.bin
   $ cd nolargefiles
 
-BUG: This should have a requires line for 'lfs'
+The requirement is added to the destination repo
 
   $ cat .hg/requires
   dotencode
   fncache
   generaldelta
+  lfs
   revlogv1
   store
+
   $ hg log -r 'all()' -G -T '{rev} {join(lfs_files, ", ")} ({desc})\n'
   o  8  (remove large_by_size.bin)
   |

--- a/tests/test-lfs.t	Sun Nov 26 14:59:39 2017 -0500
+++ b/tests/test-lfs.t	Wed Nov 22 22:38:50 2017 -0500
@@ -557,3 +557,82 @@
   repo: repo8
   repo: repo9
   repo: repo10
+
+lfs -> normal -> lfs round trip conversions are possible.  The threshold for the
+lfs destination is specified here because it was originally listed in the local
+.hgrc, and the global one is too high to trigger lfs usage.  For lfs -> normal,
+there's no 'lfs' destination repo requirement.  For normal -> lfs, there is.
+
+XXX: There's not a great way to ensure that the conversion to normal files
+actually converts _everything_ to normal.  The extension needs to be loaded for
+the source, but there's no way to disable it for the destination.  The best that
+can be done is to raise the threshold so that lfs isn't used on the destination.
+It doesn't like using '!' to unset the value on the command line.
+
+  $ hg --config extensions.convert= --config lfs.threshold=1000M \
+  >    convert repo8 convert_normal
+  initializing destination convert_normal repository
+  scanning source...
+  sorting...
+  converting...
+  2 a
+  1 b
+  0 meta
+  $ grep 'lfs' convert_normal/.hg/requires
+  [1]
+  $ hg --cwd convert_normal debugdata a1 0
+  THIS-IS-LFS-BECAUSE-10-BYTES
+
+  $ hg --config extensions.convert= --config lfs.threshold=10B \
+  >    convert convert_normal convert_lfs
+  initializing destination convert_lfs repository
+  scanning source...
+  sorting...
+  converting...
+  2 a
+  1 b
+  0 meta
+  $ hg --cwd convert_lfs debugdata a1 0
+  version https://git-lfs.github.com/spec/v1
+  oid sha256:5bb8341bee63b3649f222b2215bde37322bea075a30575aa685d8f8d21c77024
+  size 29
+  x-is-binary 0
+  $ grep 'lfs' convert_lfs/.hg/requires
+  lfs
+
+This convert is trickier, because it contains deleted files (via `hg mv`)
+
+  $ hg --config extensions.convert= --config lfs.threshold=1000M \
+  >    convert repo3 convert_normal2
+  initializing destination convert_normal2 repository
+  scanning source...
+  sorting...
+  converting...
+  4 commit with lfs content
+  3 renames
+  2 large to small, small to large
+  1 random modifications
+  0 switch large and small again
+  $ grep 'lfs' convert_normal2/.hg/requires
+  [1]
+  $ hg --cwd convert_normal2 debugdata large 0
+  LONGER-THAN-TEN-BYTES-WILL-TRIGGER-LFS
+
+  $ hg --config extensions.convert= --config lfs.threshold=10B \
+  >    convert convert_normal2 convert_lfs2
+  initializing destination convert_lfs2 repository
+  scanning source...
+  sorting...
+  converting...
+  4 commit with lfs content
+  3 renames
+  2 large to small, small to large
+  1 random modifications
+  0 switch large and small again
+  $ grep 'lfs' convert_lfs2/.hg/requires
+  lfs
+  $ hg --cwd convert_lfs2 debugdata large 0
+  version https://git-lfs.github.com/spec/v1
+  oid sha256:66100b384bf761271b407d79fc30cdd0554f3b2c5d944836e936d584b88ce88e
+  size 39
+  x-is-binary 0