--- a/mercurial/commands.py	Tue Jun 23 13:56:53 2015 -0400
+++ b/mercurial/commands.py	Mon Jun 22 09:59:48 2015 -0700
@@ -21,7 +21,7 @@
 import dagparser, context, simplemerge, graphmod, copies
 import random
 import setdiscovery, treediscovery, dagutil, pvec, localrepo
-import phases, obsolete, exchange, bundle2
+import phases, obsolete, exchange, bundle2, repair
 import ui as uimod
 
 table = {}
@@ -2728,6 +2728,11 @@
     finally:
         wlock.release()
 
+@command('debugrebuildfncache', [], '')
+def debugrebuildfncache(ui, repo):
+    """rebuild the fncache file"""
+    repair.rebuildfncache(ui, repo)
+
 @command('debugrename',
     [('r', 'rev', '', _('revision to debug'), _('REV'))],
     _('[-r REV] FILE'))

--- a/mercurial/repair.py	Tue Jun 23 13:56:53 2015 -0400
+++ b/mercurial/repair.py	Mon Jun 22 09:59:48 2015 -0700
@@ -227,3 +227,72 @@
             vfs.unlink(chgrpfile)
 
     repo.destroyed()
+
+def rebuildfncache(ui, repo):
+    """Rebuilds the fncache file from repo history.
+
+    Missing entries will be added. Extra entries will be removed.
+    """
+    repo = repo.unfiltered()
+
+    if 'fncache' not in repo.requirements:
+        ui.warn(_('(not rebuilding fncache because repository does not '
+                  'support fncache\n'))
+        return
+
+    lock = repo.lock()
+    try:
+        fnc = repo.store.fncache
+        # Trigger load of fncache.
+        if 'irrelevant' in fnc:
+            pass
+
+        oldentries = set(fnc.entries)
+        newentries = set()
+        seenfiles = set()
+
+        repolen = len(repo)
+        for rev in repo:
+            ui.progress(_('changeset'), rev, total=repolen)
+
+            ctx = repo[rev]
+            for f in ctx.files():
+                # This is to minimize I/O.
+                if f in seenfiles:
+                    continue
+                seenfiles.add(f)
+
+                i = 'data/%s.i' % f
+                d = 'data/%s.d' % f
+
+                if repo.store._exists(i):
+                    newentries.add(i)
+                if repo.store._exists(d):
+                    newentries.add(d)
+
+        ui.progress(_('changeset'), None)
+
+        addcount = len(newentries - oldentries)
+        removecount = len(oldentries - newentries)
+        for p in sorted(oldentries - newentries):
+            ui.write(_('removing %s\n') % p)
+        for p in sorted(newentries - oldentries):
+            ui.write(_('adding %s\n') % p)
+
+        if addcount or removecount:
+            ui.write(_('%d items added, %d removed from fncache\n') %
+                     (addcount, removecount))
+            fnc.entries = newentries
+            fnc._dirty = True
+
+            tr = repo.transaction('fncache')
+            try:
+                fnc.write(tr)
+                tr.close()
+            finally:
+                tr.release()
+        else:
+            ui.write(_('fncache already up to date\n'))
+    finally:
+        lock.release()
+

--- a/tests/test-completion.t	Tue Jun 23 13:56:53 2015 -0400
+++ b/tests/test-completion.t	Mon Jun 22 09:59:48 2015 -0700
@@ -96,6 +96,7 @@
   debugpushkey
   debugpvec
   debugrebuilddirstate
+  debugrebuildfncache
   debugrename
   debugrevlog
   debugrevspec
@@ -254,6 +255,7 @@
   debugpushkey: 
   debugpvec: 
   debugrebuilddirstate: rev
+  debugrebuildfncache: 
   debugrename: rev
   debugrevlog: changelog, manifest, dir, dump
   debugrevspec: optimize

--- a/tests/test-fncache.t	Tue Jun 23 13:56:53 2015 -0400
+++ b/tests/test-fncache.t	Mon Jun 22 09:59:48 2015 -0700
@@ -283,3 +283,98 @@
   1 files, 1 changesets, 1 total revisions
   $ cat .hg/store/fncache
   data/y.i
+
+  $ cd ..
+
+debugrebuildfncache does nothing unless repo has fncache requirement
+
+  $ hg --config format.usefncache=false init nofncache
+  $ cd nofncache
+  $ hg debugrebuildfncache
+  (not rebuilding fncache because repository does not support fncache
+
+  $ cd ..
+
+debugrebuildfncache works on empty repository
+
+  $ hg init empty
+  $ cd empty
+  $ hg debugrebuildfncache
+  fncache already up to date
+  $ cd ..
+
+debugrebuildfncache on an up to date repository no-ops
+
+  $ hg init repo
+  $ cd repo
+  $ echo initial > foo
+  $ echo initial > .bar
+  $ hg commit -A -m initial
+  adding .bar
+  adding foo
+
+  $ cat .hg/store/fncache | sort
+  data/.bar.i
+  data/foo.i
+
+  $ hg debugrebuildfncache
+  fncache already up to date
+
+debugrebuildfncache restores deleted fncache file
+
+  $ rm -f .hg/store/fncache
+  $ hg debugrebuildfncache
+  adding data/.bar.i
+  adding data/foo.i
+  2 items added, 0 removed from fncache
+
+  $ cat .hg/store/fncache | sort
+  data/.bar.i
+  data/foo.i
+
+Rebuild after rebuild should no-op
+
+  $ hg debugrebuildfncache
+  fncache already up to date
+
+A single missing file should get restored, an extra file should be removed
+
+  $ cat > .hg/store/fncache << EOF
+  > data/foo.i
+  > data/bad-entry.i
+  > EOF
+
+  $ hg debugrebuildfncache
+  removing data/bad-entry.i
+  adding data/.bar.i
+  1 items added, 1 removed from fncache
+
+  $ cat .hg/store/fncache | sort
+  data/.bar.i
+  data/foo.i
+
+  $ cd ..
+
+Try a simple variation without dotencode to ensure fncache is ignorant of encoding
+
+  $ hg --config format.dotencode=false init nodotencode
+  $ cd nodotencode
+  $ echo initial > foo
+  $ echo initial > .bar
+  $ hg commit -A -m initial
+  adding .bar
+  adding foo
+
+  $ cat .hg/store/fncache | sort
+  data/.bar.i
+  data/foo.i
+
+  $ rm .hg/store/fncache
+  $ hg debugrebuildfncache
+  adding data/.bar.i
+  adding data/foo.i
+  2 items added, 0 removed from fncache
+
+  $ cat .hg/store/fncache | sort
+  data/.bar.i
+  data/foo.i

--- a/tests/test-help.t	Tue Jun 23 13:56:53 2015 -0400
+++ b/tests/test-help.t	Mon Jun 22 09:59:48 2015 -0700
@@ -796,6 +796,8 @@
    debugrebuilddirstate
                  rebuild the dirstate as it would look like for the given
                  revision
+   debugrebuildfncache
+                 rebuild the fncache file
    debugrename   dump rename information
    debugrevlog   show data and statistics about a revlog
    debugrevspec  parse and apply a revision specification