fncache: make debugrebuildfncache not fail on broken fncache

The code reading the fncache changed in 5.0, to complain if the file
is not \n terminated. This makes apparent the fact that the fncache
gets corrupted.

Make it possible to recover, instead of having `hg
debugrebuildfncache` failing by saying `(run hg debugrebuildfncache)`.

The corruption itself is most likely due to hg not using fsync in
general, and so various bad things can happen. Here, the reported
problems happened when running out of disk space. So I suspect that
because the fncache is much bigger than the average commit/pull, when
running out of disk space, the bulk of the pull may succeed, but the
new fncache may get half-written and still renamed into place.

Differential Revision: https://phab.mercurial-scm.org/D6722

mercurial/repair.py

@@ -392,9 +392,7 b' def rebuildfncache(ui, repo):'
 
     with repo.lock():
         fnc = repo.store.fncache
-        # Trigger load of fncache.
-        if 'irrelevant' in fnc:
-            pass
+        fnc.ensureloaded(warn=ui.warn)
 
         oldentries = set(fnc.entries)
         newentries = set()

mercurial/store.py

@@ -458,7 +458,15 b' class fncache(object):'
         # set of new additions to fncache
         self.addls = set()
 
-    def _load(self):
+    def ensureloaded(self, warn=None):
+        '''read the fncache file if not already read.
+
+        If the file on disk is corrupted, raise. If warn is provided,
+        warn and keep going instead.'''
+        if self.entries is None:
+            self._load(warn)
+
+    def _load(self, warn=None):
         '''fill the entries from the fncache file'''
         self._dirty = False
         try:
@@ -482,19 +490,26 b' class fncache(object):'
                 pass
 
         if chunk:
-            raise error.Abort(_("fncache does not ends with a newline"),
-                              hint=_("use 'hg debugrebuildfncache' to rebuild"
-                                     " the fncache"))
-        self._checkentries(fp)
+            msg = _("fncache does not ends with a newline")
+            if warn:
+                warn(msg + '\n')
+            else:
+                raise error.Abort(msg,
+                                  hint=_("use 'hg debugrebuildfncache' to "
+                                         "rebuild the fncache"))
+        self._checkentries(fp, warn)
         fp.close()
 
-    def _checkentries(self, fp):
+    def _checkentries(self, fp, warn):
         """ make sure there is no empty string in entries """
         if '' in self.entries:
             fp.seek(0)
             for n, line in enumerate(util.iterfile(fp)):
                 if not line.rstrip('\n'):
                     t = _('invalid entry in fncache, line %d') % (n + 1)
+                    if warn:
+                        warn(t + '\n')
+                    else:
                     raise error.Abort(t)
 
     def write(self, tr):

tests/test-fncache.t

@@ -435,16 +435,18 b' A single missing file should get restore'
   data/.bar.i
   data/foo.i
 
-debugrebuildfncache fails to recover from truncated line in fncache
+debugrebuildfncache recovers from truncated line in fncache
 
   $ printf a > .hg/store/fncache
   $ hg debugrebuildfncache
-  abort: fncache does not ends with a newline
-  (use 'hg debugrebuildfncache' to rebuild the fncache)
-  [255]
+  fncache does not ends with a newline
+  adding data/.bar.i
+  adding data/foo.i
+  2 items added, 0 removed from fncache
 
   $ cat .hg/store/fncache | sort
-  a
+  data/.bar.i
+  data/foo.i
 
   $ cd ..