repoview: fix corrupted hiddencache crash Mercurial (issue5042)

Before this patch if the hiddencache existed but was empty, it would crash
mercurial. This patch adds exception handling when reading the hiddencache to
avoid the issue.
When encountering a corrupted cache file we print a devel warning. There would
be no point in issuing a normal warning as the user wouldn't be able to do
anything about the situation.

The warning looks like:

devel-warn: corrupted hidden cache, removing it at: /path/to/repoview.py

mercurial/repoview.py

@@ -150,6 +150,13 def tryreadcache(repo, hideable):
                 count = len(data) / 4
                 hidden = frozenset(struct.unpack('>%ii' % count, data))
         return hidden
+    except struct.error:
+        repo.ui.debug('corrupted hidden cache\n')
+        # No need to fix the content as it will get rewritten
+        return None
+    except (IOError, OSError):
+        repo.ui.debug('cannot read hidden cache\n')
+        return None
     finally:
         if fh:
             fh.close()

tests/test-obsolete.t

@@ -951,6 +951,23 Test heads computation on pending index
   $ hg amendtransient
   [1, 3]
 
+Check that corrupted hidden cache does not crash
+
+  $ printf "" > .hg/cache/hidden
+  $ hg log -r . -T '{node}' --debug
+  corrupted hidden cache
+  8fd96dfc63e51ed5a8af1bec18eb4b19dbf83812 (no-eol)
+  $ hg log -r . -T '{node}' --debug
+  8fd96dfc63e51ed5a8af1bec18eb4b19dbf83812 (no-eol)
+
+Check that wrong hidden cache permission does not crash
+
+  $ chmod 000 .hg/cache/hidden
+  $ hg log -r . -T '{node}' --debug
+  cannot read hidden cache
+  error writing hidden changesets cache
+  8fd96dfc63e51ed5a8af1bec18eb4b19dbf83812 (no-eol)
+
 Test cache consistency for the visible filter
 1) We want to make sure that the cached filtered revs are invalidated when
 bookmarks change