tags: behave better if a tags cache entry is partially written

This is done by discarding any partial cache entry, instead of
filling the partial cache entry with 0xff before.

Differential Revision: https://phab.mercurial-scm.org/D8095

mercurial/tags.py

@@ -720,15 +720,18 b' class hgtagsfnodescache(object):'
 
         self._dirtyoffset = None
 
-        if rawlen < wantedlen:
-            self._dirtyoffset = rawlen
-            self._raw.extend(b'\xff' * (wantedlen - rawlen))
-        elif rawlen > wantedlen:
+        rawlentokeep = min(wantedlen, (rawlen / _fnodesrecsize) * _fnodesrecsize)
+        if rawlen > rawlentokeep:
             # There's no easy way to truncate array instances. This seems
             # slightly less evil than copying a potentially large array slice.
-            for i in range(rawlen - wantedlen):
+            for i in range(rawlen - rawlentokeep):
                 self._raw.pop()
-            self._dirtyoffset = len(self._raw)
+            rawlen = len(self._raw)
+            self._dirtyoffset = rawlen
+        if rawlen < wantedlen:
+            if self._dirtyoffset is None:
+                self._dirtyoffset = rawlen
+            self._raw.extend(b'\xff' * (wantedlen - rawlen))
 
     def getfnode(self, node, computemissing=True):
         """Obtain the filenode of the .hgtags file at a specified revision.

tests/test-tags.t

@@ -371,25 +371,19 b' Extra junk data at the end should get ov'
   1970/01/01 00:00:00 bob @8dbfe60eff306a54259cfe007db9e330e7ecf866 (5000)> tags exited 0 after * seconds (glob)
   1970/01/01 00:00:00 bob @8dbfe60eff306a54259cfe007db9e330e7ecf866 (5000)> blackbox -l 6
 
-Junk data + missing cache entries results in hg further corrupting the
-cache, then failing.
+On junk data + missing cache entries, hg also overwrites the junk.
 
   $ rm -f .hg/cache/tags2-visible
   $ truncate .hg/cache/hgtagsfnodes1 -s -10
   $ hg debugtagscache | tail -2
   4 0c192d7d5e6b78a714de54a2e9627952a877e25a 0c04f2a8af31de17fab7422878ee5a2dadbc943d
-  5 8dbfe60eff306a54259cfe007db9e330e7ecf866 0c04f2a8af31de17fab7ffffffffffffffffffff
+  5 8dbfe60eff306a54259cfe007db9e330e7ecf866 missing/invalid
   $ hg tags
-  abort: data/.hgtags.i@0c04f2a8af31: no match found!
-  [255]
+  tip                                5:8dbfe60eff30
+  bar                                1:78391a272241
   $ hg debugtagscache | tail -2
   4 0c192d7d5e6b78a714de54a2e9627952a877e25a 0c04f2a8af31de17fab7422878ee5a2dadbc943d
-  5 8dbfe60eff306a54259cfe007db9e330e7ecf866 0c04f2a8af31de17fab7ffffffffffffffffffff
-# fix up the cache
-  $ rm .hg/cache/hgtagsfnodes1
-  $ hg tags -q
-  tip
-  bar
+  5 8dbfe60eff306a54259cfe007db9e330e7ecf866 0c04f2a8af31de17fab7422878ee5a2dadbc943d
 
 #if unix-permissions no-root
 Errors writing to .hgtags fnodes cache are silently ignored