tags: don't crash if unable to write tag cache

tags: don't crash if unable to write tag cache

This happens with hgweb in real life, if the httpd user is unable to
write in the repository directory. Another case is doing 'hg incoming'
on a repository in the filesystem owned by someone else.

--- a/mercurial/tags.py	Wed Aug 19 12:51:07 2009 +0200
+++ b/mercurial/tags.py	Tue Aug 18 22:07:43 2009 -0400
@@ -301,7 +301,10 @@
 
 def _writetagcache(ui, repo, heads, tagfnode, cachetags):
 
-    cachefile = repo.opener('tags.cache', 'w', atomictemp=True)
+    try:
+        cachefile = repo.opener('tags.cache', 'w', atomictemp=True)
+    except (OSError, IOError):
+        return
     _debug(ui, 'writing cache file %s\n' % cachefile.name)
 
     realheads = repo.heads()            # for sanity checks below

--- a/tests/test-tags	Wed Aug 19 12:51:07 2009 +0200
+++ b/tests/test-tags	Tue Aug 18 22:07:43 2009 -0400
@@ -37,9 +37,17 @@
 hg identify
 
 # repeat with cold tag cache
+echo "% identify with cold cache"
 rm -f .hg/tags.cache
 hg identify
 
+# and again, but now unable to write tag cache
+echo "% identify with unwritable cache"
+rm -f .hg/tags.cache
+chmod 555 .hg
+hg identify
+chmod 755 .hg
+
 echo "% create a branch"
 echo bb > a
 hg status

--- a/tests/test-tags.out	Wed Aug 19 12:51:07 2009 +0200
+++ b/tests/test-tags.out	Tue Aug 18 22:07:43 2009 -0400
@@ -13,6 +13,9 @@
 tip                                1:b9154636be93
 first                              0:acb14030fe0a
 b9154636be93 tip
+% identify with cold cache
+b9154636be93 tip
+% identify with unwritable cache
 b9154636be93 tip
 % create a branch
 M a