Intermittent errors creating mock root cache tarball

[Paul Howarth]

I have a buildsystem that targets a number of different distribution 
releases, and so I get to rebuild a root cache quite often. Quite 
frequently, the creation of the root cache tarball fails and causes the 
package build that triggered the root cache creation to fail. However, 
simply repeating the build invariably succeeds, and mock uses the 
supposedly failed cache tarball from the previous build without problems.

I've not looked at this in detail because the workaround has been so 
easy but yesterday I decided to take a look at it. I think there are two 
issues.

Firstly, the cause of the tarball creation failure. Looking at the root 
log, it appeared to be a change in one of the files whilst it was being 
archived by tar.

DEBUG util.py, Line: 234:  tar:
./usr/lib/locale/locale-archive: file changed as we read it

The same problem with the same file was menioned in a report dating back 
two years on fedora-devel-list:
http://www.redhat.com/archives/fedora-devel-list/2007-November/msg02599.html

More googling revealed a possible cause of the problem:
http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg190963.html

So I tried forcing a "sync" before creating the tarball and lo and 
behold, the problem went away. I've created at least 20 root caches 
since making this change and all worked fine, which I'm very confident 
wouldn't have been the case without the "sync". So here's the change I made:

--- /usr/lib/python2.6/site-packages/mock/plugins/root_cache.py.orig 
2009-09-02 19:08:54.000000000 +0100
+++ /usr/lib/python2.6/site-packages/mock/plugins/root_cache.py 
2009-11-18 15:20:04.353035160 +0000
@@ -110,6 +110,7 @@
              # never rebuild cache unless it was a clean build.
              if self.rootObj.chrootWasCleaned:
                  self.state("creating cache")
+                mock.util.do(["sync"], shell=False)
                  mock.util.do(
                      ["tar"] + self.compressArgs + ["-cf", 
self.rootCacheFile,
                       "-C", self.rootObj.makeChrootPath(), "."],

The second problem is I think that if the "tar" process to create the 
tarball fails (and hence causes the resulting build to fail), the cache 
should be invalidated so that the next build doesn't use that 
presumably-broken tarball. As it happens, a faulty copy of 
/usr/lib/locale/locale-archive doesn't seem to cause any problems during 
my builds but that may just be my good fortune.

Cheers, Paul.