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Bug 871212

Summary: [RFE] Atomic persist config file
Product: Red Hat Enterprise Linux 6 Reporter: Alon Bar-Lev <alonbl>
Component: ovirt-nodeAssignee: Joey Boggs <jboggs>
Status: CLOSED ERRATA QA Contact: Virtualization Bugs <virt-bugs>
Severity: low Docs Contact:
Priority: medium    
Version: 6.3CC: abaron, acathrow, bazulay, bsarathy, chetan, cshao, dougsland, gouyang, hadong, iheim, jboggs, leiwang, mburns, ovirt-maint, ycui
Target Milestone: rcKeywords: FutureFeature, Improvement
Target Release: ---   
Hardware: Unspecified   
OS: Unspecified   
Whiteboard:
Fixed In Version: ovirt-node-2.5.0-12.el6 Doc Type: Enhancement
Doc Text:
It is now possible to atomically persist configuration files. This feature allows users to update persisted files without overwriting the files' content and potentially breaking the system with misconfigured code.
Story Points: ---
Clone Of: Environment:
Last Closed: 2013-02-28 16:44:16 UTC Type: Bug
Regression: --- Mount Type: ---
Documentation: --- CRM:
Verified Versions: Category: ---
oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
Embargoed:

Description Alon Bar-Lev 2012-10-29 23:08:47 UTC
Hello,

Currently there is no way to perform atomic store of configuration file. The persistence API of ovirtnode.ovirtfunctions can fail in many stages and leave system in unpredictable state.

I suggest an API like:

persist(file, destination)

1. cp file /config/destination.tmp
2. mv /config/destination.tmp /config/destination
3. rebind /config/destination destination

This way we have either the content of the new or old file.

---

BTW:

Looking at code, please try to do as much as you can in python. I don't understand the reason of calling system utilities such as:

system("mkdir -p /config/" + dirname)
->os.makedirs(os.path.join('/config', dirname))

system("rm -f /config"+ filename)
->shutil.rmtree(os.path.join('/config', dirname))

system_closefds("echo "+filename+" >> /config/files")
->with open('/config/files', 'a') as f:
    f.write("%s\n" % filename)

ret = system_closefds("grep -q \"^$" + filename +"$\" " + \
                " /config/files 2> /dev/null")
->with open('/config/files', 'r') as f:
    if filename in f:
        ....
etc...

Using python only reduces the blocks that can fail, provide better control on code flow and much better perform.

In these small cases you must call external program, please try to avoid system() as it may route output and input of caller which is an unexpected behaviour of an API. Use subprocess.Popen().

Thank you.

Comment 16 errata-xmlrpc 2013-02-28 16:44:16 UTC
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2013-0556.html