GSoC - week 10+11

Last week I've been working on improving the LVM plugin thinpool sharing capabilities. I didn't explain LVM thin-provisioning before, so I'll do it now to rationalize what I'm doing.
With classical volumes you have a volume with given size and it always occupies the whole space that was given to it at the time of creation, which means that when you have more volumes, you usually can't use the space very efficiently because some of the volumes aren't used up to the capacity whereas others are full. Resizing them is a costly and risky operation. With LVM thin-provisioning there's one volume called thinpool, which provides space to thin volumes that are created within. The thin volumes take only as much space as they need and they don't have physical size limit (unless you set virtual size limit). That means that if the space is not used by one volume it can be used by another.
Previously there was one thinpool per configuration which corresponded to one buildroot. It could have snapshots, but there was still one buildroot that could be used at the moment. Now you can use mock's unique-ext mechanism to spawn more buildroots from single config. Unique-ext was there even before I started making changes, but now I implemented proper support for it in the LVM plugin. It's a feature that was designed mostly for buildsystems, but I think it can also be very useful for regular users who want to have more builds running at the same time. With LVM plugin the thinpool and all the snapshots are shared between the unique-exts, which means you can have multiple builds sharing the same cache and each one can be based on different snapshot. The naming scheme had to be changed to have multiple working volumes where the builds are executed. Mock implements locking of the initial volume creation, so if you launch two builds from the same config and there wasn't any snapshot before, only one of the processes will create the initial snapshot with base packages. The other process will block for that time, because otherwise you'll end up with two identical snapshots and that would be a waste of resources.
Other sharing mechanism that is now implemented is sharing the thinpool among independent configs. Then the snapshots aren't shared because the configs can be entirely different (for example different versions of Fedora), but you can have only one big logical volume (the thinpool) for all mock related stuff, which can save a lot of space for people that often use many different configs. You can set it with config_opts['pool_name'] = 'my-pool' and then all the configs with pool_name set to the same name will share the same thinpool.
Other than that I was mostly fixing bugs and communicating with upstream.

This week I've been on Flock and it has been amazing. There were some talks that are relevant for mock, most notably State of Copr build service, which will probably use some of the new features of mock in the future and Env&Stacks WG plans, which also mentioned mock improvements as one of their areas of interest.

GSoC - Mock improvements - week 9

This week I've been mostly focusing on minor improvements and documentation (manpages). Almost all my changes were already submitted upstream and if everything goes well, you can expect a new release of mock to be available in rawhide in the near future. I merged changes from the ready branch to master so now they should differ only in minor things. (Sorry for duplicates in git history, I didn't realize that beforehand)

Support for Mikolaj's nosync external library was added and the old implementations that existed as a part of mock were dropped. You can enable it by setting
config_opts['nosync'] = True
and you have to install the nosync library (mock doesn't require it in the specfile, because it's not available everywhere). If the target is multilib, you need both aritectures of the library to be installed in order to have a preload library for both types of executables. If you don't, it will print a warning and won't be activated. If you can't install both versions and still want to use it, set
config_opts['nosync_force'] = True
but expect a lot of (harmless) warnings from ld.so.  The library is available in rawhide (your mirrors might not have picked it yet)

LVM plugin was moved to separate subpackage and conditionaly disabled on RHEL 6, since it requires lvm2-python-libs and newer kernel and glibc (for setns). One of the things that I needed to sacrifice when I was making the LVM plugin was the IPC namespace unsharing, which mock uses for a long time. The problem was that lvcreate and other commands deadlocked on unitialized semaphore in the new namespace, so I temporarily disabled it and hoped I'll find a solution later. And I did, I wrapped all functions that manipulate LVM in function that calls setns to get back to global IPC namespace and after the command is done, it call setns again to get back to mock's IPC namespace.

One of the other problems I encountered is Python's (2.7) handling of SIGPIPE signal. It sets it to ignored and doesn't set it back to default when it executes a new process, so a shell launched from Python 2 (by Popen, or execve) doesn't always behave the same as regular shell.
Example in shell:
$ cat /dev/zero | head -c5
# cat got SIGPIPE and exited without error

$ python -c 'import subprocess as s; s.call(["bash"])'
$ cat /dev/zero | head -c5
cat: write error: Broken pipe
# SIGPIPE was ignored and cat got EPIPE from write()
It can be fixed by calling
signal.signal(signal.SIGPIPE, signal.SIG_DFL)
in Popen's preexec_fn argument.

For cat it's just an example and it didn't make much difference. But if you put tee between the cat and head, it will loop indefinitely instead of exiting after first 5 bytes. And there are lots of scripts out there relying on the standard behavior. It actually bit me in one of my other programs, so I thought it's worth sharing and I also fixed it in mock.

GSoC - Mock improvements - week 8

Good news, we're merging my changes upstream. It's been a lot of changes and the code wasn't always in the best shape, so I didn't want to submit it before all major features are implemented. Mirek Suchy agreed he'll do the code review and merge the changes. Big thanks to him for that :)
I've setup a new branch, rebased it on top of current upstream and tried to revisit all my code and get rid of changes that were reverted/superseded, or are not appropriate for merging yet.  I squashed fixup commits to their originl counterparts to reduce the number of commits and changed lines.
The changes that weren't submitted are:

• C nofsync library, because Mikolaj made a more robust nosync library that is packaged separately, and therefore supersedes the bundled one.
  Link: https://github.com/kjn/nosync/
  I did the review: https://bugzilla.redhat.com/show_bug.cgi?id=1118850
  That way mock can stay noarch, which gets rid of lots of packaging issues. And also saves me a lot problems with autoconf/automake. There is no support for it yet, because I need to figure out how to make it work correctly in multilib environment.
• nofsync DNF plugin - it's an ugly DNF hack and I consider it superseded by aforementioned nosync library
• noverify plugin - it's also a DNF hack, I will make a RFE for optional verification in DNF upstream instead

Everything else was submitted including the LVM plugin.  The merging branch is not pushed on github because I frequently need to make changes by interactive rebasing and force pushing the branch each time kind of defeats the purpose of SCM.
Other than that I was mostly fixing bugs, the only new features are the possibility of specifying additional commandline options to rpmbuild, such as --rpmfcdebug with --rpmbuild-opts option and ability to override command executable paths for rpm, rpmbuild, yum, yum-builddep and dnf, in order to be able to use different version of the tools than the system-wide version.

GSoC 2014 - week 7

Hi again, I'm sorry I didn't post last week, because I've been on a vacation.
Here's what I've done this week:

Passing additional options to underlying tools
rpmbuild has an option --short-circuit that skips stages of build preceding the specified one. It doesn't build a complete RPM package, but it's very handy for debugging builds that fail, especially in the install section. But this option is not accessible from within mock and I already mentioned in my proposal that I want to make it available. The option is also called --short-circuit and it accepts an argument - either build, install, or binary, representing the build phase that will be the first while the preceding phases would be skipped.
Example invocation:
$ mock rnv-1.7.11-6.fc21.src.rpm --short-circuit install

For Yum or DNF some of the options that are often used when user invokes the package manager directly also weren't available in mock. --enablerepo and --disablerepo are very common ones and now they are also supported by mock - they're directly passed to the underlying package manager.
Example invocation:
$ mock --install xmvn maven-local --enablerepo jenkins-xmvn --enablerepo jenkins-javapackages
The repos of course have to be present in the yum.conf in mock config.

Python 3 support
I started working on porting mock to Python 3. This doesn't mean that mock will run on Python 3 only, I'm trying to preserve compatibility with Python 2.6 without the need to have two version of mock for each. I changed the trace_decorator to use regular Python decorators instead of peak.utils.decorate and dropped dependency on the decoratortools package. There are slight changes in traceLog's output, that I don't consider important, but if someone did, it could be solved by using python-decorator package, which is available for both versions. There are some features that are still untested, but the regularly used functionality is already working. Rebuilding RPMs, SRPMs, working in shell, manipulating packages is tested. The plugins, that are enabled by default (yum-cache, root-cache, ccache, selinux) also work. What doesn't work is the LVM plugin, because it uses lvm2-python-libs, which doesn't have a Python 3 version yet. Same applies to mockchain, which uses urlgrabber. To try mock with Python 3, either change your system default Python implementation or manually hardcode python3 as the interpreter to the shebang in /usr/sbin/xmock.

GSoC week 5

Nofsync
I created another implementation of nofsync plugin (disables fsync(), makes it much faster), this time in python as DNF plugin that disables fsyncing in the YumDB. It is a small bit slower than the C library using LD_PRELOAD, because it doesn't eliminate fsyncs made from scriptlets (by gtk-update-icon-cache and such). But it's much simpler from packaging perspective (mock can stay noarch) and could be actually upstreamable (in dnf), because there are some other use cases, where you don't try to recover from hardware failure anyway - for example anaconda. If the power goes down, you probably don't try to resume existing installation. And this could make it faster (nofsync makes package installation approximately 3 times faster).
To compare the two implementations, set either
config_opts['nofsync'] = 'python'
or
config_opts['nofsync'] = 'ld_preload'
Default is python, to disable it, set the option to something else (empty string)

LVM support
Last week I implemented base for LVM plugin for mock using regular snapshots. This week I rewrote the plugin to use LVM Thin snapshots, which offer better performance, flexibility and share the space with the original volume and other snapshots, therefore don't waste much space. I created basic commands that can be used to manipulate the snapshots.
Example workflow:
I'll try to demonstrate how building different packages can be faster with LVM plugin. Let's repeat the configuration options necessary to set it up:
config_opts['plugin_conf']['root_cache_enable'] = False
config_opts['plugin_conf']['lvm_root_enable'] = True
config_opts['plugin_conf']['lvm_root_opts'] = {
    'volume_group': 'my-volume-group',
}
You can now also specify 'mount_options', which will be passed to -o option of mount. To set size to larger than the default 2GB, use for example 'size': '4G' (it is passed to lvcreate's -L option, so it can be any string lvcreate will understand). Now let's initialize it:
$ mock --init
Mock will now create thin pool with given size, create a logical volume in it, mount it and install the base packages into it. After the initialization is done, it creates a new snapshot named 'postinit', which will be then used to rollback changes during --clean (which is by default also executed as part of --rebuild). Now try to install some packages you often use for building your own packages. I'm a Java packager and almost every Java package in Fedora requires maven-local to build.
$ mock --install maven-local
But now since I want to rebuild more Java packages, I'd like to make snapshot of the buildroot.
$ mock --snapshot mvn
This creates a new snapshot of the current state and sets it as the default. We can list snapshots of current buildroot with --list-snapshots command (the default snapshot is prefixed with asterisk)
$ mock --list-snapshots
Snapshots for mock-devel:
  postinit
* mvn

So let's rebuild something
$ mock --rebuild jetty-9.2.1-1.fc21.src.rpm
$ mock --rebuild jetty-schemas-3.1-3.fc21.src.rpm
Because the 'mvn' snapshot was set as the default, it means that each clean executed as part of the rebuild command didn't return to the state in 'postinit', but to the state in 'mvn' snapshot. And that was the reason we wanted LVM support in the first place - it didn't have to install 300+MB of maven-local's dependencies again (with original mock, this would probably take more than 3 minutes) but still the buildroot was cleaned of the packages pulled in by previous build. We could then install some additional packages, for example eclipse, and make a snapshot that can be used to build eclipse plugins.
Now let's pretend there has been an update to my 'rnv' package, which is in C and doesn't use maven-local.
$ mock --rollback-to postinit
$ mock --list-snapshots
  mvn
* postinit
Now 'postinit' snapshot was set as default and buildroot has been restored to the state it was in when 'postinit' snapshot was taken (after initialization, no maven-local there). The 'mvn' snapshot is retained and we can switch back again using --rollback-to mvn.
So now I can rebuild my hypothetical rnv update. If I decide that I don't need the 'mvn' snapshot anymore, I can remove it with
$ mock --remove-snapshot mvn
You cannot remove 'postinit' snapshot. To remove all logical volumes belonigng to the buildroot, use mock --scrub lvm
 
So that's it. You can create as many snapshots as you want (and snapshots of snapshots) and keep a hierarchy of them to build packages that have different sets of BuildRequires.
Few more details:

• The real snapshot names passed to LVM commands have root name prefixed to avoid clashes with other buildroots or volumes that don't belong to mock at all. It also checks whether the snapshots belong to mock's thinpool.
• The volume group needs to be provided by user, mock won't create one. It won't touch anything else besides the thinpool, so it should be quite safe if it uses the same volume group as you system (I have it like that).
• The command names suck. I know. I'll try to provide short options for them.
• If you try the version in my jenkins repository, everything is renamed to xmock including the command - to allow it to exist alongside original mock.

GSoC - week 4

Last week I had exams at the university and that left me with less time for work. But I made some progress anyway.

Mock performance
Mock builds usually take a considerable amount of time and there is nothing much that can be done about the speed of actual building but the package installation can be improved. Last time I created noverify plugin which provided considerable speed up and my mentor recomended to try removing fsync calls during the package installation. I do that by making a small library in C containg only empty fsync() and fdatasync() functions and copying it into the buidroot. Then using LD_PRELOAD to make it replace actual libc implementation of this calls. My mentor measured the performance differences on his kvm virtual machine and the results are amazing - times installing @buildsys-build and maven-local (look at the wall clock time):

Standard yum:
User time (seconds): 55.66
System time (seconds): 5.78
Percent of CPU this job got: 25%
Elapsed (wall clock) time (h:mm:ss or m:ss): 4:02.03

Standard dnf:
User time (seconds): 49.61
System time (seconds): 5.68
Percent of CPU this job got: 23%
Elapsed (wall clock) time (h:mm:ss or m:ss): 3:50.94

With noverify plugin:
User time (seconds): 47.85
System time (seconds): 5.32
Percent of CPU this job got: 36%
Elapsed (wall clock) time (h:mm:ss or m:ss): 2:25.25
Maximum resident set size (kbytes): 150248

With noverify plugin, fsync() and fdatasync() disabled:
User time (seconds): 46.38
System time (seconds): 4.97
Percent of CPU this job got: 87%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:58.56
Maximum resident set size (kbytes): 150260

That's more than 4x faster and could be valuable improvement for both packagers and koji builders.

LVM plugin
I started implementing the basis for LVM support. Now it can already use LVM snapshot instead of root cache. To enable it, put the following in your config:
config_opts['plugin_conf']['root_cache_enable'] = False
config_opts['plugin_conf']['lvm_root_enable'] = True
config_opts['plugin_conf']['lvm_root_opts'] = {
    'volume_group': 'mock-vg'
}
where mock-vg is the name (not path) of the volume group you want mock to use for creating new volumes. There are other configuration options possible - filesystem, size, snapshot_size, mkfs_args. Root cache is disabled because it would be redundant. When started, it creates a logical volume, mounts it and after it's initialized, it makes a snapshot and mounts the snapshot instead of the original. Then all following builds alter only the snapshot and when clean command is executed (usually at the beginning of new build) the snapshot is deleted and replaced with new one. I originally tried to implement it the other way around - making a snapshot and still working with the original volume and then merging it when cleaning. But it was very slow - the merging took more than 10s. The current approach is fast enough - cleaning is just deleting a snapshot and creating new one which happens almost instantly (compared to deleting buildroot and unpacking root cache).

The next week I'll try to implement more advanced features of the LVM plugin - snapshot management which would allow having a hierarchy of snapshots with different preinstalled packages facilitating faster workflow for packagers working with more diverse set of packages.

GSoC - week 3

This week I've been mostly continuing the parts I started the week before. I performed more drastical refactoring - splitting the main monolithic class (Root) that does basically everything except few specific features that are delegated to other modules. I moved rest of the buildroot-building code to Buildroot class I created before for this purpose. The state logging and plugin loading/calling was decoupled to separate classes. The package management code resides in PackageManager class and Yum/Dnf subclasses. I renamed the former Root class to Commands which now does just what the name suggests - executes commands, such as build, buildsrpm, in buildroot. I've adapted plugins to the new model and customized the initialization to prepare field for the LVM backend, which will of course be implemented as a plugin (but doing that without hardcoding some parts into core will require enhancing the plugin API).

Jenkins
I've requested a Jenkins project for my improved version of mock which is now available at http://jenkins.cloud.fedoraproject.org/job/mock/ and provides built packages (and repository) at http://jenkins.cloud.fedoraproject.org/job/mock/ws/RPMS. I build it using the spec file in the mock source tree but before building I inject a script into %prep which replaces all occurences of 'mock' with 'xmock' (including filenames). Thus, the package can be installed alongside upstream mock without any conflicts. Just everything is renamed to xmock - the binary, the config directory, also the group. So feel free to test it :-)
(But be careful, mock is running as root. If I make a mistake it may have consequences on your system)

DNF
I've already got feedback from my mentor (Mikolaj Izdebski). The bad news is that dnf installroot support doesn't work for him althought it works perfectly fine for me. The problem is that for him, dnf doesn't load the config from the installroot and uses the system-wide one. I've read the corresponding part of dnf's source code and I don't see any reason why it might fail to find the configuration. If installroot is defined and the config file within is readable (tested with access(2)) it will be used. I'm yet to find why it doesn't work for him.

Interactive output
I've modified mock to print output of building and package management commands. One thing my mentor suggested was to get more output from dnf/yum. Currently, neither dnf nor yum output anything when they're synchronizing repos or downloading packages, only when installing. The reason is that they check whether the output is a terminal (with os.isatty()), which is not - in case of mock, it's a pipe. I had to trick it to them to think the output is a tty. I solved it using a pseudoterminal instead of a pipe and now os.isatty() happily returns True in the child processes and dnf now prints everything including the progressbars when downloading. But since it uses carriage retruns and backspaces to erase parts of already printed output it also introduced lot of mess to the logs, because in text files those characters aren't interpreted the same way as on terminal. So I also had to modify the output logging to get rid of these.
Note: to get reasonable output from yum/dnf, it's debuglevel has to be set to at least 2 in the config (current default is 1)

Skipping package verification
Another feature he requested was speeding up mock by skipping verification of packages when installing. Neither yum nor dnf have an option to disable those, but since they're written in Python, there's always a hackish way to accomplish what you need. There are two solutions to this problem: 1. create a plugin that modifies yum/dnf to not verify packages 2. create a wrapper module that will modify them. I chose number 1 and implemented it for dnf for now. I created a simple plugin that, once loaded, rebinds the dnf's method verify_transaction to a no-op lambda. Then I just copy it into buildroot and inject the plugin path to dnf.conf. It can be toggled with config option 'noverify' (enabled by default).

Other
Also, I fixed some corner case behavior. Now it doesn't fail when you delete the /var/lib/mock directory. Previously it recreated it with wrong permissions making it hard to detect, why it failed and how to make it work again (user had to manually set the setgid bit on the directory). Also it prints warning when incorrectly executed as regular user without  setuid wrapper (Previously it printed OSError: Success).

Future
I've been exploring the possibility of mock executing commands in a contained environment and possibly not doing most of it's work as root. I've discovered some interesting things about Linux namespaces that might quite change the way mock works. I will try to make a follow up post about this soon.