[libvirt] [RFC PATCH 0/3] Implement mockup capabilities cache in QEMU tests

[Cole Robinson]

On 08/18/2015 05:40 AM, Pavel Fedin wrote:
> Since commit e8d55172544c1fafe31a9e09346bdebca4f0d6f9 qemu driver checks
> emulator capabilities during domain XML post-parse. However, test suite
> does not initialize it, therefore a condition to skip all checks if there
> is no cache supplied was added. This is actually a hack, whose sole
> purpose is to make existing test suite working. Additionally, it prevents
> from writing new tests for this particular functionality.
> 
> This series attempts to solve this problem by implementing proper cache
> mockup in test suite. The main idea is to create a cache in standard way
> and put there a pre-defined capabilities set (which tests already have).
> 
> The main problem here is to know emulator binary name, which is contained
> in the source XML. However, we have to create our cache before reading the
> XML. The simplest way to resolve this is to assume particular binary name
> from test name. Currently tests which assume cross-architecture binary are
> all prefixed with the architecture name (with one exception of "keywrap"
> tests which all assume /usr/bin/qemu-system-s390x and do not have "s390-"
> prefix in their name).
> 
> This scheme works fine, unless we use "native" emulator binary. Here we
> have a mess. Most newer tests use /usr/bin/qemu, however there is a large
> number of tests which use /usr/libexec/qemu-kvm or /usr/bin/kvm (i guess
> these are leftovers from the epoch when qemu-kvm was a separate fork of
> qemu). This is currently not handled in any way, and these tests may
> report errors due to missing binaries (because virQEMUCapsCacheLookup()
> attempts to populate the cache automatically by querying the binary if
> not already known).
> 
> There are several possible ways to resolve this:
> a) Add all possible names as aliases for /usr/bin/qemu
> b) Forbid to use oldstyle names at all in these tests
> c) Declare some prefix like "kvm-" for those tests who want to use
>    /usr/libexec/qemu-kvm. Again, this would ban /usr/bin/kvm and
>    /usr/bin/qemu-kvm (if not using aliases like in (b)
> d) Hardcode (optional) emulator name per test. IMHO a bad idea because
>    number of tests is huge.
> e) Do some preparsing of the XML and extract binary name from it. Again,
>    i disliked it for not being simple enough.
> 

With the current approach I think e) is the way to go... yeah it's redundant
parsing but better than trying to maintain a whitelist or reformat a bunch of
test cases to match this. And for example these patches as is cause breakage
in other tests, like qemuargv2xml, and I think it's related to the incomplete
whitelist.

Using libvirt helpers it shouldn't be too hard to parse out the emulator path,
basically just virXMLParseStringCtxt and then

emulator = virXPathString("string(./devices/emulator[1])", ctxt);

Maybe other people have better ideas about how to mock this out though.

> I also thought about an alternate implementation which would patch
> postParseCallback and insert own function there which builds a cache. At
> this point binary name is already known from the XML. However, such a
> design looks like an ugly  hack by itself, so i stopped going in this
> direction.
> 

Yeah that sounds quite intense. Maybe there's some way to override a function
with our own definition from the test suite, but that's outside my realm of
knowledge.

Actually populating the cache does exercise the most qemu code though, so
that's one benefit.

Another minor bit to consider is possibly adding an assert in
qemu_capabilities.c to ensure that we never try to actually probe the host FS
from the test suite, since we don't want the test suite to accidentally become
dependent on host state. Maybe there's a test suite environment variable we
can check for.

- Cole