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Re: [Qemu-block] [PATCH 7/7] iotests: Disable 126 for some vmdk subforma

From: John Snow
Subject: Re: [Qemu-block] [PATCH 7/7] iotests: Disable 126 for some vmdk subformats
Date: Tue, 13 Aug 2019 18:26:49 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 8/13/19 10:00 AM, Max Reitz wrote:
> On 12.08.19 23:33, John Snow wrote:
>> On 7/25/19 11:57 AM, Max Reitz wrote:
>>> Several vmdk subformats do not work with iotest 126, so disable them.
>>> (twoGbMaxExtentSparse actually should work, but fixing that is a bit
>>> difficult.  The problem is that the vmdk descriptor file will contain a
>>> referenc to "image:base.vmdk", which the block layer cannot open because
>> reference
>>> it does not know the protocol "image".  This is not trivial to solve,
>>> because I suppose real protocols like "http://"; should be supported.
>>> Making vmdk treat all paths with a potential protocol prefix that the
>>> block layer does not recognize as plain files seems a bit weird,
>>> though.  Ignoring this problem does not seem too bad.)
>>> Signed-off-by: Max Reitz <address@hidden>
>>> ---
>>>  tests/qemu-iotests/126 | 6 ++++++
>>>  1 file changed, 6 insertions(+)
>>> diff --git a/tests/qemu-iotests/126 b/tests/qemu-iotests/126
>>> index 9b0dcf9255..8e55d7c843 100755
>>> --- a/tests/qemu-iotests/126
>>> +++ b/tests/qemu-iotests/126
>>> @@ -33,6 +33,12 @@ status=1 # failure is the default!
>>>  # Needs backing file support
>>>  _supported_fmt qcow qcow2 qed vmdk
>>> +# (1) Flat vmdk images do not support backing files
>>> +# (2) Split vmdk images simply fail this test right now.  Fixing that
>>> +#     is left for another day.
>> Which one? :)
> Hmmmm?  Fixing refers to #2.  #1 is not a bug or missing feature, it’s
> just how it is.  (This test needs backing files, so...)
> If you mean “which are which“, then the ones with *Flat are flat images
> (:-)), and the ones with twoGbMaxExtent* are split.

"Which day" ;)

>>> +_unsupported_imgopts "subformat=monolithicFlat" \
>>> +                     "subformat=twoGbMaxExtentFlat" \
>>> +                     "subformat=twoGbMaxExtentSparse"
>>>  # This is the default protocol (and we want to test the difference between
>>>  # colons which separate a protocol prefix from the rest and colons which 
>>> are
>>>  # just part of the filename, so we cannot test protocols which require a 
>>> prefix)
>> What exactly fails?
> Interestingly I only now noticed that the test passes with “vmdk: Use
> bdrv_dirname() for relative extent paths” (patch 2) reverted...
>>                     Does the VMDK driver see `image:` and think it's a
>> special filename it needs to handle and fails to do so?
> No.  Whenever the block layer sees a parsee filename[1] with a colon
> before a slash, it thinks everything before the colon is a protocol
> prefix.  For example:

Actually, I think we're on the same page here. I maybe meant to type
"block layer" instead of "VMDK driver", but it does look like it does
special processing on this sort of filename that breaks in this case.

> $ qemu-img info foo:bar
> qemu-img: Could not open 'foo:bar': Unknown protocol 'foo'
> This test is precisely for this.  How can you specify an image filename
> that has a colon in it (without using -blockdev)?  One way is to prepend
> it with “./”, the other is “file:”.
> Now with split VMDKs, we must write something in the header file to
> reference the extents.  What vmdk does for an image like
> “image:foo.vmdk” is it writes “image:foo-s001.vmdk” there.
> When it tries to open that extent, what happens depends on whether
> “vmdk: Use bdrv_dirname() for relative extent paths” (patch 2) is applied:
> --- Before that patch ---
> vmdk takes the descriptor filename, which, thanks to some magic in the
> block layer, is always “./image:foo.vmdk”, even when you gave it as
> “file:image:foo.vmdk” (the “file:” is stripped because it does nothing,
> generally, and the “./” is then prepended because of the false protocol
> prefix “image:”).
> It then invokes path_combine() with that path and the path given in the
> descriptor file (“image:foo-s001.vmdk”).  This yields
> “./image:foo-s001.vmdk”, which actually works.
> --- After that patch ---
> OK, what I messed up is that I just took the extent path to be an
> absolute path if it has a protocol prefix.  (Because that’s how we
> usually do it.)  Turns out that vmdk never did that, and path_combine()
> actually completely ignores protocol prefixes in the relative filename.
> I suppose I could do the same and just drop the path_has_protocol() from
> patch 2.  But that’d be a bit broken, as I wrote in the commit
> message...  If the descriptor file refers to an extent on
>http://example.com/extent.vmdk”, I suppose that should not be
> interpreted as a relative path, but actually work...
> But anyway, I guess if it’s a bit broken already, I might just keep it
> that way.
> tl;dr: Turns out patch 2 broke this test, because it (accidentally)
> tried to fix something that I consider broken.  If I just keep it broken
> (I didn’t know it was), this test will continue to work and probably
> nobody will care because, well, it already is broken and nobody cares.

So which kinda-broken thing are you making the case for? Are you
re-spinning in light of your discovery or... are we fine with the state
of things as they land here?

(Sorry, it wasn't clear to me which way you were leaning.)


> Max
> [1] By this I mean whether it is piped through .bdrv_parse_filename().
> If you specifying something with -hda or -drive file=, it will be.
> These are filenames like nbd://localhost:10809 or blkdebug:conf:image.
> If you pass a filename through QMP, that is, with -blockdev or
> blockdev-add, it will not be parsed.  It will be given to the block
> driver as is.  Protocol prefixes and other filename magic are ignored
> (you need to explicitly specify the driver anyway).

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