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Re: [Qemu-block] [libvirt] [libvirt RFC PATCH 02/10] util: storage: Add

From: Eric Blake
Subject: Re: [Qemu-block] [libvirt] [libvirt RFC PATCH 02/10] util: storage: Add parser for qemu's "json" backing pseudo-protocol
Date: Tue, 26 Jul 2016 14:28:23 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 07/15/2016 07:46 AM, Peter Krempa wrote:
> Add a modular parser that will allow to parse 'json' backing definitions
> that are supported by qemu. The initial implementation adds support for
> the 'file' driver.

Might be nice for this commit message to show an actual json: string
that it now accepts.

> ---
>  src/util/virstoragefile.c | 87 
> +++++++++++++++++++++++++++++++++++++++++++----
>  tests/virstoragetest.c    |  8 +++++
>  2 files changed, 88 insertions(+), 7 deletions(-)

At some point, I wonder if we can use qemu's QMP introspection to write
a meta-modular parser (basically, once QMP blockdev-add is
fully-functional, introspection will give a self-describing layout of
all possible combinations that QMP will accept, and therefore parsing
the introspection output would give a reasonable approach to all
possible json: strings that a given qemu will understand).  But that's
definitely more work, and blockdev-add is not fully working in qemu yet,
so hand-duplicating common parses for now at least gets us further than
libvirt's current stance of outright choking on json:.

> @@ -2514,10 +2515,80 @@ virStorageSourceParseBackingColon(virStorageSourcePtr 
> src,
>  }
> +static int
> +virStorageSourceParseBackingJSONPath(virStorageSourcePtr src,
> +                                     virJSONValuePtr json,
> +                                     int type)
> +{
> +    const char *path;
> +
> +    if (!(path = virJSONValueObjectGetString(json, "file.filename"))) {
> +        virReportError(VIR_ERR_INVALID_ARG, "%s",
> +                       _("missing 'filename' field in JSON backing volume "
> +                         "definition"));

Do we really want to require libvirt to support
"file.filename":"/path/to/foo" compressed naming, or do we want a more
hierarchical "file":{"filename":"/path/to/foo"} layout?  Or should both
be supported?  I also wonder if the virJSON code should be enhanced to
make it easier to parse deep information, via something similar to xpath
notation (other shortcuts that might be nice is "key[0]" as shorthand
for accessing element0 out of "key":[element0, element1]).

Part of the problem is that qemu is doing so much gross under-the-hood
conversion from pure QAPI into QemuOpts (which is completely flat, so we
have hacks like "file.filename"), rather than converting from QemuOpts
into QAPI, and so now libvirt has to match that grossness.

> +++ b/tests/virstoragetest.c
> @@ -1353,6 +1353,14 @@ mymain(void)
>                         "<source protocol='nbd' name='blah'>\n"
>                         "  <host name='example.org' port='6000'/>\n"
>                         "</source>\n");
> +    TEST_BACKING_PARSE(5, "json:", NULL);
> +    TEST_BACKING_PARSE(6, "json:asdgsdfg", NULL);
> +    TEST_BACKING_PARSE(7, "json:{}", NULL);
> +    TEST_BACKING_PARSE(8, "json: { \"file.driver\":\"blah\"}", NULL);
> +    TEST_BACKING_PARSE(9, "json:{\"file.driver\":\"file\"}", NULL);
> +    TEST_BACKING_PARSE(10, "json:{\"file.driver\":\"file\", "
> +                                 "\"file.filename\":\"/path/to/file\"}",
> +                       "<source file='/path/to/file'/>\n");

Okay, the testsuite shows what you now parse.  Again, I'm wondering if
qemu should first be improved, and/or whether it already allows:

"json:{\"file\":{\"driver\":\"file\", \"filename\":\"/path/to/file\"}}"

as equivalent to the "file.driver" and "file.filename" at the flat
level.  Or put another way, if qemu accepts more than one JSON
representation because of the way it crumples and flattens JSON into
QDict, maybe libvirt should first have a way to canonicalize into
preferred form.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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