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Alternates are sum types like unions, but use the JSON type on the
wire / QType in QObject instead of an explicit tag. That's why we
require alternate members to have distinct QTypes.
The recently introduced keyval_parse() (commit d454dbe) can only
produce string scalars. The qobject_input_visitor_new_keyval() input
visitor mostly hides the difference, so code using a QObject input
visitor doesn't have to care whether its input was parsed from JSON or
KEY=VALUE,... The difference leaks for alternates, as noted in commit
0ee9ae7: a non-string, non-enum scalar alternate value can't currently
be expressed.
In part, this is just our insufficiently sophisticated implementation.
Consider alternate type 'GuestFileWhence'. It has an integer member
and a 'QGASeek' member. The latter is an enumeration with values
'set', 'cur', 'end'. The meaning of b=set, b=cur, b=end, b=0, b=1 and
so forth is perfectly obvious. However, our current implementation
falls apart at run time for b=0, b=1, and so forth. Fixable, but not
today; add a test case and a TODO comment.
Now consider an alternate type with a string and an integer member.
What's the meaning of a=42? Is it the string "42" or the integer 42?
Whichever meaning you pick makes the other inexpressible. This isn't
just an implementation problem, it's fundamental. Our current
implementation will pick string.
So far, we haven't needed such alternates. To make sure we stop and
think before we add one that cannot sanely work with keyval_parse(),
let's require alternate members to have sufficiently distinct
representation in KEY=VALUE,... syntax:
* A string member clashes with any other scalar member
* An enumeration member clashes with bool members when it has value
'on' or 'off'.
* An enumeration member clashes with numeric members when it has a
value that starts with '-', '+', or a decimal digit. This is a
rather lazy approximation of the actual number syntax accepted by
the visitor.
Note that enumeration values starting with '-' and '+' are rejected
elsewhere already, but better safe than sorry.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1495471335-23707-5-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
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Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1490014548-15083-5-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
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Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1490014548-15083-3-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
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Additionally permit non-negative integers as key components. A
dictionary's keys must either be all integers or none. If all keys
are integers, convert the dictionary to a list. The set of keys must
be [0,N].
Examples:
* list.1=goner,list.0=null,list.1=eins,list.2=zwei
is equivalent to JSON [ "null", "eins", "zwei" ]
* a.b.c=1,a.b.0=2
is inconsistent: a.b.c clashes with a.b.0
* list.0=null,list.2=eins,list.2=zwei
has a hole: list.1 is missing
Similar design flaw as for objects: there is no way to denote an empty
list. While interpreting "key absent" as empty list seems natural
(removing a list member from the input string works when there are
multiple ones, so why not when there's just one), it doesn't work:
"key absent" already means "optional list absent", which isn't the
same as "empty list present".
Update the keyval object visitor to use this a.0 syntax in error
messages rather than the usual a[0].
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1488317230-26248-25-git-send-email-armbru@redhat.com>
[Off-by-one fix squashed in, as per Kevin's review]
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
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Until now, key components are separated by '.'. This leaves little
room for evolving the syntax, and is incompatible with the __RFQDN_
prefix convention for downstream extensions.
Since key components will be commonly used as QAPI member names by the
QObject input visitor, we can just as well borrow the QAPI naming
rules here: letters, digits, hyphen and period starting with a letter,
with an optional __RFQDN_ prefix for downstream extensions.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <1488317230-26248-20-git-send-email-armbru@redhat.com>
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keyval_parse() parses KEY=VALUE,... into a QDict. Works like
qemu_opts_parse(), except:
* Returns a QDict instead of a QemuOpts (d'oh).
* Supports nesting, unlike QemuOpts: a KEY is split into key
fragments at '.' (dotted key convention; the block layer does
something similar on top of QemuOpts). The key fragments are QDict
keys, and the last one's value is updated to VALUE.
* Each key fragment may be up to 127 bytes long. qemu_opts_parse()
limits the entire key to 127 bytes.
* Overlong key fragments are rejected. qemu_opts_parse() silently
truncates them.
* Empty key fragments are rejected. qemu_opts_parse() happily
accepts empty keys.
* It does not store the returned value. qemu_opts_parse() stores it
in the QemuOptsList.
* It does not treat parameter "id" specially. qemu_opts_parse()
ignores all but the first "id", and fails when its value isn't
id_wellformed(), or duplicate (a QemuOpts with the same ID is
already stored). It also screws up when a value contains ",id=".
* Implied value is not supported. qemu_opts_parse() desugars "foo" to
"foo=on", and "nofoo" to "foo=off".
* An implied key's value can't be empty, and can't contain ','.
I intend to grow this into a saner replacement for QemuOpts. It'll
take time, though.
Note: keyval_parse() provides no way to do lists, and its key syntax
is incompatible with the __RFQDN_ prefix convention for downstream
extensions, because it blindly splits at '.', even in __RFQDN_. Both
issues will be addressed later in the series.
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1488317230-26248-4-git-send-email-armbru@redhat.com>
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