Commit 39a65e2c authored by Eric Blake's avatar Eric Blake Committed by Markus Armbruster
Browse files

qapi: Document introspection stability considerations 



We are not ready (and might never be ready) to declare
introspection stable between releases. Clients written to
control multiple versions of qemu, and desiring to know
whether a particular member is supported for a given
command, must be prepared to locate that member in spite
of qapi changes that may affect the member's location or
type within the overall object, even though such changes
did not break QMP wire back-compatibility.

Signed-off-by: default avatarEric Blake <eblake@redhat.com>
Message-Id: <1447264202-19554-1-git-send-email-eblake@redhat.com>
Signed-off-by: default avatarMarkus Armbruster <armbru@redhat.com>
parent c257779e

docs/qapi-code-gen.txt

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@@ -514,6 +514,17 @@ exactly the server (QEMU) supports. 
For this purpose, QMP provides introspection via command

query-qmp-schema.  QGA currently doesn't support introspection.



While Client JSON Protocol wire compatibility should be maintained

between qemu versions, we cannot make the same guarantees for

introspection stability.  For example, one version of qemu may provide

a non-variant optional member of a struct, and a later version rework

the member to instead be non-optional and associated with a variant.

Likewise, one version of qemu may list a member with open-ended type

'str', and a later version could convert it to a finite set of strings

via an enum type; or a member may be converted from a specific type to

an alternate that represents a choice between the original type and

something else.



query-qmp-schema returns a JSON array of SchemaInfo objects.  These

objects together describe the wire ABI, as defined in the QAPI schema.

There is no specified order to the SchemaInfo objects returned; a

qapi/introspect.json

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@@ -22,6 +22,15 @@ 
# what's there), not interface specification.  The specification is in

# the QAPI schema.

#

# Furthermore, while we strive to keep the QMP wire format

# backwards-compatible across qemu versions, the introspection output

# is not guaranteed to have the same stability.  For example, one

# version of qemu may list an object member as an optional

# non-variant, while another lists the same member only through the

# object's variants; or the type of a member may change from a generic

# string into a specific enum or from one specific type into an

# alternate that includes the original type alongside something else.

#

# Returns: array of @SchemaInfo, where each element describes an

# entity in the ABI: command, event, type, ...

#