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: Eric Blake <eblake@redhat.com>
Message-Id: <1447264202-19554-1-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index f9fa6f3d96..ceb9a782d0 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -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