[Qemu-devel] [PATCH v9 09/27] qapi-introspect: Document lack of sorting

[Eric Blake]

qapi-code-gen.txt already claims that types, commands, and
events share a common namespace; set this in stone by further
documenting that our introspection output will never have
collisions with the same name tied to more than one meta-type.

Our largest QMP enum currently has 125 values, our largest
object type has 27 members, and the mean for each is less than
10.  These sizes are small enough that the per-element overhead
of O(log n) binary searching probably outweighs the speed
possible with direct O(n) linear searching (a better algorithm
with more overhead will only beat a leaner naive algorithm only
as you scale to larger input sizes).

Arguably, the overall SchemaInfo array could be sorted by name;
there, we currently have 531 entities, large enough for a binary
search to be faster than linear.  However, remember that we have
mutually-recursive types, which means there is no topological
ordering that will allow clients to learn all information about
that type in a single linear pass; thus clients will want to do
random access over the data, and they will probably read the
introspection output into a hashtable for O(1) lookup rather
than O(log n) binary searching, at which point, pre-sorting our
introspection output doesn't help the client.

Finally, I am not sure how to guarantee that python sorts strings
for the C locale even if the generator is run under a different
locale.  Our current output order is deterministic (based on the
order present in .json files, even if that order has no inherent
bearing on how a client will present their QMP commands), while
sorting data risks non-determinism if the generator is subject to
any locale differences.

For these reasons, we simply document that clients should not
rely on any particular order of items in introspection output.
Additionally, this gives us the freedom to later change our mind,
so that we could rearrange output in a different order than the
way it was listed in the .json file, without breaking any client
that was already aware it had to do a linear search.

Document these conventions, so that clients will know what can
and cannot be relied on.

Signed-off-by: Eric Blake <address@hidden>

---
v9: retitle; and swing in the opposite direction: give the user
no guarantee about order
v8: no change
v7: tweak commit wording
v6: no change from v5
---
 docs/qapi-code-gen.txt | 19 +++++++++++++++----
 qapi/introspect.json   | 21 ++++++++++++++++-----
 2 files changed, 31 insertions(+), 9 deletions(-)

diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt
index ba29bc6..20e6907 100644
--- a/docs/qapi-code-gen.txt
+++ b/docs/qapi-code-gen.txt
@@ -516,6 +516,10 @@ query-qmp-schema.  QGA currently doesn't support 
introspection.

 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
+client must search for a particular name throughout the entire array
+to learn more about that name, but is at least guaranteed that there
+will be no collisions between type, command, and event names.

 However, the SchemaInfo can't reflect all the rules and restrictions
 that apply to QMP.  It's interface introspection (figuring out what's
@@ -596,7 +600,9 @@ any.  Each element is a JSON object with members "name" 
(the member's
 name), "type" (the name of its type), and optionally "default".  The
 member is optional if "default" is present.  Currently, "default" can
 only have value null.  Other values are reserved for future
-extensions.
+extensions.  The "members" array is in no particular order; clients
+must search every member when learning whether a particular member is
+supported.

 Example: the SchemaInfo for MyType from section Struct types

@@ -610,7 +616,9 @@ Example: the SchemaInfo for MyType from section Struct types
 "variants" is a JSON array describing the object's variant members.
 Each element is a JSON object with members "case" (the value of type
 tag this element applies to) and "type" (the name of an object type
-that provides the variant members for this type tag value).
+that provides the variant members for this type tag value).  The
+"variants" array is in no particular order, and is not guaranteed to
+list cases in the same order as the corresponding "tag" enum type.

 Example: the SchemaInfo for flat union BlockdevOptions from section
 Union types
@@ -651,7 +659,8 @@ Union types
 The SchemaInfo for an alternate type has meta-type "alternate", and
 variant member "members".  "members" is a JSON array.  Each element is
 a JSON object with member "type", which names a type.  Values of the
-alternate type conform to exactly one of its member types.
+alternate type conform to exactly one of its member types.  There is
+no guarantee on the order in which "members" will be listed.

 Example: the SchemaInfo for BlockRef from section Alternate types

@@ -673,7 +682,9 @@ Example: the SchemaInfo for ['str']
       "element-type": "str" }

 The SchemaInfo for an enumeration type has meta-type "enum" and
-variant member "values".
+variant member "values".  The values are listed in no particular
+order; clients must search every value when learning whether a
+particular value is supported.

 Example: the SchemaInfo for MyEnum from section Enumeration types

diff --git a/qapi/introspect.json b/qapi/introspect.json
index cc50dc6..f86d552 100644
--- a/qapi/introspect.json
+++ b/qapi/introspect.json
@@ -25,6 +25,10 @@
 # Returns: array of @SchemaInfo, where each element describes an
 # entity in the ABI: command, event, type, ...
 #
+# The order of the various SchemaInfo is unspecified; however, all
+# names are guaranteed to be unique (no name will be duplicated with
+# different meta-types).
+#
 # Note: the QAPI schema is also used to help define *internal*
 # interfaces, by defining QAPI types.  These are not part of the QMP
 # wire ABI, and therefore not returned by this command.
@@ -78,7 +82,8 @@
 #        Commands and events have the name defined in the QAPI schema.
 #        Unlike command and event names, type names are not part of
 #        the wire ABI.  Consequently, type names are meaningless
-#        strings here.
+#        strings here, although they are still guaranteed unique
+#        regardless of @meta-type.
 #
 # All references to other SchemaInfo are by name.
 #
@@ -130,7 +135,9 @@
 #
 # Additional SchemaInfo members for meta-type 'enum'.
 #
-# @values: the enumeration type's values.
+# @values: the enumeration type's values.  The values are in no particular
+#          order, so clients must search every value when learning if a
+#          particular value is present.
 #
 # Values of this type are JSON string on the wire.
 #
@@ -158,14 +165,18 @@
 #
 # Additional SchemaInfo members for meta-type 'object'.
 #
-# @members: the object type's (non-variant) members.
+# @members: the object type's (non-variant) members.  The members are
+#           in no particular order, so clients must search every member
+#           when learning if a particular member is present.
 #
 # @tag: #optional the name of the member serving as type tag.
 #       An element of @members with this name must exist.
 #
 # @variants: #optional variant members, i.e. additional members that
 #            depend on the type tag's value.  Present exactly when
-#            @tag is present.
+#            @tag is present.  The variants are in no particular order,
+#            and may even differ from the order of the values of the
+#            enum type of the @tag.
 #
 # Values of this type are JSON object on the wire.
 #
@@ -219,7 +230,7 @@
 #
 # Additional SchemaInfo members for meta-type 'alternate'.
 #
-# @members: the alternate type's members.
+# @members: the alternate type's members, in no particular order.
 #           The members' wire encoding is distinct, see
 #           docs/qapi-code-gen.txt section Alternate types.
 #
-- 
2.4.3