From f5455044201747fd72531f5e8c1b1e9c56573d9c Mon Sep 17 00:00:00 2001 From: Eric Blake Date: Thu, 5 Nov 2015 23:35:36 -0700 Subject: [PATCH] qapi-introspect: Document lack of sorting 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. It doesn't help that sorting can be subjective if you introduce locales into the mix (I'm not experienced enough with Python to know for sure, but at least it looks like it defaults to sorting in the C locale even when run under a different locale). And while our current introspection output is deterministic (because we visit entities in a sorted order), we may want to change that order in the future (such as using OrderedDict to stick to .json declaration order). For these reasons, we simply document that clients should not rely on any particular order of items in introspection output. And since it is now a documented part of the contract, we have the freedom to later rearrange output if needed, without worrying about breaking well-written clients. Signed-off-by: Eric Blake Message-Id: <1446791754-23823-13-git-send-email-eblake@redhat.com> [Commit message tweaked] Signed-off-by: Markus Armbruster --- docs/qapi-code-gen.txt | 19 +++++++++++++++---- qapi/introspect.json | 17 ++++++++++++----- 2 files changed, 27 insertions(+), 9 deletions(-) diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt index ba29bc634a..f9fa6f3d96 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 the entire object 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 the entire enum 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 cc50dc6bcb..e7c4c3e998 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 +135,7 @@ # # Additional SchemaInfo members for meta-type 'enum'. # -# @values: the enumeration type's values. +# @values: the enumeration type's values, in no particular order. # # Values of this type are JSON string on the wire. # @@ -158,14 +163,16 @@ # # Additional SchemaInfo members for meta-type 'object'. # -# @members: the object type's (non-variant) members. +# @members: the object type's (non-variant) members, in no particular order. # # @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 +226,7 @@ # # 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. #