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From: | Eric Blake |
Subject: | Re: [Qemu-devel] [PATCH v2 1/2] qapi: allow empty branches in flat unions |
Date: | Tue, 29 May 2018 15:43:29 -0500 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 |
On 05/29/2018 11:41 AM, Anton Nefedov wrote:
It often happens that just a few discriminator values imply extra data in a flat union. Existing checks did not make possible to leave other values uncovered. Such cases had to be worked around by either stating a dummy (empty) type or introducing another (subset) discriminator enumeration. Both options create redundant entities in qapi files for little profit. With this patch it is not necessary anymore to add designated union fields for every possible value of a discriminator enumeration. Signed-off-by: Anton Nefedov <address@hidden> --- docs/devel/qapi-code-gen.txt | 7 ++++--- scripts/qapi/common.py | 11 ++++------- scripts/qapi/types.py | 4 +++- scripts/qapi/visit.py | 17 +++++++++++++---- tests/qapi-schema/flat-union-incomplete-branch.err | 1 - tests/qapi-schema/flat-union-incomplete-branch.exit | 2 +- tests/qapi-schema/flat-union-incomplete-branch.out | 14 ++++++++++++++
Generally, when converting a negative test to a positive test, I prefer to instead drop the negative test and put the positive test as part of tests/qapi-schema/qapi-schema-test.json - because that file has the additional coverage of proving that the compiler is still happy with the generated code (we don't compile any of the other positive tests).
7 files changed, 39 insertions(+), 17 deletions(-) diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt index b9b6eab..a3ea450 100644 --- a/docs/devel/qapi-code-gen.txt +++ b/docs/devel/qapi-code-gen.txt @@ -496,9 +496,10 @@ Resulting in these JSON objects:Notice that in a flat union, the discriminator name is controlled bythe user, but because it must map to a base member with enum type, the -code generator can ensure that branches exist for all values of the -enum (although the order of the keys need not match the declaration of -the enum). In the resulting generated C data types, a flat union is +code generator ensures that branches match the existing values of the +enum. The order of the keys need not match the declaration of the enum. +The keys need not cover all possible enum values.
Maybe mention that omitted enum values are still valid branches that merely add no additional members to the data type.
+In the resulting generated C data types, a flat union is represented as a struct with the base members included directly, and then a union of structures for each branch of the struct.
Otherwise seems reasonable to me. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
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