On Thu, Sep 15, 2022 at 10:42:54PM +0200, Markus Armbruster wrote: > In QAPI, absent optional members are distinct from any present value. > We thus represent an optional schema member FOO as two C members: a > FOO with the member's type, and a bool has_FOO. Likewise for function > arguments. > > However, has_FOO is actually redundant for a pointer-valued FOO, which > can be null only when has_FOO is false, i.e. has_FOO == !!FOO. Except > for arrays, where we a null FOO can also be a present empty array. > > The redundant has_FOO are a nuisance to work with. Improve the > generator to elide them. Uses of has_FOO need to be replaced as > follows. > > Tests of has_FOO become the equivalent comparison of FOO with null. > For brevity, this is commonly done by implicit conversion to bool. > > Assignments to has_FOO get dropped. > > Likewise for arguments to has_FOO parameters. > > Beware: code may violate the invariant has_FOO == !!FOO before the > transformation, and get away with it. The above transformation can > then break things. Two cases: > > * Absent: if code ignores FOO entirely when !has_FOO (except for > freeing it if necessary), even non-null / uninitialized FOO works. > Such code is known to exist. > > * Present: if code ignores FOO entirely when has_FOO, even null FOO > works. Such code should not exist. > > In both cases, replacing tests of has_FOO by FOO reverts their sense. > We have to fix the value of FOO then. > > To facilitate review of the necessary updates to handwritten code, add > means to opt out of this change, and opt out for all QAPI schema > modules where the change requires updates to handwritten code. The > next few commits will remove these opt-outs in reviewable chunks, then > drop the means to opt out. > > Signed-off-by: Markus Armbruster <arm...@redhat.com> > --- > docs/devel/qapi-code-gen.rst | 5 +-- > docs/devel/writing-monitor-commands.rst | 14 ++++---- > scripts/qapi/commands.py | 2 +- > scripts/qapi/events.py | 2 +- > scripts/qapi/gen.py | 2 +- > scripts/qapi/schema.py | 47 +++++++++++++++++++++++++ > scripts/qapi/types.py | 2 +- > scripts/qapi/visit.py | 17 +++++++-- > 8 files changed, 76 insertions(+), 15 deletions(-) >
Reviewed-by: Daniel P. Berrangé <berra...@redhat.com> With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
