Daniel P. Berrangé <berra...@redhat.com> writes: > On Thu, Mar 16, 2023 at 01:31:04PM +0100, Markus Armbruster wrote: >> Philippe Mathieu-Daudé <phi...@linaro.org> writes: >> >> > Per the C++ standard, empty enum are ill-formed. Do not generate
The C standard. The C++ standard doesn't apply here :) >> > them in order to avoid: >> > >> > In file included from qga/qga-qapi-emit-events.c:14: >> > qga/qga-qapi-emit-events.h:20:1: error: empty enum is invalid >> > 20 | } qga_QAPIEvent; >> > | ^ >> > >> > Reported-by: Markus Armbruster <arm...@redhat.com> >> > Signed-off-by: Philippe Mathieu-Daudé <phi...@linaro.org> >> >> Two failures in "make check-qapi-schema" (which is run by "make check"): >> >> 1. Positive test case qapi-schema-test >> >> --- /work/armbru/qemu/bld-x86/../tests/qapi-schema/qapi-schema-test.out >> +++ >> @@ -19,7 +19,6 @@ >> member enum2: EnumOne optional=True >> member enum3: EnumOne optional=False >> member enum4: EnumOne optional=True >> -enum MyEnum >> object Empty1 >> object Empty2 >> base Empty1 >> >> You forgot to update expected test output. No big deal. >> >> 2. Negative test case union-empty >> >> --- /work/armbru/qemu/bld-x86/../tests/qapi-schema/union-empty.err >> +++ >> @@ -1,2 +1,2 @@ >> -union-empty.json: In union 'Union': >> -union-empty.json:4: union has no branches >> +union-empty.json: In struct 'Base': >> +union-empty.json:3: member 'type' uses unknown type 'Empty' >> stderr: >> qapi-schema-test FAIL >> union-empty FAIL >> >> The error message regresses. >> >> I can see two ways to fix this: >> >> (A) You can't just drop empty enumeration types on the floor. To not >> generate code for them, you need to skip them wherever we >> generate code for enumeration types. >> >> (B) Outlaw empty enumeration types. >> >> I recommend to give (B) a try, it's likely simpler. > > Possible trap-door with (B), if we have any enums where *every* > member is conditionalized on a CONFIG_XXX rule, there might be > certain build scenarios where an enum suddenly becomes empty. True. Scratch the idea. Trap-door also applies to (A): we can still end up with empty enums. (C) Always emit a dummy member. This is actually what we do now: typedef enum OnOffAuto { ON_OFF_AUTO_AUTO = 1, ON_OFF_AUTO_ON = 2, ON_OFF_AUTO_OFF = 3, ON_OFF_AUTO__MAX, <--- the dummy } OnOffAuto; But the next patch changes it to typedef enum OnOffAuto { ON_OFF_AUTO_AUTO, ON_OFF_AUTO_ON, ON_OFF_AUTO_OFF, #define ON_OFF_AUTO__MAX 3 } OnOffAuto; Two problems, actually. One, we lose the dummy. We could add one back like typedef enum OnOffAuto { ON_OFF_AUTO__DUMMY = 0, ON_OFF_AUTO_AUTO = 0, ON_OFF_AUTO_ON, ON_OFF_AUTO_OFF, #define ON_OFF_AUTO__MAX 3 } OnOffAuto; But all of this falls apart with conditional members! Example 1 (taken from qapi/block-core.json): { 'enum': 'BlockdevAioOptions', 'data': [ 'threads', 'native', { 'name': 'io_uring', 'if': 'CONFIG_LINUX_IO_URING' } ] } Generates now: typedef enum BlockdevAioOptions { BLOCKDEV_AIO_OPTIONS_THREADS, BLOCKDEV_AIO_OPTIONS_NATIVE, #if defined(CONFIG_LINUX_IO_URING) BLOCKDEV_AIO_OPTIONS_IO_URING, #endif /* defined(CONFIG_LINUX_IO_URING) */ BLOCKDEV_AIO_OPTIONS__MAX, } BlockdevAioOptions; BLOCKDEV_AIO_OPTIONS__MAX is 3 if defined(CONFIG_LINUX_IO_URING), else 2. After the next patch: typedef enum BlockdevAioOptions { BLOCKDEV_AIO_OPTIONS_THREADS, BLOCKDEV_AIO_OPTIONS_NATIVE, #if defined(CONFIG_LINUX_IO_URING) BLOCKDEV_AIO_OPTIONS_IO_URING, #endif /* defined(CONFIG_LINUX_IO_URING) */ #define BLOCKDEV_AIO_OPTIONS__MAX 3 } BlockdevAioOptions; Now it's always 3. Example 2 (same with members reordered): { 'enum': 'BlockdevAioOptions', 'data': [ { 'name': 'io_uring', 'if': 'CONFIG_LINUX_IO_URING' }, 'threads', 'native' ] } Same problem for __MAX, additional problem for __DUMMY: typedef enum BlockdevAioOptions { BLOCKDEV_AIO_OPTIONS__DUMMY = 0, #if defined(CONFIG_LINUX_IO_URING) BLOCKDEV_AIO_OPTIONS_IO_URING = 0, #endif /* defined(CONFIG_LINUX_IO_URING) */ BLOCKDEV_AIO_OPTIONS_THREADS, BLOCKDEV_AIO_OPTIONS_NATIVE, #define BLOCKDEV_AIO_OPTIONS__MAX 3 } BlockdevAioOptions; If CONFIG_LINUX_IO_URING is off, the enum starts at 1 instead of 0. Arrays indexed by the enum start with a hole. Code using them is probably not prepared for holes. *Sigh*