Daniel P. Berrangé <berra...@redhat.com> writes: > On Tue, May 10, 2022 at 01:34:03PM +0100, Daniel P. Berrangé wrote: >> On Tue, May 10, 2022 at 02:02:56PM +0200, Markus Armbruster wrote: >> > > For a minimum viable use case, this doesn't feel all that difficult, as >> > > conceptually instead of deleting the field from QAPI, we just need to >> > > annotate it to say when it was deleted from the QEMU side. The QAPI >> > > generator for internal QEMU usage, can omit any fields annotated as >> > > deleted in QAPI schema. The QAPI generator for external app usage, >> > > can (optionally) be told to include deleted fields ranging back to >> > > a given version number. So apps can chooses what degree of compat >> > > they wish to retain. >> > >> > Consider this evolution of command block_resize >> >> To help us understand, I'll illustrate some possible interfaces >> in both Go and Python, since that covers dynamic and static >> languages >> >> > * Initially, it has a mandatory argument @device[*]. >> >> Python definition: >> >> def block_resize(device, size) >> >> Caller: >> >> block_resize('dev0', 1*GiB) >> >> >> Golang definition >> >> type BlockResizeCommand struct { >> Device string >> Size int >> } >> >> Caller >> >> cmd := &BlockResizeCommand{ >> Device: "dev0", >> Size: 1 * GiB, >> } >> >> > * An alternative way to specify the command's object emerges: new >> > argument @node-name. Both old @device and new @node-name become >> > optional, and exactly one of them must be specified. This is commit >> > 3b1dbd11a6 "qmp: Allow block_resize to manipulate bs graph nodes." >> >> Python definition. Tricky, as non-optional params must be before >> optional params, but size is naturally the last arg. One option >> is to pointlessly mark 'size' as optional >> >> def block_resize(device=None, node_name=None, size=None) >> >> Caller >> >> block_resize(device="dev0", size=1*GiB) >> block_resize(node_name="devnode0", size=1*GiB) >> >> >> In golang definition >> >> type BlockResizeArguments struct { >> Device string >> NodeName string >> Size int >> } >> >> Caller choice of >> >> cmd := &BlockResizeCommand{ >> Device: "dev0", >> Size: 1 * GiB, >> } >> >> cmd := &BlockResizeCommand{ >> NodeName: "devnode0", >> Size: 1 * GiB, >> } >> >> >> Neither case can easily prevent passing Device and NodeName >> at same time. >> >> > * At some future date, the old way gets deprecated: argument @device >> > acquires feature @deprecated. >> >> Ok, no change needed to the APIs in either case. Possibly have >> code emit a warning if a deprecated field is set. >> >> > * Still later, the old way gets removed: @device is deleted, and >> > @node-name becomes mandatory. >> >> Again no change needed to APIs, but QEMU will throw back an >> error if the wrong one is used. >> >> > What is the proper version-spanning interface? >> > >> > I figure it's both arguments optional, must specify the right one for >> > the version of QEMU actually in use. This spans versions, but it fails >> > to abstract from them. >> >> Yep, I think that's inevitable in this scenario. THe plus side >> is that apps that want to span versions can do so. The downside >> is that apps that don't want smarts to span version, may loose >> compile time warnings about use of the now deleted field. > > Having said that, a different way to approach the problem is to expose > the versioning directly in the generated code. > > Consider a QAPI with versioning info about the fields > > { 'command': 'block_resize', > 'since': '5.0.0', > 'data': { 'device': ['type': 'str', 'until': '5.2.0' ], > '*device': ['type': 'str', 'since': '5.2.0', 'until': '7.0.0' ], > '*node-name': ['type': 'str', 'since': '5.2.0', 'until: '7.0.0' > ], > 'node-name': ['type': 'str', 'since': '7.0.0' ], > 'size': 'int' } } > > Meaning > > * Introduced in 5.0.0, with 'device' mandatory > * In 5.2.0, 'device' becomes optional, with optional 'node-name' as > alternative > * In 7.0.0, 'device' is deleted, and 'node-name' becomes mandatory > > Now consider the Go structs > > In 5.0.0 we can generate: > > type BlockResizeArguments struct { > V500 *BlockResizeArguments500 > } > > type BlockResizeArgumentsV1 struct { > Device string > Size int > } > > app can use > > dev := "dev0" > cmd := BlockResizeArguments{ > V500: &BlockResizeArguments500{ > Device: dev, > Size: 1 * GiB > } > } > > > In 5.2.0 we can now generate > > type BlockResizeArguments struct { > V500 *BlockResizeArgumentsV500 > V520 *BlockResizeArgumentsV520 > } > > type BlockResizeArgumentsV500 struct { > Device string > Size int > } > > type BlockResizeArgumentsV520 struct { > Device *string > NodeName *string > Size int > } > > > App can use the same as before, or switch to one of > > dev := "dev0" > cmd := BlockResizeArguments{ > V520: &BlockResizeArguments520{ > Device: &dev, > Size: 1 * GiB > } > } > > or > > node := "nodedev0" > cmd := BlockResizeArguments{ > V520: &BlockResizeArguments520{ > NodeName: &node, > Size: 1 * GiB > } > } > > > > In 7.0.0 we can now generate > > > type BlockResizeArguments struct { > V500 *BlockResizeArgumentsV500 > V520 *BlockResizeArgumentsV520 > V700 *BlockResizeArgumentsV700 > } > > type BlockResizeArgumentsV500 struct { > Device string > Size int > } > > type BlockResizeArgumentsV520 struct { > Device *string > NodeName *string > Size int > } > > type BlockResizeArgumentsV700 struct { > NodeName string > Size int > } > > > > App can use the same as before, or switch to > > node := "nodedev0" > cmd := BlockResizeArguments{ > V700: &BlockResizeArguments700{ > NodeName: node, > Size: 1 * GiB > } > } > > > This kind of per-command/type versioning is not uncommon when defining API > protocols/interfaces.
1. At every release, put a copy of the QAPI schema in the freezer. 2. For every copy, generate Go types with a suitable name suffix. Collect all the name stems. 3. For each name stem, define a Go struct that contains all the suffixed Go types with that stem. Look Ma, no cluttering the QAPI schema with a full history! Also no complicating the schema language to provide the means for that.