conversion-gen is a tool for auto-generating functions that convert between internal and external types. A general conversion code generation task involves three sets of packages: (1) a set of packages containing internal types, (2) a single package containing the external types, and (3) a single destination package (i.e., where the generated conversion functions go, and where the developer-authored conversion functions are). The packages containing the internal types play the role known as "peer packages" in the general code-generation framework of Kubernetes.
For each conversion task, `conversion-gen` will generate functions that efficiently convert between same-name types in the two (internal, external) packages. The generated functions include ones named
for each such pair of types --- both with (pkg1,pkg2) = (internal,external) and (pkg1,pkg2) = (external,internal). Additionally: if the destination package does not contain one in a non-generated file then a function named
is also generated and it simply calls the `autoConvert...` function. The generated conversion functions use standard value assignment wherever possible. For compound types, the generated conversion functions call the `Convert...` functions for the subsidiary types. Thus developers can override the behavior for selected types. For a top-level object type (i.e., the type of an object that will be input to an apiserver), for such an override to be used by the apiserver the developer-maintained conversion functions must also be registered by invoking the `AddConversionFuncs` method of the relevant `Scheme` object from k8s.io/apimachinery/pkg/runtime.
`conversion-gen` will scan its `--input-dirs`, looking at the package defined in each of those directories for comment tags that define a conversion code generation task. A package requests conversion code generation by including one or more comment in the package's `doc.go` file (currently anywhere in that file is acceptable, but the recommended location is above the `package` statement), of the form:
This introduces a conversion task, for which the destination package is the one containing the file with the tag and the tag identifies a package containing internal types. If there is also a tag of the form
then it identifies the package containing the external types; otherwise they are in the destination package.
For each conversion code generation task, the full set of internal packages (AKA peer packages) consists of the ones specified in the `k8s:conversion-gen` tags PLUS any specified in the `--base-peer-dirs` and `--extra-peer-dirs` flags on the command line.
When generating for a package, individual types or fields of structs may opt out of Conversion generation by specifying a comment on the of the form: