implement module compatibility check

[thaJeztah]

• relates to implement module compatibility check moby/moby#46941
• relates to add //go:build directives to prevent downgrading to go1.16 language #4719

This package imports all "importable" packages, i.e., packages that:

• are not applications ("main")
• are not internal
• and that have non-test go-files

We do this to verify that our code can be consumed as a dependency in "module mode". When using a dependency that does not have a go.mod (i.e.; is not a "module"), go implicitly generates a go.mod. Lacking information from the dependency itself, it assumes "go1.16" language (see DefaultGoModVersion). Starting with Go1.21, go downgrades the language version used for such dependencies, which means that any language feature used that is not supported by go1.16 results in a compile error;

# github.com/docker/cli/cli/context/store
/go/pkg/mod/github.com/docker/cli@v25.0.0-beta.2+incompatible/cli/context/store/storeconfig.go:6:24: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
/go/pkg/mod/github.com/docker/cli@v25.0.0-beta.2+incompatible/cli/context/store/store.go:74:12: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)

These errors do NOT occur when using GOPATH mode, nor do they occur when using "pseudo module mode" (the "-mod=mod -modfile=vendor.mod" approach used in this repository).

As a workaround for this situation, we must include "//go:build" comments in any file that uses newer go-language features (such as the "any" type or the "min()", "max()" builtins).

From the go toolchain docs (https://go.dev/doc/toolchain):

The go line for each module sets the language version the compiler enforces
when compiling packages in that module. The language version can be changed
on a per-file basis by using a build constraint.

For example, a module containing code that uses the Go 1.21 language version
should have a go.mod file with a go line such as go 1.21 or go 1.21.3.
If a specific source file should be compiled only when using a newer Go
toolchain, adding //go:build go1.22 to that source file both ensures that
only Go 1.22 and newer toolchains will compile the file and also changes
the language version in that file to Go 1.22.

This file is a generated module that imports all packages provided in the repository, which replicates an external consumer using our code as a dependency in go-module mode, and verifies all files in those packages have the correct "//go:build " set.

To test this package:

make shell
make -C ./internal/gocompat/
make: Entering directory '/go/src/github.com/docker/cli/internal/gocompat'
GO111MODULE=off go generate .
GO111MODULE=on go mod tidy
GO111MODULE=on go test -v
# github.com/docker/cli/templates
../../templates/templates.go:13:17: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
# github.com/docker/cli/cli/compose/template
../../cli/compose/template/template.go:98:45: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/template/template.go:105:27: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/template/template.go:141:28: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
# github.com/docker/cli/cli/compose/types
../../cli/compose/types/types.go:53:22: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/types/types.go:86:34: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/types/types.go:105:22: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/types/types.go:137:34: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/types/types.go:211:20: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/types/types.go:343:35: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/types/types.go:442:40: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/types/types.go:469:24: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/types/types.go:490:24: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/types/types.go:587:28: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/types/types.go:442:40: too many errors
# github.com/docker/cli/cli/context/store
../../cli/context/store/storeconfig.go:6:24: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/context/store/store.go:74:12: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/context/store/store.go:75:23: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/context/store/metadatastore.go:43:58: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/context/store/metadatastore.go:48:22: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/context/store/metadatastore.go:80:30: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
# github.com/docker/cli/cli/command/idresolver
../../cli/command/idresolver/idresolver.go:6:2: "github.com/docker/docker/api/types" imported and not used
../../cli/command/idresolver/idresolver.go:7:2: "github.com/docker/docker/api/types/swarm" imported and not used
../../cli/command/idresolver/idresolver.go:9:2: "github.com/pkg/errors" imported and not used
../../cli/command/idresolver/idresolver.go:28:49: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/command/idresolver/idresolver.go:58:53: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
# github.com/docker/cli/cli/compose/schema
../../cli/compose/schema/schema.go:20:46: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/schema/schema.go:27:53: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/schema/schema.go:45:32: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
../../cli/compose/schema/schema.go:66:33: predeclared any requires go1.18 or later (-lang was set to go1.16; check go.mod)
FAIL	gocompat [build failed]
make: *** [Makefile:3: verify] Error 1
make: Leaving directory '/go/src/github.com/docker/cli/internal/gocompat'

- What I did

- How I did it

- How to verify it

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[laurazard]

This is going to get rid of so many headaches in the future! Thanks @thaJeztah :')

[thaJeztah]

Was still deciding whether we should merge this one as-is (but we don't have a flow yet to run it cleanly, which could be some stage in the Dockerfile), or if we could make this work without that; per moby/moby#46941 (comment)

Good news! go vet will be able to perform the module compatibility check itself starting in Go 1.23.