implement module compatibility check

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/[email protected]+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/[email protected]+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 <go language version>" 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'

[DefaultGoModVersion]: https://github.com/golang/go/blob/58c28ba286dd0e98fe4cca80f5d64bbcb824a685/src/cmd/go/internal/gover/version.go#L15-L24
[2]: https://go.dev/doc/toolchain

Signed-off-by: Sebastiaan van Stijn <[email protected]>

internal/gocompat/.gitignore

@@ -0,0 +1,4 @@
+go.mod
+go.sum
+main.go
+main_test.go

internal/gocompat/Makefile

@@ -0,0 +1,12 @@
+.PHONY: verify
+verify: generate
+	GO111MODULE=on go test -v
+
+.PHONY: generate
+generate: clean
+	GO111MODULE=off go generate .
+	GO111MODULE=on go mod tidy
+
+.PHONY: clean
+clean:
+	@rm -f go.mod go.sum main.go main_test.go

internal/gocompat/generate.go

@@ -0,0 +1,6 @@
+package main
+
+//go:generate go run modulegenerator.go
+
+// make sure the modfile package is vendored.
+import _ "golang.org/x/mod/modfile"

internal/gocompat/modulegenerator.go

@@ -0,0 +1,193 @@
+//go:build ignore
+
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"strings"
+	"text/template"
+
+	"golang.org/x/mod/modfile"
+)
+
+func main() {
+	if err := generateApp(); err != nil {
+		log.Fatal(err)
+	}
+	if err := generateModule(); err != nil {
+		log.Fatal(err)
+	}
+}
+
+func generateApp() error {
+	cmd := exec.Command("go", "list", "-find", "-f", `{{- if ne .Name "main"}}{{if .GoFiles}}{{.ImportPath}}{{end}}{{end -}}`, "../../...")
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return err
+	}
+
+	var pkgs []string
+	for _, p := range strings.Split(string(out), "\n") {
+		if strings.TrimSpace(p) == "" || strings.Contains(p, "/internal") {
+			continue
+		}
+		pkgs = append(pkgs, p)
+	}
+	tmpl, err := template.New("main").Parse(appTemplate)
+	if err != nil {
+		return err
+	}
+
+	var buf bytes.Buffer
+	err = tmpl.Execute(&buf, appContext{Generator: cmd.String(), Packages: pkgs})
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile("main_test.go", buf.Bytes(), 0o644)
+}
+
+func generateModule() error {
+	content, err := os.ReadFile("../../go.mod")
+	if err != nil {
+		if !os.IsNotExist(err) {
+			return err
+		}
+		content = []byte("module github.com/docker/cli\n")
+		if err := os.WriteFile("../../go.mod", content, 0o644); err != nil {
+			return err
+		}
+		// Let's be nice, and remove the go.mod if we created it.
+		// FIXME(thaJeztah): we need to clean up the go.mod after running the test, but need to know if we created it (or if it was an existing go.mod)
+		// defer os.Remove("../../go.mod")
+	} else {
+		log.Println("WARN: go.mod exists in the repository root!")
+		log.Println("WARN: Using your go.mod instead of our generated version -- this may misbehave!")
+	}
+	mod, err := modfile.Parse("../../go.mod", content, nil)
+	if err != nil {
+		return err
+	}
+	if mod.Go != nil && mod.Go.Version != "" {
+		return fmt.Errorf("main go.mod must not contain a go version")
+	}
+	content, err = os.ReadFile("../../vendor.mod")
+	if err != nil {
+		return err
+	}
+	mod, err = modfile.Parse("../../vendor.mod", content, nil)
+	if err != nil {
+		return err
+	}
+	if err := mod.AddModuleStmt("gocompat"); err != nil {
+		return err
+	}
+	if err := mod.AddReplace("github.com/docker/cli", "", "../../", ""); err != nil {
+		return err
+	}
+	if err := mod.AddGoStmt("1.21"); err != nil {
+		return err
+	}
+	out, err := mod.Format()
+	if err != nil {
+		return err
+	}
+	tmpl, err := template.New("mod").Parse(modTemplate)
+	if err != nil {
+		return err
+	}
+
+	gen, _ := os.Executable()
+
+	var buf bytes.Buffer
+	err = tmpl.Execute(&buf, appContext{Generator: gen, Dependencies: string(out)})
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile("go.mod", buf.Bytes(), 0o644)
+}
+
+type appContext struct {
+	Generator    string
+	Packages     []string
+	Dependencies string
+}
+
+const appTemplate = `// Code generated by "{{ .Generator }}". DO NOT EDIT.
+
+package main_test
+
+import (
+	"testing"
+
+	// Import all importable packages, i.e., packages that:
+	//
+	// - are not applications ("main")
+	// - are not internal
+	// - and that have non-test go-files
+{{- range .Packages }}
+ 	_ "{{ . }}"
+{{- end}}
+)
+
+// This file import 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/[email protected]+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/[email protected]+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 <go language version>" set.
+//
+// [DefaultGoModVersion]: https://github.com/golang/go/blob/58c28ba286dd0e98fe4cca80f5d64bbcb824a685/src/cmd/go/internal/gover/version.go#L15-L24
+// [2]: https://go.dev/doc/toolchain
+func TestModuleCompatibllity(t *testing.T) {
+	t.Log("all packages have the correct go version specified through //go:build")
+}
+`
+
+const modTemplate = `// Code generated by "{{ .Generator }}". DO NOT EDIT.
+
+{{.Dependencies}}
+`

internal/tools.go

@@ -0,0 +1,4 @@
+package internal
+
+// make sure the modfile package is vendored.
+import _ "golang.org/x/mod/modfile"

vendor.mod

@@ -37,6 +37,7 @@ require (
 	github.com/theupdateframework/notary v0.7.1-0.20210315103452-bf96a202a09a
 	github.com/tonistiigi/go-rosetta v0.0.0-20200727161949-f79598599c5d
 	github.com/xeipuuv/gojsonschema v1.2.0
+	golang.org/x/mod v0.11.0
 	golang.org/x/sync v0.3.0
 	golang.org/x/sys v0.13.0
 	golang.org/x/term v0.13.0
@@ -80,7 +81,6 @@ require (
 	go.opentelemetry.io/otel/metric v1.19.0 // indirect
 	go.opentelemetry.io/otel/trace v1.19.0 // indirect
 	golang.org/x/crypto v0.14.0 // indirect
-	golang.org/x/mod v0.11.0 // indirect
 	golang.org/x/net v0.17.0 // indirect
 	golang.org/x/time v0.3.0 // indirect
 	golang.org/x/tools v0.10.0 // indirect