Use Go cache if modules are in found in the cache

In module mode, when all module dependencies are given with canonical
versions and the version is found in the cache, set GOPROXY to
file://$(go env GOMODCACHE)/cache/download so that "go get" skips
querying the proxy.

Related: golang/go#43646 (proposal for
GOPROXY=cache)

Author: dolmen

main.go

@@ -31,12 +31,14 @@ import (
 	"time"
 
 	"golang.org/x/mod/module"
+	"golang.org/x/mod/semver"
 	goimp "golang.org/x/tools/imports"
 )
 
 type imports struct {
-	packages map[string]string // alias => import path
-	modules  map[string]string // module path => version
+	packages   map[string]string // alias => import path
+	modules    map[string]string // module path => version
+	onlySemVer bool
 }
 
 func (*imports) String() string {
@@ -71,6 +73,7 @@ func (imp *imports) Set(s string) error {
 			imp.modules = make(map[string]string)
 		}
 		imp.modules[path] = version
+		imp.onlySemVer = imp.onlySemVer && semver.IsValid(version) && version == semver.Canonical(version)
 	} else if alias == "" {
 		alias = "  " + path // special alias
 	}
@@ -104,7 +107,7 @@ func runSilent(cmd *exec.Cmd) error {
 
 func runX(cmd *exec.Cmd) error {
 	// Inject -x in go commands
-	if cmd.Args[0] == "go" {
+	if cmd.Args[0] == "go" && cmd.Args[1] != "env" {
 		cmd.Args = append([]string{"go", cmd.Args[1], "-x"}, cmd.Args[2:]...)
 	}
 	fmt.Printf("%s\n", cmd.Args)
@@ -118,6 +121,25 @@ func runTime(cmd *exec.Cmd) error {
 	return cmd.Run()
 }
 
+var goCmd = "go"
+
+func getGOMODCACHE(env []string) (string, error) {
+	var out bytes.Buffer
+	cmd := exec.Command(goCmd, "env", "GOMODCACHE")
+	cmd.Stderr = os.Stderr
+	cmd.Stdout = &out
+	cmd.Env = env
+	err := run(cmd)
+	if err != nil {
+		return "", err
+	}
+	b := bytes.TrimRight(out.Bytes(), "\r\n")
+	if len(b) == 0 {
+		return "", errors.New("can't retrieve GOMODCACHE")
+	}
+	return string(b), nil
+}
+
 func main() {
 	err := _main()
 	if exit, ok := err.(*exec.ExitError); ok && exit.ExitCode() > 0 {
@@ -129,7 +151,8 @@ func main() {
 
 func _main() error {
 	imports := imports{
-		packages: map[string]string{"  ": "os"},
+		packages:   map[string]string{"  ": "os"},
+		onlySemVer: true,
 	}
 	flag.Var(&imports, "i", "import package: [alias=]import-path")
 
@@ -193,6 +216,16 @@ func _main() error {
 	var dir, origDir string
 
 	if moduleMode {
+		// "go get" is not yet as smart as we want, so let's help
+		// https://go.dev/issue/43646
+		preferCache := imports.onlySemVer
+		var gomodcache string
+		if preferCache {
+			var err error
+			gomodcache, err = getGOMODCACHE(env)
+			preferCache = err == nil
+		}
+
 		var err error
 		if dir, err = os.MkdirTemp("", "goeval*"); err != nil {
 			log.Fatal(err)
@@ -216,15 +249,28 @@ func _main() error {
 		gogetArgs = append(gogetArgs, "get", "--")
 		for mod, ver := range imports.modules {
 			gogetArgs = append(gogetArgs, mod+"@"+ver)
+			if preferCache {
+				// Keep preferCache as long as we find modules in the cache
+				_, err := os.Stat(gomodcache + "/cache/download/" + mod + "/@v/" + ver + ".mod")
+				preferCache = err == nil
+			}
 		}
 		for _, path := range imports.packages {
 			if _, seen := imports.modules[path]; !seen {
 				gogetArgs = append(gogetArgs, path)
 			}
 		}
 
+		// fmt.Println("preferCache", preferCache)
+
 		cmd := exec.Command("go", gogetArgs...)
-		cmd.Env = env
+		if preferCache {
+			// As we found all modules in the cache, tell "go get" to not use the proxy.
+			// See https://go.dev/issue/43646
+			cmd.Env = append(env, "GOPROXY=file://"+filepath.ToSlash(gomodcache)+"/cache/download")
+		} else {
+			cmd.Env = env
+		}
 		cmd.Dir = dir
 		cmd.Stdin = nil
 		cmd.Stdout = nil