Address review comments

Rather than having arguments for ListModulePath,
create separate functions for listing in the module
cache and vendor directory. Create an unexported
function in dev-tools/mage which calls the appropriate
one depending on the value of UseVendor.

dev-tools/mage/common.go

@@ -49,6 +49,8 @@ import (
 	"github.com/magefile/mage/sh"
 	"github.com/magefile/mage/target"
 	"github.com/pkg/errors"
+
+	"github.com/elastic/beats/v7/dev-tools/mage/gotool"
 )
 
 // Expand expands the given Go text/template string.
@@ -791,3 +793,13 @@ func ParseVersion(version string) (major, minor, patch int, err error) {
 	patch, _ = strconv.Atoi(data["patch"])
 	return
 }
+
+// listModuleDir calls gotool.ListModuleVendorDir or
+// gotool.ListModuleCacheDir, depending on the value of
+// UseVendor.
+func listModuleDir(modpath string) (string, error) {
+	if UseVendor {
+		return gotool.ListModuleVendorDir(modpath)
+	}
+	return gotool.ListModuleCacheDir(modpath)
+}

dev-tools/mage/godaemon.go

@@ -22,8 +22,6 @@ import (
 	"log"
 	"os"
 	"path/filepath"
-
-	"github.com/elastic/beats/v7/dev-tools/mage/gotool"
 )
 
 var (
@@ -44,19 +42,13 @@ func BuildGoDaemon() error {
 			"only be executed within the golang-crossbuild docker environment.")
 	}
 
-	// Locate tsg/go-daemon.
-	var args []string
-	if UseVendor {
-		args = append(args, "-mod=vendor")
-	}
-	godaemonDir, err := gotool.ListModulePath("github.com/tsg/go-daemon", args...)
+	// Test if binaries are up-to-date.
+	godaemonDir, err := listModuleDir("github.com/tsg/go-daemon")
 	if err != nil {
 		return err
 	}
-
-	// Test if binaries are up-to-date.
-	output := MustExpand("build/golang-crossbuild/god-{{.Platform.GOOS}}-{{.Platform.Arch}}")
 	input := filepath.Join(godaemonDir, "src", "god.c")
+	output := MustExpand("build/golang-crossbuild/god-{{.Platform.GOOS}}-{{.Platform.Arch}}")
 	if IsUpToDate(output, input) {
 		log.Println(">>> buildGoDaemon is up-to-date for", Platform.Name)
 		return nil

dev-tools/mage/gomod.go

@@ -75,7 +75,7 @@ func Vendor() error {
 
 	// copy packages which require the whole tree
 	for _, p := range copyAll {
-		path, err := gotool.ListModulePath(p.name)
+		path, err := gotool.ListModuleVendorDir(p.name)
 		if err != nil {
 			return err
 		}

dev-tools/mage/gotool/go.go

@@ -93,18 +93,29 @@ func ListTestFiles(pkg string) ([]string, error) {
 	return getLines(callGo(nil, "list", "-f", tmpl, pkg))
 }
 
-// ListModulePath returns the path to the specified module.
-//
-// Additional parameters like "-mod=vendor" may be passed
-// to control behaviour.
-func ListModulePath(pkg string, args ...string) (string, error) {
+// ListModuleCacheDir returns the module cache directory containing
+// the specified module. If the module does not exist in the cache,
+// an error will be returned.
+func ListModuleCacheDir(pkg string) (string, error) {
+	return listModuleDir(pkg, false)
+}
+
+// ListModuleVendorDir returns the vendor directory containing the
+// specified module. If the module has not been vendored, an error
+// will be returned.
+func ListModuleVendorDir(pkg string) (string, error) {
+	return listModuleDir(pkg, true)
+}
+
+func listModuleDir(pkg string, vendor bool) (string, error) {
 	env := map[string]string{
-		// make sure to look in the module cache,
-		// unless otherwise configured in args.
+		// Make sure GOFLAGS does not influence behaviour.
 		"GOFLAGS": "",
 	}
-	args = append([]string{"-m", "-f", "{{.Dir}}"}, args...)
-	args = append(args, pkg)
+	args := []string{"-m", "-f", "{{.Dir}}", pkg}
+	if vendor {
+		args = append(args, "-mod=vendor")
+	}
 	lines, err := getLines(callGo(env, "list", args...))
 	if err != nil {
 		return "", err

dev-tools/mage/settings.go

@@ -284,11 +284,7 @@ func findElasticBeatsDir() (string, error) {
 	if repo.IsElasticBeats() {
 		return repo.RootDir, nil
 	}
-	var args []string
-	if UseVendor {
-		args = append(args, "-mod=vendor")
-	}
-	return gotool.ListModulePath("github.com/elastic/beats/v7")
+	return listModuleDir("github.com/elastic/beats/v7")
 }
 
 var (

generator/common/beatgen/setup/setup.go

@@ -112,7 +112,7 @@ func CopyVendor() error {
 		return err
 	}
 
-	path, err := gotool.ListModulePath("github.com/elastic/beats/v7")
+	path, err := gotool.ListModuleCacheDir("github.com/elastic/beats/v7")
 	if err != nil {
 		return err
 	}