fix race condition on magefile

Fix a race condition when calling `go-licence-detector`. The original
code waited for a goroutine to write to `go-licence-detector`'s stdin
before starting the command, so the mage target was hanging.

This commit fixes it, adds logging (when verbose set) to the command
run as well as improves the error message.

magefile.go

@@ -8,6 +8,7 @@
 package main
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"io"
@@ -111,8 +112,22 @@ func Notice() error {
 		return errors.Wrap(err, "failed running go list, please fix the issues reported")
 	}
 	fmt.Println(">> fmt - go run")
-	cmd := exec.Command("go", "run", "go.elastic.co/go-licence-detector", "-includeIndirect", "-rules", "dev-tools/notice/rules.json", "-overrides", "dev-tools/notice/overrides.json", "-noticeTemplate", "dev-tools/notice/NOTICE.txt.tmpl",
-		"-noticeOut", "NOTICE.txt", "-depsOut", "\"\"")
+	goLicenceDetectorCMD := []string{"go",
+		"run",
+		"go.elastic.co/go-licence-detector",
+		"-includeIndirect",
+		"-rules",
+		"dev-tools/notice/rules.json",
+		"-overrides",
+		"dev-tools/notice/overrides.json",
+		"-noticeTemplate",
+		"dev-tools/notice/NOTICE.txt.tmpl",
+		"-noticeOut",
+		"NOTICE.txt",
+		"-depsOut",
+		"\"\""}
+	printCMD(goLicenceDetectorCMD)
+	cmd := exec.Command(goLicenceDetectorCMD[0], goLicenceDetectorCMD[1:]...)
 	stdin, err := cmd.StdinPipe()
 	if err != nil {
 		return errors.Wrap(err, "failed running go run, please fix the issues reported")
@@ -126,10 +141,10 @@ func Notice() error {
 			fmt.Println(err)
 		}
 	}()
+	out, err := cmd.CombinedOutput()
 	wg.Wait()
-	_, err = cmd.CombinedOutput()
 	if err != nil {
-		return errors.Wrap(err, "failed combined output, please fix the issues reported")
+		return fmt.Errorf("calling go-licence-detector returned an error: '%w'. Its output is: '%s'", err, string(out))
 	}
 	return nil
 }
@@ -925,3 +940,21 @@ func majorMinor() string {
 	}
 	return ""
 }
+
+// printCMD prints the command in the same format than when
+// using the functions from the `sh` package. It also respects
+// the mage verbose flag
+func printCMD(cmd []string) {
+	if !mg.Verbose() {
+		return
+	}
+
+	buff := &bytes.Buffer{}
+
+	fmt.Fprintf(buff, "exec: %s", cmd[0])
+	for _, arg := range cmd[1:] {
+		fmt.Fprintf(buff, " %q", arg)
+	}
+
+	fmt.Println(buff.String())
+}