osutil: use system API to get the username when env vars are empty (#6246)

As Golang supports getting usernames via the standard library, this function
is now used to get the username if the environment variables are empty.
Using the standard library as a fallback is intended to keep compability
to existing implementations that rely on the environment variables dictating
the current username.

internal/osutil/osutil.go

@@ -6,6 +6,7 @@ package osutil
 
 import (
 	"os"
+	"os/user"
 )
 
 // IsFile returns true if given path exists as a file (i.e. not a directory).
@@ -33,12 +34,20 @@ func IsExist(path string) bool {
 	return err == nil || os.IsExist(err)
 }
 
-// CurrentUsername returns the current system user via environment variables.
+// CurrentUsername returns the current system user
 func CurrentUsername() string {
 	username := os.Getenv("USER")
 	if len(username) > 0 {
 		return username
 	}
 
-	return os.Getenv("USERNAME")
+	username = os.Getenv("USERNAME")
+	if len(username) > 0 {
+		return username
+	}
+
+	if user, err := user.Current(); err == nil {
+		username = user.Username
+	}
+	return username
 }

internal/osutil/osutil_test.go

@@ -5,6 +5,7 @@
 package osutil
 
 import (
+	"os"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -83,3 +84,17 @@ func TestCurrentUsername(t *testing.T) {
 	// Make sure it does not blow up
 	CurrentUsername()
 }
+
+func TestCurrentUsernamePrefersEnvironmentVariable(t *testing.T) {
+	// Some users/scripts expect that they can change the current username via environment variables
+	if userBak, ok := os.LookupEnv("USER"); ok {
+		defer os.Setenv("USER", userBak)
+	} else {
+		defer os.Unsetenv("USER")
+	}
+
+	if err := os.Setenv("USER", "__TESTING::USERNAME"); err != nil {
+		t.Skip("Could not set the USER environment variable:", err)
+	}
+	assert.Equal(t, "__TESTING::USERNAME", CurrentUsername())
+}