module-transaction: Add GetUser() method that prompts an user if non-set

We can now finally test this properly both using a mock and through the
interactive module that will do the request for us in various conditions.

module-transaction.go

@@ -1,11 +1,20 @@
 // Package pam provides a wrapper for the PAM application API.
 package pam
 
+/*
+#cgo CFLAGS: -Wall -std=c99
+#cgo LDFLAGS: -lpam
+
+#include <security/pam_modules.h>
+#include <stdlib.h>
+#include <stdint.h>
+*/
 import "C"
 
 import (
 	"errors"
 	"fmt"
+	"unsafe"
 )
 
 // ModuleTransaction is an interface that a pam module transaction
@@ -16,6 +25,7 @@ type ModuleTransaction interface {
 	PutEnv(nameVal string) error
 	GetEnv(name string) string
 	GetEnvList() (map[string]string, error)
+	GetUser(prompt string) (string, error)
 }
 
 // ModuleHandlerFunc is a function type used by the ModuleHandler.
@@ -89,3 +99,31 @@ func (m *moduleTransaction) InvokeHandler(handler ModuleHandlerFunc,
 	m.lastStatus.Store(status)
 	return err
 }
+
+type moduleTransactionIface interface {
+	getUser(outUser **C.char, prompt *C.char) C.int
+}
+
+func (m *moduleTransaction) getUser(outUser **C.char, prompt *C.char) C.int {
+	return C.pam_get_user(m.handle, outUser, prompt)
+}
+
+// getUserImpl is the default implementation for GetUser, but kept as private so
+// that can be used to test the pam package
+func (m *moduleTransaction) getUserImpl(iface moduleTransactionIface,
+	prompt string) (string, error) {
+	var user *C.char
+	var cPrompt = C.CString(prompt)
+	defer C.free(unsafe.Pointer(cPrompt))
+	err := m.handlePamStatus(iface.getUser(&user, cPrompt))
+	if err != nil {
+		return "", err
+	}
+	return C.GoString(user), nil
+}
+
+// GetUser is similar to GetItem(User), but it would start a conversation if
+// no user is currently set in PAM.
+func (m *moduleTransaction) GetUser(prompt string) (string, error) {
+	return m.getUserImpl(m, prompt)
+}