Allow to define confdir

PAM has a pam_start_confdir() which allows to define the configuration
directory where all services are located.
This is useful to define your own service on tests in particular, so
that you can control your stack and be independant of the host when
running them.
Allow defining this configuration directory, with a new StartConfDir
function.
Also, allow pre-checking for the API availability with
CheckPamHasStartConfdir().

transaction.go

@@ -6,9 +6,12 @@ package pam
 //#cgo CFLAGS: -Wall -std=c99
 //#cgo LDFLAGS: -lpam
 //void init_pam_conv(struct pam_conv *conv, long c);
+//int pam_start_confdir(const char *service_name, const char *user, const struct pam_conv *pam_conversation, const char *confdir, pam_handle_t **pamh) __attribute__ ((weak));
+//int check_pam_start_confdir(void);
 import "C"
 
 import (
+	"errors"
 	"runtime"
 	"strings"
 	"unsafe"
@@ -85,9 +88,33 @@ func transactionFinalizer(t *Transaction) {
 // Start initiates a new PAM transaction. Service is treated identically to
 // how pam_start treats it internally.
 //
-// All application calls to PAM begin with Start (or StartFunc). The returned
+// All application calls to PAM begin with Start*. The returned
 // transaction provides an interface to the remainder of the API.
 func Start(service, user string, handler ConversationHandler) (*Transaction, error) {
+	return start(service, user, handler, "")
+}
+
+// StartFunc registers the handler func as a conversation handler.
+func StartFunc(service, user string, handler func(Style, string) (string, error)) (*Transaction, error) {
+	return Start(service, user, ConversationFunc(handler))
+}
+
+// StartConfDir initiates a new PAM transaction. Service is treated identically to
+// how pam_start treats it internally.
+// confdir allows to define where all pam services are defined. This is used to provide
+// custom paths for tests.
+//
+// All application calls to PAM begin with Start*. The returned
+// transaction provides an interface to the remainder of the API.
+func StartConfDir(service, user string, handler ConversationHandler, confDir string) (*Transaction, error) {
+	if !CheckPamHasStartConfdir() {
+		return nil, errors.New("StartConfDir() was used, but the pam version on the system is not recent enough")
+	}
+
+	return start(service, user, handler, confDir)
+}
+
+func start(service, user string, handler ConversationHandler, confDir string) (*Transaction, error) {
 	t := &Transaction{
 		conv: &C.struct_pam_conv{},
 		c:    cbAdd(handler),
@@ -101,18 +128,19 @@ func Start(service, user string, handler ConversationHandler) (*Transaction, err
 		u = C.CString(user)
 		defer C.free(unsafe.Pointer(u))
 	}
-	t.status = C.pam_start(s, u, t.conv, &t.handle)
+	if confDir == "" {
+		t.status = C.pam_start(s, u, t.conv, &t.handle)
+	} else {
+		c := C.CString(confDir)
+		defer C.free(unsafe.Pointer(c))
+		t.status = C.pam_start_confdir(s, u, t.conv, c, &t.handle)
+	}
 	if t.status != C.PAM_SUCCESS {
 		return nil, t
 	}
 	return t, nil
 }
 
-// StartFunc registers the handler func as a conversation handler.
-func StartFunc(service, user string, handler func(Style, string) (string, error)) (*Transaction, error) {
-	return Start(service, user, ConversationFunc(handler))
-}
-
 func (t *Transaction) Error() string {
 	return C.GoString(C.pam_strerror(t.handle, C.int(t.status)))
 }
@@ -304,3 +332,8 @@ func (t *Transaction) GetEnvList() (map[string]string, error) {
 	C.free(unsafe.Pointer(p))
 	return env, nil
 }
+
+// CheckPamHasStartConfdir return if pam on system supports pam_system_confdir
+func CheckPamHasStartConfdir() bool {
+	return C.check_pam_start_confdir() == 0
+}