gdbusserver: retry binding to unix addresses only when possible

This adds the condition that a new unix address is tried only when there
is enough space to append random character(s) before the address reach
the length of UNIX_PATH_MAX.

Before this change, creating a GDBusServer with unix addresses longer
than UNIX_PATH_MAX that existed in the filesystem would never return. A
test that expects G_IO_ERROR_ADDRESS_IN_USE in such scenario was added.

gio/gdbusserver.c

@@ -45,12 +45,14 @@
 #include "ginputstream.h"
 #include "giostream.h"
 #include "gmarshal-internal.h"
+#include "gnetworking.h"
 
 #ifdef G_OS_UNIX
 #include <unistd.h>
 #endif
 #ifdef G_OS_WIN32
 #include <io.h>
+#include "giowin32-afunix.h"
 #endif
 
 #include "gunixsocketaddress.h"
@@ -62,6 +64,10 @@
    G_DBUS_SERVER_FLAGS_AUTHENTICATION_ALLOW_ANONYMOUS | \
    G_DBUS_SERVER_FLAGS_AUTHENTICATION_REQUIRE_SAME_USER)
 
+#ifndef UNIX_PATH_MAX
+#define UNIX_PATH_MAX G_SIZEOF_MEMBER (struct sockaddr_un, sun_path)
+#endif
+
 /**
  * GDBusServer:
  *
@@ -699,10 +705,12 @@ try_unix (GDBusServer  *server,
       gint n;
       GString *s;
       GError *local_error;
+      gsize orig_path_len = 0; // length before random characters are added
 
     retry:
       s = g_string_new (tmpdir != NULL ? tmpdir : dir);
       g_string_append (s, "/dbus-");
+      orig_path_len = s->len;
       for (n = 0; n < 8; n++)
         g_string_append_c (s, random_ascii ());
 
@@ -720,8 +728,11 @@ try_unix (GDBusServer  *server,
         {
           if (local_error->domain == G_IO_ERROR && local_error->code == G_IO_ERROR_ADDRESS_IN_USE)
             {
-              g_error_free (local_error);
+              if (orig_path_len < UNIX_PATH_MAX - 2) /* random_ascii + NULL byte */
-              goto retry;
+                {
+                  g_error_free (local_error);
+                  goto retry;
+                }
             }
           g_propagate_error (error, local_error);
           goto out;

gio/tests/gdbus-server-auth.c

@@ -537,6 +537,50 @@ test_server_auth_sha1_tcp (void)
   do_test_server_auth (INTEROP_FLAGS_SHA1 | INTEROP_FLAGS_TCP);
 }
 
+static void
+test_server_path_in_use (void)
+{
+#ifdef G_OS_UNIX
+#ifndef UNIX_PATH_MAX
+#define UNIX_PATH_MAX G_SIZEOF_MEMBER (struct sockaddr_un, sun_path)
+#endif
+  GError *error = NULL;
+  GString *path = NULL;
+  gchar *listenable_address = NULL;
+  GDBusServer *server = NULL;
+  gchar *guid = NULL;
+  gchar *escaped = NULL;
+
+  path = g_string_new (g_get_tmp_dir ());
+  guid = g_dbus_generate_guid ();
+  while (path->len < UNIX_PATH_MAX - 1)
+    g_string_append_c (path, 'x');
+  escaped = g_dbus_address_escape_value (path->str);
+  listenable_address = g_strdup_printf ("unix:%s=%s", "dir", escaped);
+  g_assert_cmpint (g_mkdir_with_parents (path->str, 0700), ==, 0);
+  g_string_free (path, TRUE);
+
+  server = g_dbus_server_new_sync (listenable_address,
+                                   G_DBUS_SERVER_FLAGS_RUN_IN_THREAD,
+                                   guid,
+                                   NULL,
+                                   NULL,
+                                   &error);
+  g_assert_error (error, G_IO_ERROR, G_IO_ERROR_ADDRESS_IN_USE);
+  g_error_free (error);
+
+  if (server != NULL)
+    g_dbus_server_stop (server);
+
+  g_clear_object (&server);
+  g_free (guid);
+  g_free (escaped);
+  g_free (listenable_address);
+#else
+  g_test_skip ("unix: addresses only work on Unix");
+#endif
+}
+
 int
 main (int   argc,
       char *argv[])
@@ -552,6 +596,7 @@ main (int   argc,
   g_test_add_func ("/gdbus/server-auth/external/require-same-user", test_server_auth_external_require_same_user);
   g_test_add_func ("/gdbus/server-auth/sha1", test_server_auth_sha1);
   g_test_add_func ("/gdbus/server-auth/sha1/tcp", test_server_auth_sha1_tcp);
+  g_test_add_func ("/gdbus/server-auth/path-in-use", test_server_path_in_use);
 
   return g_test_run();
 }