[glib/wip/smcv/big-dbus-write-with-fds: 1/2] gdbus: Cope with sending fds in a message that takes multiple writes

[Simon McVittie <smcv src gnome org>]

commit d7f12aceb777ee959952150518a2ea61c42227d4
Author: Simon McVittie <smcv collabora com>
Date:   Wed Oct 28 11:04:47 2020 +0000

    gdbus: Cope with sending fds in a message that takes multiple writes
    
    Suppose we are sending a 5K message with fds, and the kernel is only
    accepting 4K with each sendmsg(). data->blob_size is 5K.
    
    The first time we get into write_message_continue_writing(),
    data->total_written is 0. We will try to write the entire message,
    plus the attached file descriptors; or if the stream doesn't support
    fd-passing (not a socket), we need to fail with
    "Tried sending a file descriptor on unsupported stream".
    
    Because the kernel didn't accept the entire message, we come back in.
    This time, we won't enter the Unix-specific block that involves sending
    fds, because now data->total_written is 4K, and it would be wrong to
    try to attach the same fds again. However, we also need to avoid
    failing with "Tried sending a file descriptor on unsupported stream"
    in this case.
    
    Resolves: https://gitlab.gnome.org/GNOME/glib/-/issues/2074
    Signed-off-by: Simon McVittie <smcv collabora com>

 gio/gdbusprivate.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)
---
diff --git a/gio/gdbusprivate.c b/gio/gdbusprivate.c
index 5c980b40b..2551e4791 100644
--- a/gio/gdbusprivate.c
+++ b/gio/gdbusprivate.c
@@ -1085,8 +1085,11 @@ write_message_continue_writing (MessageToWriteData *data)
   else
     {
 #ifdef G_OS_UNIX
-      if (fd_list != NULL)
+      if (data->total_written == 0 && fd_list != NULL)
         {
+          /* We were trying to write byte 0 of the message, which needs
+           * the fd list to be attached to it, but this connection doesn't
+           * support doing that. */
           g_task_return_new_error (task,
                                    G_IO_ERROR,
                                    G_IO_ERROR_FAILED,