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Suppose we are sending a 5K message with fds (so data->blob points to 5K of data, data->blob_size is 5K, and fd_list is non-null), but the kernel is only accepting up to 4K with each sendmsg(). The first time we get into write_message_continue_writing(), data->total_written will be 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. We just want to write out the data of the rest of the message, starting from (blob + total_written) (in this exaple, the last 1K). Resolves: #2074 Signed-off-by: Simon McVittie <smcv@collabora.com>
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