Re: [Libguestfs] [nbdkit PATCH] sh: Allow pwrite to not consume all data

On Thu, Aug 31, 2023 at 10:52:59AM +0100, Richard W.M. Jones wrote: I look at it as: the plugin decides HOW to process the bytes that the client requested to write. A traditional plugin will store the data so it can be read back later (in which case, failing to read all of stdin better result in the plugin returning failure). But during testing, a plugin can declare "I am a data sink; I don't care what bytes you write, my behavior is to ignore them and focus on something else" (for example, it can be convenient to throw out a quick sh plugin that has a side effect of counting .pwrite calls). Such a plugin is going to confuse any client that expects to read back what it just wrote, but when you are testing the protocol with a client that only sends writes (and doesn't even attempt to send read commands, because it knows the writes aren't persistent), that is just fine. In other words, as long as your plugin documents that writes are not persistent, then returning success without draining stdin should be acceptable; just the same as a C plugin .pwrite that ignores the input buffer can still return success. Of course, until newer nbdkit has propagated to systems we care about, any unit tests using sh or eval plugins as a data sink still have to inject 'cat >/dev/null' to drain stdin, in order to pass under heavy load when EPIPE even happens in the first place (I was surprised at how hard it is to lose the race and actually get an EPIPE message from nbdkit talking to the sh plugin script on my machine). -- Eric Blake, Principal Software Engineer Red Hat, Inc. Virtualization: qemu.org | libguestfs.org