implement short read/write, EOF handling (#415)

Report any short reads to callers as ECONNRESET, which is the closest we can
meaningfully get right now. This also fixes get_next_command(), which previously
wasn't checking for short reads at all.

When we fail to send or recv from the socket due to the client disappearing in
some manner, call into vfu_reset_ctx() to clean up the connection fd, allowing a
subsequent vfu_attach_ctx() to work.

If we get 0 bytes from recv[msg](), this is reported by the transport as ENOMSG,
and is a normal EOF condition.

We can also get ECONNRESET: this can happen when we've written unacknowledged
data to the socket, the client side socket is closed, and we try a subsequent
read.

Finally, we can get a short read or write. Our handling of these still has
issues, but for now we'll presume this means the client has gone too. It may
in fact be due to a client bug - if it failed to write enough data - but right
now, we can't easily tell that.

Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@nutanix.com>

1 files changed, 7 insertions, 3 deletions

diff --git a/include/libvfio-user.h b/include/libvfio-user.h
index 1048236..925550a 100644
--- a/include/libvfio-user.h
+++ b/include/libvfio-user.h
@@ -137,18 +137,22 @@ int
 vfu_get_poll_fd(vfu_ctx_t *vfu_ctx);
 
 /**
- * Polls the vfu_ctx and processes the command recieved from client.
+ * Polls the vfu_ctx and processes the command received from client.
  * - Blocking vfu_ctx:
  *   Blocks until new request is received from client and continues processing
  *   the requests. Exits only in case of error or if the client disconnects.
  * - Non-blocking vfu_ctx(LIBVFIO_USER_FLAG_ATTACH_NB):
  *   Processes one request from client if it's available, otherwise it
- *   immediatelly returns and the caller is responsible for periodically
+ *   immediately returns and the caller is responsible for periodically
  *   calling again.
  *
  * @vfu_ctx: The libvfio-user context to poll
  *
- * @returns 0 on success, -1 on error. Sets errno.
+ * @returns 0 on success, -1 on error, with errno set as follows:
+ *
+ * EAGAIN/EWOULDBLOCK: no more commands to process
+ * ENOTCONN: client closed connection, vfu_attach_ctx() should be called again
+ * Other errno values are also possible.
  */
 int
 vfu_run_ctx(vfu_ctx_t *vfu_ctx);