[Libguestfs] [libnbd PATCH 1/6] api: Add nbd_aio_in_flight

[Eric Blake]

On 6/29/19 8:28 AM, Eric Blake wrote:
> Some clients need to know when it is safe to issue NBD_CMD_DISC, or to
> decide whether calling poll(POLLIN) will block indefinitely because
> the server isn't expected to respond.  Make this easier to learn by
> tracking the count of commands we have queued up to send, as well as
> the count of commands where we are waiting on the server's response.
> 
> Update tests/aio-parallel* and examples/batched-read-write to use
> nbd's own in-flight counter instead of reimplementing it ourselves.
> 
> Note that h->in_flight is only ever updated while the lock is held;
> but we may want to consider also making it atomic and therefore
> readable as a lock-less function.
> ---

> +++ b/lib/aio.c
> @@ -23,6 +23,7 @@
>  #include <stdbool.h>
>  #include <errno.h>
>  #include <inttypes.h>
> +#include <assert.h>
> 
>  #include "internal.h"
> 
> @@ -84,6 +85,8 @@ nbd_unlocked_aio_command_completed (struct nbd_handle *h,
>      prev_cmd->next = cmd->next;
>    else
>      h->cmds_done = cmd->next;
> +  h->in_flight--;
> +  assert (h->in_flight >= 0);

We guard against underflow...


> +++ b/lib/rw.c

> @@ -236,6 +241,7 @@ nbd_internal_command_common (struct nbd_handle *h,
        nbd_internal_run (h, cmd_issue) == -1)
>        return -1;
>    }
> 
> +  h->in_flight++;

...but even though we inserted in the list, we fail to increment if
nbd_internal_run() encountered an error. Also, the count is too low if
the server managed to get a reply to us without us blocking. Obvious fix
pushed.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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