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

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. ---

@@ -2012,6 +2018,22 @@ C<nbd_aio_command_completed> to actually retire the command and learn whether the command was successful."; }; + "aio_in_flight", { + default_call with + args = []; ret = RInt; + permitted_states = [ Connected; Closed; Dead ]; + (* XXX is_locked = false ? *) + shortdesc = "check how many aio commands are still in flight"; + longdesc = "\ +Return the number of in-flight aio commands that are still awaiting a +response from the server before they can be retired. If this returns +a non-zero value when requesting a disconnect from the server (see +C<nbd_aio_disconnect> and C<nbd_shutdown>), libnbd does not try to +wait for those commands to complete gracefully; if the server strands +commands while shutting down, C<nbd_aio_command_completed> will not +be able to report status on those commands.";

+++ 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);