ObjectCacher: optionally make writex always non-blocking

Add a callback argument to writex, and a finisher to run the
callbacks. Move the check for dirty+tx > max_dirty into a helper that
can be called from a wrapper around the callbacks from writex, or from
the current place in _wait_for_write().

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librbd: make aio_writes to the cache always non-blocking by default

When the ObjectCacher's writex blocks, it affects the thread requesting
the aio, which can cause starvation for other I/O when used by QEMU.

Preserve the old behavior via a config option in case this has any
bad side-effects, like too much memory usage under heavy write loads.

Fixes: #4091
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
ObjectCacher: always complete flush_set() callback

This removes the last remnants of
b5e9995f59d363ba00d9cac413d9b754ee44e370. If there's nothing to flush,
immediately call the callback instead of deleting it. Callers were
assuming they were responsible for completing the callback whenever
flush_set() returned true, and always called complete(0) in this
case. Simplify the interface and just do this in flush_set(), so that
it always calls the callback.

Since C_GatherBuilder deletes its finisher if there are no subs,
only set its finisher when subs are present. This way we can still
call ->complete() for the callback.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
ObjectCacher: remove NULL checks in flush_set()

Callers will always pass a callback, so assert this and remove the
checks for it being NULL.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
ObjectCacher: remove unneeded var from flush_set()

The gather will only have subs if there is something to flush. Remove
the safe variable, which indicates the same thing, and convert the
conditionals that used it to an else branch. Movinig gather.activate()
inside the has_subs() check has no effect since activate() does
nothing when there are no subs.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librados: increase version number to indicated new functions

Listing snaps and listing watchers were added to the C++ api.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librados: add async flush interface

Sometimes you don't want flush to block, and can't modify
already scheduled aio_writes. This will be useful for a
librbd async flush interface.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librados: add versions of a couple functions taking explicit snap args

Usually the snapid to read from or the snapcontext to send with a write
are determined implicitly by the IoCtx the operations are done on.

This makes it difficult to have multiple ops in flight to the same
IoCtx using different snapcontexts or reading from different snapshots,
particularly when more than one operation may be needed past the initial
scheduling.

Add versions of aio_read, aio_sparse_read, and aio_operate
that don't depend on the snap id or snapcontext stored in the IoCtx,
but get them from the caller. Specifying this information for each
operation can be a more useful interface in general, but for now just
add it for the methods used by librbd.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librbd: add an is_complete() method to AioCompletions

Mainly this is useful for testing, like flushing and checking that
all pending writes are complete after the flush finishes.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librbd: use the same IoCtx for each request

Before we were duplicating the IoCtx for each new request since they
could have a different snapshot context or read from a different
snapshot id. Since librados now supports setting these explicitly
for a given request, do that instead.

Since librados tracks outstanding requests on a per-IoCtx basis, this
also fixes a bug that causes flush() without caching to ignore
all the outstanding requests, since they were to separate,
duplicate IoCtxs.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librbd: add an async flush

At this point it's a simple wrapper around the ObjectCacher or
librados.

This is needed for QEMU so that its main thread can continue while a
flush is occurring. Since this will be backported, don't update the
librbd version yet, just add a #define that QEMU and others can use to
detect the presence of aio_flush().

Refs: #3737
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librbd: increment version for new functions is_complete() and aio_flush()

This done in a separate commit since the increased version number
should not be backported.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>