curl: Fix coroutine waking

If we wake a coroutine from a different context, we must ensure that it
will yield exactly once (now or later), awaiting that wake.

curl’s current .ret == -EINPROGRESS loop may lead to the coroutine not
yielding if the request finishes before the loop gets run.  To fix it,
we must drop the loop and yield exactly once, if we need to yield.

Finding out that latter part ("if we need to yield") makes it a bit
complicated: Requests may be served from a cache internal to the curl
block driver, or fail before being submitted.  In these cases, we must
not yield.  However, if we find a matching but still ongoing request in
the cache, we will have to await that, i.e. still yield.

To address this, move the yield inside of the respective functions:
- Inside of curl_find_buf() when awaiting ongoing concurrent requests,
- Inside of curl_setup_preadv() when having created a new request.

Rename curl_setup_preadv() to curl_do_preadv() to reflect this.

(Can be reproduced with multiqueue by adding a usleep(100000) before the
`while (acb.ret == -EINPROGRESS)` loop.)

Also, add a comment why aio_co_wake() is safe regardless of whether the
coroutine and curl_multi_check_completion() run in the same context.

Cc: qemu-stable@nongnu.org
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20251110154854.151484-6-hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>