On Fri, Jul 28, 2023 at 06:17:50PM +0100, Richard W.M. Jones wrote:
> This plugin operation might need to do some real work (instead of just
> fetching a number from memory), and so it might have to be retried.
>
> In particular, changes to the curl plugin make .get_size into a
> heavyweight operation, where previously it was done as a side-effect
> of .open. And so we must allow .get_size to be retried independent of
> .open.
> ---
> filters/retry-request/retry-request.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
It feels a bit odd that .open isn't calling .get_size to cache it, but
it's hard to state whether that's a bug in the filter (for not
pre-caching .get_size), in the plugin (for not having .get_size ready
to go by the end of .open), in whatever other filter is stacked on top
of retry-request and not caching .get_size during its .open, or not a
bug at all. Thus, while I'm slightly worried that this patch may be
papering over something instead of addressing root cause, I can't
pinpoint anything in specific where we might be going against our
documentation, and I can live with this going in.
>
> diff --git a/filters/retry-request/retry-request.c
> b/filters/retry-request/retry-request.c
> index e5b8344cd..8e3dd8246 100644
> --- a/filters/retry-request/retry-request.c
> +++ b/filters/retry-request/retry-request.c
> @@ -141,6 +141,18 @@ retry_request_open (nbdkit_next_open *next,
> nbdkit_context *nxdata,
> return r == 0 ? NBDKIT_HANDLE_NOT_NEEDED : NULL;
> }
>
> +static int64_t
> +retry_request_get_size (nbdkit_next *next, void *handle)
> +{
> + int64_t r;
> + int *err = &errno; /* used by the RETRY_* macros */
This is the reason why I'm reluctant to say whether this is the right
approach - if .get_size() is encountered during a .pread, there is no
guarantee that we pass a correct errno value back if the pread fails
because .get_size failed. Ensuring that .get_size is cached during
.open guarantees that we have a valid size for all subsequent
operations, without needing to worry about what happens to errno.
> +
> + RETRY_START("get_size")
> + r = next->get_size (next);
> + RETRY_END;
> + return r;
> +}
> +
> static int
> retry_request_pread (nbdkit_next *next,
> void *handle, void *buf, uint32_t count, uint64_t
> offset,
> @@ -267,6 +279,7 @@ static struct nbdkit_filter filter = {
> .config = retry_request_config,
> .config_help = retry_request_config_help,
> .open = retry_request_open,
> + .get_size = retry_request_get_size,
> .pread = retry_request_pread,
> .pwrite = retry_request_pwrite,
> .trim = retry_request_trim,
> --
> 2.41.0
>
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>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization: qemu.org | libguestfs.org
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