Re: [Libguestfs] [PATCH nbdkit v2 6/9] retry-request: Allow get_size operation to be retried
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