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Re: [Qemu-devel] [PATCH v2] block/curl: Don't lose original error when a
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH v2] block/curl: Don't lose original error when a connection fails. |
Date: |
Wed, 8 Jul 2015 12:23:37 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 03.07.2015 um 14:35 hat Markus Armbruster geschrieben:
> "Richard W.M. Jones" <address@hidden> writes:
>
> > Currently if qemu is connected to a curl source (eg. web server), and
> > the web server fails / times out / dies, you always see a bogus EIO
> > "Input/output error".
> >
> > For example, choose a large file located on any local webserver which
> > you control:
> >
> > $ qemu-img convert -p http://example.com/large.iso /tmp/test
> >
> > Once it starts copying the file, stop the webserver and you will see
> > qemu-img fail with:
> >
> > qemu-img: error while reading sector 61440: Input/output error
> >
> > This patch does two things: Firstly print the actual error from curl
> > so it doesn't get lost. Secondly, change EIO to EPROTO. EPROTO is a
> > POSIX.1 compatible errno which more accurately reflects that there was
> > a protocol error, rather than some kind of hardware failure.
> >
> > After this patch is applied, the error changes to:
> >
> > $ qemu-img convert -p http://example.com/large.iso /tmp/test
> > qemu-img: curl: transfer closed with 469989 bytes remaining to read
> > qemu-img: error while reading sector 16384: Protocol error
> >
> > Signed-off-by: Richard W.M. Jones <address@hidden>
> > Reviewed-by: Stefan Hajnoczi <address@hidden>
> > ---
> > block/curl.c | 9 ++++++++-
> > 1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/block/curl.c b/block/curl.c
> > index 3a2b63e..2fd7c06 100644
> > --- a/block/curl.c
> > +++ b/block/curl.c
> > @@ -22,6 +22,7 @@
> > * THE SOFTWARE.
> > */
> > #include "qemu-common.h"
> > +#include "qemu/error-report.h"
> > #include "block/block_int.h"
> > #include "qapi/qmp/qbool.h"
> > #include "qapi/qmp/qstring.h"
> > @@ -298,6 +299,12 @@ static void curl_multi_check_completion(BDRVCURLState
> > *s)
> > /* ACBs for successful messages get completed in curl_read_cb
> > */
> > if (msg->data.result != CURLE_OK) {
> > int i;
> > +
> > + /* Don't lose the original error message from curl, since
> > + * it contains extra data.
> > + */
> > + error_report("curl: %s", state->errmsg);
> > +
> > for (i = 0; i < CURL_NUM_ACB; i++) {
> > CURLAIOCB *acb = state->acb[i];
> >
>
> Printing an error message, then returning an error code is problematic.
>
> It works when the caller is going to print its own error message to the
> same destination. Callee produces a specific error message devoid of
> context, caller produces an unspecific one with hopefully more context.
> Better than just one of them. Worse than a single specific error with
> context, but that can't be done with just a "return errno code"
> interface.
>
> It's kind of wrong when the caller reports its own error somewhere else,
> e.g. to a monitor. Still, when barfing extra info to stderr is the best
> we can do, it's better than nothing.
>
> It's more wrong when the caller handles the error quietly. I guess
> that's never the case here, but I can't be sure without a lot more
> sleuthing. Perhaps Kevin or Stefan can judge this immediately.
I'm not worried too much about requests made by the monitor or during
startup. I don't like the error_report() there, but having a more
specific error message on stderr is better than having nothing.
The case that bothers me more is guest requests. Depending on the
werror/rerror settings, this may allow the guest to flood the log file
with curl error messages.
> > @@ -305,7 +312,7 @@ static void curl_multi_check_completion(BDRVCURLState
> > *s)
> > continue;
> > }
> >
> > - acb->common.cb(acb->common.opaque, -EIO);
> > + acb->common.cb(acb->common.opaque, -EPROTO);
> > qemu_aio_unref(acb);
> > state->acb[i] = NULL;
> > }
>
> To understand impact exactly, we'd need to figure out where the changed
> error code gets consumed. However, I don't expect consumers to check
> the actual error code. A quick grep for comparisons with EIO or -EIO
> finds nothing related to block I/O, except for nbd_trip() checking the
> value of nbd_co_receive_request(), and that's unrelated.
Yes, I wouldn't expect any problems caused by this change.
Kevin