Re: [Qemu-devel] [PATCH 02/18] block: add error parameter to bdrv_snapshot_create() and related functions

[Luiz Capitulino <lcapitulino@redhat.com>]

On Thu, 06 Sep 2012 11:07:54 +0200
Pavel Hrdina <phrdina@redhat.com> wrote:

> On 08/30/2012 04:47 PM, Luiz Capitulino wrote:
> > On Wed, 15 Aug 2012 09:41:43 +0200
> > Pavel Hrdina <phrdina@redhat.com> wrote:
> >
> >> Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
> >> ---
> >>   block.c                | 25 +++++++++++++++++--------
> >>   block.h                |  3 ++-
> >>   block/qcow2-snapshot.c |  9 ++++++++-
> >>   block/qcow2.h          |  4 +++-
> >>   block/rbd.c            | 20 ++++++++++++++------
> >>   block/sheepdog.c       | 17 +++++++++--------
> >>   block_int.h            |  3 ++-
> >>   qemu-img.c             |  2 +-
> >>   savevm.c               |  2 +-
> >>   9 files changed, 57 insertions(+), 28 deletions(-)
> >>
> >> diff --git a/block.c b/block.c
> >> index 016858b..8bc49b7 100644
> >> --- a/block.c
> >> +++ b/block.c
> >> @@ -2661,16 +2661,25 @@ BlockDriverState *bdrv_snapshots(void)
> >>   }
> >>   
> >>   int bdrv_snapshot_create(BlockDriverState *bs,
> >> -                         QEMUSnapshotInfo *sn_info)
> >> +                         QEMUSnapshotInfo *sn_info,
> >> +                         Error **errp)
> >>   {
> >>       BlockDriver *drv = bs->drv;
> >> -    if (!drv)
> >> -        return -ENOMEDIUM;
> >> -    if (drv->bdrv_snapshot_create)
> >> -        return drv->bdrv_snapshot_create(bs, sn_info);
> >> -    if (bs->file)
> >> -        return bdrv_snapshot_create(bs->file, sn_info);
> >> -    return -ENOTSUP;
> >> +    int ret;
> >> +
> >> +    if (!drv) {
> >> +        error_set(errp, QERR_DEVICE_HAS_NO_MEDIUM, bdrv_get_device_name(bs));
> > We should only use QERR_ macros for the errors listed in the ErrorClass enum
> > (except GenericError), all other errors should generally use error_setg(), like
> > this:
> >
> >   error_setg(errp, "device '%s' has no medium);
> 
> I agree that we should use error_setg() for all GenericError, but I 
> think that would be better have macros for errors which are used 
> many-times . For example:
> 
> #define QERR_ERRMSG_DEVICE_HAS_NO_MEDIUM "Device '%s' has no medium"
> 
> and use error_setg(errp, QERR_ERRMSG_DEVICE_HAS_NO_MEDIUM, 
> bdrv_get_device_name(bs));

Hmm, no. The best way to refactor code that use the same error message
like the one above is to move the whole operation (with error handling
included) to a wrapper and use the wrapper instead.

Adding macros for error messages is a Bad Thing because soon or later
someone will create a new macro for a very similar but different error
message (which will lead to macro explosion) or will use an existing
macro that doesn't fit well to the failure being reported.

> 
> I think that the consistency of error messages is important and an error 
> message should be the same for every error occurrence.
> >
> >> +        ret = -ENOMEDIUM;
> > And, usually, we should get rid of errno propagation. There are two cases here:
> >
> >   1. errno is propagated up so that upper layers can print a decent error
> >      message to the user.
> >
> >      In this case, it's safe to eliminate errno. error_setg() will store a
> >      decent message already and the Error object can be propagated up.
> >
> >   2. errno is propagated up so that upper layers can distinguish among
> >      error causes and take different actions accordingly.
> >
> >      Doesn't seem to be the case of bdrv_snapshot_create() (ie. errno is only
> >      used to communicate the error to the user). However, I'm pretty sure that
> >      such usage exists in qemu and the error API will break it, as most of our
> >      errors are generic.
> >
> >      I see two solutions to this problem:
> >
> >         A. Add specific errors to ErrorClass. I don't like this very much,
> >            as it's possible that such errors are going to be useful only internally.
> >
> >         B. Add two new functions:
> >
> >              void error_sete(Error **err, ErrorClass err_class, int errno, const char *fmt, ...);
> >              int error_get_errno(const Error **err);
> >
> >           So that we can maintain errno when it's used to communicate
> >           error cause among functions.
> >
> >> +    } else if (drv->bdrv_snapshot_create) {
> >> +        ret = drv->bdrv_snapshot_create(bs, sn_info, errp);
> >> +    } else if (bs->file) {
> >> +        ret = bdrv_snapshot_create(bs->file, sn_info, errp);
> >> +    } else {
> >> +        error_set(errp, QERR_NOT_SUPPORTED);
> >> +        ret = -ENOTSUP;
> >> +    }
> >> +
> >> +    return ret;
> >>   }
> >>   
> >>   int bdrv_snapshot_goto(BlockDriverState *bs,
> >> diff --git a/block.h b/block.h
> >> index 2e2be11..92e782b 100644
> >> --- a/block.h
> >> +++ b/block.h
> >> @@ -296,7 +296,8 @@ int bdrv_can_snapshot(BlockDriverState *bs);
> >>   int bdrv_is_snapshot(BlockDriverState *bs);
> >>   BlockDriverState *bdrv_snapshots(void);
> >>   int bdrv_snapshot_create(BlockDriverState *bs,
> >> -                         QEMUSnapshotInfo *sn_info);
> >> +                         QEMUSnapshotInfo *sn_info,
> >> +                         Error **errp);
> >>   int bdrv_snapshot_goto(BlockDriverState *bs,
> >>                          const char *snapshot_id);
> >>   int bdrv_snapshot_delete(BlockDriverState *bs, const char *snapshot_id);
> >> diff --git a/block/qcow2-snapshot.c b/block/qcow2-snapshot.c
> >> index 4e7c93b..cf86dae 100644
> >> --- a/block/qcow2-snapshot.c
> >> +++ b/block/qcow2-snapshot.c
> >> @@ -25,6 +25,7 @@
> >>   #include "qemu-common.h"
> >>   #include "block_int.h"
> >>   #include "block/qcow2.h"
> >> +#include "qerror.h"
> >>   
> >>   typedef struct QEMU_PACKED QCowSnapshotHeader {
> >>       /* header is 8 byte aligned */
> >> @@ -312,7 +313,9 @@ static int find_snapshot_by_id_or_name(BlockDriverState *bs, const char *name)
> >>   }
> >>   
> >>   /* if no id is provided, a new one is constructed */
> >> -int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
> >> +int qcow2_snapshot_create(BlockDriverState *bs,
> >> +                          QEMUSnapshotInfo *sn_info,
> >> +                          Error **errp)
> >>   {
> >>       BDRVQcowState *s = bs->opaque;
> >>       QCowSnapshot *new_snapshot_list = NULL;
> >> @@ -331,6 +334,8 @@ int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
> >>   
> >>       /* Check that the ID is unique */
> >>       if (find_snapshot_by_id(bs, sn_info->id_str) >= 0) {
> >> +        error_set(errp, QERR_INVALID_PARAMETER_VALUE,
> >> +                  "name", "non-existing id identifier");
> >>           return -EEXIST;
> >>       }
> >>   
> >> @@ -415,6 +420,8 @@ fail:
> >>       g_free(sn->name);
> >>       g_free(l1_table);
> >>   
> >> +    error_set(errp, QERR_GENERIC_ERROR, ret);
> >> +
> >>       return ret;
> >>   }
> >>   
> >> diff --git a/block/qcow2.h b/block/qcow2.h
> >> index b4eb654..854bd12 100644
> >> --- a/block/qcow2.h
> >> +++ b/block/qcow2.h
> >> @@ -308,7 +308,9 @@ int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
> >>   int qcow2_zero_clusters(BlockDriverState *bs, uint64_t offset, int nb_sectors);
> >>   
> >>   /* qcow2-snapshot.c functions */
> >> -int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info);
> >> +int qcow2_snapshot_create(BlockDriverState *bs,
> >> +                          QEMUSnapshotInfo *sn_info,
> >> +                          Error **errp);
> >>   int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id);
> >>   int qcow2_snapshot_delete(BlockDriverState *bs, const char *snapshot_id);
> >>   int qcow2_snapshot_list(BlockDriverState *bs, QEMUSnapshotInfo **psn_tab);
> >> diff --git a/block/rbd.c b/block/rbd.c
> >> index 5a0f79f..7bc42f0 100644
> >> --- a/block/rbd.c
> >> +++ b/block/rbd.c
> >> @@ -16,6 +16,7 @@
> >>   #include "qemu-common.h"
> >>   #include "qemu-error.h"
> >>   #include "block_int.h"
> >> +#include "qerror.h"
> >>   
> >>   #include <rbd/librbd.h>
> >>   
> >> @@ -817,12 +818,15 @@ static int qemu_rbd_truncate(BlockDriverState *bs, int64_t offset)
> >>   }
> >>   
> >>   static int qemu_rbd_snap_create(BlockDriverState *bs,
> >> -                                QEMUSnapshotInfo *sn_info)
> >> +                                QEMUSnapshotInfo *sn_info,
> >> +                                Error **errp)
> >>   {
> >>       BDRVRBDState *s = bs->opaque;
> >> -    int r;
> >> +    int ret;
> >>   
> >>       if (sn_info->name[0] == '\0') {
> >> +        error_set(errp, QERR_INVALID_PARAMETER_VALUE,
> >> +                  "name", "some tag identifier");
> >>           return -EINVAL; /* we need a name for rbd snapshots */
> >>       }
> >>   
> >> @@ -832,17 +836,21 @@ static int qemu_rbd_snap_create(BlockDriverState *bs,
> >>        */
> >>       if (sn_info->id_str[0] != '\0' &&
> >>           strcmp(sn_info->id_str, sn_info->name) != 0) {
> >> +        error_set(errp, QERR_INVALID_PARAMETER_VALUE,
> >> +                  "name", "id and tag to equal");
> >>           return -EINVAL;
> >>       }
> >>   
> >>       if (strlen(sn_info->name) >= sizeof(sn_info->id_str)) {
> >> +        error_set(errp, QERR_INVALID_PARAMETER_VALUE,
> >> +                  "name", "shorter than 127 chars");
> >>           return -ERANGE;
> >>       }
> >>   
> >> -    r = rbd_snap_create(s->image, sn_info->name);
> >> -    if (r < 0) {
> >> -        error_report("failed to create snap: %s", strerror(-r));
> >> -        return r;
> >> +    ret = rbd_snap_create(s->image, sn_info->name);
> >> +    if (ret < 0) {
> >> +        error_set(errp, QERR_GENERIC_ERROR, ret);
> >> +        return ret;
> > You should always be careful when dropping error_report(), most of the time you
> > also have to change callers. In this case, bdrv_snapshot_create() is called
> > by qemu-img, which should be changed to print the error from the Error object.
> >
> >>       }
> >>   
> >>       return 0;
> >> diff --git a/block/sheepdog.c b/block/sheepdog.c
> >> index a04ad99..fc51e04 100644
> >> --- a/block/sheepdog.c
> >> +++ b/block/sheepdog.c
> >> @@ -17,6 +17,7 @@
> >>   #include "qemu_socket.h"
> >>   #include "block_int.h"
> >>   #include "bitops.h"
> >> +#include "qerror.h"
> >>   
> >>   #define SD_PROTO_VER 0x01
> >>   
> >> @@ -1730,7 +1731,9 @@ static int coroutine_fn sd_co_flush_to_disk(BlockDriverState *bs)
> >>       return 0;
> >>   }
> >>   
> >> -static int sd_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
> >> +static int sd_snapshot_create(BlockDriverState *bs,
> >> +                              QEMUSnapshotInfo *sn_info,
> >> +                              Error **errp)
> >>   {
> >>       BDRVSheepdogState *s = bs->opaque;
> >>       int ret, fd;
> >> @@ -1743,9 +1746,8 @@ static int sd_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
> >>               s->name, sn_info->vm_state_size, s->is_snapshot);
> >>   
> >>       if (s->is_snapshot) {
> >> -        error_report("You can't create a snapshot of a snapshot VDI, "
> >> -                     "%s (%" PRIu32 ").", s->name, s->inode.vdi_id);
> >> -
> >> +        error_set(errp, QERR_INVALID_PARAMETER_VALUE,
> >> +                  "name", " not VDI snapshot");
> >>           return -EINVAL;
> >>       }
> >>   
> >> @@ -1767,15 +1769,12 @@ static int sd_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
> >>       ret = write_object(fd, (char *)&s->inode, vid_to_vdi_oid(s->inode.vdi_id),
> >>                          s->inode.nr_copies, datalen, 0, 0, s->cache_enabled);
> >>       if (ret < 0) {
> >> -        error_report("failed to write snapshot's inode.");
> >>           goto cleanup;
> >>       }
> >>   
> >>       ret = do_sd_create(s->name, s->inode.vdi_size, s->inode.vdi_id, &new_vid, 1,
> >>                          s->addr, s->port);
> >>       if (ret < 0) {
> >> -        error_report("failed to create inode for snapshot. %s",
> >> -                     strerror(errno));
> >>           goto cleanup;
> >>       }
> >>   
> >> @@ -1785,7 +1784,6 @@ static int sd_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
> >>                         s->inode.nr_copies, datalen, 0, s->cache_enabled);
> >>   
> >>       if (ret < 0) {
> >> -        error_report("failed to read new inode info. %s", strerror(errno));
> >>           goto cleanup;
> >>       }
> >>   
> >> @@ -1795,6 +1793,9 @@ static int sd_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info)
> >>   
> >>   cleanup:
> >>       closesocket(fd);
> >> +    if (ret < 0) {
> >> +        error_set(errp, QERR_GENERIC_ERROR, ret);
> >> +    }
> >>       return ret;
> >>   }
> >>   
> >> diff --git a/block_int.h b/block_int.h
> >> index 4452f6f..b76c943 100644
> >> --- a/block_int.h
> >> +++ b/block_int.h
> >> @@ -206,7 +206,8 @@ struct BlockDriver {
> >>                                    const uint8_t *buf, int nb_sectors);
> >>   
> >>       int (*bdrv_snapshot_create)(BlockDriverState *bs,
> >> -                                QEMUSnapshotInfo *sn_info);
> >> +                                QEMUSnapshotInfo *sn_info,
> >> +                                Error **errp);
> >>       int (*bdrv_snapshot_goto)(BlockDriverState *bs,
> >>                                 const char *snapshot_id);
> >>       int (*bdrv_snapshot_delete)(BlockDriverState *bs, const char *snapshot_id);
> >> diff --git a/qemu-img.c b/qemu-img.c
> >> index b41e670..c798d66 100644
> >> --- a/qemu-img.c
> >> +++ b/qemu-img.c
> >> @@ -1267,7 +1267,7 @@ static int img_snapshot(int argc, char **argv)
> >>           sn.date_sec = tv.tv_sec;
> >>           sn.date_nsec = tv.tv_usec * 1000;
> >>   
> >> -        ret = bdrv_snapshot_create(bs, &sn);
> >> +        ret = bdrv_snapshot_create(bs, &sn, NULL);
> >>           if (ret) {
> >>               error_report("Could not create snapshot '%s': %d (%s)",
> >>                   snapshot_name, ret, strerror(-ret));
> >> diff --git a/savevm.c b/savevm.c
> >> index 0ea10c9..1b67af6 100644
> >> --- a/savevm.c
> >> +++ b/savevm.c
> >> @@ -2165,7 +2165,7 @@ void do_savevm(Monitor *mon, const QDict *qdict)
> >>           if (bdrv_can_snapshot(bs1)) {
> >>               /* Write VM state size only to the image that contains the state */
> >>               sn->vm_state_size = (bs == bs1 ? vm_state_size : 0);
> >> -            ret = bdrv_snapshot_create(bs1, sn);
> >> +            ret = bdrv_snapshot_create(bs1, sn, NULL);
> >>               if (ret < 0) {
> >>                   monitor_printf(mon, "Error while creating snapshot on '%s'\n",
> >>                                  bdrv_get_device_name(bs1));
>