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Re: [Qemu-devel] [PATCH 6/6] migration: Pass Error ** argument to {save,


From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH 6/6] migration: Pass Error ** argument to {save, load}_vmstate
Date: Fri, 28 Apr 2017 17:19:03 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux)

"Dr. David Alan Gilbert" <address@hidden> writes:

> * Juan Quintela (address@hidden) wrote:
>> This way we use the "normal" way of printing errors for hmp commands.
>> 
>> --
>> Paolo suggestion
>> 
>> Signed-off-by: Juan Quintela <address@hidden>
>
> <snip>
>
>> +int save_vmstate(const char *name, Error **errp)
>>  {
>>      BlockDriverState *bs, *bs1;
>>      QEMUSnapshotInfo sn1, *sn = &sn1, old_sn1, *old_sn = &old_sn1;
>> @@ -2076,29 +2076,27 @@ int save_vmstate(const char *name)
>>      uint64_t vm_state_size;
>>      qemu_timeval tv;
>>      struct tm tm;
>> -    Error *local_err = NULL;
>>      AioContext *aio_context;
>>  
>>      if (!bdrv_all_can_snapshot(&bs)) {
>> -        error_report("Device '%s' is writable but does not support 
>> snapshots",
>> -                     bdrv_get_device_name(bs));
>> +        error_setg(errp, "Device '%s' is writable but does not support "
>> +                   "snapshots", bdrv_get_device_name(bs));
>>          return ret;
>>      }
>>  
>>      /* Delete old snapshots of the same name */
>>      if (name) {
>> -        ret = bdrv_all_delete_snapshot(name, &bs1, &local_err);
>> +        ret = bdrv_all_delete_snapshot(name, &bs1, errp);
>>          if (ret < 0) {
>> -            error_reportf_err(local_err,
>> -                              "Error while deleting snapshot on device 
>> '%s': ",
>> -                              bdrv_get_device_name(bs1));
>> +            error_prepend(errp, "Error while deleting snapshot on device "
>> +                          "'%s': ", bdrv_get_device_name(bs1));
>
> I thought the rule was that you had to use a local_err and use 
> error_propagate?
> (I hate that rule)

Quote qapi/error.h:

 * Receive an error and pass it on to the caller:
 *     Error *err = NULL;
 *     foo(arg, &err);
 *     if (err) {
 *         handle the error...
 *         error_propagate(errp, err);
 *     }
 * where Error **errp is a parameter, by convention the last one.
 *
 * Do *not* "optimize" this to
 *     foo(arg, errp);
 *     if (*errp) { // WRONG!
 *         handle the error...
 *     }
 * because errp may be NULL!
 *
 * But when all you do with the error is pass it on, please use
 *     foo(arg, errp);
 * for readability.

Neglects to mention you can error_prepend() and still avoid
error_propagate(), but the comment is long enough as it is.

>>              return ret;
>>          }
>>      }
>>  
>>      bs = bdrv_all_find_vmstate_bs();
>>      if (bs == NULL) {
>> -        error_report("No block device can accept snapshots");
>> +        error_setg(errp, "No block device can accept snapshots");
>>          return ret;
>>      }
>>      aio_context = bdrv_get_aio_context(bs);
>> @@ -2107,7 +2105,7 @@ int save_vmstate(const char *name)
>>  
>>      ret = global_state_store();
>>      if (ret) {
>> -        error_report("Error saving global state");
>> +        error_setg(errp, "Error saving global state");
>>          return ret;
>>      }
>>      vm_stop(RUN_STATE_SAVE_VM);
>> @@ -2139,21 +2137,20 @@ int save_vmstate(const char *name)
>>      /* save the VM state */
>>      f = qemu_fopen_bdrv(bs, 1);
>>      if (!f) {
>> -        error_report("Could not open VM state file");
>> +        error_setg(errp, "Could not open VM state file");
>>          goto the_end;
>>      }
>> -    ret = qemu_savevm_state(f, &local_err);
>> +    ret = qemu_savevm_state(f, errp);
>>      vm_state_size = qemu_ftell(f);
>>      qemu_fclose(f);
>>      if (ret < 0) {
>> -        error_report_err(local_err);
>>          goto the_end;
>>      }
>>  
>>      ret = bdrv_all_create_snapshot(sn, bs, vm_state_size, &bs);
>>      if (ret < 0) {
>> -        error_report("Error while creating snapshot on '%s'",
>> -                     bdrv_get_device_name(bs));
>> +        error_setg(errp, "Error while creating snapshot on '%s'",
>> +                   bdrv_get_device_name(bs));
>>          goto the_end;
>>      }
>>  
>> @@ -2226,7 +2223,7 @@ void qmp_xen_load_devices_state(const char *filename, 
>> Error **errp)
>>      migration_incoming_state_destroy();
>>  }
>>  
>> -int load_vmstate(const char *name)
>> +int load_vmstate(const char *name, Error **errp)
>>  {
>>      BlockDriverState *bs, *bs_vm_state;
>>      QEMUSnapshotInfo sn;
>> @@ -2236,20 +2233,22 @@ int load_vmstate(const char *name)
>>      MigrationIncomingState *mis = migration_incoming_get_current();
>>  
>>      if (!bdrv_all_can_snapshot(&bs)) {
>> -        error_report("Device '%s' is writable but does not support 
>> snapshots",
>> -                     bdrv_get_device_name(bs));
>> +        error_setg(errp,
>> +                   "Device '%s' is writable but does not support snapshots",
>> +                   bdrv_get_device_name(bs));
>>          return -ENOTSUP;
>>      }
>>      ret = bdrv_all_find_snapshot(name, &bs);
>>      if (ret < 0) {
>> -        error_report("Device '%s' does not have the requested snapshot 
>> '%s'",
>> -                     bdrv_get_device_name(bs), name);
>> +        error_setg(errp,
>> +                   "Device '%s' does not have the requested snapshot '%s'",
>> +                   bdrv_get_device_name(bs), name);
>>          return ret;
>>      }
>>  
>>      bs_vm_state = bdrv_all_find_vmstate_bs();
>>      if (!bs_vm_state) {
>> -        error_report("No block device supports snapshots");
>> +        error_setg(errp, "No block device supports snapshots");
>>          return -ENOTSUP;
>>      }
>>      aio_context = bdrv_get_aio_context(bs_vm_state);
>> @@ -2261,8 +2260,8 @@ int load_vmstate(const char *name)
>>      if (ret < 0) {
>>          return ret;
>>      } else if (sn.vm_state_size == 0) {
>> -        error_report("This is a disk-only snapshot. Revert to it offline "
>> -            "using qemu-img.");
>> +        error_setg(errp, "This is a disk-only snapshot. Revert to it "
>> +                   " offline using qemu-img");
>>          return -EINVAL;
>>      }
>>  
>> @@ -2271,7 +2270,7 @@ int load_vmstate(const char *name)
>>  
>>      ret = bdrv_all_goto_snapshot(name, &bs);
>>      if (ret < 0) {
>> -        error_report("Error %d while activating snapshot '%s' on '%s'",
>> +        error_setg(errp, "Error %d while activating snapshot '%s' on '%s'",
>>                       ret, name, bdrv_get_device_name(bs));
>>          return ret;
>>      }
>> @@ -2279,7 +2278,7 @@ int load_vmstate(const char *name)
>>      /* restore the VM state */
>>      f = qemu_fopen_bdrv(bs_vm_state, 0);
>>      if (!f) {
>> -        error_report("Could not open VM state file");
>> +        error_setg(errp, "Could not open VM state file");
>>          return -EINVAL;
>>      }
>>  
>> @@ -2293,7 +2292,7 @@ int load_vmstate(const char *name)
>>  
>>      migration_incoming_state_destroy();
>>      if (ret < 0) {
>> -        error_report("Error %d while loading VM state", ret);
>> +        error_setg(errp, "Error %d while loading VM state", ret);
>>          return ret;
>>      }
>>  
>> diff --git a/replay/replay-snapshot.c b/replay/replay-snapshot.c
>> index 8cced46..fdc4353 100644
>> --- a/replay/replay-snapshot.c
>> +++ b/replay/replay-snapshot.c
>> @@ -62,14 +62,16 @@ void replay_vmstate_register(void)
>>  
>>  void replay_vmstate_init(void)
>>  {
>> +    Error *err = NULL;
>> +
>>      if (replay_snapshot) {
>>          if (replay_mode == REPLAY_MODE_RECORD) {
>> -            if (save_vmstate(replay_snapshot) != 0) {
>> +            if (save_vmstate(replay_snapshot, &err) != 0) {
>>                  error_report("Could not create snapshot for icount record");
>>                  exit(1);
>>              }
>>          } else if (replay_mode == REPLAY_MODE_PLAY) {
>> -            if (load_vmstate(replay_snapshot) != 0) {
>> +            if (load_vmstate(replay_snapshot, &err) != 0) {
>>                  error_report("Could not load snapshot for icount replay");
>>                  exit(1);
>>              }
>
> How do either of the contents of the 'err' get reported - they're not
> printed at all are they?
> (I don't like error_abort, I'd rather see the other message as well).

I guess you mean &error_fatal.

Consider error_report_err().

>
> Dave
>
>> diff --git a/vl.c b/vl.c
>> index 0b4ed52..8b3ec2e 100644
>> --- a/vl.c
>> +++ b/vl.c
>> @@ -4681,7 +4681,9 @@ int main(int argc, char **argv, char **envp)
>>      if (replay_mode != REPLAY_MODE_NONE) {
>>          replay_vmstate_init();
>>      } else if (loadvm) {
>> -        if (load_vmstate(loadvm) < 0) {
>> +        Error *local_err = NULL;
>> +        if (load_vmstate(loadvm, &local_err) < 0) {
>> +            error_report_err(local_err);
>>              autostart = 0;
>>          }
>>      }
>> -- 
>> 2.9.3
>> 
>> 
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK



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