Re: [Qemu-block] [Qemu-devel] [RFC] transactions: add transaction-wide property

[John Snow]

So here's the current status of this blob:

- Markus supports the idea of a transaction-wide property, but hasn't
reviewed this particular RFC.
- Eric seemed supportive of a transaction-wide property, but hasn't
chimed in to this thread yet.
- Stefan was not sure what this patch was trying to accomplish.
- Fam appears to think that the concept of transactional properties is
worth pursuing and offered some light review on this RFC.

So let's recap a little bit, in a Markus-style breakdown to help bring
everyone back up to speed, Because I would desperately like some input
from the Qapibarons.

== Recap ==

We're trying to add transactional commands to make the incremental
backup QMP primitives (namely dirty bitmap clear and drive-backup with
sync=incremental) useful in an atomic fashion. Currently, you can't
clear a bitmap and start a backup simultaneously, so it's hard to
guarantee the safety of these things.

The primary problem with mixing jobs and transactions is that if a job
later fails, what do we do with the other actions that succeeded?

We need to be able to cancel jobs all together for failures, but we
don't want to break backwards compatibility with workflows that didn't
expect one failed backup to cancel a different backup.

So we need to add a parameter _somewhere_. First attempts put it as a
new parameter into the QMP command, second attempts put it in an
Action-only extension to the QMP command structure.

I propose we stick it on as an argument to the Transaction command
directly. No worrying about if one action has the transactional-cancel
argument and another doesn't, no managing the difference between QMP and
Action versions of command argument structures.

== What this patch is trying to do ==

This RFC patch applies to Fam's V7, and removes his per-action
transactional-cancel arguments and installs instead a transaction-wide
property, named "allow-partial," which I intended to be short for "Allow
partial completion."

The way it does this is meant to emulate a forthcoming feature by Eric
Blake. Instead of adding an "allow-partial" argument directly to
Transactions, it does this:

- Add an optional TransactionProperties structure to Transaction
- Add an optional allow-partial bool to this structure

This way, the transactional arguments are permanently "boxed" and we can
add/remove them at our leisure without disturbing the C prototypes any
further. This may not be necessary after Eric and Markus' qapi/qmp
refactoring decathlon.

The "default" creates allow-partial as "true," the current behavior. Any
Actions that do not support the allow-partial boolean set to false will
report an error immediately during the prepare phase of the transaction.

This way, if this property is set at the transactional level but an
action doesn't "support" or understand that property, it can fail the
transaction to prevent undefined behavior. This restriction can be
potentially lifted in the future on a per-action basis without harm to
backwards compatibility.

This approach is simple and intuitive: you are instructing all actions
as part of a transaction to live or die together. If they are incapable
of complying, they fail up-front and outright. It reduces the C
management burden and it is taxonomically obvious.

>From this approach, we leave ourselves expansion options, as well:
We can always add per-action overrides to transaction-wide properties in
the future if we so desire, but don't have to do that now.

== What happens next? ==

I believe Fam has completed work on his contribution to this set and is
handing the reins back over to me. I would like to push for the
transactional property.

I will prepare a new iteration that assumes transaction-wide properties
are the "way to go" with the understanding that if there is significant
ruckus and unhappiness we can always roll back to Fam's last version if
we all get tired of reading these long emails that I type.

== Potential Problems ==

(Because you know I can't help myself.)

As discussed on-list with Fam as a response to his review of this RFC,
there might be room to change this property from a simple boolean to a
tri-state:

sync-cancel: {none, jobs, all}

where none/false implies no change from the current behavior.
all/true implies that all actions are to live and die together.
"jobs" might imply that every action that creates a job is to
participate in the group cancellation, but any action that does not can
ignore it.

This would require some introspection to the client to be meaningful
("Which actions are jobs?") but it is probably the most powerful.

Thinking about it, I think what I'll do is this:

sync-cancel: {none, all}

And if we want "jobs" at a later date, it's just an enum addition away.

== Any questions? ==

:D

--js

On 09/24/2015 05:40 PM, John Snow wrote:
> This replaces the per-action property as in Fam's series.
> Instead, we have a transaction-wide property that is shared
> with each action.
> 
> At the action level, if a property supplied transaction-wide
> is disagreeable, we return error and the transaction is aborted.
> 
> RFC:
> 
> Where this makes sense: Any transactional actions that aren't
> prepared to accept this new paradigm of transactional behavior
> can error_setg and return.
> 
> Where this may not make sense: consider the transactions which
> do not use BlockJobs to perform their actions, i.e. they are
> synchronous during the transactional phase. Because they either
> fail or succeed so early, we might say that of course they can
> support this property ...
> 
> ...however, consider the case where we create a new bitmap and
> perform a full backup, using allow_partial=false. If the backup
> fails, we might well expect the bitmap to be deleted because a
> member of the transaction ultimately/eventually failed. However,
> the bitmap creation was not undone because it does not have a
> pending/delayed abort/commit action -- those are only for jobs
> in this implementation.
> 
> How do we fix this?
> 
> (1) We just say "No, you can't use the new block job transaction
>     completion mechanic in conjunction with these commands,"
> 
> (2) We make Bitmap creation/resetting small, synchronous blockjobs
>     that can join the BlockJobTxn
> 
> Signed-off-by: John Snow <address@hidden>
> ---
>  blockdev.c             | 87 
> ++++++++++++++++++++++++++++++++++++--------------
>  blockjob.c             |  2 +-
>  qapi-schema.json       | 15 +++++++--
>  qapi/block-core.json   | 26 ---------------
>  qmp-commands.hx        |  2 +-
>  tests/qemu-iotests/124 | 12 +++----
>  6 files changed, 83 insertions(+), 61 deletions(-)
> 
> diff --git a/blockdev.c b/blockdev.c
> index 45a9fe7..02b1a83 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1061,7 +1061,7 @@ static void blockdev_do_action(int kind, void *data, 
> Error **errp)
>      action.data = data;
>      list.value = &action;
>      list.next = NULL;
> -    qmp_transaction(&list, errp);
> +    qmp_transaction(&list, false, NULL, errp);
>  }
>  
>  void qmp_blockdev_snapshot_sync(bool has_device, const char *device,
> @@ -1286,6 +1286,7 @@ struct BlkActionState {
>      TransactionAction *action;
>      const BlkActionOps *ops;
>      BlockJobTxn *block_job_txn;
> +    TransactionProperties *txn_props;
>      QSIMPLEQ_ENTRY(BlkActionState) entry;
>  };
>  
> @@ -1322,6 +1323,12 @@ static void internal_snapshot_prepare(BlkActionState 
> *common,
>      name = internal->name;
>  
>      /* 2. check for validation */
> +    if (!common->txn_props->allow_partial) {
> +        error_setg(errp,
> +                   "internal_snapshot does not support allow_partial = 
> false");
> +        return;
> +    }
> +
>      blk = blk_by_name(device);
>      if (!blk) {
>          error_set(errp, ERROR_CLASS_DEVICE_NOT_FOUND,
> @@ -1473,6 +1480,12 @@ static void external_snapshot_prepare(BlkActionState 
> *common,
>      }
>  
>      /* start processing */
> +    if (!common->txn_props->allow_partial) {
> +        error_setg(errp,
> +                   "external_snapshot does not support allow_partial = 
> false");
> +        return;
> +    }
> +
>      state->old_bs = bdrv_lookup_bs(has_device ? device : NULL,
>                                     has_node_name ? node_name : NULL,
>                                     &local_err);
> @@ -1603,14 +1616,11 @@ static void drive_backup_prepare(BlkActionState 
> *common, Error **errp)
>      DriveBackupState *state = DO_UPCAST(DriveBackupState, common, common);
>      BlockDriverState *bs;
>      BlockBackend *blk;
> -    DriveBackupTxn *backup_txn;
>      DriveBackup *backup;
> -    BlockJobTxn *txn = NULL;
>      Error *local_err = NULL;
>  
>      assert(common->action->kind == TRANSACTION_ACTION_KIND_DRIVE_BACKUP);
> -    backup_txn = common->action->drive_backup;
> -    backup = backup_txn->base;
> +    backup = common->action->drive_backup->base;
>  
>      blk = blk_by_name(backup->device);
>      if (!blk) {
> @@ -1624,11 +1634,6 @@ static void drive_backup_prepare(BlkActionState 
> *common, Error **errp)
>      state->aio_context = bdrv_get_aio_context(bs);
>      aio_context_acquire(state->aio_context);
>  
> -    if (backup_txn->has_transactional_cancel &&
> -        backup_txn->transactional_cancel) {
> -        txn = common->block_job_txn;
> -    }
> -
>      do_drive_backup(backup->device, backup->target,
>                      backup->has_format, backup->format,
>                      backup->sync,
> @@ -1637,7 +1642,7 @@ static void drive_backup_prepare(BlkActionState 
> *common, Error **errp)
>                      backup->has_bitmap, backup->bitmap,
>                      backup->has_on_source_error, backup->on_source_error,
>                      backup->has_on_target_error, backup->on_target_error,
> -                    txn, &local_err);
> +                    common->block_job_txn, &local_err);
>      if (local_err) {
>          error_propagate(errp, local_err);
>          return;
> @@ -1686,16 +1691,13 @@ static void do_blockdev_backup(const char *device, 
> const char *target,
>  static void blockdev_backup_prepare(BlkActionState *common, Error **errp)
>  {
>      BlockdevBackupState *state = DO_UPCAST(BlockdevBackupState, common, 
> common);
> -    BlockdevBackupTxn *backup_txn;
>      BlockdevBackup *backup;
>      BlockDriverState *bs, *target;
>      BlockBackend *blk;
> -    BlockJobTxn *txn = NULL;
>      Error *local_err = NULL;
>  
>      assert(common->action->kind == TRANSACTION_ACTION_KIND_BLOCKDEV_BACKUP);
> -    backup_txn = common->action->blockdev_backup;
> -    backup = backup_txn->base;
> +    backup = common->action->blockdev_backup->base;
>  
>      blk = blk_by_name(backup->device);
>      if (!blk) {
> @@ -1720,17 +1722,12 @@ static void blockdev_backup_prepare(BlkActionState 
> *common, Error **errp)
>      }
>      aio_context_acquire(state->aio_context);
>  
> -    if (backup_txn->has_transactional_cancel &&
> -        backup_txn->transactional_cancel) {
> -        txn = common->block_job_txn;
> -    }
> -
>      do_blockdev_backup(backup->device, backup->target,
>                         backup->sync,
>                         backup->has_speed, backup->speed,
>                         backup->has_on_source_error, backup->on_source_error,
>                         backup->has_on_target_error, backup->on_target_error,
> -                       txn, &local_err);
> +                       common->block_job_txn, &local_err);
>      if (local_err) {
>          error_propagate(errp, local_err);
>          return;
> @@ -1777,6 +1774,13 @@ static void 
> block_dirty_bitmap_add_prepare(BlkActionState *common,
>      BlockDirtyBitmapState *state = DO_UPCAST(BlockDirtyBitmapState,
>                                               common, common);
>  
> +    if (!common->txn_props->allow_partial) {
> +        error_setg(errp,
> +                   "block_dirty_bitmap_add does not support "
> +                   "allow_partial = false");
> +        return;
> +    }
> +
>      action = common->action->block_dirty_bitmap_add;
>      /* AIO context taken and released within qmp_block_dirty_bitmap_add */
>      qmp_block_dirty_bitmap_add(action->node, action->name,
> @@ -1812,6 +1816,13 @@ static void 
> block_dirty_bitmap_clear_prepare(BlkActionState *common,
>                                               common, common);
>      BlockDirtyBitmap *action;
>  
> +    if (!common->txn_props->allow_partial) {
> +        error_setg(errp,
> +                   "block_dirty_bitmap_clear does not support "
> +                   "allow_partial = false");
> +        return;
> +    }
> +
>      action = common->action->block_dirty_bitmap_clear;
>      state->bitmap = block_dirty_bitmap_lookup(action->node,
>                                                action->name,
> @@ -1914,21 +1925,45 @@ static const BlkActionOps actions[] = {
>      }
>  };
>  
> +/**
> + * Allocate a TransactionProperties structure if necessary, and fill
> + * that structure with desired defaults if they are unset.
> + */
> +static TransactionProperties 
> *get_transaction_properties(TransactionProperties *props)
> +{
> +    if (!props) {
> +        props = g_new0(TransactionProperties, 1);
> +    }
> +
> +    if (!props->has_allow_partial) {
> +        props->allow_partial = true;
> +    }
> +
> +    return props;
> +}
> +
>  /*
>   * 'Atomic' group operations.  The operations are performed as a set, and if
>   * any fail then we roll back all operations in the group.
>   */
> -void qmp_transaction(TransactionActionList *dev_list, Error **errp)
> +void qmp_transaction(TransactionActionList *dev_list,
> +                     bool hasTransactionProperties,
> +                     struct TransactionProperties *props,
> +                     Error **errp)
>  {
>      TransactionActionList *dev_entry = dev_list;
> -    BlockJobTxn *block_job_txn;
> +    BlockJobTxn *block_job_txn = NULL;
>      BlkActionState *state, *next;
>      Error *local_err = NULL;
>  
>      QSIMPLEQ_HEAD(snap_bdrv_states, BlkActionState) snap_bdrv_states;
>      QSIMPLEQ_INIT(&snap_bdrv_states);
>  
> -    block_job_txn = block_job_txn_new();
> +    /* Does this transaction get *canceled* as a group on failure? */
> +    props = get_transaction_properties(props);
> +    if (props->allow_partial == false) {
> +        block_job_txn = block_job_txn_new();
> +    }
>  
>      /* drain all i/o before any operations */
>      bdrv_drain_all();
> @@ -1950,6 +1985,7 @@ void qmp_transaction(TransactionActionList *dev_list, 
> Error **errp)
>          state->ops = ops;
>          state->action = dev_info;
>          state->block_job_txn = block_job_txn;
> +        state->txn_props = props;
>          QSIMPLEQ_INSERT_TAIL(&snap_bdrv_states, state, entry);
>  
>          state->ops->prepare(state, &local_err);
> @@ -1982,6 +2018,9 @@ exit:
>          }
>          g_free(state);
>      }
> +    if (!hasTransactionProperties) {
> +        g_free(props);
> +    }
>      block_job_txn_unref(block_job_txn);
>  }
>  
> diff --git a/blockjob.c b/blockjob.c
> index 91e8d3c..00146ff 100644
> --- a/blockjob.c
> +++ b/blockjob.c
> @@ -512,7 +512,7 @@ static void block_job_txn_ref(BlockJobTxn *txn)
>  
>  void block_job_txn_unref(BlockJobTxn *txn)
>  {
> -    if (--txn->refcnt == 0) {
> +    if (txn && --txn->refcnt == 0) {
>          g_free(txn);
>      }
>  }
> diff --git a/qapi-schema.json b/qapi-schema.json
> index 8769099..0f28311 100644
> --- a/qapi-schema.json
> +++ b/qapi-schema.json
> @@ -1505,14 +1505,20 @@
>  { 'union': 'TransactionAction',
>    'data': {
>         'blockdev-snapshot-sync': 'BlockdevSnapshot',
> -       'drive-backup': 'DriveBackupTxn',
> -       'blockdev-backup': 'BlockdevBackupTxn',
> +       'drive-backup': 'DriveBackup',
> +       'blockdev-backup': 'BlockdevBackup',
>         'abort': 'Abort',
>         'blockdev-snapshot-internal-sync': 'BlockdevSnapshotInternal',
>         'block-dirty-bitmap-add': 'BlockDirtyBitmapAdd',
>         'block-dirty-bitmap-clear': 'BlockDirtyBitmap'
>     } }
>  
> +{ 'struct': 'TransactionProperties',
> +  'data': {
> +      '*allow-partial': 'bool'
> +  }
> +}
> +
>  ##
>  # @transaction
>  #
> @@ -1533,7 +1539,10 @@
>  # Since 1.1
>  ##
>  { 'command': 'transaction',
> -  'data': { 'actions': [ 'TransactionAction' ] } }
> +  'data': { 'actions': [ 'TransactionAction' ],
> +            '*properties': 'TransactionProperties'
> +          }
> +}
>  
>  ##
>  # @human-monitor-command:
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 24db5c2..83742ab 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -749,19 +749,6 @@
>              '*on-target-error': 'BlockdevOnError' } }
>  
>  ##
> -# @DriveBackupTxn
> -#
> -# @transactional-cancel: #optional whether failure or cancellation of other
> -#                        block jobs with @transactional-cancel true in the 
> same
> -#                        transaction causes the whole group to cancel. 
> Default
> -#                        is false.
> -#
> -# Since: 2.5
> -##
> -{ 'struct': 'DriveBackupTxn', 'base': 'DriveBackup',
> -  'data': {'*transactional-cancel': 'bool' } }
> -
> -##
>  # @BlockdevBackup
>  #
>  # @device: the name of the device which should be copied.
> @@ -797,19 +784,6 @@
>              '*on-target-error': 'BlockdevOnError' } }
>  
>  ##
> -# @BlockdevBackupTxn
> -#
> -# @transactional-cancel: #optional whether failure or cancellation of other
> -#                        block jobs with @transactional-cancel true in the 
> same
> -#                        transaction causes the whole group to cancel. 
> Default
> -#                        is false
> -#
> -# Since: 2.5
> -##
> -{ 'struct': 'BlockdevBackupTxn', 'base': 'BlockdevBackup',
> -  'data': {'*transactional-cancel': 'bool' } }
> -
> -##
>  # @blockdev-snapshot-sync
>  #
>  # Generates a synchronous snapshot of a block device.
> diff --git a/qmp-commands.hx b/qmp-commands.hx
> index c388274..7c1fed9 100644
> --- a/qmp-commands.hx
> +++ b/qmp-commands.hx
> @@ -1262,7 +1262,7 @@ EQMP
>      },
>      {
>          .name       = "transaction",
> -        .args_type  = "actions:q",
> +        .args_type  = "actions:q,properties:q?",
>          .mhandler.cmd_new = qmp_marshal_transaction,
>      },
>  
> diff --git a/tests/qemu-iotests/124 b/tests/qemu-iotests/124
> index 33d00ef..028b346 100644
> --- a/tests/qemu-iotests/124
> +++ b/tests/qemu-iotests/124
> @@ -456,14 +456,13 @@ class TestIncrementalBackup(iotests.QMPTestCase):
>          transaction = [
>              transaction_drive_backup(drive0['id'], target0, 
> sync='incremental',
>                                       format=drive0['fmt'], mode='existing',
> -                                     bitmap=dr0bm0.name,
> -                                     transactional_cancel=True),
> +                                     bitmap=dr0bm0.name),
>              transaction_drive_backup(drive1['id'], target1, 
> sync='incremental',
>                                       format=drive1['fmt'], mode='existing',
> -                                     bitmap=dr1bm0.name,
> -                                     transactional_cancel=True),
> +                                     bitmap=dr1bm0.name)
>          ]
> -        result = self.vm.qmp('transaction', actions=transaction)
> +        result = self.vm.qmp('transaction', actions=transaction,
> +                             properties={'allow-partial':False} )
>          self.assert_qmp(result, 'return', {})
>  
>          # Observe that drive0's backup is cancelled and drive1 completes with
> @@ -485,7 +484,8 @@ class TestIncrementalBackup(iotests.QMPTestCase):
>          target1 = self.prepare_backup(dr1bm0)
>  
>          # Re-run the exact same transaction.
> -        result = self.vm.qmp('transaction', actions=transaction)
> +        result = self.vm.qmp('transaction', actions=transaction,
> +                             properties={'allow-partial':False})
>          self.assert_qmp(result, 'return', {})
>  
>          # Both should complete successfully this time.
>