Re: [Qemu-devel] [PATCH 03/11] block: add transactional callbacks feature

[Max Reitz <mreitz@redhat.com>]

On 2015-03-04 at 23:15, John Snow wrote:
> The goal here is to add a new method to transactions that allows
> developers to specify a callback that will get invoked only once
> all jobs spawned by a transaction are completed, allowing developers
> the chance to perform actions conditionally pending complete success
> or complete failure.
>
> In order to register the new callback to be invoked, a user must request
> a callback pointer and closure by calling new_transaction_wrapper, which
> creates a wrapper around a closure and callback that would originally have
> been passed to e.g. backup_start().
>
> The function will return a function pointer and a new closure to be passed
> instead. The transaction system will effectively intercept these callbacks
> and execute the desired actions upon reception of all intercepted callbacks.
>
> This means that the registered callbacks will be called after all other
> transaction actions that requested a callback have completed. The feature
> has no knowledge of jobs spawned without informing the BlkTransactionList.
>
> For an example of how to use the feature, please skip ahead to:
> 'block: drive_backup transaction callback support' which serves as an example
> for how to hook in drive_backup (or any block job launched by transactions).
>
>
> Note 1: Defining a callback method alone is not sufficient to have the new
>          method invoked. You must call new_transaction_wrapper AND ensure the
>          callback it returns to you is used as the callback for the job.
>
> Note 2: You can use this feature for any system that registers completions of
>          an action via a callback of the form (void *opaque, int ret), not just
>          block job callbacks.
>
> Note 3: new_blk_transaction has no users in this patch, but will in
>          the next patch where it will become static and local to blockdev.c.
>
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>   blockdev.c | 225 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 223 insertions(+), 2 deletions(-)
>
> diff --git a/blockdev.c b/blockdev.c
> index 5120af1..3153ee7 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1207,6 +1207,8 @@ static BdrvDirtyBitmap *block_dirty_bitmap_lookup(const char *node,
>   /* New and old BlockDriverState structs for atomic group operations */
>   
>   typedef struct BlkTransactionState BlkTransactionState;
> +typedef struct BlkTransactionList BlkTransactionList;
> +typedef struct BlkTransactionData BlkTransactionData;
>   
>   /* Only prepare() may fail. In a single transaction, only one of commit() or
>      abort() will be called, clean() will always be called if it present. */
> @@ -1221,6 +1223,8 @@ typedef struct BdrvActionOps {
>       void (*abort)(BlkTransactionState *common);
>       /* Clean up resource in the end, can be NULL. */
>       void (*clean)(BlkTransactionState *common);
> +    /* Execute this after +all+ jobs in the transaction finish */
> +    void (*cb)(BlkTransactionState *common);
>   } BdrvActionOps;
>   
>   /*
> @@ -1231,9 +1235,220 @@ typedef struct BdrvActionOps {
>   struct BlkTransactionState {
>       TransactionAction *action;
>       const BdrvActionOps *ops;
> +    BlkTransactionList *list;
> +    void *opaque;
> +    /* Allow external users (callbacks) to reference this obj past .clean() */
> +    int refcount;
> +    /* All transactions in the current group */
>       QSIMPLEQ_ENTRY(BlkTransactionState) entry;
> +    /* Transactions in the current group with callbacks */
> +    QSIMPLEQ_ENTRY(BlkTransactionState) list_entry;

"list_entry" seems very generic and it's hard for me to see a 
fundamental difference to just "entry". How about "cb_entry", or 
"cb_list_entry", or "cb_group_entry" or something like that?

>   };
>   
> +struct BlkTransactionList {
> +    int jobs;     /* Effectively: A refcount */

Good to know, but I'd like the reason why it's called like this to be 
here anyway ("Number of jobs remaining" or I don't know).

> +    int status;   /* Cumulative retcode */

The only places I currently find "retcode" in are linux-user/signal.c 
and tests/image-fuzzer/runner.py. How about the more common "return 
value" (or simply "cumulative status")?

("retcode" reads a bit like "retcon" to me, that's why I had to think 
about it for a second)

> +    QSIMPLEQ_HEAD(actions, BlkTransactionState) actions;
> +};
> +
> +typedef struct BlkTransactionData {
> +    void *opaque; /* Data given to encapsulated callback */
> +    int ret;      /* Return code given to encapsulated callback */
> +} BlkTransactionData;
> +
> +typedef struct BlkTransactionWrapper {
> +    void *opaque; /* Data to be given to encapsulated callback */
> +    void (*callback)(void *opaque, int ret); /* Encapsulated callback */
> +} BlkTransactionWrapper;

I find it pretty difficult to figure out what these objects are each 
for. Care to add comments for that, too?

> +
> +static BlkTransactionList *new_blk_transaction_list(void)
> +{
> +    BlkTransactionList *btl = g_malloc0(sizeof(*btl));

Maybe use g_new0(BlkTransactionList, 1); (It's typesafe! And who doesn't 
love typesafety?).

(Optional, though, the foo = malloc(sizeof(*foo)) pattern is pretty 
wide-spread, and as far as I remember, Markus purposely did not replace 
it by foo = g_new(Foo, 1))

> +
> +    /* Implicit 'job' for qmp_transaction itself */
> +    btl->jobs = 1;

Well, if it is a refcount, just call it that way...

(Update: Okay, I see why you didn't call it that way. Maybe the comment 
needs improvement, like "The transaction itself is a job, too, that 
needs to be completed before the callbacks are called")

> +    QSIMPLEQ_INIT(&btl->actions);
> +    return btl;
> +}
> +
> +static BlkTransactionState *blk_put_transaction_state(BlkTransactionState *bts)
> +{
> +    bts->refcount--;
> +    if (bts->refcount == 0) {
> +        g_free(bts);

How about removing it from the lists it's in? Doesn't appear to be 
necessary, but I'd find it cleaner.

> +        return NULL;
> +    }
> +    return bts;
> +}
> +
> +static void del_blk_transaction_list(BlkTransactionList *btl)
> +{
> +    BlkTransactionState *bts, *bts_next;
> +
> +    /* The list should in normal cases be empty,
> +     * but in case someone really just wants to kibosh the whole deal: */

Thank you for teaching me a new word.

> +    QSIMPLEQ_FOREACH_SAFE(bts, &btl->actions, list_entry, bts_next) {
> +        g_free(bts->opaque);

Urp... Are you sure? :-/

I'd rather have some callback for destroying the object... Apparently 
this will (for now) be always useless overhead, because that callback is 
only calling g_free(), but it's an opaque pointer, so it's not really up 
to you to do anything with it other than passing it around.

> +        blk_put_transaction_state(bts);
> +    }
> +
> +    g_free(btl);
> +}
> +
> +static void blk_run_transaction_callbacks(BlkTransactionList *btl)
> +{
> +    BlkTransactionState *bts, *bts_next;
> +
> +    QSIMPLEQ_FOREACH_SAFE(bts, &btl->actions, list_entry, bts_next) {
> +        if (bts->ops->cb) {
> +            bts->ops->cb(bts);
> +        }
> +
> +        /* Free the BlkTransactionData */
> +        g_free(bts->opaque);

Again... Urp. If you know it's a BlkTransactionData object, please make 
it a BlkTransactionData pointer and don't call it "opaque" if it's not 
so opaque in the end.

> +        bts->opaque = NULL;
> +        blk_put_transaction_state(bts);
> +    }
> +
> +    QSIMPLEQ_INIT(&btl->actions);
> +}
> +
> +static BlkTransactionList *put_blk_transaction_list(BlkTransactionList *btl)
> +{
> +    btl->jobs--;
> +    if (btl->jobs == 0) {

Okay, this is interesting, because on the one hand it makes it clear why 
you did not call it "refcount", while on the other hand this is clearly 
a pattern which looks very much like handling a refcount.

Maybe you could call the function differently, like 
"blk_transaction_list_job_completed()" or something. It's longer, but I 
think it reduces the change of people staring at this thinking "Why is 
'jobs' actually a refcount?".

> +        blk_run_transaction_callbacks(btl);
> +        del_blk_transaction_list(btl);
> +        return NULL;
> +    }
> +    return btl;
> +}
> +
> +static void blk_transaction_complete(BlkTransactionState *common)
> +{
> +    BlkTransactionList *btl = common->list;
> +
> +    /* Add this action into the pile to be completed */
> +    QSIMPLEQ_INSERT_TAIL(&btl->actions, common, list_entry);
> +
> +    /* Inform the list that we have a completion;
> +     * possibly run all the pending actions. */
> +    put_blk_transaction_list(btl);
> +}
> +
> +/**
> + * Intercept a callback that was issued due to a transactional action.
> + */
> +static void transaction_callback(void *opaque, int ret)
> +{
> +    BlkTransactionState *common = opaque;
> +    BlkTransactionWrapper *btw = common->opaque;
> +
> +    /* Prepare data for ops->cb() */
> +    BlkTransactionData *btd = g_malloc0(sizeof(*btd));

g_new0(BlkTransactionData, 1);

> +    btd->opaque = btw->opaque;
> +    btd->ret = ret;
> +
> +    /* Transaction state now tracks OUR data */
> +    common->opaque = btd;

Sorry, but I really have a hard time following this opaqueness... Again, 
if you can, please clarify what the objects are for, and it would be 
very nice to separate truly opaque objects from these internally used 
objects (which are managed by you and thus are to be managed by you 
(g_free()), because reusing void * pointers for different kinds of 
objects like this makes my brain go strawberry.

> +
> +    /* Keep track of the amalgamated return code */
> +    common->list->status |= ret;

Hm, I guess you're expecting ret to be -errno. In that case, you'd 
probably rather want "if (ret && (!common->list->status || 
common->list->status == -ENOSPC)) { common->list->status = ret; }" or 
something like that, because bit-OR-ing different -errno values will 
probably not turn out so great.

> +
> +    /* Deliver the intercepted callback FIRST */
> +    btw->callback(btw->opaque, ret);
> +    blk_transaction_complete(common);
> +    g_free(btw);
> +}
> +
> +typedef void (CallbackFn)(void *opaque, int ret);
> +
> +/* Temporary. Removed in the next patch. */

Actually, no. :-)

(remove in patch 7)

Why are you making them non-static in the first place? I see both 
functions mentioned in patch 3 and patch 7 only (except for context in 
patch 6).

> +CallbackFn *new_transaction_wrapper(BlkTransactionState *common,
> +                                    void *opaque,
> +                                    void (*callback)(void *, int),
> +                                    void **new_opaque);
> +
> +void undo_transaction_wrapper(BlkTransactionState *common);
> +
> +/**
> + * Create a new transactional callback wrapper.
> + *
> + * Given a callback and a closure, generate a new
> + * callback and closure that will invoke the
> + * given callback with the given closure.
> + *
> + * After all wrappers in the transactional group have
> + * been processed, each action's .cb() method will be
> + * invoked.
> + *
> + * @common The transactional state to set a callback for.
> + * @opaque A closure intended for the encapsulated callback.
> + * @callback The callback we are encapsulating.
> + * @new_opaque The closure to be used instead of @opaque.
> + *
> + * @return The callback to be used instead of @callback.
> + */
> +CallbackFn *new_transaction_wrapper(BlkTransactionState *common,
> +                                           void *opaque,
> +                                           CallbackFn *callback,
> +                                           void **new_opaque)
> +{
> +    BlkTransactionWrapper *btw = g_malloc0(sizeof(*btw));

g_new0(BlkTransactionWrapper, 1);

> +    assert(new_opaque);
> +
> +    /* Stash the original callback information */
> +    btw->opaque = opaque;
> +    btw->callback = callback;
> +    common->opaque = btw;
> +
> +    /* The BTS will serve as our new closure */
> +    *new_opaque = common;
> +    common->refcount++;
> +
> +    /* Inform the transaction BTL to expect one more return */
> +    common->list->jobs++;
> +
> +    /* Lastly, the actual callback function to handle the interception. */
> +    return transaction_callback;
> +}
> +
> +/**
> + * Undo any actions performed by the above call.
> + */
> +void undo_transaction_wrapper(BlkTransactionState *common)
> +{
> +    BlkTransactionList *btl = common->list;
> +    BlkTransactionState *bts;
> +    BlkTransactionData *btd;
> +
> +    /* Stage 0: Wrapper was never created: */
> +    if (common->opaque == NULL && common->refcount == 1) {
> +        return;
> +    }
> +
> +    /* Stage 2: Job already completed or was canceled.
> +     * Force an error in the callback data and just invoke the completion
> +     * handler to perform appropriate cleanup for us.
> +     */
> +    QSIMPLEQ_FOREACH(bts, &btl->actions, list_entry) {
> +        if (bts == common) {
> +            btd = common->opaque;
> +            /* Force error for callback */
> +            btd->ret = -1;

No -errno?

> +            common->ops->cb(common);
> +            QSIMPLEQ_REMOVE(&btl->actions, common,
> +                            BlkTransactionState, list_entry);
> +            goto cleanup;
> +        }
> +    }
> +    /* Stage 1: Callback created, but job never launched */
> +    put_blk_transaction_list(common->list);
> + cleanup:
> +    g_free(common->opaque);

urp

> +    blk_put_transaction_state(common);
> +}
> +
>   /* internal snapshot private data */
>   typedef struct InternalSnapshotState {
>       BlkTransactionState common;
> @@ -1775,7 +1990,7 @@ static const BdrvActionOps actions[] = {
>           .instance_size = sizeof(DriveBackupState),
>           .prepare = drive_backup_prepare,
>           .abort = drive_backup_abort,
> -        .clean = drive_backup_clean,
> +        .clean = drive_backup_clean

Probably not intended, I guess.

>       },
>       [TRANSACTION_ACTION_KIND_BLOCKDEV_BACKUP] = {
>           .instance_size = sizeof(BlockdevBackupState),
> @@ -1815,10 +2030,12 @@ void qmp_transaction(TransactionActionList *dev_list, Error **errp)
>   {
>       TransactionActionList *dev_entry = dev_list;
>       BlkTransactionState *state, *next;
> +    BlkTransactionList *btl;
>       Error *local_err = NULL;
>   
>       QSIMPLEQ_HEAD(snap_bdrv_states, BlkTransactionState) snap_bdrv_states;
>       QSIMPLEQ_INIT(&snap_bdrv_states);
> +    btl = new_blk_transaction_list();
>   
>       /* drain all i/o before any operations */
>       bdrv_drain_all();
> @@ -1837,8 +2054,10 @@ void qmp_transaction(TransactionActionList *dev_list, Error **errp)
>           assert(ops->instance_size > 0);
>   
>           state = g_malloc0(ops->instance_size);
> +        state->refcount = 1;
>           state->ops = ops;
>           state->action = dev_info;
> +        state->list = btl;
>           QSIMPLEQ_INSERT_TAIL(&snap_bdrv_states, state, entry);
>   
>           state->ops->prepare(state, &local_err);
> @@ -1869,8 +2088,10 @@ exit:
>           if (state->ops->clean) {
>               state->ops->clean(state);
>           }
> -        g_free(state);
> +        blk_put_transaction_state(state);
>       }
> +
> +    put_blk_transaction_list(btl);
>   }
>   
>   

Sorry, as I said, I need more context on the objects and very much would 
like a separation between truly opaque pointers and not-so-opaque 
pointers, before I can really understand what's going on (or I put more 
effort into it, but since it's always more probable to belong to the 
majority, I guess I won't be the only one getting confused).

Maybe the next patches will clear it up, I don't know.

Max