gstreamer/subprojects/gst-docs/markdown/additional/design/element-sink.md

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# Sink elements
Sink elements consume data and normally have no source pads.
Typical sink elements include:
- audio/video renderers
- network sinks
- filesinks
Sinks are harder to construct than other element types as they are
treated specially by the GStreamer core.
## state changes
A sink always returns `ASYNC` from the state change to `PAUSED`, this
includes a state change from `READY→PAUSED` and `PLAYING→PAUSED`. The reason
for this is that this way we can detect when the first buffer or event
arrives in the sink when the state change completes.
A sink should block on the first `EOS` event or buffer received in the
`READY→PAUSED` state before committing the state to `PAUSED`.
`FLUSHING` events have to be handled out of sync with the buffer flow and
take no part in the preroll procedure.
Events other than `EOS` do not complete the preroll stage.
## sink overview
- TODO: `PREROLL_LOCK` can be removed and we can safely use the `STREAM_LOCK`.
```c
# Commit the state. We return TRUE if we can continue
# streaming, FALSE in the case we go to a READY or NULL state.
# if we go to PLAYING, we don't need to block on preroll.
commit
{
LOCK
switch (pending)
case PLAYING:
need_preroll = FALSE
break
case PAUSED:
break
case READY:
case NULL:
return FALSE
case VOID:
return TRUE
# update state
state = pending
next = VOID
pending = VOID
UNLOCK
return TRUE
}
# Sync an object. We have to wait for the element to reach
# the PLAYING state before we can wait on the clock.
# Some items do not need synchronisation (most events) so the
# get_times method returns FALSE (not syncable)
# need_preroll indicates that we are not in the PLAYING state
# and therefore need to commit and potentially block on preroll
# if our clock_wait got interrupted we commit and block again.
# The reason for this is that the current item being rendered is
# not yet finished and we can use that item to finish preroll.
do_sync (obj)
{
# get timing information for this object
syncable = get_times (obj, &start, &stop)
if (!syncable)
return OK;
again:
while (need_preroll)
if (need_commit)
need_commit = FALSE
if (!commit)
return FLUSHING
if (need_preroll)
# release PREROLL_LOCK and wait. prerolled can be observed
# and will be TRUE
prerolled = TRUE
PREROLL_WAIT (releasing PREROLL_LOCK)
prerolled = FALSE
if (flushing)
return FLUSHING
if (valid (start || stop))
PREROLL_UNLOCK
end_time = stop
ret = wait_clock (obj,start)
PREROLL_LOCK
if (flushing)
return FLUSHING
# if the clock was unscheduled, we redo the
# preroll
if (ret == UNSCHEDULED)
goto again
}
# render a prerollable item (EOS or buffer). It is
# always called with the PREROLL_LOCK helt.
render_object (obj)
{
ret = do_sync (obj)
if (ret != OK)
return ret;
# preroll and syncing done, now we can render
render(obj)
}
| # sinks that sync on buffer contents do like this
| while (more_to_render)
| ret = render
| if (ret == interrupted)
| prerolled = TRUE
render (buffer) ----->| PREROLL_WAIT (releasing PREROLL_LOCK)
| prerolled = FALSE
| if (flushing)
| return FLUSHING
|
# queue a prerollable item (EOS or buffer). It is
# always called with the PREROLL_LOCK helt.
# This function will commit the state when receiving the
# first prerollable item.
# items are then added to the rendering queue or rendered
# right away if no preroll is needed.
queue (obj, prerollable)
{
if (need_preroll)
if (prerollable)
queuelen++
# first item in the queue while we need preroll
# will complete state change and call preroll
if (queuelen == 1)
preroll (obj)
if (need_commit)
need_commit = FALSE
if (!commit)
return FLUSHING
# then see if we need more preroll items before we
# can block
if (need_preroll)
if (queuelen <= maxqueue)
queue.add (obj)
return OK
# now clear the queue and render each item before
# rendering the current item.
while (queue.hasItem)
render_object (queue.remove())
render_object (obj)
queuelen = 0
}
# various event functions
event
EOS:
# events must complete preroll too
STREAM_LOCK
PREROLL_LOCK
if (flushing)
return FALSE
ret = queue (event, TRUE)
if (ret == FLUSHING)
return FALSE
PREROLL_UNLOCK
STREAM_UNLOCK
break
SEGMENT:
# the segment must be used to clip incoming
# buffers. Then go into the queue as non-prerollable
# items used for syncing the buffers
STREAM_LOCK
PREROLL_LOCK
if (flushing)
return FALSE
set_clip
ret = queue (event, FALSE)
if (ret == FLUSHING)
return FALSE
PREROLL_UNLOCK
STREAM_UNLOCK
break
FLUSH_START:
# set flushing and unblock all that is waiting
event ----> subclasses can interrupt render
PREROLL_LOCK
flushing = TRUE
unlock_clock
PREROLL_SIGNAL
PREROLL_UNLOCK
STREAM_LOCK
lost_state
STREAM_UNLOCK
break
FLUSH_END:
# unset flushing and clear all data and eos
STREAM_LOCK
event
PREROLL_LOCK
queue.clear
queuelen = 0
flushing = FALSE
eos = FALSE
PREROLL_UNLOCK
STREAM_UNLOCK
break
# the chain function checks the buffer falls within the
# configured segment and queues the buffer for preroll and
# rendering
chain
STREAM_LOCK
PREROLL_LOCK
if (flushing)
return FLUSHING
if (clip)
queue (buffer, TRUE)
PREROLL_UNLOCK
STREAM_UNLOCK
state
switch (transition)
READY_PAUSED:
# no datapassing is going on so we always return ASYNC
ret = ASYNC
need_commit = TRUE
eos = FALSE
flushing = FALSE
need_preroll = TRUE
prerolled = FALSE
break
PAUSED_PLAYING:
# we grab the preroll lock. This we can only do if the
# chain function is either doing some clock sync, we are
# waiting for preroll or the chain function is not being called.
PREROLL_LOCK
if (prerolled || eos)
ret = OK
need_commit = FALSE
need_preroll = FALSE
if (eos)
post_eos
else
PREROLL_SIGNAL
else
need_preroll = TRUE
need_commit = TRUE
ret = ASYNC
PREROLL_UNLOCK
break
PLAYING_PAUSED:
---> subclass can interrupt render
# we grab the preroll lock. This we can only do if the
# chain function is either doing some clock sync
# or the chain function is not being called.
PREROLL_LOCK
need_preroll = TRUE
unlock_clock
if (prerolled || eos)
ret = OK
else
ret = ASYNC
PREROLL_UNLOCK
break
PAUSED_READY:
---> subclass can interrupt render
# we grab the preroll lock. Set to flushing and unlock
# everything. This should exit the chain functions and stop
# streaming.
PREROLL_LOCK
flushing = TRUE
unlock_clock
queue.clear
queuelen = 0
PREROLL_SIGNAL
ret = OK
PREROLL_UNLOCK
break
```