gst-plugins-rs/generic/threadshare/src/queue/imp.rs

// Copyright (C) 2018 Sebastian Dröge <sebastian@centricular.com>
//
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Library General Public
// License as published by the Free Software Foundation; either
// version 2 of the License, or (at your option) any later version.
//
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Library General Public License for more details.
//
// You should have received a copy of the GNU Library General Public
// License along with this library; if not, write to the
// Free Software Foundation, Inc., 51 Franklin Street, Suite 500,
// Boston, MA 02110-1335, USA.
//
// SPDX-License-Identifier: LGPL-2.1-or-later

use futures::channel::oneshot;
use futures::future::BoxFuture;
use futures::prelude::*;

use gst::glib;
use gst::prelude::*;
use gst::subclass::prelude::*;

use once_cell::sync::Lazy;

use std::collections::VecDeque;
use std::sync::Mutex;
use std::time::Duration;
use std::{u32, u64};

use crate::runtime::prelude::*;
use crate::runtime::{Context, PadSink, PadSinkRef, PadSinkWeak, PadSrc, PadSrcRef, Task};

use crate::dataqueue::{DataQueue, DataQueueItem};

const DEFAULT_MAX_SIZE_BUFFERS: u32 = 200;
const DEFAULT_MAX_SIZE_BYTES: u32 = 1024 * 1024;
const DEFAULT_MAX_SIZE_TIME: gst::ClockTime = gst::ClockTime::SECOND;
const DEFAULT_CONTEXT: &str = "";
const DEFAULT_CONTEXT_WAIT: Duration = Duration::ZERO;

#[derive(Debug, Clone)]
struct Settings {
    max_size_buffers: u32,
    max_size_bytes: u32,
    max_size_time: gst::ClockTime,
    context: String,
    context_wait: Duration,
}

impl Default for Settings {
    fn default() -> Self {
        Settings {
            max_size_buffers: DEFAULT_MAX_SIZE_BUFFERS,
            max_size_bytes: DEFAULT_MAX_SIZE_BYTES,
            max_size_time: DEFAULT_MAX_SIZE_TIME,
            context: DEFAULT_CONTEXT.into(),
            context_wait: DEFAULT_CONTEXT_WAIT,
        }
    }
}

#[derive(Debug)]
struct PendingQueue {
    more_queue_space_sender: Option<oneshot::Sender<()>>,
    scheduled: bool,
    items: VecDeque<DataQueueItem>,
}

impl PendingQueue {
    fn notify_more_queue_space(&mut self) {
        self.more_queue_space_sender.take();
    }
}

#[derive(Clone)]
struct QueuePadSinkHandler;

impl PadSinkHandler for QueuePadSinkHandler {
    type ElementImpl = Queue;

    fn sink_chain(
        self,
        pad: PadSinkWeak,
        elem: super::Queue,
        buffer: gst::Buffer,
    ) -> BoxFuture<'static, Result<gst::FlowSuccess, gst::FlowError>> {
        async move {
            let pad = pad.upgrade().expect("PadSink no longer exists");
            gst::log!(CAT, obj: pad.gst_pad(), "Handling {:?}", buffer);
            let imp = elem.imp();
            imp.enqueue_item(DataQueueItem::Buffer(buffer)).await
        }
        .boxed()
    }

    fn sink_chain_list(
        self,
        pad: PadSinkWeak,
        elem: super::Queue,
        list: gst::BufferList,
    ) -> BoxFuture<'static, Result<gst::FlowSuccess, gst::FlowError>> {
        async move {
            let pad = pad.upgrade().expect("PadSink no longer exists");
            gst::log!(CAT, obj: pad.gst_pad(), "Handling {:?}", list);
            let imp = elem.imp();
            imp.enqueue_item(DataQueueItem::BufferList(list)).await
        }
        .boxed()
    }

    fn sink_event(&self, pad: &PadSinkRef, imp: &Queue, event: gst::Event) -> bool {
        gst::debug!(CAT, obj: pad.gst_pad(), "Handling non-serialized {:?}", event);

        if let gst::EventView::FlushStart(..) = event.view() {
            if let Err(err) = imp.task.flush_start().await_maybe_on_context() {
                gst::error!(CAT, obj: pad.gst_pad(), "FlushStart failed {:?}", err);
                gst::element_imp_error!(
                    imp,
                    gst::StreamError::Failed,
                    ("Internal data stream error"),
                    ["FlushStart failed {:?}", err]
                );
                return false;
            }
        }

        gst::log!(CAT, obj: pad.gst_pad(), "Forwarding non-serialized {:?}", event);
        imp.src_pad.gst_pad().push_event(event)
    }

    fn sink_event_serialized(
        self,
        pad: PadSinkWeak,
        elem: super::Queue,
        event: gst::Event,
    ) -> BoxFuture<'static, bool> {
        async move {
            let pad = pad.upgrade().expect("PadSink no longer exists");
            gst::log!(CAT, obj: pad.gst_pad(), "Handling serialized {:?}", event);

            let imp = elem.imp();

            if let gst::EventView::FlushStop(..) = event.view() {
                if let Err(err) = imp.task.flush_stop().await_maybe_on_context() {
                    gst::error!(CAT, obj: pad.gst_pad(), "FlushStop failed {:?}", err);
                    gst::element_imp_error!(
                        imp,
                        gst::StreamError::Failed,
                        ("Internal data stream error"),
                        ["FlushStop failed {:?}", err]
                    );
                    return false;
                }
            }

            gst::log!(CAT, obj: pad.gst_pad(), "Queuing serialized {:?}", event);
            imp.enqueue_item(DataQueueItem::Event(event)).await.is_ok()
        }
        .boxed()
    }

    fn sink_query(&self, pad: &PadSinkRef, imp: &Queue, query: &mut gst::QueryRef) -> bool {
        gst::log!(CAT, obj: pad.gst_pad(), "Handling {:?}", query);

        if query.is_serialized() {
            // FIXME: How can we do this?
            gst::log!(CAT, obj: pad.gst_pad(), "Dropping serialized {:?}", query);
            false
        } else {
            gst::log!(CAT, obj: pad.gst_pad(), "Forwarding {:?}", query);
            imp.src_pad.gst_pad().peer_query(query)
        }
    }
}

#[derive(Clone, Debug)]
struct QueuePadSrcHandler;

impl PadSrcHandler for QueuePadSrcHandler {
    type ElementImpl = Queue;

    fn src_event(&self, pad: &PadSrcRef, imp: &Queue, event: gst::Event) -> bool {
        gst::log!(CAT, obj: pad.gst_pad(), "Handling {:?}", event);

        use gst::EventView;
        match event.view() {
            EventView::FlushStart(..) => {
                if let Err(err) = imp.task.flush_start().await_maybe_on_context() {
                    gst::error!(CAT, obj: pad.gst_pad(), "FlushStart failed {:?}", err);
                }
            }
            EventView::FlushStop(..) => {
                if let Err(err) = imp.task.flush_stop().await_maybe_on_context() {
                    gst::error!(CAT, obj: pad.gst_pad(), "FlushStop failed {:?}", err);
                    gst::element_imp_error!(
                        imp,
                        gst::StreamError::Failed,
                        ("Internal data stream error"),
                        ["FlushStop failed {:?}", err]
                    );
                    return false;
                }
            }
            _ => (),
        }

        gst::log!(CAT, obj: pad.gst_pad(), "Forwarding {:?}", event);
        imp.sink_pad.gst_pad().push_event(event)
    }

    fn src_query(&self, pad: &PadSrcRef, imp: &Queue, query: &mut gst::QueryRef) -> bool {
        gst::log!(CAT, obj: pad.gst_pad(), "Handling {:?}", query);

        if let gst::QueryViewMut::Scheduling(q) = query.view_mut() {
            let mut new_query = gst::query::Scheduling::new();
            let res = imp.sink_pad.gst_pad().peer_query(&mut new_query);
            if !res {
                return res;
            }

            gst::log!(CAT, obj: pad.gst_pad(), "Upstream returned {:?}", new_query);

            let (flags, min, max, align) = new_query.result();
            q.set(flags, min, max, align);
            q.add_scheduling_modes(
                &new_query
                    .scheduling_modes()
                    .iter()
                    .cloned()
                    .filter(|m| m != &gst::PadMode::Pull)
                    .collect::<Vec<_>>(),
            );
            gst::log!(CAT, obj: pad.gst_pad(), "Returning {:?}", q.query_mut());
            return true;
        }

        gst::log!(CAT, obj: pad.gst_pad(), "Forwarding {:?}", query);
        imp.sink_pad.gst_pad().peer_query(query)
    }
}

#[derive(Debug)]
struct QueueTask {
    element: super::Queue,
    dataqueue: DataQueue,
}

impl QueueTask {
    fn new(element: super::Queue, dataqueue: DataQueue) -> Self {
        QueueTask { element, dataqueue }
    }

    async fn push_item(&self, item: DataQueueItem) -> Result<(), gst::FlowError> {
        let queue = self.element.imp();

        if let Some(pending_queue) = queue.pending_queue.lock().unwrap().as_mut() {
            pending_queue.notify_more_queue_space();
        }

        match item {
            DataQueueItem::Buffer(buffer) => {
                gst::log!(CAT, obj: &self.element, "Forwarding {:?}", buffer);
                queue.src_pad.push(buffer).await.map(drop)
            }
            DataQueueItem::BufferList(list) => {
                gst::log!(CAT, obj: &self.element, "Forwarding {:?}", list);
                queue.src_pad.push_list(list).await.map(drop)
            }
            DataQueueItem::Event(event) => {
                gst::log!(CAT, obj: &self.element, "Forwarding {:?}", event);
                queue.src_pad.push_event(event).await;
                Ok(())
            }
        }
    }
}

impl TaskImpl for QueueTask {
    type Item = DataQueueItem;

    fn start(&mut self) -> BoxFuture<'_, Result<(), gst::ErrorMessage>> {
        async move {
            gst::log!(CAT, obj: &self.element, "Starting task");

            let queue = self.element.imp();
            let mut last_res = queue.last_res.lock().unwrap();

            self.dataqueue.start();

            *last_res = Ok(gst::FlowSuccess::Ok);

            gst::log!(CAT, obj: &self.element, "Task started");
            Ok(())
        }
        .boxed()
    }

    fn try_next(&mut self) -> BoxFuture<'_, Result<DataQueueItem, gst::FlowError>> {
        async move {
            self.dataqueue
                .next()
                .await
                .ok_or_else(|| panic!("DataQueue stopped while Task is Started"))
        }
        .boxed()
    }

    fn handle_item(&mut self, item: DataQueueItem) -> BoxFuture<'_, Result<(), gst::FlowError>> {
        async {
            let res = self.push_item(item).await;
            let queue = self.element.imp();
            match res {
                Ok(()) => {
                    gst::log!(CAT, obj: &self.element, "Successfully pushed item");
                    *queue.last_res.lock().unwrap() = Ok(gst::FlowSuccess::Ok);
                }
                Err(gst::FlowError::Flushing) => {
                    gst::debug!(CAT, obj: &self.element, "Flushing");
                    *queue.last_res.lock().unwrap() = Err(gst::FlowError::Flushing);
                }
                Err(gst::FlowError::Eos) => {
                    gst::debug!(CAT, obj: &self.element, "EOS");
                    *queue.last_res.lock().unwrap() = Err(gst::FlowError::Eos);
                    queue.src_pad.push_event(gst::event::Eos::new()).await;
                }
                Err(err) => {
                    gst::error!(CAT, obj: &self.element, "Got error {}", err);
                    gst::element_error!(
                        &self.element,
                        gst::StreamError::Failed,
                        ("Internal data stream error"),
                        ["streaming stopped, reason {}", err]
                    );
                    *queue.last_res.lock().unwrap() = Err(err);
                }
            }

            res
        }
        .boxed()
    }

    fn stop(&mut self) -> BoxFuture<'_, Result<(), gst::ErrorMessage>> {
        async move {
            gst::log!(CAT, obj: &self.element, "Stopping task");

            let queue = self.element.imp();
            let mut last_res = queue.last_res.lock().unwrap();

            self.dataqueue.stop();
            self.dataqueue.clear();

            if let Some(mut pending_queue) = queue.pending_queue.lock().unwrap().take() {
                pending_queue.notify_more_queue_space();
            }

            *last_res = Err(gst::FlowError::Flushing);

            gst::log!(CAT, obj: &self.element, "Task stopped");
            Ok(())
        }
        .boxed()
    }

    fn flush_start(&mut self) -> BoxFuture<'_, Result<(), gst::ErrorMessage>> {
        async move {
            gst::log!(CAT, obj: &self.element, "Starting task flush");

            let queue = self.element.imp();
            let mut last_res = queue.last_res.lock().unwrap();

            self.dataqueue.clear();

            if let Some(mut pending_queue) = queue.pending_queue.lock().unwrap().take() {
                pending_queue.notify_more_queue_space();
            }

            *last_res = Err(gst::FlowError::Flushing);

            gst::log!(CAT, obj: &self.element, "Task flush started");
            Ok(())
        }
        .boxed()
    }
}

#[derive(Debug)]
pub struct Queue {
    sink_pad: PadSink,
    src_pad: PadSrc,
    task: Task,
    dataqueue: Mutex<Option<DataQueue>>,
    pending_queue: Mutex<Option<PendingQueue>>,
    last_res: Mutex<Result<gst::FlowSuccess, gst::FlowError>>,
    settings: Mutex<Settings>,
}

static CAT: Lazy<gst::DebugCategory> = Lazy::new(|| {
    gst::DebugCategory::new(
        "ts-queue",
        gst::DebugColorFlags::empty(),
        Some("Thread-sharing queue"),
    )
});

impl Queue {
    /* Try transfering all the items from the pending queue to the DataQueue, then
     * the current item. Errors out if the DataQueue was full, or the pending queue
     * is already scheduled, in which case the current item should be added to the
     * pending queue */
    fn queue_until_full(
        &self,
        dataqueue: &DataQueue,
        pending_queue: &mut Option<PendingQueue>,
        item: DataQueueItem,
    ) -> Result<(), DataQueueItem> {
        match pending_queue {
            None => dataqueue.push(item),
            Some(PendingQueue {
                scheduled: false,
                ref mut items,
                ..
            }) => {
                let mut failed_item = None;
                while let Some(item) = items.pop_front() {
                    if let Err(item) = dataqueue.push(item) {
                        failed_item = Some(item);
                    }
                }

                if let Some(failed_item) = failed_item {
                    items.push_front(failed_item);

                    Err(item)
                } else {
                    dataqueue.push(item)
                }
            }
            _ => Err(item),
        }
    }

    /* Schedules emptying of the pending queue. If there is an upstream
     * TaskContext, the new task is spawned, it is otherwise
     * returned, for the caller to block on */
    async fn schedule_pending_queue(&self) {
        loop {
            let more_queue_space_receiver = {
                let dataqueue = self.dataqueue.lock().unwrap();
                if dataqueue.is_none() {
                    return;
                }
                let mut pending_queue_grd = self.pending_queue.lock().unwrap();

                gst::log!(CAT, imp: self, "Trying to empty pending queue");

                if let Some(pending_queue) = pending_queue_grd.as_mut() {
                    let mut failed_item = None;
                    while let Some(item) = pending_queue.items.pop_front() {
                        if let Err(item) = dataqueue.as_ref().unwrap().push(item) {
                            failed_item = Some(item);
                        }
                    }

                    if let Some(failed_item) = failed_item {
                        pending_queue.items.push_front(failed_item);
                        let (sender, receiver) = oneshot::channel();
                        pending_queue.more_queue_space_sender = Some(sender);

                        receiver
                    } else {
                        gst::log!(CAT, imp: self, "Pending queue is empty now");
                        *pending_queue_grd = None;
                        return;
                    }
                } else {
                    gst::log!(CAT, imp: self, "Flushing, dropping pending queue");
                    return;
                }
            };

            gst::log!(CAT, imp: self, "Waiting for more queue space");
            let _ = more_queue_space_receiver.await;
        }
    }

    async fn enqueue_item(&self, item: DataQueueItem) -> Result<gst::FlowSuccess, gst::FlowError> {
        let wait_fut = {
            let dataqueue = self.dataqueue.lock().unwrap();
            let dataqueue = dataqueue.as_ref().ok_or_else(|| {
                gst::error!(CAT, imp: self, "No DataQueue");
                gst::FlowError::Error
            })?;

            let mut pending_queue = self.pending_queue.lock().unwrap();

            if let Err(item) = self.queue_until_full(dataqueue, &mut pending_queue, item) {
                if pending_queue
                    .as_ref()
                    .map(|pq| !pq.scheduled)
                    .unwrap_or(true)
                {
                    if pending_queue.is_none() {
                        *pending_queue = Some(PendingQueue {
                            more_queue_space_sender: None,
                            scheduled: false,
                            items: VecDeque::new(),
                        });
                    }

                    let schedule_now = !matches!(
                        item,
                        DataQueueItem::Event(ref ev) if ev.type_() != gst::EventType::Eos,
                    );

                    pending_queue.as_mut().unwrap().items.push_back(item);

                    gst::log!(
                        CAT,
                        imp: self,
                        "Queue is full - Pushing first item on pending queue"
                    );

                    if schedule_now {
                        gst::log!(CAT, imp: self, "Scheduling pending queue now");
                        pending_queue.as_mut().unwrap().scheduled = true;

                        let wait_fut = self.schedule_pending_queue();
                        Some(wait_fut)
                    } else {
                        gst::log!(CAT, imp: self, "Scheduling pending queue later");
                        None
                    }
                } else {
                    pending_queue.as_mut().unwrap().items.push_back(item);
                    None
                }
            } else {
                None
            }
        };

        if let Some(wait_fut) = wait_fut {
            gst::log!(CAT, imp: self, "Blocking until queue has space again");
            wait_fut.await;
        }

        *self.last_res.lock().unwrap()
    }

    fn prepare(&self) -> Result<(), gst::ErrorMessage> {
        gst::debug!(CAT, imp: self, "Preparing");

        let settings = self.settings.lock().unwrap().clone();

        let dataqueue = DataQueue::new(
            &self.instance().clone().upcast(),
            self.src_pad.gst_pad(),
            if settings.max_size_buffers == 0 {
                None
            } else {
                Some(settings.max_size_buffers)
            },
            if settings.max_size_bytes == 0 {
                None
            } else {
                Some(settings.max_size_bytes)
            },
            if settings.max_size_time.is_zero() {
                None
            } else {
                Some(settings.max_size_time)
            },
        );

        *self.dataqueue.lock().unwrap() = Some(dataqueue.clone());

        let context =
            Context::acquire(&settings.context, settings.context_wait).map_err(|err| {
                gst::error_msg!(
                    gst::ResourceError::OpenRead,
                    ["Failed to acquire Context: {}", err]
                )
            })?;

        self.task
            .prepare(QueueTask::new(self.instance().clone(), dataqueue), context)
            .block_on()?;

        gst::debug!(CAT, imp: self, "Prepared");

        Ok(())
    }

    fn unprepare(&self) {
        gst::debug!(CAT, imp: self, "Unpreparing");

        self.task.unprepare().block_on().unwrap();

        *self.dataqueue.lock().unwrap() = None;
        *self.pending_queue.lock().unwrap() = None;

        *self.last_res.lock().unwrap() = Ok(gst::FlowSuccess::Ok);

        gst::debug!(CAT, imp: self, "Unprepared");
    }

    fn stop(&self) -> Result<(), gst::ErrorMessage> {
        gst::debug!(CAT, imp: self, "Stopping");
        self.task.stop().await_maybe_on_context()?;
        gst::debug!(CAT, imp: self, "Stopped");
        Ok(())
    }

    fn start(&self) -> Result<(), gst::ErrorMessage> {
        gst::debug!(CAT, imp: self, "Starting");
        self.task.start().await_maybe_on_context()?;
        gst::debug!(CAT, imp: self, "Started");
        Ok(())
    }
}

#[glib::object_subclass]
impl ObjectSubclass for Queue {
    const NAME: &'static str = "GstTsQueue";
    type Type = super::Queue;
    type ParentType = gst::Element;

    fn with_class(klass: &Self::Class) -> Self {
        Self {
            sink_pad: PadSink::new(
                gst::Pad::from_template(&klass.pad_template("sink").unwrap(), Some("sink")),
                QueuePadSinkHandler,
            ),
            src_pad: PadSrc::new(
                gst::Pad::from_template(&klass.pad_template("src").unwrap(), Some("src")),
                QueuePadSrcHandler,
            ),
            task: Task::default(),
            dataqueue: Mutex::new(None),
            pending_queue: Mutex::new(None),
            last_res: Mutex::new(Ok(gst::FlowSuccess::Ok)),
            settings: Mutex::new(Settings::default()),
        }
    }
}

impl ObjectImpl for Queue {
    fn properties() -> &'static [glib::ParamSpec] {
        static PROPERTIES: Lazy<Vec<glib::ParamSpec>> = Lazy::new(|| {
            vec![
                glib::ParamSpecString::builder("context")
                    .nick("Context")
                    .blurb("Context name to share threads with")
                    .default_value(Some(DEFAULT_CONTEXT))
                    .build(),
                glib::ParamSpecUInt::builder("context-wait")
                    .nick("Context Wait")
                    .blurb("Throttle poll loop to run at most once every this many ms")
                    .maximum(1000)
                    .default_value(DEFAULT_CONTEXT_WAIT.as_millis() as u32)
                    .build(),
                glib::ParamSpecUInt::builder("max-size-buffers")
                    .nick("Max Size Buffers")
                    .blurb("Maximum number of buffers to queue (0=unlimited)")
                    .default_value(DEFAULT_MAX_SIZE_BUFFERS)
                    .build(),
                glib::ParamSpecUInt::builder("max-size-bytes")
                    .nick("Max Size Bytes")
                    .blurb("Maximum number of bytes to queue (0=unlimited)")
                    .default_value(DEFAULT_MAX_SIZE_BYTES)
                    .build(),
                glib::ParamSpecUInt64::builder("max-size-time")
                    .nick("Max Size Time")
                    .blurb("Maximum number of nanoseconds to queue (0=unlimited)")
                    .maximum(u64::MAX - 1)
                    .default_value(DEFAULT_MAX_SIZE_TIME.nseconds())
                    .build(),
            ]
        });

        PROPERTIES.as_ref()
    }

    fn set_property(&self, _id: usize, value: &glib::Value, pspec: &glib::ParamSpec) {
        let mut settings = self.settings.lock().unwrap();
        match pspec.name() {
            "max-size-buffers" => {
                settings.max_size_buffers = value.get().expect("type checked upstream");
            }
            "max-size-bytes" => {
                settings.max_size_bytes = value.get().expect("type checked upstream");
            }
            "max-size-time" => {
                settings.max_size_time = value.get::<u64>().unwrap().nseconds();
            }
            "context" => {
                settings.context = value
                    .get::<Option<String>>()
                    .expect("type checked upstream")
                    .unwrap_or_else(|| DEFAULT_CONTEXT.into());
            }
            "context-wait" => {
                settings.context_wait = Duration::from_millis(
                    value.get::<u32>().expect("type checked upstream").into(),
                );
            }
            _ => unimplemented!(),
        }
    }

    fn property(&self, _id: usize, pspec: &glib::ParamSpec) -> glib::Value {
        let settings = self.settings.lock().unwrap();
        match pspec.name() {
            "max-size-buffers" => settings.max_size_buffers.to_value(),
            "max-size-bytes" => settings.max_size_bytes.to_value(),
            "max-size-time" => settings.max_size_time.nseconds().to_value(),
            "context" => settings.context.to_value(),
            "context-wait" => (settings.context_wait.as_millis() as u32).to_value(),
            _ => unimplemented!(),
        }
    }

    fn constructed(&self) {
        self.parent_constructed();

        let obj = self.instance();
        obj.add_pad(self.sink_pad.gst_pad()).unwrap();
        obj.add_pad(self.src_pad.gst_pad()).unwrap();
    }
}

impl GstObjectImpl for Queue {}

impl ElementImpl for Queue {
    fn metadata() -> Option<&'static gst::subclass::ElementMetadata> {
        static ELEMENT_METADATA: Lazy<gst::subclass::ElementMetadata> = Lazy::new(|| {
            gst::subclass::ElementMetadata::new(
                "Thread-sharing queue",
                "Generic",
                "Simple data queue",
                "Sebastian Dröge <sebastian@centricular.com>",
            )
        });

        Some(&*ELEMENT_METADATA)
    }

    fn pad_templates() -> &'static [gst::PadTemplate] {
        static PAD_TEMPLATES: Lazy<Vec<gst::PadTemplate>> = Lazy::new(|| {
            let caps = gst::Caps::new_any();

            let sink_pad_template = gst::PadTemplate::new(
                "sink",
                gst::PadDirection::Sink,
                gst::PadPresence::Always,
                &caps,
            )
            .unwrap();

            let src_pad_template = gst::PadTemplate::new(
                "src",
                gst::PadDirection::Src,
                gst::PadPresence::Always,
                &caps,
            )
            .unwrap();

            vec![sink_pad_template, src_pad_template]
        });

        PAD_TEMPLATES.as_ref()
    }

    fn change_state(
        &self,
        transition: gst::StateChange,
    ) -> Result<gst::StateChangeSuccess, gst::StateChangeError> {
        gst::trace!(CAT, imp: self, "Changing state {:?}", transition);

        match transition {
            gst::StateChange::NullToReady => {
                self.prepare().map_err(|err| {
                    self.post_error_message(err);
                    gst::StateChangeError
                })?;
            }
            gst::StateChange::PausedToReady => {
                self.stop().map_err(|_| gst::StateChangeError)?;
            }
            gst::StateChange::ReadyToNull => {
                self.unprepare();
            }
            _ => (),
        }

        let success = self.parent_change_state(transition)?;

        if transition == gst::StateChange::ReadyToPaused {
            self.start().map_err(|_| gst::StateChangeError)?;
        }

        Ok(success)
    }
}