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gstreamer/markdown/design/states.md
Reynaldo H. Verdejo Pinochet 6632904686 design: states: add missing markup
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States

Both elements and pads can be in different states. The states of the pads are linked to the state of the element so the design of the states is mainly focused around the element states.

An element can be in 4 states. NULL, READY, PAUSED and PLAYING. When an element is initially instantiated, it is in the NULL state.

State definitions

  • NULL: This is the initial state of an element.

  • READY: The element should be prepared to go to PAUSED.

  • PAUSED: The element should be ready to accept and process data. Sink elements, however, only accept one buffer and then block.

  • PLAYING: The same as PAUSED except for live sources and sinks. Sinks accept and render data. Live sources produce data.

We call the sequence NULL→PLAYING an upwards state change and PLAYING→NULL a downwards state change.

State transitions

the following state changes are possible:

  • NULL -> READY:

    • The element must check if the resources it needs are available. Device sinks and sources typically try to probe the device to constrain their caps.
    • The element opens the device, this is needed if the previous step requires the device to be opened.

  • READY -> PAUSED:

    • The element pads are activated in order to receive data in PAUSED. Streaming threads are started.
    • Some elements might need to return ASYNC and complete the state change when they have enough information. It is a requirement for sinks to return ASYNC and complete the state change when they receive the first buffer or EOS event (preroll). Sinks also block the dataflow when in PAUSED.
    • A pipeline resets the running_time to 0.
    • Live sources return NO_PREROLL and don't generate data.

  • PAUSED -> PLAYING:

    • Most elements ignore this state change.
    • The pipeline selects a clock and distributes this to all the children before setting them to PLAYING. This means that it is only allowed to synchronize on the clock in the PLAYING state.
    • The pipeline uses the clock and the running_time to calculate the base_time. This base_time is distributed to all children when performing the state change.
    • Sink elements stop blocking on the preroll buffer or event and start rendering the data.
    • Sinks can post the EOS message in the PLAYING state. It is not allowed to post EOS when not in the PLAYING state.
    • While streaming in PAUSED or PLAYING elements can create and remove sometimes pads.
    • Live sources start generating data and return SUCCESS.

  • PLAYING -> PAUSED:

    • Most elements ignore this state change.
    • The pipeline calculates the running_time based on the last selected clock and the base_time. It stores this information to continue playback when going back to the PLAYING state.
    • Sinks unblock any clock wait calls.
    • When a sink does not have a pending buffer to play, it returns ASYNC from this state change and completes the state change when it receives a new buffer or an EOS event.
    • Any queued EOS messages are removed since they will be reposted when going back to the PLAYING state. The EOS messages are queued in GstBins.
    • Live sources stop generating data and return NO_PREROLL.

  • PAUSED -> READY:

    • Sinks unblock any waits in the preroll.
    • Elements unblock any waits on devices
    • Chain or get_range() functions return FLUSHING.
    • The element pads are deactivated so that streaming becomes impossible and all streaming threads are stopped.
    • The sink forgets all negotiated formats
    • Elements remove all sometimes pads

  • READY -> NULL:

    • Elements close devices
    • Elements reset any internal state.

State variables

An element has 4 state variables that are protected with the object LOCK:

  • STATE
  • STATE_NEXT
  • STATE_PENDING
  • STATE_RETURN

The STATE always reflects the current state of the element. The STATE_NEXT reflects the next state the element will go to. The STATE_PENDING always reflects the required state of the element. The STATE_RETURN reflects the last return value of a state change.

The STATE_NEXT and STATE_PENDING can be VOID_PENDING if the element is in the right state.

An element has a special lock to protect against concurrent invocations of set_state(), called the STATE_LOCK.

Setting state on elements

The state of an element can be changed with _element_set_state(). When changing the state of an element all intermediate states will also be set on the element until the final desired state is set.

The set_state() function can return 3 possible values:

  • GST_STATE_FAILURE: The state change failed for some reason. The plugin should have posted an error message on the bus with information.

  • GST_STATE_SUCCESS: The state change is completed successfully.

  • GST_STATE_ASYNC: The state change will complete later on. This can happen when the element needs a long time to perform the state change or for sinks that need to receive the first buffer before they can complete the state change (preroll).

  • GST_STATE_NO_PREROLL: The state change is completed successfully but the element will not be able to produce data in the PAUSED state.

In the case of an ASYNC state change, it is possible to proceed to the next state before the current state change completes, however, the element will only get to this next state before completing the previous ASYNC state change. After receiving an ASYNC return value, you can use element_get_state() to poll the status of the element. If the polling returns SUCCESS, the element completed the state change to the last requested state with set_state().

When setting the state of an element, the STATE_PENDING is set to the required state. Then the state change function of the element is called and the result of that function is used to update the STATE and STATE_RETURN fields, STATE_NEXT, STATE_PENDING and STATE_RETURN fields. If the function returned ASYNC, this result is immediately returned to the caller.

Getting the state of elements

The get_state() function takes 3 arguments, two pointers that will hold the current and pending state and one GstClockTime that holds a timeout value. The function returns a GstElementStateReturn.

  • If the element returned SUCCESS to the previous _set_state() function, this function will return the last state set on the element and VOID_PENDING in the pending state value. The function returns GST_STATE_SUCCESS.

  • If the element returned NO_PREROLL to the previous _set_state() function, this function will return the last state set on the element and VOID_PENDING in the pending state value. The function returns GST_STATE_NO_PREROLL.

  • If the element returned FAILURE to the previous _set_state() call, this function will return FAILURE with the state set to the current state of the element and the pending state set to the value used in the last call of _set_state().

  • If the element returned ASYNC to the previous _set_state() call, this function will wait for the element to complete its state change up to the amount of time specified in the GstClockTime.

    • If the element does not complete the state change in the specified amount of time, this function will return ASYNC with the state set to the current state and the pending state set to the pending state.

    • If the element completes the state change within the specified timeout, this function returns the updated state and VOID_PENDING as the pending state.

    • If the element aborts the ASYNC state change due to an error within the specified timeout, this function returns FAILURE with the state set to last successful state and pending set to the last attempt. The element should also post an error message on the bus with more information about the problem.

States in GstBin

A GstBin manages the state of its children. It does this by propagating the state changes performed on it to all of its children. The _set_state() function on a bin will call the _set_state() function on all of its children, that are not already in the target state or in a change state to the target state.

The children are iterated from the sink elements to the source elements. This makes sure that when changing the state of an element, the downstream elements are in the correct state to process the eventual buffers. In the case of a downwards state change, the sink elements will shut down first which makes the upstream elements shut down as well since the _push() function returns a GST_FLOW_FLUSHING error.

If all the children return SUCCESS, the function returns SUCCESS as well.

If one of the children returns FAILURE, the function returns FAILURE as well. In this state it is possible that some elements successfully changed state. The application can check which elements have a changed state, which were in error and which were not affected by iterating the elements and calling _get_state() on the elements.

If after calling the state function on all children, one of the children returned ASYNC, the function returns ASYNC as well.

If after calling the state function on all children, one of the children returned NO_PREROLL, the function returns NO_PREROLL as well.

If both NO_PREROLL and ASYNC children are present, NO_PREROLL is returned.

The current state of the bin can be retrieved with _get_state().

If the bin is performing an ASYNC state change, it will automatically update its current state fields when it receives state messages from the children.

Implementing states in elements

Upward state change

Upward state changes always return ASYNC either if the STATE_PENDING is reached or not.

Element:

  • A -> B => SUCCESS

    • commit state

  • A -> B => ASYNC

    • no commit state
    • element commits state ASYNC

  • A -> B while ASYNC

    • update STATE_PENDING state
    • no commit state
    • no change_state() called on element

Bin:

  • A->B: all elements SUCCESS

    • commit state

  • A->B: some elements ASYNC

    • no commit state
    • listen for commit messages on bus
    • for each commit message, poll elements, this happens in another thread.
    • if no ASYNC elements, commit state, continue state change to STATE_PENDING

Downward state change

Downward state changes only return ASYNC if the final state is ASYNC. This is to make sure that it’s not needed to wait for an element to complete the preroll or other ASYNC state changes when one only wants to shut down an element.

Element:

A -> B => SUCCESS

  • commit state

A -> B => ASYNC not final state

  • commit state on behalf of element

A -> B => ASYNC final state

  • element will commit ASYNC

Bin:

A -> B -> SUCCESS

  • commit state

A -> B -> ASYNC not final state

  • commit state on behalf of element, continue state change

A -> B => ASYNC final state

  • no commit state
  • listen for commit messages on bus
  • for each commit message, poll elements
  • if no ASYNC elements, commit state

Locking overview (element)

  • Element committing SUCCESS

    • STATE_LOCK is taken in set_state()

    • change state is called if SUCCESS, commit state is called

    • commit state calls change_state() to next state change.

    • if final state is reached, stack unwinds and result is returned to set_state() and caller.

set_state(element)       change_state (element)   commit_state

    |                         |                       |
    |                         |                       |
STATE_LOCK                    |                       |
    |                         |                       |
    |------------------------>|                       |
    |                         |                       |
    |                         |                       |
    |                         | (do state change)     |
    |                         |                       |
    |                         |                       |
    |                         | if `SUCCESS`            |
    |                         |---------------------->|
    |                         |                       | post message
    |                         |                       |
    |                         |<----------------------| if (!final) change_state (next)
    |                         |                       | else SIGNAL
    |                         |                       |
    |                         |                       |
    |                         |                       |
    |<------------------------|                       |
    |     `SUCCESS`
    |
STATE_UNLOCK
    |
  `SUCCESS`

  • Element committing ASYNC

    • STATE_LOCK is taken in set_state()

    • change state is called and returns ASYNC

    • ASYNC returned to the caller.

    • element takes LOCK in streaming thread.

    • element calls commit_state in streaming thread.

    • commit state calls change_state() to next state change.

set_state(element)       change_state (element)     stream_thread      commit_state (element)

    |                         |                          |                  |
    |                         |                          |                  |
STATE_LOCK                    |                          |                  |
    |                         |                          |                  |
    |------------------------>|                          |                  |
    |                         |                          |                  |
    |                         |                          |                  |
    |                         | (start_task)             |                  |
    |                         |                          |                  |
    |                         |                     STREAM_LOCK             |
    |                         |                          |...               |
    |<------------------------|                          |                  |
    |     ASYNC                                     STREAM_UNLOCK           |
STATE_UNLOCK                                             |                  |
    |                .....sync........               STATE_LOCK             |
  ASYNC                                                  |----------------->|
                                                         |                  |
                                                         |                  |---> post_message()
                                                         |                  |---> if (!final) change_state (next)
                                                         |                  |     else SIGNAL
                                                         |<-----------------|
                                                     STATE_UNLOCK
                                                         |
                                                    STREAM_LOCK
                                                         | ...
                                                    STREAM_UNLOCK

Remarks

set_state() cannot be called from multiple threads at the same time. The STATE_LOCK prevents this.

State variables are protected with the LOCK.

Calling set_state() while get_state() is called should unlock the get_state() with an error. The cookie will do that.

set_state(element)

STATE_LOCK

LOCK
update current, next, pending state
cookie++
UNLOCK

change_state

STATE_UNLOCK