gstreamer/docs/random/wtay/eos-19012001

EOS as implemented on Jan 21 2001:

1) terminology
--------------

EOS: end of stream. The problem we are trying to solve here is 
     detecting when the processing of a piece of media data has
     ended. 

     This problem can get complicated because the pipeline might
     contain an arbitrary number of threads and other ASYNC and
     DECOUPLED elements like queues. We are trying to solve the
     obvious cases first by making some assumptions (see limitations)
     in order to avoid the exponentially growing complexity.

     The trick is to detect when all the elements/subbins/threads
     and done processing their data. This involves monitoring the
     threads and elements.

     Catching the EOS signals from an element is easy. We are focusing
     here on the EOS signal propagation to the manager bin up to the
     toplevel bin.

EOS call: the pads have a method gst_pad_set_eos () that will
     be called by an element that cannot send any more data on the
     pad. 

EOS signals: elements fires the EOS signal when all it's pads are in
    EOS.

chains: at plan generation, the bin will find the elements it has to
    manage. The elements that are managed together are called a chain.
    This is typically a set of elements that need input from their
    peer element before they can output data. When one of those elements
    cannot provide more data and goes into EOS, the other elements
    are basically worthless and there is no point in trying to schedule
    them anymore.

    Chains are typically broken up on DECOUPLED elements. Those elements
    have no clear relation between their input/output behaviour, like
    the queue.

EOS denial: An element can deny an EOS call by returning FALSE on the
    overridden EOS call. This behaviour is typical for an element that 
    performs _get_region on its sinkpad and basically does not care
    about the EOS because it knows it will pull a specific region
    anyway.
    

2) EOS implementation
---------------------

EOS is currently implemented by selectively disabling scheduling of
the chains. This procedure continues untill a bin/thread has no more
chains left to schedule, at which point it will fire the EOS signal.

A gboolean was added to the chain structure to indicate if this chain
need scheduling. Initially this will gboolean will be set to TRUE.

the gst_bin_iterate_func will only schedule those chains that have 
their need_scheduling flag to TRUE. 

All elements are treated as potential EOS signal providers. When we add
the elements to a chain, we attach the eos signal to them and hand
the chain they were added to as an argument to the signal handler.
When an element goes to EOS, we mark the chain as need_scheduling=FALSE.
This removes the chain from the scheduling loop.

Since plain bins in bins are flattened during the chain creation, we do 
not need to worry about the bin EOS. 

Other elements that do their own managing (like threads) are treated
differently, they are added to a list of EOS-providers. Since they
have their own chains and use the same iterate_func, they will eventually
fire EOS on their own when they run out of schedulable chains. 
The EOS signal of the EOS providers is caught by the parent bin which 
will then remove the bin from the list of possible EOS providers.

Combining the EOS providers list and the chains, the bin will fire
and EOS signal when 1) it has no more chains to schedule, 2) all its
EOS providers have signaled EOS.

3) queue EOS handling
---------------------

The queue overrides the eos call and performs the following:

Set the sinkpad to EOS and signal its internal g_cond to unlock any
waiting threads on the srcpad.

The scrpad _get function performs the EOS call when no more buffers
are queued and the sinkpad is in EOS. This causes the EOS call to
propagate downstream and effectively causes all chains and threads
to become EOS.


4) limitations
--------------

We assume a chain is a single schedualable entity. Rescheduling of
the bins and chains are not performed. 

No provisions for changing the state of the elements in EOS, although 
this probably isn't hard to do.

No provisions for undoing the EOS state. This is probably related to
the state change, where a chain should become schedulable again when the
element goes back to the PLAYING state.