Optional properties in caps:

During the 0.8 series of GStreamer, we regularly felt the need to add
properties to caps, thereby sometimes breaking specific caps negotiation
cases or annoying developers. This document outlines problems it could
lead to and tries to explain in which cases properties can be added and
when they can't. It also explains when properties can be optional (both
temporarily and permanently) and when they cannot.

--

There's two cases where optional properties could be added:
1) to fix an issue that makes any case fail
2) to fix an issue that makes some cases fail

Case 1 can be compared to not providing extra_data in caps for WMA. The caps
are defined, but it will never work because you cannot decode WMA audio
without the sequence header. In this case, adding the property breaks
caps compatibility, but it is still allowed because there is no regression
and it fixes a bug. No optional property should be added here, it should be
made a required property directly. Another example here is channel-positions
for channels>2.

Case 2 is more complex. There's various subcases:
a] not providing this property means ANY (or don't care, or unknown, each
   of which is another way of saying ANY)
b] not providing this property means a specific value
 .] adding this property will lead to unwanted behaviour
 .] adding this property will not lead to unwanted behaviour

An example case for 2a is the buffer-frames property in float audio or the
frames property in MPEG audio. Buffer-frames is 0 (which should be removed)
means ANY. The reason that it should not be zero is because connecting an
element with buffer-frames=SOME_VALUE should be allowed to connect to any
element out there that has no buffer-frames requirement. The opposite is
true reversely: an element with no buffer-frames property should never be
allowed to connect to any element requesting a specific buffer-frames value.
For MPEG audio, it is TRUE likewise. Mathematically, buffer-frames=0 does not
exist. It implies ANY. Similarly, framed=FALSE cannot exist, because it
implies framed={FALSE,TRUE} (in words: an element cannot require non-framed
MPEG audio, because framed MPEG audio is a subset of non-framed MPEG audio
and thus valid input). In all those cases for 2a, optional properties are
fine. Subtraction will not work, but as explained, values are subsets of
another value and thus subtraction is irrelevant (because the mathematical
value of the subtraction has no real value).
This same principle is true for rate/channels on (for example) MPEG audio.
Our caps negotiation already allows for all of this, and optional properties
are already being used for this.

2b is complex, since subtraction actually has a value here, and addition
of such properties may lead to regressions or crashes. Let's give another
two examples: stride (for raw video; not providing this value implies 4-byte
aligned video) and pixel-aspect-ratio (default value being 1/1). Adding p-a-r
was in this case an example of 2bII, whereas stride is 2bI. The reason for
this is simple: adding pixel-aspect-ratio to some element and not to others
could lead to misunderstanding size. However, this is not a regression,
because not adding it altogether would lead to the same misunderstanding.
In both cases, the result would be wrongly sized video. Therefore, there
is no regression and there is a bugfix, so this is fine. Obviously, the
optional property is in this case very specifically a temporary solution.
As soon as we can, this property should become obligatory.
Stride is different, because elements have implicit associated behaviour
based on the previous beahviour. This hebaviour could break if some elements
do implement stride and others still don't. Therefore, adding an optional
property as a temporary hack is, in this case, not a good idea and should
be disallowed. A proper fix should be done in the same timeframe as 2bII. In
this case, optional properties should not be added. Another example of this
case was adding channel-positions to audio caps with channels=1,2, which
was rejected for the same reason: it would break a perfectly-working set
of rules in a stable series.

--

Obviously, with all of the above, people will start fighting about which
group their specific properties change belongs to. General consensus is
the only way to get around that problem. Long live politics.