It's a programming error to not do so and would cause all kinds of
problems in the caller that assumed its own buffer to have been filled.

This new API allow resuming a task if it was paused, while leaving it to
stopped stated if it was stopped or not started yet. This new API can be
useful for callback driver workflow, where you basically want to pause and
resume the task when buffers are notified while avoiding the race with a
gst_task_stop() coming from another thread.

Going through each state on the way back down to GST_STATE_NULL
can cause deadlocks, for example:

gst-launch-1.0 audiotestsrc ! valve drop=true ! autoaudiosink

ctrl + C

Hangs forever when going to PAUSED, because the "final" state is
ASYNC, and the sink blocks waiting for a preroll buffer.

Going straight to NULL addresses this issue, and also helps
making teardown faster when piping sparse streams to a
sync sink.

When running in pull mode (for e.g. mp3 reading),
baseparse currently reads 64KB from upstream, then mp3parse
consumes typically around 417/418 bytes of it. Then
on the next loop, it will read a full fresh 64KB again,
which is a big waste.

Fix the read loop to use the available cache buffer first
before going for more data, until the cache drops to < 1KB.

Fixes #518

current_buf_mem_idx stands for the index of memory of the corresponding
buffer which is scheduled to be written in the next iteration.
If all memory objects were scheduled to be written in the current
iteration, reset the index to zero so that starting from the first
memory object of the next buffer.

If size of vector is greater than one, we are allocating zero-size
memory and trying invalid memcpy operation

Elements that pass a seek with INSTANT_RATE flag to
gst_segment_do_seek() haven't been updated and we should
refuse the seek.

Don't assume that a given seek succeeds - check the return result.

Previously the default and full modes were the same. Now the default
mode is like before: it accumulates all buffers in a buffer list until
the threshold is reached and then writes them all out, potentially in
multiple writes.

The new full mode works by always copying memory to a single memory area
and writing everything out with a single write once the threshold is
reached.

If buffer lists with too many buffers would be written before, a stack
overflow would happen because of memory linear with the number of
GstMemory would be allocated on the stack. This could happen for example
when filesink is configured with a very big buffer size.

Instead now move the buffer and buffer list writing into the helper
functions and at most write IOV_MAX memories at once. Anything bigger
than that wouldn't be passed to writev() anyway and written differently
in the previous code, so this also potentially speeds up writing for
these cases.

For example the following pipeline would crash with a stackoverflow:
gst-launch-1.0 audiotestsrc ! filesink buffer-size=1073741824 location=/dev/null

What may happen is that during the course of processing a buffer,
all of the pads in a flow combiner may disappear.  In this case, we
would return NOT_LINKED.  Instead return whatever the input flow return
was.

This way we can avoid to process parameters if log is not going
to be printed.

Everywhere else places the period outside.

The old code would leave a dangling pointer in oldstr_ptr if two threads
attempted to take the same structure into the same location at the same
time:

1. First "oldstr == newstr" check (before the loop) fails.
2. Compare-and-exchange fails, due to a second thread completing the
   same gst_structure_take.
3. Second "oldstr == newstr" check (in the loop) succeeds, loop breaks.
4. "oldstr" check succeeds, old structure gets freed.
5. oldstr_ptr now contains a dangling pointer.

This shouldn't happen in code that handles ownership sanely, so check
that we don't try to do this and complain loudly.

Also simplify the function by using a do-while loop, like
gst_mini_object_take.

!413

Avoids a few compiler warnings:

../subprojects/gstreamer/gst/gstdeviceproviderfactory.c: In function ‘gst_device_provider_factory_finalize’:
../subprojects/gstreamer/gst/gstdeviceproviderfactory.c:96:12: warning: assignment discards ‘volatile’ qualifier from pointer target type [-Wdiscarded-qualifiers]
   96 |   provider = g_atomic_pointer_get (&factory->provider);
      |            ^
../subprojects/gstreamer/gst/gstdeviceproviderfactory.c: In function ‘gst_device_provider_factory_get’:
../subprojects/gstreamer/gst/gstdeviceproviderfactory.c:276:19: warning: assignment discards ‘volatile’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  276 |   device_provider = g_atomic_pointer_get (&newfactory->provider);
      |                   ^
../subprojects/gstreamer/gst/gstdeviceproviderfactory.c:309:21: warning: assignment discards ‘volatile’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  309 |     device_provider = g_atomic_pointer_get (&newfactory->provider);
      |

!414

Also add a unit test for this.

Fixes #524

When this `_strnlen` internal method was added, strnlen (in glibc)
was not available yet (appeared in 2.10 it was released that same
year).

If available, use the much more optimized strnlen

The type checks at the end of `gst_value_intersect` to call the flagset
intersection are relatively expensive.

If we already know that:
* There was a compare function but it didn't return GST_VALUE_EQUAL
* AND none of the registered intersect functions failed

Then we know they can't intersect and can return early.

Trims ~20% of the instruction calls

For subtracting a list from another, the previous implementation would
do a double subtraction of one from another (which would create temporary
arrays/values which would then be discarded). Instead iterate and do
the comparision directly.

For intersecting a list with another, we can directly iterate both at
once and therefore avoid doing a *full* check of all values of the list
against all other values of the list.

This tries to inline as much as possible array/list and its contents
in order to avoid double allocation/freeing. This also improves the
locality of data.

The internal value is still API/ABI compatible with the *public*
GArray structure. This allows READ-ONLY backwards compatibility with
any external users that assume that the content of a list/array value
is backed by a GArray.

In case the buffer is not writable, the parent (the BufferList) is not
removed before calling func. So if it is changed, the parent (the BufferList)
of the previous buffer should be removed after calling func.

For all the structure creation using valist/varargs we calculate
the number of fields we will need to store. This ensures all callers
will end up with a single allocation.

Instead of having 3 allocations:
* One for GstStructure
* One for GArray
* One for the array *within* GArray

We try to limit this to a single allocation, inlining everything. This
reduces the number of micro-allocations and improves locality of data
access.

Having a NULL caps in a GValue is legal and we should handle it
properly for comparisons.

Before that commit `{test, }` wouldn't be accepted as an array
because of the trailing coma, the commit fixes that.

At the same time, the code has been refactored to avoid special casing
the first element of the list, making `{,}` or `<,>` valid lists.

Looks like it's been removed from glib.devhelp2 on Fedora 31.

Fix #508

This allows an element to not require implementing transform or
transform_ip.

There can only be a single waiter: on the streaming thread.

There can only be a single waiter: on the streaming thread.

... and immediately return FLUSHING from the streaming thread instead of
waiting potentially forever.

Fixes gstreamer/gstreamer#516

We kept the start time around and subtracted it everywhere for "easy of
debugging", but we don't do anything like this anywhere else and it
only complicates the code unnecessarily.

fixate() will return empty caps if it gets empty caps passed and assert
early if any caps are provided as there's no meaningful way of fixating
any caps.

truncate() and simplify() will return the input caps in case of
any/empty caps as before, but slightly optimized and as documented
behaviour.

Also add tests for this and a few other operations behaviour on
empty/any caps.

This is not an API breakage, as implementors are already
expected to return a GstFlowReturn

Required to avoid broken log string on Windows but missed
in the commit of 493a3261