So that it's tried before sndio. Many Linux systems seem to have libsndio
installed as a hard dependency from SDL2, but not the server component to
actually make it work. And because there's no way to test if it's usable
without opening a device, the backend would get selected over ALSA and
subsequently fail to work. As a result, Linux users that don't use PipeWire or
PulseAudio would have to explicitly configure for ALSA.
The write vector represents the chunks queued for OpenSL, and the write pointer
only increments when OpenSL fills in more samples. So requeueing from the start
of the write vector is requeueing chunks that are already queued, which is
obviously wrong. It instead needs to queue chunks that become available as
they're read.
Because the ring buffer holds more elements than can be written to at a givem
time, the read vector does not represent the next writable chunks to queue.
Advancing the read pointer increases the number of writable elements, so
instead of queueing from the read vector or the start of the write vector,
queue from the end of the write vector given the number of chunks read.
Newer versions of PipeWire may add things to public structures. For example,
pw_buffer::requested added in 0.3.49. Building against 0.3.49 or newer, but
then running with 0.3.48 could result in invalid accesses since the returned
pw_buffer objects are shorter than the definition says to expect, creating
undefined behavior. Even if explicit access to the additional fields is
protected by a runtime check, the language allows the compiler to assume a
pointer to a pw_buffer object contains a complete pw_buffer, allowing the
optimizer to access the field earlier than the check (with the check only
controlling if the value gets used).
Another example is pw_time, which had a few fields added in 0.3.50 along with a
function, pw_stream_get_time_n, that provides the size of the pw_time struct
the application is using (so the library knows what version of the struct it
has to fill in). If a later version adds a new field, running it with an older
version will either fail (due to the library getting a size larger than it
knows about) or silently leave the newer fields as garbage.
Building against 0.3.50 or newer will require that version or newer at runtime.
Building against a version before 0.3.50 will work with newer versions.
It's treated as 5.1 + 2 aux channels. This allows AL_DIRECT_CHANNELS_SOFT to
behave better, not forwarding rear left/right channel inputs to lower front and
upper rear, and allows reporting a more appropriate output mode to the app
instead of 7.1.
The values are identical to the matching loopback channel configuration enums,
so for example ALC_5POINT1_SOFT is interchangable with ALC_SURROUND_5_1_SOFT,
this is simply providing a cleaner styling for potential future modes.
Stopping Oboe capture drops uncaptured samples, whereas OpenAL keeps them. Also
make sure the reported available count doesn't go backwards without reading.
