android: reject a zero sample rate or channel count in oto_oboe_Play

[kumagi]

What issue is this addressing?

No issue filed yet. Found during an audit of the Android binding.

What type of issue is this addressing?

bug

What this PR does | solves

Stream::Play (internal/oboe/binding_android.cpp) stores channel_num_ and sample_rate_ without validation, and later code divides by them:

// OpenLocked / PrepareBuffersLocked
buffer_size_in_bytes_ / channel_num_ / 4
num_frames * 1000000 / sample_rate_

If the caller ever passes 0 (e.g. a NewContextOptions with SampleRate/ChannelCount left at their zero value reaching the binding unchecked), this faults with SIGFPE on the audio thread instead of returning a configuration error. Note the Go side does not currently reject zero values either, so the binding is the last line of defense.

The fix

Validate both arguments up front and return a descriptive error string, matching the existing const char * error contract of oto_oboe_Play:

if (sample_rate <= 0) {
    return "oto_oboe_Play: sample rate must be positive";
}
if (channel_num <= 0) {
    return "oto_oboe_Play: channel count must be positive";
}

The error propagates back to newContext in driver_android.go and is reported through c.err.Join as usual.

[hajimehoshi]

Could we put this validation in the common NewContext entry point, before setting contextCreated, so it applies to all platforms and returns an error synchronously?

There is already a sampleRate <= 0 check in durationToBufferSize, but its early return for duration == 0 bypasses that check when the default buffer size is used. NewContext should unconditionally reject SampleRate <= 0 and validate ChannelCount as 1 or 2, matching its documented contract.

This would also avoid sending these configuration errors through Android's asynchronous initialization error path, which currently records the error without closing ready, leaving callers waiting on that channel indefinitely.

Posted by Codex on behalf of @hajimehoshi.