New proposal for fixing race conditions

Overview
In order to solve the "data race" issue in WebAudio (bug 22725), this proposal restricts access to the internal PCM data of an AudioBuffer. This does not require AudioBuffers to be immutable, but that modifying their contents occurs through an API which enforces no-data-race semantics.

Since this will involve breaking API changes, this proposal incorporates suggestions from Issue 48 to simplify the AudioBuffer interface, and will also adopt a constructor, as opposed to a factory method.

The new constructor provides an upgrade path for existing pages while not breaking those pages. The factory method can continue to create the "legacy" AudioBuffer in prefixed implementations.

Note: I still prefer the "Immutable ArrayBuffer" proposal.

New interfaces
AudioBufferChannel
This interface represents a single channel of an AudioBuffer.

interface AudioBufferChannel {
    readonly attribute unsigned long length;

    void set(Float32Array array, optional unsigned long offset);
    Float32Array slice(long begin, optional long end);
};

Attributes
length

Length of the PCM audio data in sample-frames.

Methods
set

Copies the Float32Array representing the PCM audio data.

An INDEX_SIZE_ERR exception must be thrown if the length ofarray plus offset exceeds the length of the available audio data.

An NO_MODIFICATION_ALLOWED_ERR exception must be thrown if the AudioBufferChannel is currently is use by a live AudioNode.

slice

Returns a copy of the underlying PCM audio data. If eitherbegin or end is negative, it refers to an index from the end of the audio data.

If end is unspecified, the copy will include all the data frombegin to the end of the audio data.

The range specified by begin and end is clamped to the valid range for the audio data. If the computed length of the copied data would be negative, it is clamped to zero.


AudioBuffer
[Constructor(unsigned long numberOfChannels, unsigned long length, float sampleRate)]
interface AudioBuffer {

    readonly attribute float sampleRate;

    // in seconds 
    readonly attribute double duration;

    readonly attribute AudioBufferChannel[] channels;
};

Attributes
sampleRate

The sample-rate for the PCM audio data in samples per second.
duration

Duration of the PCM audio data in seconds.
channels

An array of AudioBufferChannel objects representing the PCM audio data for each audio channel.

This array is of fixed length and is read-only.


Alternatives
Immutable ArrayBuffer
To make ArrayBuffer immutable, the set() method would be removed from ArrayBufferChannel.

interface ArrayBufferChannel {
    readonly attribute unsigned long length;
    Float32Array slice(long begin, optional long end);
};
This would require a different constructor method on ArrayBuffer.

[Constructor(Float32Array[] arrays, float sampleRate)]
This would also require the outputBuffer attribute on AudioProcessingEvent to drop the 'readonly' declaration.

partial interface AudioProcessingEvent {
    attribute AudioBuffer outputBuffer;
};

Backwards Compatibility Not normative
WebKitAudioBuffer
interface WebKitAudioBuffer : AudioBuffer {
    readonly attribute length;

    readonly attribute long numberOfChannels;

    Float32Array getChannelData(unsigned long channel);
};

WebKitAudioContext
partial interface WebKitAudioContext {
    WebKitAudioBuffer createBuffer(unsigned long numberOfChannels, unsigned long length, float sampleRate); 
};
WebKit will continue to support WebKitAudioBuffer objects in the prefixed version of the API for every attribute or parameter which currently specifies AudioBuffer. WebKit will drop WebKitAudioBuffer support in the unprefixed API.