class ConsumerStage

Executes a single mix job that consumes `period` worth of audio data. This method should be

called at some time during range `R = [mix_job_start_time, mix_job_start_time + period)`,

relative to our reference clock, with the expectation that RunMixJob should complete before

`R.end`. This expectation is not checked -- it's the caller's responsibility to check for

overruns and underflows as desired.

Put differently, within RunMixJob, all calls to `ref_clock->now()` should return a value in

range `R`. The range `R` bounds the "actual current time" according to this consumer's

reference clock, not the presentation times of audio frames to consume. As described below, the

mix job actually consumes frames presented outside of range `R` -- either after `R` (in output

pipelines) or before `R` (in input pipelines).

## Output pipelines

Since this job may run until `mix_job_start_time + period`, we cannot consume any frames that

will be presented before `T = mix_job_start_time + period + downstream_delay`. Hence, we

consume frames in the range `[T, T + period)`.

In the diagram below, each `+` represents a frame ordered by presentation time, with

presentation time increasing to the right. The frames marked with `X` are consumed during the

mix job at `mix_job_start_time`.

```

downstream

|

<

-- period -->|

<

--- delay --->|

<

-- period -->| ...

frames: ++++++++++++++++++++++++++++++++++XXXXXXXXXXXXXXX+++++

^ ^ ^

mix job mix job T

start time end time

```

## Input pipelines

Since this job starts running as early as `mix_job_start_time`, we cannot consume any frames

that are presented after `T = mix_job_start_time - upstream_delay`. Hence, we consume frames in

the range `[T - period, T)`.

In the diagram below, each `+` represents a frame ordered by presentation time, with

presentation time increasing to the right. The frames marked with `X` are consumed during the

mix job at `mix_job_start_time`.

```

downstream

|

<

-- period -->|

<

--- delay --->|

<

-- period -->| ...

frames: +++XXXXXXXXXXXXXXX++++++++++++++++++++++++++++++++++++

^ ^ ^

T mix job mix job

start time end time

```

Sets the delay introduced by our downstream external output device, such as a physical speaker.

This is Node::max_downstream_output_pipeline_delay().

REQUIRED: must have `pipeline_direction == kOutput`

Sets the delay introduced by our source input pipeline, not including any delays introduced by

this ConsumerStage. This is Node::max_upstream_input_pipeline_delay() in our *source* node.

REQUIRED: must have `pipeline_direction == kInput`