Processors are a binary API that works off of content streams.

The incoming content stream will be provided to the binary via `STDIN`

and the stream processor is expected to output the processed stream on

`STDOUT`. If errors are encountered, errors MUST be returned via `STDERR`

with a non-zero exit status.

Additional information can be provided to stream processors via a payload.

Payloads are marshaled as `protobuf.Any` types and can wrap any type of

serialized data structure.

On Unix systems, the payload, if available, is provided on `fd 3` for the process.

On Windows systems, the payload, if available, is provided via a named pipe with the

pipe's path set as the value of the environment variable `STREAM_PROCESSOR_PIPE`.

To configure stream processors for containerd, entries in the config file need to be made.

The `stream_processors` field is an array so that users can chain together multiple processors

to mutate content streams.

Processor Fields:

* `id` - ID of the processor, used for passing a specific payload to the processor.

* `accepts` - Accepted media-types for the processor that it can handle.

* `returns` - The media-type that the processor returns.

* `path` - Path to the processor binary.

* `args` - Arguments passed to the processor binary.

[[stream_processors]]

id = "io.containerd.processor.v1.pigz"

accepts = ["application/vnd.docker.image.rootfs.diff.tar.gzip"]

returns = "application/vnd.oci.image.layer.v1.tar"

path = "unpigz"

args = ["-d", "-c"]