Merge pull request containerd#3518 from crosbymichael/processors

Add docs for stream processors

docs/stream_processors.md

@@ -0,0 +1,42 @@
+# Stream Processors
+
+## Processor API
+
+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`.
+
+## Configuration
+
+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.
+
+```toml
+[[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"]
+```