overlay: Extend documentation comment for layout

Let's expand the existing doc comment, as it's really
useful to understand the implementation in order to
debug or improve things.

We also clearly say first that these are implementation details.

Signed-off-by: Colin Walters <walters@verbum.org>

drivers/overlay/overlay.go

@@ -1,3 +1,76 @@
+// Package overlay uses the overlay union filesystem for containers
+// with diff directories for each layer.
+//
+// # Requirements
+//
+// This version of the overlay driver requires at least Linux kernel
+// 4.0.0 in order to support mounting multiple diff directories.
+//
+// # Layout stability
+//
+// The documentation below describes the current data layout; it
+// is however an internal implementation of this package, and
+// may change incompatibly in the future.  Do not directly parse
+// or access the files or directories described here; only access
+// them via the higher level Go APIs (or a higher level CLI/API
+// which use the Go APIs, such as `podman`).
+//
+// # The "overlay/" directory
+//
+// This directory holds layers; the sha256 digest here is a
+// "chain ID" identifying the precise sequence of parent1→parent2→…→child
+// layer IDs; and with zstd:chunked, also identifying the contents of the
+// layer depending on how we actually created it.
+// I.e. a layer with the same DiffID can have several physical objects in this directory.
+// More on "chain ID" in [opencontainers ChainID].
+//
+// Each layer has at least a "diff" directory and "link" file.
+// If there is also a "lower" file when there are diff layers
+// below as well as "merged" and "work" directories. The "diff" directory
+// has the upper layer of the overlay and is used to capture any
+// changes to the layer. The "lower" file contains all the lower layer
+// mounts separated by ":" and ordered from uppermost to lowermost
+// layers. The overlay itself is mounted in the "merged" directory,
+// and the "work" dir is needed for overlay to work.
+//
+// The "link" file for each layer contains a unique string for the layer.
+// Under the "l" directory at the root there will be a symbolic link
+// with that unique string pointing the "diff" directory for the layer.
+// The symbolic links are used to reference lower layers in the "lower"
+// file and on mount. The links are used to shorten the total length
+// of a layer reference without requiring changes to the layer identifier
+// or root directory. Mounts are always done relative to root and
+// referencing the symbolic links in order to ensure the number of
+// lower directories can fit in a single page for making the mount
+// syscall. A hard upper limit of 500 lower layers is enforced to ensure
+// that mounts do not fail due to length.
+//
+// # The "overlay-layers" directory
+//
+// Created by [storage.getLayerStoreLocked].
+//
+// Each layer will have a `${layerid}.tar-split.gz` file containing the
+// original input tar stream without file content; this is necessary
+// to be able to reconstruct the original tar stream exactly after
+// a layer content has been unpacked into `diff/`.  More in [tar-split].
+//
+// Other files in this directory:
+//
+// - layers.json: Global metadata associated with layers
+// - volatile-layers.json: Global metadata, but will not persist across reboot
+// - layers.lock: Protects changes to this directory
+//
+// # The "overlay-containers" directory
+//
+// Created by [storage.load].  This directory holds running container images.
+//
+// - containers.json: Global metadata for containers
+// - volatile-containers.json: Global metadata, but will not persist across reboot
+// - containers.lock: Protects changes to this directory
+//
+// [opencontainers ChainID]: https://github.com/opencontainers/image-spec/blob/main/config.md#layer-chainid
+// [tar-split]: https://github.com/vbatts/tar-split
+
 //go:build linux
 // +build linux
 
@@ -51,33 +124,6 @@ const (
 	mountProgramFlagFile = ".has-mount-program"
 )
 
-// This backend uses the overlay union filesystem for containers
-// with diff directories for each layer.
-
-// This version of the overlay driver requires at least kernel
-// 4.0.0 in order to support mounting multiple diff directories.
-
-// Each container/image has at least a "diff" directory and "link" file.
-// If there is also a "lower" file when there are diff layers
-// below as well as "merged" and "work" directories. The "diff" directory
-// has the upper layer of the overlay and is used to capture any
-// changes to the layer. The "lower" file contains all the lower layer
-// mounts separated by ":" and ordered from uppermost to lowermost
-// layers. The overlay itself is mounted in the "merged" directory,
-// and the "work" dir is needed for overlay to work.
-
-// The "link" file for each layer contains a unique string for the layer.
-// Under the "l" directory at the root there will be a symbolic link
-// with that unique string pointing the "diff" directory for the layer.
-// The symbolic links are used to reference lower layers in the "lower"
-// file and on mount. The links are used to shorten the total length
-// of a layer reference without requiring changes to the layer identifier
-// or root directory. Mounts are always done relative to root and
-// referencing the symbolic links in order to ensure the number of
-// lower directories can fit in a single page for making the mount
-// syscall. A hard upper limit of 500 lower layers is enforced to ensure
-// that mounts do not fail due to length.
-
 const (
 	linkDir    = "l"
 	stagingDir = "staging"