From b76a500f2eff4395824cceb280d7b92974ddff8c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Motiejus=20Jak=C5=A1tys?= <motiejus@jakstys.lt>
Date: Mon, 24 May 2021 00:11:58 +0300
Subject: [PATCH] wip lxc-config

---
 lxc-config/BUILD        |  8 ++++
 lxc-config/lxcconfig.go | 92 +++++++++++++++++++++++++++++++++++++++++
 rootfs/rootfs.go        |  2 +-
 3 files changed, 101 insertions(+), 1 deletion(-)
 create mode 100644 lxc-config/BUILD
 create mode 100644 lxc-config/lxcconfig.go

diff --git a/lxc-config/BUILD b/lxc-config/BUILD
new file mode 100644
index 0000000..e360101
--- /dev/null
+++ b/lxc-config/BUILD
@@ -0,0 +1,8 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["lxcconfig.go"],
+    importpath = "github.com/motiejus/code/undocker/lxc-config",
+    visibility = ["//visibility:public"],
+)
diff --git a/lxc-config/lxcconfig.go b/lxc-config/lxcconfig.go
new file mode 100644
index 0000000..6a529e9
--- /dev/null
+++ b/lxc-config/lxcconfig.go
@@ -0,0 +1,92 @@
+package lxcconfig
+
+import (
+	"archive/tar"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"strings"
+)
+
+const _manifestJSON = "manifest.json"
+
+// dockerManifest is manifest.json
+type dockerManifest []struct {
+	Config string `json:"Config"`
+}
+
+// dockerConfig returns interesting configs for the container. user/group are
+// skipped, since Docker allows specifying them by name, which would require
+// peeking into the container image's /etc/passwd to resolve the names to ints.
+type dockerConfig struct {
+	Architecture string `json:"architecture"`
+	Config       struct {
+		Entrypoint []string `json:"Entrypoint"`
+		Cmd        []string `json:"Cmd"`
+		Env        []string `json:"Env"`
+		WorkingDir string   `json:"WorkingDir"`
+	} `json:"config"`
+}
+
+var (
+	errBadManifest = errors.New("bad or missing manifest.json")
+)
+
+func getDockerConfig(in io.ReadSeeker) (dockerConfig, error) {
+	tr := tar.NewReader(in)
+	// get offsets to all json files in the archive
+	jsonOffsets := map[string]int64{}
+	for {
+		hdr, err := tr.Next()
+		if err == io.EOF {
+			break
+		}
+		if hdr.Typeflag != tar.TypeReg {
+			continue
+		}
+		if !strings.HasSuffix(".json", hdr.Name) {
+			continue
+		}
+		here, err := in.Seek(0, io.SeekCurrent)
+		if err != nil {
+			return dockerConfig{}, err
+		}
+		jsonOffsets[hdr.Name] = here
+	}
+
+	// manifest is the docker manifest in the image
+	var manifest dockerManifest
+	if err := parseJSON(in, jsonOffsets, _manifestJSON, &manifest); err != nil {
+		return dockerConfig{}, err
+	}
+	if len(manifest) == 0 {
+		return dockerConfig{}, errBadManifest
+	}
+
+	var config dockerConfig
+	if err := parseJSON(in, jsonOffsets, manifest[0].Config, &config); err != nil {
+		return dockerConfig{}, err
+	}
+
+	return config, nil
+}
+
+func parseJSON(in io.ReadSeeker, offsets map[string]int64, fname string, c interface{}) error {
+	configOffset, ok := offsets[fname]
+	if !ok {
+		return fmt.Errorf("file %s not found", fname)
+	}
+	if _, err := in.Seek(configOffset, io.SeekStart); err != nil {
+		return fmt.Errorf("seek to %s: %w", fname, err)
+	}
+	tr := tar.NewReader(in)
+	if _, err := tr.Next(); err != nil {
+		return err
+	}
+	dec := json.NewDecoder(tr)
+	if err := dec.Decode(c); err != nil {
+		return fmt.Errorf("decode %s: %w", fname, err)
+	}
+	return nil
+}
diff --git a/rootfs/rootfs.go b/rootfs/rootfs.go
index 4848eb0..41839b8 100644
--- a/rootfs/rootfs.go
+++ b/rootfs/rootfs.go
@@ -61,7 +61,7 @@ func RootFS(in io.ReadSeeker, out io.Writer) (err error) {
 		case filepath.Clean(hdr.Name) == _manifestJSON:
 			dec := json.NewDecoder(tr)
 			if err := dec.Decode(&manifest); err != nil {
-				return fmt.Errorf("parse %s: %w", _manifestJSON, err)
+				return fmt.Errorf("decode %s: %w", _manifestJSON, err)
 			}
 		case strings.HasSuffix(hdr.Name, _layerSuffix):
 			here, err := in.Seek(0, io.SeekCurrent)