unify apk installed db for base layer (#3879)

src/cmd/linuxkit/moby/apk_tarwriter.go

@@ -0,0 +1,86 @@
+package moby
+
+import (
+	"archive/tar"
+	"bytes"
+)
+
+// apkTarWriter apk-aware tar writer that consolidates installed database, so that
+// it can be called multiple times and will do the union of all such databases,
+// rather than overwriting the previous one.
+const apkInstalledPath = "lib/apk/db/installed"
+
+type apkTarWriter struct {
+	*tar.Writer
+	dbs     [][]byte
+	current *bytes.Buffer
+}
+
+func newAPKTarWriter(w *tar.Writer) *apkTarWriter {
+	return &apkTarWriter{
+		Writer: w,
+	}
+}
+
+func (a *apkTarWriter) WriteHeader(hdr *tar.Header) error {
+	if a.current != nil {
+		a.dbs = append(a.dbs, a.current.Bytes())
+		a.current = nil
+	}
+	if hdr.Name == apkInstalledPath {
+		a.current = new(bytes.Buffer)
+	}
+	return a.Writer.WriteHeader(hdr)
+}
+func (a *apkTarWriter) Write(b []byte) (int, error) {
+	if a.current != nil {
+		a.current.Write(b)
+	}
+	return a.Writer.Write(b)
+}
+
+func (a *apkTarWriter) Close() error {
+	// before closing, write out the union of all the databases
+	if a.current != nil {
+		a.dbs = append(a.dbs, a.current.Bytes())
+		a.current = nil
+	}
+	if err := a.WriteAPKDB(); err != nil {
+		return err
+	}
+	return a.Writer.Close()
+}
+
+func (a *apkTarWriter) WriteAPKDB() error {
+	if len(a.dbs) > 1 {
+		// consolidate the databases
+		// calculate the size of the new database
+		var size int
+		for _, db := range a.dbs {
+			size += len(db)
+			size += 2 // 2 trailing newlines for each db
+		}
+		hdr := &tar.Header{
+			Name:     apkInstalledPath,
+			Mode:     0o644,
+			Uid:      0,
+			Gid:      0,
+			Typeflag: tar.TypeReg,
+			Size:     int64(size),
+		}
+		if err := a.Writer.WriteHeader(hdr); err != nil {
+			return err
+		}
+		for _, db := range a.dbs {
+			if _, err := a.Writer.Write(db); err != nil {
+				return err
+			}
+			if _, err := a.Writer.Write([]byte{'\n', '\n'}); err != nil {
+				return err
+			}
+		}
+	}
+	// once complete, clear the databases
+	a.dbs = nil
+	return nil
+}

src/cmd/linuxkit/moby/build.go

@@ -168,13 +168,17 @@ func Build(m Moby, w io.Writer, opts BuildOpts) error {
 	if len(m.Init) != 0 {
 		log.Infof("Add init containers:")
 	}
+	apkTar := newAPKTarWriter(iw)
 	for _, ii := range m.initRefs {
 		log.Infof("Process init image: %s", ii)
-		err := ImageTar(ii, "", iw, resolvconfSymlink, opts)
+		err := ImageTar(ii, "", apkTar, resolvconfSymlink, opts)
 		if err != nil {
 			return fmt.Errorf("Failed to build init tarball from %s: %v", ii, err)
 		}
 	}
+	if err := apkTar.WriteAPKDB(); err != nil {
+		return err
+	}
 
 	if len(m.Onboot) != 0 {
 		log.Infof("Add onboot containers:")