feat(server): Implement creation of layer tarballs in the server

This will create, upload and hash the layer tarballs in one disk read.

tools/nixery/server/builder/archive.go

@@ -0,0 +1,92 @@
+package builder
+
+// This file implements logic for walking through a directory and creating a
+// tarball of it.
+//
+// The tarball is written straight to the supplied reader, which makes it
+// possible to create an image layer from the specified store paths, hash it and
+// upload it in one reading pass.
+
+import (
+	"archive/tar"
+	"io"
+	"log"
+	"os"
+	"path/filepath"
+
+	"github.com/google/nixery/layers"
+)
+
+// Create a new tarball from each of the paths in the list and write the tarball
+// to the supplied writer.
+func tarStorePaths(l *layers.Layer, w io.Writer) error {
+	t := tar.NewWriter(w)
+
+	for _, path := range l.Contents {
+		err := filepath.Walk(path, tarStorePath(t))
+		if err != nil {
+			return err
+		}
+	}
+
+	if err := t.Close(); err != nil {
+		return err
+	}
+
+	log.Printf("Created layer for '%s'\n", l.Hash())
+	return nil
+}
+
+func tarStorePath(w *tar.Writer) filepath.WalkFunc {
+	return func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+
+		// If the entry is not a symlink or regular file, skip it.
+		if info.Mode()&os.ModeSymlink == 0 && !info.Mode().IsRegular() {
+			return nil
+		}
+
+		// the symlink target is read if this entry is a symlink, as it
+		// is required when creating the file header
+		var link string
+		if info.Mode()&os.ModeSymlink != 0 {
+			link, err = os.Readlink(path)
+			if err != nil {
+				return err
+			}
+		}
+
+		header, err := tar.FileInfoHeader(info, link)
+		if err != nil {
+			return err
+		}
+
+		// The name retrieved from os.FileInfo only contains the file's
+		// basename, but the full path is required within the layer
+		// tarball.
+		header.Name = path
+		if err = w.WriteHeader(header); err != nil {
+			return err
+		}
+
+		// At this point, return if no file content needs to be written
+		if !info.Mode().IsRegular() {
+			return nil
+		}
+
+		f, err := os.Open(path)
+		if err != nil {
+			return err
+		}
+
+		if _, err := io.Copy(w, f); err != nil {
+			return err
+		}
+
+		f.Close()
+
+		return nil
+	}
+}