cmd/pkg: Build a build context from 'sources'

This is the final piece. If 'sources' are defined, tar up
the sources and rewrite them accordingly. Pass it as build
build context to 'docker'.

This allows building from something like this:

  ├── etc
  │   ├── foo
  └── foo
      ├── Dockerfile
          ├── build.yml
          └── main.go

With 'build.yml':

  image: foo
  extra-sources:
    - ../etc:etc

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

src/cmd/linuxkit/pkglib/build.go

@@ -1,12 +1,18 @@
 package pkglib
 
 import (
+	"archive/tar"
 	"encoding/json"
 	"fmt"
+	"io"
 	"os"
+	"path"
+	"path/filepath"
 	"runtime"
+	"strings"
 
 	"github.com/linuxkit/linuxkit/src/cmd/linuxkit/version"
+	log "github.com/sirupsen/logrus"
 )
 
 type buildOpts struct {
@@ -141,6 +147,8 @@ func (p Pkg) Build(bos ...BuildOpt) error {
 		args = append(args, "--label=org.mobyproject.linuxkit.version="+version.Version)
 		args = append(args, "--label=org.mobyproject.linuxkit.revision="+version.GitCommit)
 
+		d.ctx = &buildCtx{sources: p.sources}
+
 		if err := d.build(p.Tag()+suffix, p.path, args...); err != nil {
 			return err
 		}
@@ -191,3 +199,59 @@ func (p Pkg) Build(bos ...BuildOpt) error {
 
 	return nil
 }
+
+type buildCtx struct {
+	sources []pkgSource
+}
+
+// Copy iterates over the sources, tars up the content after rewriting the paths.
+// It assumes that sources is sane, ie is well formed and the first part is an absolute path
+// and that it exists. NewFromCLI() ensures that.
+func (c *buildCtx) Copy(w io.WriteCloser) error {
+	tw := tar.NewWriter(w)
+	defer func() {
+		tw.Close()
+		w.Close()
+	}()
+
+	for _, s := range c.sources {
+		log.Debugf("Adding to build context: %s -> %s", s.src, s.dst)
+
+		f := func(p string, i os.FileInfo, err error) error {
+			if err != nil {
+				return fmt.Errorf("ctx: Walk error on %s: %v", p, err)
+			}
+
+			h, err := tar.FileInfoHeader(i, "")
+			if err != nil {
+				return fmt.Errorf("ctx: Converting FileInfo for %s: %v", p, err)
+			}
+			h.Name = path.Join(s.dst, strings.TrimPrefix(p, s.src))
+			if err := tw.WriteHeader(h); err != nil {
+				return fmt.Errorf("ctx: Writing header for %s: %v", p, err)
+			}
+
+			if !i.Mode().IsRegular() {
+				return nil
+			}
+
+			f, err := os.Open(p)
+			if err != nil {
+				return fmt.Errorf("ctx: Open %s: %v", p, err)
+			}
+			defer f.Close()
+
+			_, err = io.Copy(tw, f)
+			if err != nil {
+				return fmt.Errorf("ctx: Writing %s: %v", p, err)
+			}
+			return nil
+		}
+
+		if err := filepath.Walk(s.src, f); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}