Add support for COPY from previous rootfs

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

builder/dockerfile/imagecontext.go

@@ -0,0 +1,121 @@
+package dockerfile
+
+import (
+	"sync"
+
+	"github.com/Sirupsen/logrus"
+	"github.com/docker/docker/builder"
+	"github.com/docker/docker/builder/remotecontext"
+	"github.com/pkg/errors"
+)
+
+// imageContexts is a helper for stacking up built image rootfs and reusing
+// them as contexts
+type imageContexts struct {
+	b     *Builder
+	list  []*imageMount
+	cache *pathCache
+}
+
+type imageMount struct {
+	id      string
+	ctx     builder.Context
+	release func() error
+}
+
+func (ic *imageContexts) new() {
+	ic.list = append(ic.list, &imageMount{})
+}
+
+func (ic *imageContexts) update(imageID string) {
+	ic.list[len(ic.list)-1].id = imageID
+}
+
+func (ic *imageContexts) validate(i int) error {
+	if i < 0 || i >= len(ic.list)-1 {
+		var extraMsg string
+		if i == len(ic.list)-1 {
+			extraMsg = " refers current build block"
+		}
+		return errors.Errorf("invalid from flag value %d%s", i, extraMsg)
+	}
+	return nil
+}
+
+func (ic *imageContexts) context(i int) (builder.Context, error) {
+	if err := ic.validate(i); err != nil {
+		return nil, err
+	}
+	im := ic.list[i]
+	if im.ctx == nil {
+		if im.id == "" {
+			return nil, errors.Errorf("could not copy from empty context")
+		}
+		p, release, err := ic.b.docker.MountImage(im.id)
+		if err != nil {
+			return nil, errors.Wrapf(err, "failed to mount %s", im.id)
+		}
+		ctx, err := remotecontext.NewLazyContext(p)
+		if err != nil {
+			return nil, errors.Wrapf(err, "failed to create lazycontext for %s", p)
+		}
+		logrus.Debugf("mounted image: %s %s", im.id, p)
+		im.release = release
+		im.ctx = ctx
+	}
+	return im.ctx, nil
+}
+
+func (ic *imageContexts) unmount() (retErr error) {
+	for _, im := range ic.list {
+		if im.release != nil {
+			if err := im.release(); err != nil {
+				logrus.Error(errors.Wrapf(err, "failed to unmount previous build image"))
+				retErr = err
+			}
+		}
+	}
+	return
+}
+
+func (ic *imageContexts) getCache(i int, path string) (interface{}, bool) {
+	if ic.cache != nil {
+		im := ic.list[i]
+		if im.id == "" {
+			return nil, false
+		}
+		return ic.cache.get(im.id + path)
+	}
+	return nil, false
+}
+
+func (ic *imageContexts) setCache(i int, path string, v interface{}) {
+	if ic.cache != nil {
+		ic.cache.set(ic.list[i].id+path, v)
+	}
+}
+
+type pathCache struct {
+	mu    sync.Mutex
+	items map[string]interface{}
+}
+
+func (c *pathCache) set(k string, v interface{}) {
+	c.mu.Lock()
+	if c.items == nil {
+		c.items = make(map[string]interface{})
+	}
+	c.items[k] = v
+	c.mu.Unlock()
+}
+
+func (c *pathCache) get(k string) (interface{}, bool) {
+	c.mu.Lock()
+	if c.items == nil {
+		c.mu.Unlock()
+		return nil, false
+	}
+	v, ok := c.items[k]
+	c.mu.Unlock()
+	return v, ok
+}