Rework cache key generation a bit. (#375)

* Rework cache key generation a bit.

Cache keys are now based on the previous commands, rather than the previous state
of the filesystem.

* Refactor command interface a bit, only cache the context for commands that use it.

pkg/executor/composite_cache.go

@@ -0,0 +1,76 @@
+/*
+Copyright 2018 Google LLC
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package executor
+
+import (
+	"crypto/sha256"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/GoogleContainerTools/kaniko/pkg/util"
+)
+
+// NewCompositeCache returns an initialized composite cache object.
+func NewCompositeCache(initial ...string) *CompositeCache {
+	c := CompositeCache{
+		keys: initial,
+	}
+	return &c
+}
+
+// CompositeCache is a type that generates a cache key from a series of keys.
+type CompositeCache struct {
+	keys []string
+}
+
+// AddKey adds the specified key to the sequence.
+func (s *CompositeCache) AddKey(k ...string) {
+	s.keys = append(s.keys, k...)
+}
+
+// Key returns the human readable composite key as a string.
+func (s *CompositeCache) Key() string {
+	return strings.Join(s.keys, "-")
+}
+
+// Hash returns the composite key in a string SHA256 format.
+func (s *CompositeCache) Hash() (string, error) {
+	return util.SHA256(strings.NewReader(s.Key()))
+}
+
+// HashDir returns a hash of the directory.
+func HashDir(p string) (string, error) {
+	sha := sha256.New()
+	if err := filepath.Walk(p, func(path string, fi os.FileInfo, err error) error {
+		if err != nil {
+			return err
+		}
+		fileHash, err := util.CacheHasher()(path)
+		if err != nil {
+			return err
+		}
+		if _, err := sha.Write([]byte(fileHash)); err != nil {
+			return err
+		}
+		return nil
+	}); err != nil {
+		return "", err
+	}
+
+	return string(sha.Sum(nil)), nil
+}