packer: Add FileCache and Cache

packer/cache.go

+package packer
+
+import (
+	"crypto/sha256"
+	"path/filepath"
+	"sync"
+)
+
+// Cache implements a caching interface where files can be stored for
+// re-use between multiple runs.
+type Cache interface {
+	// Lock takes a key and returns the path where the file can be written to.
+	// Packer guarantees that no other process will write to this file while
+	// the lock is held.
+	//
+	// The cache will block and wait for the lock.
+	Lock(string) (string, error)
+
+	// Unlock will unlock a certain cache key. Be very careful that this
+	// is only called once per lock obtained.
+	Unlock(string)
+
+	// RLock returns the path to a key in the cache and locks it for reading.
+	// The second return parameter is whether the key existed or not.
+	// This will block if any locks are held for writing. No lock will be
+	// held if the key doesn't exist.
+	RLock(string) (string, bool)
+
+	// RUnlock will unlock a key for reading.
+	RUnlock(string)
+}
+
+// FileCache implements a Cache by caching the data directly to a cache
+// directory.
+type FileCache struct {
+	CacheDir string
+	l        sync.Mutex
+	rw       map[string]*sync.RWMutex
+}
+
+func (f *FileCache) Lock(key string) (string, error) {
+	hashKey := f.hashKey(key)
+	rw := f.rwLock(hashKey)
+	rw.Lock()
+
+	return filepath.Join(f.CacheDir, hashKey), nil
+}
+
+func (f *FileCache) Unlock(key string) {
+	hashKey := f.hashKey(key)
+	rw := f.rwLock(hashKey)
+	rw.Unlock()
+}
+
+func (f *FileCache) RLock(key string) (string, bool) {
+	hashKey := f.hashKey(key)
+	rw := f.rwLock(hashKey)
+	rw.RLock()
+
+	return filepath.Join(f.CacheDir, hashKey), true
+}
+
+func (f *FileCache) RUnlock(key string) {
+	hashKey := f.hashKey(key)
+	rw := f.rwLock(hashKey)
+	rw.RUnlock()
+}
+
+func (f *FileCache) hashKey(key string) string {
+	sha := sha256.New()
+	sha.Write([]byte(key))
+	return string(sha.Sum(nil))
+}
+
+func (f *FileCache) rwLock(hashKey string) *sync.RWMutex {
+	f.l.Lock()
+	defer f.l.Unlock()
+
+	if f.rw == nil {
+		f.rw = make(map[string]*sync.RWMutex)
+	}
+
+	if result, ok := f.rw[hashKey]; ok {
+		return result
+	}
+
+	var result sync.RWMutex
+	f.rw[hashKey] = &result
+	return &result
+}

packer/cache_test.go

+package packer
+
+import (
+	"io/ioutil"
+	"os"
+	"testing"
+)
+
+func TestFileCache_Implements(t *testing.T) {
+	var raw interface{}
+	raw = &FileCache{}
+	if _, ok := raw.(Cache); !ok {
+		t.Fatal("FileCache must be a Cache")
+	}
+}
+
+func TestFileCache(t *testing.T) {
+	cacheDir, err := ioutil.TempDir("", "packer")
+	if err != nil {
+		t.Fatalf("error creating temporary dir: %s", err)
+	}
+	defer os.RemoveAll(cacheDir)
+
+	cache := &FileCache{CacheDir: cacheDir}
+	path, err := cache.Lock("foo")
+	if err != nil {
+		t.Fatalf("error locking: %s", err)
+	}
+
+	err = ioutil.WriteFile(path, []byte("data"), 0666)
+	if err != nil {
+		t.Fatalf("error writing: %s", err)
+	}
+
+	cache.Unlock("foo")
+
+	path, ok := cache.RLock("foo")
+	if !ok {
+		t.Fatal("cache says key doesn't exist")
+	}
+	defer cache.RUnlock("foo")
+
+	data, err := ioutil.ReadFile(path)
+	if err != nil {
+		t.Fatalf("error reading file: %s", err)
+	}
+
+	if string(data) != "data" {
+		t.Fatalf("unknown data: %s", data)
+	}
+}