fail prepare if hash not found

post-processor/checksum/post-processor.go

@@ -37,6 +37,17 @@ type outputPathTemplate struct {
 	HashType    string
 }
 
+func getHashMap() map[string]func() hash.Hash {
+	return map[string]func() hash.Hash{
+		"md5":    func() hash.Hash { return md5.New() },
+		"sha1":   func() hash.Hash { return sha1.New() },
+		"sha224": func() hash.Hash { return sha256.New224() },
+		"sha256": func() hash.Hash { return sha256.New() },
+		"sha384": func() hash.Hash { return sha512.New384() },
+		"sha512": func() hash.Hash { return sha512.New() },
+	}
+}
+
 func (p *PostProcessor) Configure(raws ...interface{}) error {
 	err := config.Decode(&p.config, &config.DecodeOpts{
 		Interpolate: true,
@@ -47,17 +58,24 @@ func (p *PostProcessor) Configure(raws ...interface{}) error {
 	if err != nil {
 		return err
 	}
+	errs := new(packer.MultiError)
 
 	if p.config.ChecksumTypes == nil {
 		p.config.ChecksumTypes = []string{"md5"}
 	}
 
+	hashMap := getHashMap()
+	for _, k := range p.config.ChecksumTypes {
+		if _, ok := hashMap[k]; !ok {
+			errs = packer.MultiErrorAppend(errs,
+				fmt.Errorf("Unrecognized checksum type: %s", k))
+		}
+	}
+
 	if p.config.OutputPath == "" {
 		p.config.OutputPath = "packer_{{.BuildName}}_{{.BuilderType}}_{{.HashType}}.checksum"
 	}
 
-	errs := new(packer.MultiError)
-
 	if err = interpolate.Validate(p.config.OutputPath, &p.config.ctx); err != nil {
 		errs = packer.MultiErrorAppend(
 			errs, fmt.Errorf("Error parsing target template: %s", err))
@@ -70,25 +88,6 @@ func (p *PostProcessor) Configure(raws ...interface{}) error {
 	return nil
 }
 
-func getHash(t string) hash.Hash {
-	var h hash.Hash
-	switch t {
-	case "md5":
-		h = md5.New()
-	case "sha1":
-		h = sha1.New()
-	case "sha224":
-		h = sha256.New224()
-	case "sha256":
-		h = sha256.New()
-	case "sha384":
-		h = sha512.New384()
-	case "sha512":
-		h = sha512.New()
-	}
-	return h
-}
-
 func (p *PostProcessor) PostProcess(ui packer.Ui, artifact packer.Artifact) (packer.Artifact, bool, error) {
 	files := artifact.Files()
 	var h hash.Hash
@@ -101,7 +100,7 @@ func (p *PostProcessor) PostProcess(ui packer.Ui, artifact packer.Artifact) (pac
 	}
 
 	for _, ct := range p.config.ChecksumTypes {
-		h = getHash(ct)
+		h = getHashMap()[ct]()
 		opTpl.HashType = ct
 		p.config.ctx.Data = &opTpl