*: refactoring to support streams

when creating a manifest from, or validating, a stream like a tar
archive, it requires thinking about some of the functions differently
than walking a directory tree.

This is the beginning of allowing for such features.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

check.go

@@ -3,7 +3,6 @@ package mtree
 import (
 	"fmt"
 	"os"
-	"path/filepath"
 	"sort"
 )
 
@@ -50,7 +49,8 @@ func Check(root string, dh *DirectoryHierarchy, keywords []string) (*Result, err
 				creator.curSet = nil
 			}
 		case RelativeType, FullType:
-			info, err := os.Lstat(e.Path())
+			filename := e.Path()
+			info, err := os.Lstat(filename)
 			if err != nil {
 				return nil, err
 			}
@@ -70,10 +70,16 @@ func Check(root string, dh *DirectoryHierarchy, keywords []string) (*Result, err
 				if keywords != nil && !inSlice(kv.Keyword(), keywords) {
 					continue
 				}
-				curKeyVal, err := keywordFunc(filepath.Join(root, e.Path()), info)
+				fh, err := os.Open(filename)
 				if err != nil {
 					return nil, err
 				}
+				curKeyVal, err := keywordFunc(filename, info, fh)
+				if err != nil {
+					fh.Close()
+					return nil, err
+				}
+				fh.Close()
 				if string(kv) != curKeyVal {
 					failure := Failure{Path: e.Path(), Keyword: kv.Keyword(), Expected: kv.Value(), Got: KeyVal(curKeyVal).Value()}
 					result.Failures = append(result.Failures, failure)