*: add an update/restore functionality

This allows for restoring some attributes of files from the state in an
mtree Manifest

Reported-by: Matthew Garrett <Matthewgarrett@google.com>
Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

update.go

@@ -0,0 +1,87 @@
+package mtree
+
+import (
+	"os"
+	"sort"
+)
+
+// DefaultUpdateKeywords is the default set of keywords that can take updates to the files on disk
+var DefaultUpdateKeywords = []Keyword{
+	"uid",
+	"gid",
+	"mode",
+	"time",
+	// TODO xattr
+}
+
+// Update attempts to set the attributes of root directory path, given the values of `keywords` in dh DirectoryHierarchy.
+func Update(root string, dh *DirectoryHierarchy, keywords []Keyword, fs FsEval) ([]InodeDelta, error) {
+	creator := dhCreator{DH: dh}
+	curDir, err := os.Getwd()
+	if err == nil {
+		defer os.Chdir(curDir)
+	}
+
+	if err := os.Chdir(root); err != nil {
+		return nil, err
+	}
+	sort.Sort(byPos(creator.DH.Entries))
+
+	results := []InodeDelta{}
+	for i, e := range creator.DH.Entries {
+		switch e.Type {
+		case SpecialType:
+			if e.Name == "/set" {
+				creator.curSet = &creator.DH.Entries[i]
+			} else if e.Name == "/unset" {
+				creator.curSet = nil
+			}
+			Debugf("%#v", e)
+			continue
+		case RelativeType, FullType:
+			e.Set = creator.curSet
+			pathname, err := e.Path()
+			if err != nil {
+				return nil, err
+			}
+
+			// filter the keywords to update on the file, from the keywords available for this entry:
+			var toCheck []KeyVal
+			toCheck = keyvalSelector(e.AllKeys(), keywords)
+			Debugf("toCheck(%q): %v", pathname, toCheck)
+
+			for _, kv := range toCheck {
+				if !InKeywordSlice(kv.Keyword(), keywords) {
+					continue
+				}
+				ukFunc, ok := UpdateKeywordFuncs[kv.Keyword()]
+				if !ok {
+					Debugf("no UpdateKeywordFunc for %s; skipping", kv.Keyword())
+					continue
+				}
+				if _, err := ukFunc(pathname, kv.Value()); err != nil {
+					results = append(results, InodeDelta{
+						diff: ErrorDifference,
+						path: pathname,
+						old:  e,
+						keys: []KeyDelta{
+							{
+								diff: ErrorDifference,
+								name: kv.Keyword(),
+								err:  err,
+							},
+						}})
+				}
+			}
+		}
+	}
+
+	return results, nil
+}
+
+// Result of an "update" returns the produced results
+type Result struct {
+	Path    string
+	Keyword Keyword
+	Got     string
+}