Support backup and restore of hardlinks

This tracks inode/device from the stat info and creates backward
compatible snapshots that allow preserving hardlinks. Backwards
compatibility is preserved by saving a virtual inode number index in the
Link field of the file entry. Since this field was previously only used
for symlinks, this won't break old versions. Additionally, the entry
data is cloned so restoration with an old version works.

Current limitations are primarility with restore. They include:
- no command line option to prevent hard link restore
- if a file has the immutable or append only flag it will be set before
hardlinks are restored, so hardlinking will fail.
- if a partial restore includes a hardlink but not the parent
directories the hardlink will fail.

These will be solved by grouping restore of hardlinks together
with file, prior to applying final metadata.

- if a file is changed and is being rewritten by a restore hardlinks are
not preserved.

src/duplicacy_entry.go

@@ -4,6 +4,8 @@
 package duplicacy
 
 import (
+	"bytes"
+	"crypto/sha256"
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
@@ -15,12 +17,10 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"syscall"
 	"time"
-	"bytes"
-	"crypto/sha256"
-
-    "github.com/vmihailenco/msgpack"
 
+	"github.com/vmihailenco/msgpack"
 )
 
 // This is the hidden directory in the repository for storing various files.
@@ -110,15 +110,36 @@ func (entry *Entry) Copy() *Entry {
 		UID: entry.UID,
 		GID: entry.GID,
 
-		StartChunk: entry.StartChunk,
+		StartChunk:  entry.StartChunk,
 		StartOffset: entry.StartOffset,
-		EndChunk: entry.EndChunk,
-		EndOffset: entry.EndOffset,
+		EndChunk:    entry.EndChunk,
+		EndOffset:   entry.EndOffset,
 
 		Attributes: entry.Attributes,
 	}
 }
 
+func (entry *Entry) HardLinkTo(target *Entry, startChunk int, endChunk int) *Entry {
+	return &Entry{
+		Path: entry.Path,
+		Size: target.Size,
+		Time: target.Time,
+		Mode: target.Mode,
+		Link: entry.Link,
+		Hash: target.Hash,
+
+		UID: target.UID,
+		GID: target.GID,
+
+		StartChunk:  startChunk,
+		StartOffset: target.StartOffset,
+		EndChunk:    endChunk,
+		EndOffset:   target.EndOffset,
+
+		Attributes: target.Attributes,
+	}
+}
+
 // CreateEntryFromJSON creates an entry from a json description.
 func (entry *Entry) UnmarshalJSON(description []byte) (err error) {
 
@@ -362,12 +383,12 @@ func (entry *Entry) EncodeMsgpack(encoder *msgpack.Encoder) error {
 
 	if entry.Attributes != nil {
 		attributes := make([]string, numberOfAttributes)
-        i := 0
-        for attribute := range *entry.Attributes {
-            attributes[i] = attribute
-            i++
-        }
-        sort.Strings(attributes)
+		i := 0
+		for attribute := range *entry.Attributes {
+			attributes[i] = attribute
+			i++
+		}
+		sort.Strings(attributes)
 		for _, attribute := range attributes {
 			err = encoder.EncodeString(attribute)
 			if err != nil {
@@ -380,7 +401,7 @@ func (entry *Entry) EncodeMsgpack(encoder *msgpack.Encoder) error {
 		}
 	}
 
-    return nil
+	return nil
 }
 
 func (entry *Entry) DecodeMsgpack(decoder *msgpack.Decoder) error {
@@ -492,14 +513,22 @@ func (entry *Entry) IsComplete() bool {
 	return entry.Size >= 0
 }
 
+func (entry *Entry) IsHardlinkedFrom() bool {
+	return entry.IsFile() && len(entry.Link) > 0 && entry.Link != "/"
+}
+
+func (entry *Entry) IsHardlinkRoot() bool {
+	return entry.IsFile() && entry.Link == "/"
+}
+
 func (entry *Entry) GetPermissions() os.FileMode {
 	return os.FileMode(entry.Mode) & fileModeMask
 }
 
 func (entry *Entry) GetParent() string {
 	path := entry.Path
-	if path != "" && path[len(path) - 1] == '/' {
-		path = path[:len(path) - 1]
+	if path != "" && path[len(path)-1] == '/' {
+		path = path[:len(path)-1]
 	}
 	i := strings.LastIndex(path, "/")
 	if i == -1 {
@@ -596,7 +625,7 @@ func ComparePaths(left string, right string) int {
 	for i := p; i < len(left); i++ {
 		c3 = left[i]
 		if c3 == '/' {
-			last1 = i == len(left) - 1
+			last1 = i == len(left)-1
 			break
 		}
 	}
@@ -606,7 +635,7 @@ func ComparePaths(left string, right string) int {
 	for i := p; i < len(right); i++ {
 		c4 = right[i]
 		if c4 == '/' {
-			last2 = i == len(right) - 1
+			last2 = i == len(right)-1
 			break
 		}
 	}
@@ -694,10 +723,27 @@ func (files FileInfoCompare) Less(i, j int) bool {
 	}
 }
 
+type listEntryLinkKey struct {
+	dev uint64
+	ino uint64
+}
+
+type ListingState struct {
+	linkIndex int
+	linkTable map[listEntryLinkKey]int // map unique inode details to initially found path
+}
+
+func NewListingState() *ListingState {
+	return &ListingState{
+		linkTable: make(map[listEntryLinkKey]int),
+	}
+}
+
 // ListEntries returns a list of entries representing file and subdirectories under the directory 'path'.  Entry paths
 // are normalized as relative to 'top'.  'patterns' are used to exclude or include certain files.
-func ListEntries(top string, path string, patterns []string, nobackupFile string, excludeByAttribute bool, listingChannel chan *Entry) (directoryList []*Entry,
-	skippedFiles []string, err error) {
+func ListEntries(top string, path string, patterns []string, nobackupFile string, excludeByAttribute bool,
+	listingState *ListingState,
+	listingChannel chan *Entry) (directoryList []*Entry, skippedFiles []string, err error) {
 
 	LOG_DEBUG("LIST_ENTRIES", "Listing %s", path)
 
@@ -777,6 +823,24 @@ func ListEntries(top string, path string, patterns []string, nobackupFile string
 			continue
 		}
 
+		var linkKey *listEntryLinkKey
+
+		if stat, ok := f.Sys().(*syscall.Stat_t); entry.IsFile() && ok && stat != nil && stat.Nlink > 1 {
+			k := listEntryLinkKey{dev: uint64(stat.Dev), ino: uint64(stat.Ino)}
+			if linkIndex, seen := listingState.linkTable[k]; seen {
+				if linkIndex == -1 {
+					LOG_DEBUG("LIST_EXCLUDE", "%s is excluded by attribute (hardlink)", entry.Path)
+					continue
+				}
+				entry.Size = 0
+				entry.Link = strconv.FormatInt(int64(linkIndex), 16)
+			} else {
+				entry.Link = "/"
+				listingState.linkTable[k] = -1
+				linkKey = &k
+			}
+		}
+
 		entry.ReadAttributes(top)
 
 		if excludeByAttribute && entry.Attributes != nil && excludedByAttribute(*entry.Attributes) {
@@ -784,6 +848,11 @@ func ListEntries(top string, path string, patterns []string, nobackupFile string
 			continue
 		}
 
+		if linkKey != nil {
+			listingState.linkTable[*linkKey] = listingState.linkIndex
+			listingState.linkIndex++
+		}
+
 		if entry.IsDir() {
 			directoryList = append(directoryList, entry)
 		} else {