Check the good stuff

- Create a new snapshot version number as this method is not backwards
compatible.
- This has some breakages with restoring. Namely if the root file
is not marked for download any hardlinked files that need to be restored
will not be linked, they will be restored as a regular file

src/duplicacy_backupmanager.go

@@ -304,7 +304,24 @@ func (manager *BackupManager) Backup(top string, quickMode bool, threads int, ta
 			remoteEntry = nil
 		}
 
-		if compareResult == 0  {
+		if localEntry.IsHardlinkedFrom() {
+			// FIXME: Sanity check?
+			// FIXME: perhaps we can make size = 0 an initial invariant of the link?
+			//
+			// Note that if the initial size was 0 then this original logic doesn't change!
+			localEntry.Size = 0
+			// targetEntry, ok := localEntryList.HardLinkTable[localEntry.Link]
+			// if !ok {
+			// 	LOG_ERROR("BACKUP_CREATE", "Hard link %s not found in entry cache for path %s", localEntry.Link, localEntry.Path)
+			// }
+			// localEntry.Size = targetEntry.Size
+			// localEntry.Hash = targetEntry.Hash
+			// localEntry.StartChunk = targetEntry.StartChunk
+			// localEntry.StartOffset = targetEntry.StartOffset
+			// localEntry.EndChunk = targetEntry.EndChunk
+			// localEntry.EndOffset = targetEntry.EndOffset
+			// LOG_DEBUG("BACKUP_CREATE", "Hard link %s to %s in initial listing", localEntry.Link, targetEntry.Path)
+		} else if compareResult == 0  {
 			// No need to check if it is in hash mode -- in that case remote listing is nil
 			if localEntry.IsSameAs(remoteEntry) && localEntry.IsFile() {
 				if localEntry.Size > 0 {
@@ -622,6 +639,11 @@ func (manager *BackupManager) Backup(top string, quickMode bool, threads int, ta
 	return true
 }
 
+type hardLinkEntry struct {
+	entry *Entry
+	willDownload bool
+}
+
 // Restore downloads the specified snapshot, compares it with what's on the repository, and then downloads
 // files that are different. 'base' is a directory that contains files at a different revision which can
 // serve as a local cache to avoid download chunks available locally.  It is perfectly ok for 'base' to be
@@ -703,8 +725,16 @@ func (manager *BackupManager) Restore(top string, revision int, inPlace bool, qu
 	var localEntry *Entry
 	localListingOK := true
 
+	hardLinkTable := make(map[string]hardLinkEntry)
+	//hardLinks := make([]*Entry, 0)
+
 	for remoteEntry := range remoteListingChannel {
 
+		if remoteEntry.IsFile() && remoteEntry.Link == "/" {
+			LOG_INFO("RESTORE_LINK", "Noting hardlinked source file %s", remoteEntry.Path)
+			hardLinkTable[remoteEntry.Path] = hardLinkEntry{remoteEntry, false}
+		}
+
 		if len(patterns) > 0 && !MatchPath(remoteEntry.Path, patterns) {
 			continue
 		}
@@ -713,6 +743,8 @@ func (manager *BackupManager) Restore(top string, revision int, inPlace bool, qu
 		var compareResult int
 
 		for {
+			// TODO: We likely need to check if a local listing file exists in the hardLinkTable for the case where one is restoring a hardlink
+			// to an existing disk file. Right now, we'll just end up downloading the file new.
 			if localEntry == nil && localListingOK {
 				localEntry, localListingOK = <- localListingChannel
 			}
@@ -730,12 +762,28 @@ func (manager *BackupManager) Restore(top string, revision int, inPlace bool, qu
 		}
 
 		if compareResult == 0 {
+			// if quickMode && localEntry.IsFile() {
 				if quickMode && localEntry.IsFile() && localEntry.IsSameAs(remoteEntry) {
 					LOG_TRACE("RESTORE_SKIP", "File %s unchanged (by size and timestamp)", localEntry.Path)
 					skippedFileSize += localEntry.Size
 					skippedFileCount++
 					localEntry = nil
 					continue
+				// checkEntry := remoteEntry
+				// if len(remoteEntry.Link) > 0 && remoteEntry.Link != "/" {
+				// 	if e, ok := hardLinkTable[remoteEntry.Link]; !ok {
+				// 		LOG_ERROR("RESTORE_LINK", "Source file %s for hardlink %s missing", remoteEntry.Link, remoteEntry.Path)
+				// 	} else {
+				// 		checkEntry = e.entry
+				// 	}
+				// }
+				// if localEntry.IsSameAs(checkEntry) {
+				// 	LOG_TRACE("RESTORE_SKIP", "File %s unchanged (by size and timestamp)", localEntry.Path)
+				// 	skippedFileSize += localEntry.Size
+				// 	skippedFileCount++
+				// 	localEntry = nil
+				// 	continue
+				// }
 			}
 			localEntry = nil
 		}
@@ -782,6 +830,21 @@ func (manager *BackupManager) Restore(top string, revision int, inPlace bool, qu
 			}
 			directoryEntries = append(directoryEntries, remoteEntry)
 		} else {
+			// if remoteEntry.Link == "/" {
+			// 	hardLinkTable[remoteEntry.Path] = hardLinkEntry{remoteEntry, true}
+			// } else if len(remoteEntry.Link) > 0 {
+			// 	if e, ok := hardLinkTable[remoteEntry.Link]; !ok {
+			// 		LOG_ERROR("RESTORE_LINK", "Source file %s for hardlink %s missing", remoteEntry.Link, remoteEntry.Path)
+			// 	} else if !e.willDownload {
+			// 		origSourcePath := e.entry.Path
+			// 		e.entry.Path = remoteEntry.Path
+			// 		remoteEntry = e.entry
+			// 		hardLinkTable[origSourcePath] = hardLinkEntry{remoteEntry, true}
+			// 	} else {
+			// 		hardLinks = append(hardLinks, remoteEntry)
+			// 		continue
+			// 	}
+			// }
 			// We can't download files here since fileEntries needs to be sorted
 			fileEntries = append(fileEntries, remoteEntry)
 			totalFileSize += remoteEntry.Size
@@ -837,6 +900,10 @@ func (manager *BackupManager) Restore(top string, revision int, inPlace bool, qu
 		stat, _ := os.Stat(fullPath)
 		if stat != nil {
 			if quickMode {
+				// cmpFile := file
+				// if file.IsFile() && len(file.Link) > 0 && file.Link != "/" {
+				// 	cmpFile = hardLinkTable[file.Link].entry
+				// }
 				if file.IsSameAsFileInfo(stat) {
 					LOG_TRACE("RESTORE_SKIP", "File %s unchanged (by size and timestamp)", file.Path)
 					skippedFileSize += file.Size
@@ -874,7 +941,6 @@ func (manager *BackupManager) Restore(top string, revision int, inPlace bool, qu
 				downloadedFileSize += file.Size
 				downloadedFiles = append(downloadedFiles, file)
 			}
-
 			continue
 		}
 
@@ -901,6 +967,15 @@ func (manager *BackupManager) Restore(top string, revision int, inPlace bool, qu
 		file.RestoreMetadata(fullPath, nil, setOwner)
 	}
 
+	// for _, linkEntry := range hardLinks {
+	// 	sourcePath := joinPath(top, hardLinkTable[linkEntry.Link].entry.Path)
+	// 	fullPath := joinPath(top, linkEntry.Path)
+	// 	LOG_INFO("DOWNLOAD_LINK", "Hard linking %s -> %s", fullPath, sourcePath)
+	// 	if err := os.Link(sourcePath, fullPath); err != nil {
+	// 		LOG_ERROR("DOWNLOAD_LINK", "Failed to create hard link %s -> %s", fullPath, sourcePath)
+	// 	}
+	// }
+
 	if deleteMode && len(patterns) == 0 {
 		// Reverse the order to make sure directories are empty before being deleted
 		for i := range extraFiles {
@@ -1053,6 +1128,9 @@ func (manager *BackupManager) UploadSnapshot(chunkOperator *ChunkOperator, top s
 	lastEndChunk := 0
 
 	uploadEntryInfoFunc := func(entry *Entry) error {
+		if entry.IsHardlinkRoot() {
+			entryList.HardLinkTable[entry.Path] = entry
+		}
 
 		if entry.IsFile() && entry.Size > 0 {
 			delta := entry.StartChunk - len(chunkHashes) + 1
@@ -1075,6 +1153,14 @@ func (manager *BackupManager) UploadSnapshot(chunkOperator *ChunkOperator, top s
 			entry.StartChunk -= lastEndChunk
 			lastEndChunk = entry.EndChunk
 			entry.EndChunk = delta
+		} else if entry.IsHardlinkedFrom() {
+			targetEntry, ok := entryList.HardLinkTable[entry.Link]
+			if !ok {
+				LOG_ERROR("SNAPSHOT_UPLOAD", "Unable to find hardlink target for %s to %s", entry.Path, entry.Link)
+			}
+			// FIXME: We will use a copy, so it is probably sufficient to skip rereading xattrs and such in the initial code
+			entry = entry.LinkTo(targetEntry)
+			LOG_DEBUG("SNAPSHOT_UPLOAD", "Uploading cloned hardlink entry for %s to %s", entry.Path, entry.Link)
 		}
 
 		buffer.Reset()

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"
-
 )
 
 // This is the hidden directory in the repository for storing various files.
@@ -119,6 +119,27 @@ func (entry *Entry) Copy() *Entry {
 	}
 }
 
+func (entry *Entry) LinkTo(target *Entry) *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:  target.StartChunk,
+		StartOffset: target.StartOffset,
+		EndChunk:    target.EndChunk,
+		EndOffset:   target.EndOffset,
+
+		Attributes: target.Attributes,
+	}
+}
+
 // CreateEntryFromJSON creates an entry from a json description.
 func (entry *Entry) UnmarshalJSON(description []byte) (err error) {
 
@@ -492,14 +513,26 @@ func (entry *Entry) IsComplete() bool {
 	return entry.Size >= 0
 }
 
+func (entry *Entry) IsFileNotHardlink() bool {
+	return entry.IsFile() && (len(entry.Link) == 0 || entry.Link == "/")
+}
+
+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] == '/' {
+	if path != "" && path[len(path)-1] == '/' {
-		path = path[:len(path) - 1]
+		path = path[:len(path)-1]
 	}
 	i := strings.LastIndex(path, "/")
 	if i == -1 {
@@ -596,7 +629,7 @@ 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 +639,7 @@ 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 +727,26 @@ func (files FileInfoCompare) Less(i, j int) bool {
 	}
 }
 
+type listEntryLinkKey struct {
+	dev uint64
+	ino uint64
+}
+
+type ListingState struct {
+	linkTable map[listEntryLinkKey]string // map unique inode details to initially found path
+}
+
+func NewListingState() *ListingState {
+	return &ListingState{
+		linkTable: make(map[listEntryLinkKey]string),
+	}
+}
+
 // 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,
+func ListEntries(top string, path string, patterns []string, nobackupFile string, excludeByAttribute bool,
-	skippedFiles []string, err error) {
+	listingState *ListingState,
+	listingChannel chan *Entry) (directoryList []*Entry, skippedFiles []string, err error) {
 
 	LOG_DEBUG("LIST_ENTRIES", "Listing %s", path)
 
@@ -784,6 +833,20 @@ func ListEntries(top string, path string, patterns []string, nobackupFile string
 			continue
 		}
 
+		if entry.IsFile() {
+			stat, ok := f.Sys().(*syscall.Stat_t)
+			if ok && stat != nil && stat.Nlink > 1 {
+				k := listEntryLinkKey{dev: uint64(stat.Dev), ino: uint64(stat.Ino)}
+				if path, ok := listingState.linkTable[k]; ok {
+					LOG_DEBUG("LIST_HARDLINK", "Detected hardlink %s to %s", entry.Path, path)
+					entry.Link = path
+				} else {
+					entry.Link = "/"
+					listingState.linkTable[k] = entry.Path
+				}
+			}
+		}
+
 		if entry.IsDir() {
 			directoryList = append(directoryList, entry)
 		} else {

src/duplicacy_entrylist.go

@@ -62,6 +62,7 @@ type EntryList struct {
 	uploadedChunkIndex int                  // counter for upload chunks
 	uploadedChunkOffset int                 // the start offset for the current modified entry
 
+	HardLinkTable map[string]*Entry
 }
 
 // Create a new entry list
@@ -78,6 +79,7 @@ func CreateEntryList(snapshotID string, cachePath string, maximumInMemoryEntries
 		maximumInMemoryEntries: maximumInMemoryEntries,
 		cachePath: cachePath,
 		Token: string(token),
+		HardLinkTable: make(map[string]*Entry),
 	}
 
 	return entryList, nil
@@ -118,7 +120,7 @@ func (entryList *EntryList)AddEntry(entry *Entry) error {
 	if !entry.IsComplete() {
 		if entry.IsDir() || entry.IsLink() {
 			entry.Size = 0
-		} else {
+		} else if !entry.IsHardlinkedFrom() {
 			modifiedEntry := ModifiedEntry {
 				Path: entry.Path,
 				Size: -1,

src/duplicacy_snapshot.go

@@ -68,6 +68,7 @@ func (snapshot *Snapshot) ListLocalFiles(top string, nobackupFile string,
 								         skippedDirectories *[]string, skippedFiles *[]string) {
 
 	var patterns []string
+	var listingState = NewListingState()
 
 	if filtersFile == "" {
 		filtersFile = joinPath(GetDuplicacyPreferencePath(), "filters")
@@ -81,7 +82,7 @@ func (snapshot *Snapshot) ListLocalFiles(top string, nobackupFile string,
 
 		directory := directories[len(directories)-1]
 		directories = directories[:len(directories)-1]
-		subdirectories, skipped, err := ListEntries(top, directory.Path, patterns, nobackupFile, excludeByAttribute, listingChannel)
+		subdirectories, skipped, err := ListEntries(top, directory.Path, patterns, nobackupFile, excludeByAttribute, listingState, listingChannel)
 		if err != nil {
 			if directory.Path == "" {
 				LOG_ERROR("LIST_FAILURE", "Failed to list the repository root: %v", err)