Fix uncleanable size=0 orphans with volume.fsck -forcePurging (#7783)

This is a follow-up fix to PR #7332 which partially addressed the issue.

The problem is that size=0 needles are in a gray area:
- IsValid() returns false for size=0 (because size must be > 0)
- IsDeleted() returns false for size=0 (because size must be < 0 or == TombstoneFileSize)

PR #7332 only fixed 2 places, but several other places still had the same bug:

1. needle_map_memory.go:doLoading - line 43 still used oldSize.IsValid()
2. needle_map_memory.go:DoOffsetLoading - used during vacuum and incremental loading
3. needle_map_leveldb.go:generateLevelDbFile - used when generating LevelDB needle maps
4. needle_map_leveldb.go:DoOffsetLoading - used during incremental loading for LevelDB
5. needle_map/compact_map.go:delete - couldn't delete size=0 entries because:
   - The condition 'size > 0' failed for size=0
   - Even if it passed, negating 0 gives 0 (not marking as deleted)

Changes:
- Changed size.IsValid() to !size.IsDeleted() in doLoading and DoOffsetLoading functions
- Fixed compact_map delete to use TombstoneFileSize for size=0 entries

Fixes #7293

weed/storage/needle_map/compact_map.go

@@ -175,9 +175,14 @@ func (cs *CompactMapSegment) get(key types.NeedleId) (*CompactNeedleValue, bool)
 // delete deletes a map entry by key. Returns the entries' previous Size, if available.
 func (cs *CompactMapSegment) delete(key types.NeedleId) types.Size {
 	if i, found := cs.bsearchKey(key); found {
-		if cs.list[i].size > 0 && cs.list[i].size.IsValid() {
+		if !cs.list[i].size.IsDeleted() {
 			ret := cs.list[i].size
-			cs.list[i].size = -cs.list[i].size
+			if cs.list[i].size == 0 {
+				// size=0 needles can't be marked deleted by negating, use tombstone
+				cs.list[i].size = types.TombstoneFileSize
+			} else {
+				cs.list[i].size = -cs.list[i].size
+			}
 			return ret
 		}
 	}

weed/storage/needle_map_leveldb.go

@@ -122,7 +122,7 @@ func generateLevelDbFile(dbFileName string, indexFile *os.File) error {
 		glog.V(0).Infof("generateLevelDbFile %s, watermark %d, num of entries:%d", dbFileName, watermark, (uint64(stat.Size())-watermark*NeedleMapEntrySize)/NeedleMapEntrySize)
 	}
 	return idx.WalkIndexFile(indexFile, watermark, func(key NeedleId, offset Offset, size Size) error {
-		if !offset.IsZero() && size.IsValid() {
+		if !offset.IsZero() && !size.IsDeleted() {
 			levelDbWrite(db, key, offset, size, false, 0)
 		} else {
 			levelDbDelete(db, key)
@@ -380,10 +380,10 @@ func (m *LevelDbNeedleMap) DoOffsetLoading(v *Volume, indexFile *os.File, startF
 			// needle is found
 			oldSize := BytesToSize(data[OffsetSize : OffsetSize+SizeSize])
 			oldOffset := BytesToOffset(data[0:OffsetSize])
-			if !offset.IsZero() && size.IsValid() {
+			if !offset.IsZero() && !size.IsDeleted() {
 				// updated needle
 				m.mapMetric.FileByteCounter += uint64(size)
-				if !oldOffset.IsZero() && oldSize.IsValid() {
+				if !oldOffset.IsZero() && !oldSize.IsDeleted() {
 					m.mapMetric.DeletionCounter++
 					m.mapMetric.DeletionByteCounter += uint64(oldSize)
 				}

weed/storage/needle_map_memory.go

@@ -40,7 +40,7 @@ func doLoading(file *os.File, nm *NeedleMap) (*NeedleMap, error) {
 			nm.FileCounter++
 			nm.FileByteCounter = nm.FileByteCounter + uint64(size)
 			oldOffset, oldSize := nm.m.Set(NeedleId(key), offset, size)
-			if !oldOffset.IsZero() && oldSize.IsValid() {
+			if !oldOffset.IsZero() && !oldSize.IsDeleted() {
 				nm.DeletionCounter++
 				nm.DeletionByteCounter = nm.DeletionByteCounter + uint64(oldSize)
 			}
@@ -112,10 +112,10 @@ func (nm *NeedleMap) DoOffsetLoading(v *Volume, indexFile *os.File, startFrom ui
 	e := idx.WalkIndexFile(indexFile, startFrom, func(key NeedleId, offset Offset, size Size) error {
 		nm.MaybeSetMaxFileKey(key)
 		nm.FileCounter++
-		if !offset.IsZero() && size.IsValid() {
+		if !offset.IsZero() && !size.IsDeleted() {
 			nm.FileByteCounter = nm.FileByteCounter + uint64(size)
 			oldOffset, oldSize := nm.m.Set(NeedleId(key), offset, size)
-			if !oldOffset.IsZero() && oldSize.IsValid() {
+			if !oldOffset.IsZero() && !oldSize.IsDeleted() {
 				nm.DeletionCounter++
 				nm.DeletionByteCounter = nm.DeletionByteCounter + uint64(oldSize)
 			}

weed/storage/types/needle_types.go

@@ -14,9 +14,18 @@ type Offset struct {
 
 type Size int32
 
+// IsDeleted checks if the needle entry has been marked as deleted (tombstoned).
+// Use this when checking if an entry should exist in the needle map.
+// Returns true for negative sizes or TombstoneFileSize.
+// Note: size=0 is NOT considered deleted - it's an anomalous but active entry.
 func (s Size) IsDeleted() bool {
 	return s < 0 || s == TombstoneFileSize
 }
+
+// IsValid checks if the needle has actual readable data.
+// Use this when checking if needle data can be read or when counting data bytes.
+// Returns true only for positive sizes (size > 0).
+// Note: size=0 returns false - such needles exist in the map but have no readable data.
 func (s Size) IsValid() bool {
 	return s > 0 && s != TombstoneFileSize
 }