Remove shadow WAL iterator

This commit removes the shadow WAL iterator and replaces it with a
fileWalSegmentIterator instead. This works since the shadow WAL now
has the same structure as the replica WAL. This reduces duplicate
code and will make it so read replication can be daisy chained in
the future.

db.go

@@ -1236,103 +1236,7 @@ func (db *DB) WALSegments(ctx context.Context, generation string) (WALSegmentIte
 
 	sort.Ints(indexes)
 
-	return newShadowWALSegmentIterator(db, generation, indexes), nil
+	return newFileWALSegmentIterator(db.ShadowWALDir(generation), generation, indexes), nil
-}
-
-type shadowWALSegmentIterator struct {
-	db         *DB
-	generation string
-	indexes    []int
-
-	infos []WALSegmentInfo
-	err   error
-}
-
-func newShadowWALSegmentIterator(db *DB, generation string, indexes []int) *shadowWALSegmentIterator {
-	return &shadowWALSegmentIterator{
-		db:         db,
-		generation: generation,
-		indexes:    indexes,
-	}
-}
-
-func (itr *shadowWALSegmentIterator) Close() (err error) {
-	return itr.err
-}
-
-func (itr *shadowWALSegmentIterator) Next() bool {
-	// Exit if an error has already occurred.
-	if itr.err != nil {
-		return false
-	}
-
-	for {
-		// Move to the next segment in cache, if available.
-		if len(itr.infos) > 1 {
-			itr.infos = itr.infos[1:]
-			return true
-		}
-		itr.infos = itr.infos[:0] // otherwise clear infos
-
-		// If no indexes remain, stop iteration.
-		if len(itr.indexes) == 0 {
-			return false
-		}
-
-		// Read segments into a cache for the current index.
-		index := itr.indexes[0]
-		itr.indexes = itr.indexes[1:]
-		f, err := os.Open(filepath.Join(itr.db.ShadowWALDir(itr.generation), FormatIndex(index)))
-		if err != nil {
-			itr.err = err
-			return false
-		}
-		defer func() { _ = f.Close() }()
-
-		fis, err := f.Readdir(-1)
-		if err != nil {
-			itr.err = err
-			return false
-		} else if err := f.Close(); err != nil {
-			itr.err = err
-			return false
-		}
-		for _, fi := range fis {
-			filename := filepath.Base(fi.Name())
-			if fi.IsDir() {
-				continue
-			}
-
-			offset, err := ParseOffset(strings.TrimSuffix(filename, ".wal.lz4"))
-			if err != nil {
-				continue
-			}
-
-			itr.infos = append(itr.infos, WALSegmentInfo{
-				Generation: itr.generation,
-				Index:      index,
-				Offset:     offset,
-				Size:       fi.Size(),
-				CreatedAt:  fi.ModTime().UTC(),
-			})
-		}
-
-		// Ensure segments are sorted within index.
-		sort.Sort(WALSegmentInfoSlice(itr.infos))
-
-		if len(itr.infos) > 0 {
-			return true
-		}
-	}
-}
-
-func (itr *shadowWALSegmentIterator) Err() error { return itr.err }
-
-func (itr *shadowWALSegmentIterator) WALSegment() WALSegmentInfo {
-	if len(itr.infos) == 0 {
-		return WALSegmentInfo{}
-	}
-	return itr.infos[0]
 }
 
 // SQLite WAL constants