restore: New optimized multithreaded implementation

Signed-off-by: Igor Fedorenko <igor@ifedorenko.com>

internal/restorer/fileswriter.go

@@ -0,0 +1,69 @@
+package restorer
+
+import (
+	"io"
+	"os"
+	"sync"
+
+	"github.com/hashicorp/golang-lru/simplelru"
+	"github.com/restic/restic/internal/debug"
+	"github.com/restic/restic/internal/errors"
+)
+
+type filesWriter struct {
+	lock       sync.Mutex             // guards concurrent access
+	inprogress map[*fileInfo]struct{} // (logically) opened file writers
+	writers    simplelru.LRUCache     // key: *fileInfo, value: *os.File
+}
+
+func newFilesWriter(count int) *filesWriter {
+	writers, _ := simplelru.NewLRU(count, func(key interface{}, value interface{}) {
+		value.(*os.File).Close()
+		debug.Log("Closed and purged cached writer for %v", key)
+	})
+	return &filesWriter{inprogress: make(map[*fileInfo]struct{}), writers: writers}
+}
+
+func (w *filesWriter) writeToFile(file *fileInfo, buf []byte) error {
+	acquireWriter := func() (io.Writer, error) {
+		w.lock.Lock()
+		defer w.lock.Unlock()
+		if wr, ok := w.writers.Get(file); ok {
+			debug.Log("Used cached writer for %s", file.path)
+			return wr.(*os.File), nil
+		}
+		var flags int
+		if _, append := w.inprogress[file]; append {
+			flags = os.O_APPEND | os.O_WRONLY
+		} else {
+			w.inprogress[file] = struct{}{}
+			flags = os.O_CREATE | os.O_TRUNC | os.O_WRONLY
+		}
+		wr, err := os.OpenFile(file.path, flags, 0600)
+		if err != nil {
+			return nil, err
+		}
+		w.writers.Add(file, wr)
+		debug.Log("Opened and cached writer for %s", file.path)
+		return wr, nil
+	}
+
+	wr, err := acquireWriter()
+	if err != nil {
+		return err
+	}
+	n, err := wr.Write(buf)
+	if err != nil {
+		return err
+	}
+	if n != len(buf) {
+		return errors.Errorf("error writing file %v: wrong length written, want %d, got %d", file.path, len(buf), n)
+	}
+	return nil
+}
+
+func (w *filesWriter) close(file *fileInfo) {
+	w.lock.Lock()
+	defer w.lock.Unlock()
+	w.writers.Remove(file)
+}