[#1462] fstree: Allow to fetch file content lazily

If we should process address based on some condition, there is no need
to read file content in memory.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/local_object_storage/blobstor/fstree/fstree.go

@@ -72,6 +72,7 @@ func addressFromString(s string) (*oid.Address, error) {
 type IterationPrm struct {
 	handler      func(addr oid.Address, data []byte) error
 	ignoreErrors bool
+	lazyHandler  func(oid.Address, func() ([]byte, error)) error
 }
 
 // WithHandler sets a function to call on each object.
@@ -79,6 +80,13 @@ func (p *IterationPrm) WithHandler(f func(addr oid.Address, data []byte) error)
 	p.handler = f
 }
 
+// WithLazyHandler sets a function to call on each object.
+// Second callback parameter opens file and reads all data to a buffer.
+// File is not opened at all unless this callback is invoked.
+func (p *IterationPrm) WithLazyHandler(f func(oid.Address, func() ([]byte, error)) error) {
+	p.lazyHandler = f
+}
+
 // WithIgnoreErrors sets a flag indicating whether errors should be ignored.
 func (p *IterationPrm) WithIgnoreErrors(ignore bool) {
 	p.ignoreErrors = ignore
@@ -124,16 +132,23 @@ func (t *FSTree) iterate(depth int, curPath []string, prm IterationPrm) error {
 			continue
 		}
 
-		data, err := os.ReadFile(filepath.Join(curPath...))
+		if prm.lazyHandler != nil {
+			err = prm.lazyHandler(*addr, func() ([]byte, error) {
+				return os.ReadFile(filepath.Join(curPath...))
+			})
+		} else {
+			var data []byte
+			data, err = os.ReadFile(filepath.Join(curPath...))
 			if err != nil {
 				if prm.ignoreErrors {
 					continue
 				}
 				return err
 			}
+			err = prm.handler(*addr, data)
+		}
 
-		if err := prm.handler(*addr, data); err != nil {
+		if err != nil {
-			// Error occurred in handler, outside of our scope, needs to be reported.
 			return err
 		}
 	}

pkg/local_object_storage/writecache/flush.go

@@ -138,13 +138,19 @@ func (c *cache) flushBigObjects() {
 			evictNum := 0
 
 			var prm fstree.IterationPrm
-			prm.WithHandler(func(addr oid.Address, data []byte) error {
+			prm.WithLazyHandler(func(addr oid.Address, f func() ([]byte, error)) error {
 				sAddr := addr.EncodeToString()
 
 				if _, ok := c.store.flushed.Peek(sAddr); ok {
 					return nil
 				}
 
+				data, err := f()
+				if err != nil {
+					c.log.Error("can't read a file", zap.Stringer("address", addr))
+					return nil
+				}
+
 				c.mtx.Lock()
 				_, compress := c.compressFlags[sAddr]
 				c.mtx.Unlock()

pkg/local_object_storage/writecache/iterate.go

@@ -53,10 +53,17 @@ func (c *cache) Iterate(prm IterationPrm) error {
 
 	var fsPrm fstree.IterationPrm
 	fsPrm.WithIgnoreErrors(prm.ignoreErrors)
-	fsPrm.WithHandler(func(addr oid.Address, data []byte) error {
+	fsPrm.WithLazyHandler(func(addr oid.Address, f func() ([]byte, error)) error {
 		if _, ok := c.flushed.Peek(addr.EncodeToString()); ok {
 			return nil
 		}
+		data, err := f()
+		if err != nil {
+			if prm.ignoreErrors {
+				return nil
+			}
+			return err
+		}
 		return prm.handler(data)
 	})