local: stricter handling of short files

internal/backend/local/local.go

@@ -2,6 +2,7 @@ package local
 
 import (
 	"context"
+	"fmt"
 	"hash"
 	"io"
 	"os"
@@ -30,6 +31,8 @@ type Local struct {
 // ensure statically that *Local implements backend.Backend.
 var _ backend.Backend = &Local{}
 
+var errTooShort = fmt.Errorf("file is too short")
+
 func NewFactory() location.Factory {
 	return location.NewLimitedBackendFactory("local", ParseConfig, location.NoPassword, limiter.WrapBackendConstructor(Create), limiter.WrapBackendConstructor(Open))
 }
@@ -110,6 +113,10 @@ func (b *Local) IsNotExist(err error) bool {
 	return errors.Is(err, os.ErrNotExist)
 }
 
+func (b *Local) IsPermanentError(err error) bool {
+	return b.IsNotExist(err) || errors.Is(err, errTooShort) || errors.Is(err, os.ErrPermission)
+}
+
 // Save stores data in the backend at the handle.
 func (b *Local) Save(_ context.Context, h backend.Handle, rd backend.RewindReader) (err error) {
 	finalname := b.Filename(h)
@@ -219,6 +226,18 @@ func (b *Local) openReader(_ context.Context, h backend.Handle, length int, offs
 		return nil, err
 	}
 
+	fi, err := f.Stat()
+	if err != nil {
+		_ = f.Close()
+		return nil, err
+	}
+
+	size := fi.Size()
+	if size < offset+int64(length) {
+		_ = f.Close()
+		return nil, errTooShort
+	}
+
 	if offset > 0 {
 		_, err = f.Seek(offset, 0)
 		if err != nil {