local: Add flag --no-preallocate - #3207

Some virtual filesystems (such as Google Drive File Stream) may
incorrectly set the actual file size equal to the preallocated space,
causing checksum and file size checks to fail.

This flag can be used to disable preallocation for local backends of
this type.

backend/local/local.go

@@ -148,6 +148,17 @@ Windows/macOS and case sensitive for everything else.  Use this flag
 to override the default choice.`,
 			Default:  false,
 			Advanced: true,
+		}, {
+			Name: "no_preallocate",
+			Help: `Disable preallocation of disk space for transferred files
+
+Preallocation of disk space helps prevent filesystem fragmentation.
+However, some virtual filesystem layers (such as Google Drive File
+Stream) may incorrectly set the actual file size equal to the
+preallocated space, causing checksum and file size checks to fail.
+Use this flag to disable preallocation.`,
+			Default:  false,
+			Advanced: true,
 		}, {
 			Name: "no_sparse",
 			Help: `Disable sparse files for multi-thread downloads
@@ -191,6 +202,7 @@ type Options struct {
 	OneFileSystem     bool                 `config:"one_file_system"`
 	CaseSensitive     bool                 `config:"case_sensitive"`
 	CaseInsensitive   bool                 `config:"case_insensitive"`
+	NoPreAllocate     bool                 `config:"no_preallocate"`
 	NoSparse          bool                 `config:"no_sparse"`
 	NoSetModTime      bool                 `config:"no_set_modtime"`
 	Enc               encoder.MultiEncoder `config:"encoding"`
@@ -1127,10 +1139,12 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 				return err
 			}
 		}
-		// Pre-allocate the file for performance reasons
-		err = file.PreAllocate(src.Size(), f)
-		if err != nil {
-			fs.Debugf(o, "Failed to pre-allocate: %v", err)
+		if !o.fs.opt.NoPreAllocate {
+			// Pre-allocate the file for performance reasons
+			err = file.PreAllocate(src.Size(), f)
+			if err != nil {
+				fs.Debugf(o, "Failed to pre-allocate: %v", err)
+			}
 		}
 		out = f
 	} else {
@@ -1217,9 +1231,11 @@ func (f *Fs) OpenWriterAt(ctx context.Context, remote string, size int64) (fs.Wr
 		return nil, err
 	}
 	// Pre-allocate the file for performance reasons
-	err = file.PreAllocate(size, out)
-	if err != nil {
-		fs.Debugf(o, "Failed to pre-allocate: %v", err)
+	if !f.opt.NoPreAllocate {
+		err = file.PreAllocate(size, out)
+		if err != nil {
+			fs.Debugf(o, "Failed to pre-allocate: %v", err)
+		}
 	}
 	if !f.opt.NoSparse && file.SetSparseImplemented {
 		sparseWarning.Do(func() {

docs/content/local.md

@@ -447,6 +447,21 @@ to override the default choice.
 - Type:        bool
 - Default:     false
 
+#### --local-no-preallocate
+
+Disable preallocation of disk space for transferred files
+
+Preallocation of disk space helps prevent filesystem fragmentation.
+However, some virtual filesystem layers (such as Google Drive File
+Stream) may incorrectly set the actual file size equal to the
+preallocated space, causing checksum and file size checks to fail.
+Use this flag to disable preallocation.
+
+- Config:      no_preallocate
+- Env Var:     RCLONE_LOCAL_NO_PREALLOCATE
+- Type:        bool
+- Default:     false
+
 #### --local-no-sparse
 
 Disable sparse files for multi-thread downloads