swift: add --swift-no-large-objects to reduce HEAD requests

Supplying the flag --swift-no-large-objects is a promise to the swift
backend that there are no dynamic or static large objects stored.

Using that knowledge rclone can speed its operations up reducing the
number of HEAD requests.

See: https://forum.rclone.org/t/handling-or-not-of-large-objects-in-swift/33389/
See: https://forum.rclone.org/t/swift-sync-checksum-calls-head-on-every-object-so-is-very-slow/30322

backend/swift/swift.go

@@ -63,6 +63,32 @@ Rclone will still chunk files bigger than chunk_size when doing normal
 copy operations.`,
 	Default:  false,
 	Advanced: true,
+}, {
+	Name: "no_large_objects",
+	Help: strings.ReplaceAll(`Disable support for static and dynamic large objects
+
+Swift cannot transparently store files bigger than 5 GiB. There are
+two schemes for doing that, static or dynamic large objects, and the
+API does not allow rclone to determine whether a file is a static or
+dynamic large object without doing a HEAD on the object. Since these
+need to be treated differently, this means rclone has to issue HEAD
+requests for objects for example when reading checksums.
+
+When |no_large_objects| is set, rclone will assume that there are no
+static or dynamic large objects stored. This means it can stop doing
+the extra HEAD calls which in turn increases performance greatly
+especially when doing a swift to swift transfer with |--checksum| set.
+
+Setting this option implies |no_chunk| and also that no files will be
+uploaded in chunks, so files bigger than 5 GiB will just fail on
+upload.
+
+If you set this option and there *are* static or dynamic large objects,
+then this will give incorrect hashes for them. Downloads will succeed,
+but other operations such as Remove and Copy will fail.
+`, "|", "`"),
+	Default:  false,
+	Advanced: true,
 }, {
 	Name:     config.ConfigEncoding,
 	Help:     config.ConfigEncodingHelp,
@@ -222,6 +248,7 @@ type Options struct {
 	EndpointType                string               `config:"endpoint_type"`
 	ChunkSize                   fs.SizeSuffix        `config:"chunk_size"`
 	NoChunk                     bool                 `config:"no_chunk"`
+	NoLargeObjects              bool                 `config:"no_large_objects"`
 	Enc                         encoder.MultiEncoder `config:"encoding"`
 }
 
@@ -1100,15 +1127,24 @@ func (o *Object) hasHeader(ctx context.Context, header string) (bool, error) {
 
 // isDynamicLargeObject checks for X-Object-Manifest header
 func (o *Object) isDynamicLargeObject(ctx context.Context) (bool, error) {
+	if o.fs.opt.NoLargeObjects {
+		return false, nil
+	}
 	return o.hasHeader(ctx, "X-Object-Manifest")
 }
 
 // isStaticLargeObjectFile checks for the X-Static-Large-Object header
 func (o *Object) isStaticLargeObject(ctx context.Context) (bool, error) {
+	if o.fs.opt.NoLargeObjects {
+		return false, nil
+	}
 	return o.hasHeader(ctx, "X-Static-Large-Object")
 }
 
 func (o *Object) isLargeObject(ctx context.Context) (result bool, err error) {
+	if o.fs.opt.NoLargeObjects {
+		return false, nil
+	}
 	result, err = o.hasHeader(ctx, "X-Static-Large-Object")
 	if result {
 		return
@@ -1464,7 +1500,7 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 	headers := m.ObjectHeaders()
 	fs.OpenOptionAddHeaders(options, headers)
 
-	if size > int64(o.fs.opt.ChunkSize) || (size == -1 && !o.fs.opt.NoChunk) {
+	if (size > int64(o.fs.opt.ChunkSize) || (size == -1 && !o.fs.opt.NoChunk)) && !o.fs.opt.NoLargeObjects {
 		_, err = o.updateChunks(ctx, in, headers, size, contentType)
 		if err != nil {
 			return err