s3,swift: Add --use-server-modtime

`--use-server-modtime` stops s3 and swift retrieving the modtime from metadata which enables a fast sync mode with the `--update` flag.

backend/s3/s3.go

@@ -945,6 +945,9 @@ func (o *Object) readMetaData() (err error) {
 // It attempts to read the objects mtime and if that isn't present the
 // LastModified returned in the http headers
 func (o *Object) ModTime() time.Time {
+	if fs.Config.UseServerModTime {
+		return o.lastModified
+	}
 	err := o.readMetaData()
 	if err != nil {
 		fs.Logf(o, "Failed to read metadata: %v", err)

backend/swift/swift.go

@@ -719,6 +719,9 @@ func (o *Object) readMetaData() (err error) {
 // It attempts to read the objects mtime and if that isn't present the
 // LastModified returned in the http headers
 func (o *Object) ModTime() time.Time {
+	if fs.Config.UseServerModTime {
+		return o.info.LastModified
+	}
 	err := o.readMetaData()
 	if err != nil {
 		fs.Debugf(o, "Failed to read metadata: %s", err)

docs/content/docs.md

@@ -799,6 +799,19 @@ This can be useful when transferring to a remote which doesn't support
 mod times directly as it is more accurate than a `--size-only` check
 and faster than using `--checksum`.
 
+### --use-server-modtime ###
+
+Some object-store backends (e.g, Swift, S3) do not preserve file modification
+times (modtime). On these backends, rclone stores the original modtime as
+additional metadata on the object. By default it will make an API call to
+retrieve the metadata when the modtime is needed by an operation.
+
+Use this flag to disable the extra API call and rely instead on the server's
+modified time. In cases such as a local to remote sync, knowing the local file
+is newer than the time it was last uploaded to the remote is sufficient. In
+those cases, this flag can speed up the process and reduce the number of API
+calls necessary.
+
 ### -v, -vv, --verbose ###
 
 With `-v` rclone will tell you about each file that is transferred and

docs/content/s3.md

@@ -213,6 +213,19 @@ This remote supports `--fast-list` which allows you to use fewer
 transactions in exchange for more memory. See the [rclone
 docs](/docs/#fast-list) for more details.
 
+### --update and --use-server-modtime ###
+
+As noted below, the modified time is stored on metadata on the object. It is
+used by default for all operations that require checking the time a file was
+last updated. It allows rclone to treat the remote more like a true filesystem,
+but it is inefficient because it requires an extra API call to retrieve the
+metadata.
+
+For many operations, the time the object was last uploaded to the remote is
+sufficient to determine if it is "dirty". By using `--update` along with
+`--use-server-modtime`, you can avoid the extra API call and simply upload
+files whose local modtime is newer than the time it was last uploaded.
+
 ### Modified time ###
 
 The modified time is stored as metadata on the object as

docs/content/swift.md

@@ -248,6 +248,19 @@ This remote supports `--fast-list` which allows you to use fewer
 transactions in exchange for more memory. See the [rclone
 docs](/docs/#fast-list) for more details.
 
+### --update and --use-server-modtime ###
+
+As noted below, the modified time is stored on metadata on the object. It is
+used by default for all operations that require checking the time a file was
+last updated. It allows rclone to treat the remote more like a true filesystem,
+but it is inefficient because it requires an extra API call to retrieve the
+metadata.
+
+For many operations, the time the object was last uploaded to the remote is
+sufficient to determine if it is "dirty". By using `--update` along with
+`--use-server-modtime`, you can avoid the extra API call and simply upload
+files whose local modtime is newer than the time it was last uploaded.
+
 ### Specific options ###
 
 Here are the command line options specific to this cloud storage

fs/config.go

@@ -67,6 +67,7 @@ type ConfigInfo struct {
 	StreamingUploadCutoff SizeSuffix
 	StatsFileNameLength   int
 	AskPassword           bool
+	UseServerModTime      bool
 }
 
 // NewConfig creates a new config with everything set to the default

fs/config/configflags/configflags.go

@@ -58,6 +58,7 @@ func AddFlags(flagSet *pflag.FlagSet) {
 	flags.BoolVarP(flagSet, &fs.Config.TrackRenames, "track-renames", "", fs.Config.TrackRenames, "When synchronizing, track file renames and do a server side move if possible")
 	flags.IntVarP(flagSet, &fs.Config.LowLevelRetries, "low-level-retries", "", fs.Config.LowLevelRetries, "Number of low level retries to do.")
 	flags.BoolVarP(flagSet, &fs.Config.UpdateOlder, "update", "u", fs.Config.UpdateOlder, "Skip files that are newer on the destination.")
+	flags.BoolVarP(flagSet, &fs.Config.UseServerModTime, "use-server-modtime", "", fs.Config.UseServerModTime, "Use server modified time instead of object metadata")
 	flags.BoolVarP(flagSet, &fs.Config.NoGzip, "no-gzip-encoding", "", fs.Config.NoGzip, "Don't set Accept-Encoding: gzip.")
 	flags.IntVarP(flagSet, &fs.Config.MaxDepth, "max-depth", "", fs.Config.MaxDepth, "If set limits the recursion depth to this.")
 	flags.BoolVarP(flagSet, &fs.Config.IgnoreSize, "ignore-size", "", false, "Ignore size when skipping use mod-time or checksum.")