Add support to toggle bandwidth limits via SIGUSR2.

Sending rclone a SIGUSR2 signal will toggle the limiter between off and
the limit set with the --bwlimit command-line option.

MANUAL.md

@@ -854,6 +854,12 @@ For example to limit bandwidth usage to 10 MBytes/s use `--bwlimit 10M`
 This only limits the bandwidth of the data transfer, it doesn't limit
 the bandwith of the directory listings etc.
 
+For Linux/Unix operating systems: rclone will toggle the bandwidth limiter on
+and off upon receipt of the SIGUSR2 signal. This feature allows the user to
+remove the bandwidth limitations of a long running rclone transfer during
+off-peak hours, and to restore it back to the value specified with --bwlimit
+again when needed.
+
 ### --checkers=N ###
 
 The number of checkers to run in parallel.  Checkers do the equality

fs/accounting.go

@@ -17,8 +17,10 @@ import (
 
 // Globals
 var (
-	Stats       = NewStats()
-	tokenBucket *tb.Bucket
+	Stats           = NewStats()
+	tokenBucket     *tb.Bucket
+	origTokenBucket = tokenBucket
+	prevTokenBucket = tokenBucket
 )
 
 // Start the token bucket if necessary
@@ -27,6 +29,12 @@ func startTokenBucket() {
 		tokenBucket = tb.NewBucket(int64(bwLimit), 100*time.Millisecond)
 		Log(nil, "Starting bandwidth limiter at %vBytes/s", &bwLimit)
 	}
+	origTokenBucket = tokenBucket
+	prevTokenBucket = tokenBucket
+
+	// Start the SIGUSR2 signal handler to toggle bandwidth.
+	// This function does nothing in windows systems.
+	startSignalHandler()
 }
 
 // stringSet holds a set of strings
@@ -333,6 +341,16 @@ func (acc *Account) read(in io.Reader, p []byte) (n int, err error) {
 
 	Stats.Bytes(int64(n))
 
+	// Log bandwidth limiter status change.
+	if tokenBucket != prevTokenBucket {
+		s := "disabled"
+		if tokenBucket != nil {
+			s = "enabled"
+		}
+		Log(nil, "Bandwidth limit %s by user", s)
+		prevTokenBucket = tokenBucket
+	}
+
 	// Limit the transfer speed if required
 	if tokenBucket != nil {
 		tokenBucket.Wait(int64(n))

fs/accounting_unix.go

@@ -0,0 +1,35 @@
+// Accounting and limiting reader
+// Unix specific functions.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd
+
+package fs
+
+import (
+	"os"
+	"os/signal"
+	"syscall"
+)
+
+// startSignalHandler() sets a signal handler to catch SIGUSR2 and toggle throttling.
+func startSignalHandler() {
+	// Don't do anything if no bandwidth limits requested.
+	if bwLimit <= 0 {
+		return
+	}
+
+	signals := make(chan os.Signal, 1)
+	signal.Notify(signals, syscall.SIGUSR2)
+
+	go func() {
+		// This runs forever, but blocks until the signal is received.
+		for {
+			<-signals
+			if tokenBucket == nil {
+				tokenBucket = origTokenBucket
+			} else {
+				tokenBucket = nil
+			}
+		}
+	}()
+}

fs/accounting_windows.go

@@ -0,0 +1,10 @@
+// Accounting and limiting reader
+// Windows specific functions.
+
+// +build windows
+
+package fs
+
+// startSignalHandler() is Unix specific and does nothing under windows
+// platforms.
+func startSignalHandler() {}