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.

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
+			}
+		}
+	}()
+}