lib/osutil: Don't attempt to reduce our niceness level (fixes #4681)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4682

lib/osutil/lowprio_linux.go

@@ -42,23 +42,52 @@ func ioprioSet(class ioprioClass, value int) error {
 // SetLowPriority lowers the process CPU scheduling priority, and possibly
 // I/O priority depending on the platform and OS.
 func SetLowPriority() error {
+	// Process zero is "self", niceness value 9 is something between 0
+	// (default) and 19 (worst priority). But then, this is Linux, so of
+	// course we get this to take care of as well:
+	//
+	// "C library/kernel differences
+	//
+	// Within  the  kernel,  nice  values are actually represented using the
+	// range 40..1 (since negative numbers are error codes) and  these  are
+	// the  values employed  by  the  setpriority() and getpriority() system
+	// calls.  The glibc wrapper functions for these system calls handle the
+	// translations  between the user-land and kernel representations of the
+	// nice value according to the formula unice = 20 - knice. (Thus, the
+	// kernel's 40..1 range corresponds to the range -20..19 as seen by user
+	// space.)"
+
+	const (
+		pidSelf       = 0
+		wantNiceLevel = 20 - 9
+	)
+
+	// Remember Linux kernel nice levels are upside down.
+	if cur, err := syscall.Getpriority(syscall.PRIO_PROCESS, 0); err == nil && cur <= wantNiceLevel {
+		// We're done here.
+		return nil
+	}
+
 	// Move ourselves to a new process group so that we can use the process
 	// group variants of Setpriority etc to affect all of our threads in one
 	// go. If this fails, bail, so that we don't affect things we shouldn't.
 	// If we are already the leader of our own process group, do nothing.
-	if pgid, err := syscall.Getpgid(0); err != nil {
+	//
+	// Oh and this is because Linux doesn't follow the POSIX threading model
+	// where setting the niceness of the process would actually set the
+	// niceness of the process, instead it just affects the current thread
+	// so we need this workaround...
+	if pgid, err := syscall.Getpgid(pidSelf); err != nil {
 		// This error really shouldn't happen
 		return errors.Wrap(err, "get process group")
 	} else if pgid != os.Getpid() {
 		// We are not process group leader. Elevate!
-		if err := syscall.Setpgid(0, 0); err != nil {
+		if err := syscall.Setpgid(pidSelf, 0); err != nil {
 			return errors.Wrap(err, "set process group")
 		}
 	}
 
-	// Process zero is "self", niceness value 9 is something between 0
-	// (default) and 19 (worst priority).
-	if err := syscall.Setpriority(syscall.PRIO_PGRP, 0, 9); err != nil {
+	if err := syscall.Setpriority(syscall.PRIO_PGRP, pidSelf, wantNiceLevel); err != nil {
 		return errors.Wrap(err, "set niceness")
 	}
 

lib/osutil/lowprio_unix.go

@@ -19,6 +19,16 @@ import (
 func SetLowPriority() error {
 	// Process zero is "self", niceness value 9 is something between 0
 	// (default) and 19 (worst priority).
-	err := syscall.Setpriority(syscall.PRIO_PROCESS, 0, 9)
+	const (
+		pidSelf       = 0
+		wantNiceLevel = 9
+	)
+
+	if cur, err := syscall.Getpriority(syscall.PRIO_PROCESS, pidSelf); err == nil && cur >= wantNiceLevel {
+		// We're done here.
+		return nil
+	}
+
+	err := syscall.Setpriority(syscall.PRIO_PROCESS, pidSelf, wantNiceLevel)
 	return errors.Wrap(err, "set niceness") // wraps nil as nil
 }