cmd/nix-tool: flag for idle priority

Mainly for usability. This is often ran on a workstation.

cmd/nix-tool/main.go

@@ -9,6 +9,7 @@ import (
 	"log"
 	"os"
 	"os/signal"
+	"runtime"
 	"slices"
 	"strings"
 	"syscall"
@@ -29,6 +30,7 @@ func main() {
 
 	var (
 		flagStore   string
+		flagIdle    bool
 		flagNixOS   bool
 		flagVerbose bool
 		flagJSON    bool
@@ -36,6 +38,13 @@ func main() {
 	c := command.New(os.Stderr, log.Printf, "nix-tool", func(args []string) error {
 		log.SetFlags(0)
 
+		if flagIdle {
+			runtime.LockOSThread()
+			if err := schedSetPolicy(0, SCHED_IDLE); err != nil {
+				return commandHandlerError(fmt.Sprintf("cannot set policy: %v", err))
+			}
+		}
+
 		var extraArgs []string
 		flagStore = strings.TrimSpace(flagStore)
 		if flagStore != string(os.PathSeparator) {
@@ -58,6 +67,7 @@ func main() {
 		return nil
 	}).
 		Flag(&flagStore, "store", command.StringFlag("/"), "Path to the nix root").
+		Flag(&flagIdle, "idle", command.BoolFlag(false), "Whether to set SCHED_IDLE policy").
 		Flag(&flagNixOS, "nixos", command.BoolFlag(false), "Interpret input as NixOS flake installable").
 		Flag(&flagVerbose, "v", command.BoolFlag(false), "Connect nix stderr").
 		Flag(&flagJSON, "json", command.BoolFlag(false), "Serialise output in JSON when applicable")

cmd/nix-tool/sched.go

@@ -0,0 +1,25 @@
+package main
+
+/*
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE // SCHED_IDLE
+#endif
+
+#include <sched.h>
+
+const struct sched_param param = { .sched_priority = 0 };
+const int NIX_TOOL_SCHED_IDLE = SCHED_IDLE;
+*/
+import "C"
+
+var (
+	SCHED_IDLE = int(C.NIX_TOOL_SCHED_IDLE)
+)
+
+func schedSetPolicy(pid, policy int) error {
+	r, err := C.sched_setscheduler(C.pid_t(pid), C.int(policy), &C.param)
+	if r != 0 {
+		return err
+	}
+	return nil
+}