container: sched policy string representation

This also uses priority obtained via sched_get_priority_min, and improves bounds checking.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/syscall.go

@@ -1,6 +1,9 @@
 package container
 
 import (
+	"encoding"
+	"strconv"
+	"sync"
 	. "syscall"
 	"unsafe"
 
@@ -43,18 +46,132 @@ func Isatty(fd int) bool {
 	return r == 0
 }
 
+// SchedPolicy denotes a scheduling policy defined in include/uapi/linux/sched.h.
+type SchedPolicy int
+
 // include/uapi/linux/sched.h
 const (
-	SCHED_NORMAL = iota
+	SCHED_NORMAL SchedPolicy = iota
 	SCHED_FIFO
 	SCHED_RR
 	SCHED_BATCH
-	_ // SCHED_ISO: reserved but not implemented yet
+	_SCHED_ISO // SCHED_ISO: reserved but not implemented yet
 	SCHED_IDLE
 	SCHED_DEADLINE
 	SCHED_EXT
+
+	_SCHED_LAST SchedPolicy = iota - 1
 )
 
+var _ encoding.TextMarshaler = _SCHED_LAST
+var _ encoding.TextUnmarshaler = new(SchedPolicy)
+
+// String returns a unique representation of policy, also used in encoding.
+func (policy SchedPolicy) String() string {
+	switch policy {
+	case SCHED_NORMAL:
+		return ""
+	case SCHED_FIFO:
+		return "fifo"
+	case SCHED_RR:
+		return "rr"
+	case SCHED_BATCH:
+		return "batch"
+	case SCHED_IDLE:
+		return "idle"
+	case SCHED_DEADLINE:
+		return "deadline"
+	case SCHED_EXT:
+		return "ext"
+
+	default:
+		return "invalid policy " + strconv.Itoa(int(policy))
+	}
+}
+
+// MarshalText performs bounds checking and returns the result of String.
+func (policy SchedPolicy) MarshalText() ([]byte, error) {
+	if policy == _SCHED_ISO || policy < 0 || policy > _SCHED_LAST {
+		return nil, EINVAL
+	}
+	return []byte(policy.String()), nil
+}
+
+// InvalidSchedPolicyError is an invalid string representation of a [SchedPolicy].
+type InvalidSchedPolicyError string
+
+func (InvalidSchedPolicyError) Unwrap() error { return EINVAL }
+func (e InvalidSchedPolicyError) Error() string {
+	return "invalid scheduling policy " + strconv.Quote(string(e))
+}
+
+// UnmarshalText is the inverse of MarshalText.
+func (policy *SchedPolicy) UnmarshalText(text []byte) error {
+	switch string(text) {
+	case "fifo":
+		*policy = SCHED_FIFO
+	case "rr":
+		*policy = SCHED_RR
+	case "batch":
+		*policy = SCHED_BATCH
+	case "idle":
+		*policy = SCHED_IDLE
+	case "deadline":
+		*policy = SCHED_DEADLINE
+	case "ext":
+		*policy = SCHED_EXT
+
+	case "":
+		*policy = 0
+		return nil
+	default:
+		return InvalidSchedPolicyError(text)
+	}
+	return nil
+}
+
+// for sched_get_priority_max and sched_get_priority_min
+var (
+	schedPriority     [_SCHED_LAST + 1][2]std.Int
+	schedPriorityErr  [_SCHED_LAST + 1][2]error
+	schedPriorityOnce [_SCHED_LAST + 1][2]sync.Once
+)
+
+// GetPriorityMax returns the maximum priority value that can be used with the
+// scheduling algorithm identified by policy.
+func (policy SchedPolicy) GetPriorityMax() (std.Int, error) {
+	schedPriorityOnce[policy][0].Do(func() {
+		priority, _, errno := Syscall(
+			SYS_SCHED_GET_PRIORITY_MAX,
+			uintptr(policy),
+			0, 0,
+		)
+		schedPriority[policy][0] = std.Int(priority)
+		if schedPriority[policy][0] < 0 {
+			schedPriorityErr[policy][0] = errno
+		}
+	})
+	return schedPriority[policy][0], schedPriorityErr[policy][0]
+}
+
+// GetPriorityMin returns the minimum priority value that can be used with the
+// scheduling algorithm identified by policy.
+func (policy SchedPolicy) GetPriorityMin() (std.Int, error) {
+	schedPriorityOnce[policy][1].Do(func() {
+		priority, _, errno := Syscall(
+			SYS_SCHED_GET_PRIORITY_MIN,
+			uintptr(policy),
+			0, 0,
+		)
+		schedPriority[policy][1] = std.Int(priority)
+		if schedPriority[policy][1] < 0 {
+			schedPriorityErr[policy][1] = errno
+		}
+	})
+	return schedPriority[policy][1], schedPriorityErr[policy][1]
+
+}
+
 // schedParam is equivalent to struct sched_param from include/linux/sched.h.
 type schedParam struct {
 	// sched_priority
@@ -74,7 +191,7 @@ type schedParam struct {
 // this if you do not have something similar in place!
 //
 // [very subtle to use correctly]: https://www.openwall.com/lists/musl/2016/03/01/4
-func schedSetscheduler(tid, policy int, param *schedParam) error {
+func schedSetscheduler(tid int, policy SchedPolicy, param *schedParam) error {
 	if r, _, errno := Syscall(
 		SYS_SCHED_SETSCHEDULER,
 		uintptr(tid),