internal/pkg: loadavg target in container environment

This exposes preferred loadavg target to the container initial process.

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

internal/pkg/pkg.go

@@ -281,6 +281,10 @@ func (c *common) GetMessage() message.Msg { return c.cache.msg }
 // value must not affect cure outcome.
 func (c *common) GetJobs() int { return c.cache.attr.Jobs }
 
+// GetLoad returns the preferred load average target, when applicable. Its
+// value must not affect cure outcome.
+func (c *common) GetLoad() int { return c.cache.attr.Load }
+
 // GetWorkDir returns a pathname to a directory which [Artifact] is expected to
 // write its output to. This is not the final resting place of the [Artifact]
 // and this pathname should not be directly referred to in the final contents.
@@ -2684,6 +2688,8 @@ type CacheAttr struct {
 	Flags int
 	// Preferred job count, when applicable.
 	Jobs int
+	// Preferred loadavg target, when applicable.
+	Load int
 
 	// Omit the [lockedfile] lock.
 	skipLock bool
@@ -2716,15 +2722,19 @@ func Open(
 		panic("attempting to open cache with incomplete variant setup")
 	}
 
-	if attr == nil {
-		attr = new(CacheAttr)
+	var a CacheAttr
+	if attr != nil {
+		a = *attr
 	}
 
-	if attr.Cures < 1 {
-		attr.Cures = runtime.NumCPU()
+	if a.Cures < 1 {
+		a.Cures = runtime.NumCPU()
 	}
-	if attr.Jobs < 1 {
-		attr.Jobs = runtime.NumCPU()
+	if a.Jobs < 1 {
+		a.Jobs = runtime.NumCPU()
+	}
+	if a.Load < 1 {
+		a.Load = runtime.NumCPU() + 2
 	}
 
 	for _, name := range []string{
@@ -2746,8 +2756,8 @@ func Open(
 	c := Cache{
 		parent: ctx,
 
-		cures: make(chan struct{}, attr.Cures),
-		attr:  *attr,
+		cures: make(chan struct{}, a.Cures),
+		attr:  a,
 
 		msg:  msg,
 		base: base,
@@ -2764,7 +2774,7 @@ func Open(
 	}
 	c.toplevel.Store(newToplevel(ctx))
 
-	if !attr.skipLock || !testing.Testing() {
+	if !a.skipLock || !testing.Testing() {
 		if unlock, err := lockedfile.MutexAt(
 			base.Append(fileLock).String(),
 		).Lock(); err != nil {
@@ -2825,7 +2835,7 @@ func Open(
 		}
 	} else if s := string(p); s == "" {
 		if extension != "" {
-			if attr.Flags&CPromoteVariant == 0 {
+			if a.Flags&CPromoteVariant == 0 {
 				c.unlock()
 				return nil, ErrWouldPromote
 			}