internal/pkg: job count in container environment

This exposes preferred job count to the container initial process.

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

internal/pkg/exec.go

@@ -27,6 +27,11 @@ import (
 // AbsWork is the container pathname [TContext.GetWorkDir] is mounted on.
 var AbsWork = fhs.AbsRoot.Append("work/")
 
+// EnvJobs is the name of the environment variable holding a decimal
+// representation of the preferred job count. Its value must not affect cure
+// outcome.
+const EnvJobs = "CURE_JOBS"
+
 // ExecPath is a slice of [Artifact] and the [check.Absolute] pathname to make
 // it available at under in the container.
 type ExecPath struct {
@@ -397,7 +402,7 @@ const SeccompPresets = std.PresetStrict &
 func (a *execArtifact) makeContainer(
 	ctx context.Context,
 	msg message.Msg,
-	flags int,
+	flags, jobs int,
 	hostNet bool,
 	temp, work *check.Absolute,
 	getArtifact GetArtifactFunc,
@@ -432,8 +437,8 @@ func (a *execArtifact) makeContainer(
 		z.Hostname = "cure-net"
 	}
 	z.Uid, z.Gid = (1<<10)-1, (1<<10)-1
-
-	z.Dir, z.Env, z.Path, z.Args = a.dir, a.env, a.path, a.args
+	z.Dir, z.Path, z.Args = a.dir, a.path, a.args
+	z.Env = slices.Concat(a.env, []string{EnvJobs + "=" + strconv.Itoa(jobs)})
 	z.Grow(len(a.paths) + 4)
 
 	for i, b := range a.paths {
@@ -563,6 +568,7 @@ func (c *Cache) EnterExec(
 	z, err = e.makeContainer(
 		ctx, c.msg,
 		c.flags,
+		c.jobs,
 		hostNet,
 		temp, work,
 		func(a Artifact) (*check.Absolute, unique.Handle[Checksum]) {
@@ -602,7 +608,7 @@ func (a *execArtifact) cure(f *FContext, hostNet bool) (err error) {
 	msg := f.GetMessage()
 	var z *container.Container
 	if z, err = a.makeContainer(
-		ctx, msg, f.cache.flags, hostNet,
+		ctx, msg, f.cache.flags, f.GetJobs(), hostNet,
 		f.GetTempDir(), f.GetWorkDir(),
 		f.GetArtifact,
 		f.cache.Ident,