internal/pkg: pass impure job count

This is cleaner than checking cpu count during cure, it is impossible to avoid impurity in both situations but this is configurable.

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

internal/pkg/pkg.go

@@ -188,6 +188,10 @@ func (c *common) Unwrap() context.Context { return c.cache.ctx }
 // GetMessage returns [message.Msg] held by the underlying [Cache].
 func (c *common) GetMessage() message.Msg { return c.cache.msg }
 
+// GetJobs returns the preferred number of jobs to run, when applicable. Its
+// value must not affect cure outcome.
+func (c *common) GetJobs() int { return c.cache.jobs }
+
 // 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.
@@ -552,6 +556,8 @@ type Cache struct {
 	base *check.Absolute
 	// Immutable cure options set by [Open].
 	flags int
+	// Immutable job count, when applicable.
+	jobs int
 
 	// Artifact to [unique.Handle] of identifier cache.
 	artifact sync.Map
@@ -1818,7 +1824,7 @@ func (c *Cache) Close() {
 // caller-supplied value, however direct calls to [Cache.Cure] is not subject
 // to this limitation.
 //
-// A cures value of 0 or lower is equivalent to the value returned by
+// A cures or jobs value of 0 or lower is equivalent to the value returned by
 // [runtime.NumCPU].
 //
 // A successful call to Open guarantees exclusive access to the on-filesystem
@@ -1828,10 +1834,10 @@ func (c *Cache) Close() {
 func Open(
 	ctx context.Context,
 	msg message.Msg,
-	flags, cures int,
+	flags, cures, jobs int,
 	base *check.Absolute,
 ) (*Cache, error) {
-	return open(ctx, msg, flags, cures, base, true)
+	return open(ctx, msg, flags, cures, jobs, base, true)
 }
 
 // open implements Open but allows omitting the [lockedfile] lock when called
@@ -1839,13 +1845,16 @@ func Open(
 func open(
 	ctx context.Context,
 	msg message.Msg,
-	flags, cures int,
+	flags, cures, jobs int,
 	base *check.Absolute,
 	lock bool,
 ) (*Cache, error) {
 	if cures < 1 {
 		cures = runtime.NumCPU()
 	}
+	if jobs < 1 {
+		jobs = runtime.NumCPU()
+	}
 
 	for _, name := range []string{
 		dirIdentifier,
@@ -1862,6 +1871,7 @@ func open(
 	c := Cache{
 		cures: make(chan struct{}, cures),
 		flags: flags,
+		jobs:  jobs,
 
 		msg:  msg,
 		base: base,