internal/pkg: exclusive artifacts

This alleviates scheduler overhead when curing many artifacts.

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

internal/pkg/pkg.go

@@ -253,6 +253,24 @@ type Artifact interface {
 	//
 	// Result must remain identical across multiple invocations.
 	Dependencies() []Artifact
+
+	// IsExclusive returns whether the [Artifact] is exclusive. Exclusive
+	// artifacts might not run in parallel with each other, and are still
+	// subject to the cures limit.
+	//
+	// Some implementations may saturate the CPU for a nontrivial amount of
+	// time. Curing multiple such implementations simultaneously causes
+	// significant CPU scheduler overhead. An exclusive artifact will generally
+	// not be cured alongside another exclusive artifact, thus alleviating this
+	// overhead.
+	//
+	// Note that [Cache] reserves the right to still cure exclusive
+	// artifacts concurrently as this is not a synchronisation primitive but
+	// an optimisation one. Implementations are forbidden from accessing global
+	// state regardless of exclusivity.
+	//
+	// Result must remain identical across multiple invocations.
+	IsExclusive() bool
 }
 
 // FloodArtifact refers to an [Artifact] requiring its entire dependency graph
@@ -472,6 +490,8 @@ type Cache struct {
 	// Synchronises access to ident and corresponding filesystem entries.
 	identMu sync.RWMutex
 
+	// Synchronises entry into exclusive artifacts for the cure method.
+	exclMu sync.Mutex
 	// Buffered I/O free list, must not be accessed directly.
 	bufioPool sync.Pool
 
@@ -1215,7 +1235,10 @@ func (e *DependencyCureError) Error() string {
 }
 
 // enterCure must be called before entering an [Artifact] implementation.
-func (c *Cache) enterCure(curesExempt bool) error {
+func (c *Cache) enterCure(a Artifact, curesExempt bool) error {
+	if a.IsExclusive() {
+		c.exclMu.Lock()
+	}
 	if curesExempt {
 		return nil
 	}
@@ -1225,15 +1248,23 @@ func (c *Cache) enterCure(curesExempt bool) error {
 		return nil
 
 	case <-c.ctx.Done():
+		if a.IsExclusive() {
+			c.exclMu.Unlock()
+		}
 		return c.ctx.Err()
 	}
 }
 
 // exitCure must be called after exiting an [Artifact] implementation.
-func (c *Cache) exitCure(curesExempt bool) {
-	if !curesExempt {
-		<-c.cures
+func (c *Cache) exitCure(a Artifact, curesExempt bool) {
+	if a.IsExclusive() {
+		c.exclMu.Unlock()
 	}
+	if curesExempt {
+		return
+	}
+
+	<-c.cures
 }
 
 // getWriter is like [bufio.NewWriter] but for bufioPool.
@@ -1456,7 +1487,7 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 		}()
 
 		var r io.ReadCloser
-		if err = c.enterCure(curesExempt); err != nil {
+		if err = c.enterCure(a, curesExempt); err != nil {
 			return
 		}
 		r, err = f.Cure(&RContext{c})
@@ -1505,7 +1536,7 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 				err = closeErr
 			}
 		}
-		c.exitCure(curesExempt)
+		c.exitCure(a, curesExempt)
 		if err != nil {
 			return
 		}
@@ -1539,11 +1570,11 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 	switch ca := a.(type) {
 	case TrivialArtifact:
 		defer t.destroy(&err)
-		if err = c.enterCure(curesExempt); err != nil {
+		if err = c.enterCure(a, curesExempt); err != nil {
 			return
 		}
 		err = ca.Cure(&t)
-		c.exitCure(curesExempt)
+		c.exitCure(a, curesExempt)
 		if err != nil {
 			return
 		}
@@ -1573,11 +1604,11 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 		}
 
 		defer f.destroy(&err)
-		if err = c.enterCure(curesExempt); err != nil {
+		if err = c.enterCure(a, curesExempt); err != nil {
 			return
 		}
 		err = ca.Cure(&f)
-		c.exitCure(curesExempt)
+		c.exitCure(a, curesExempt)
 		if err != nil {
 			return
 		}