internal/pkg: exclusive artifacts

This alleviates scheduler overhead when curing many artifacts.

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

7 files changed, 79 insertions, 29 deletions

diff --git a/internal/pkg/exec.go b/internal/pkg/exec.go
index 3d3f0b65..0c43cff0 100644
--- a/internal/pkg/exec.go
+++ b/internal/pkg/exec.go
@@ -105,6 +105,9 @@ type execArtifact struct {
 	// equivalent to execTimeoutDefault. This value is never encoded in Params
 	// because it cannot affect outcome.
 	timeout time.Duration
+
+	// Caller-supplied exclusivity value, returned as is by IsExclusive.
+	exclusive bool
 }
 
 var _ fmt.Stringer = new(execArtifact)
@@ -123,7 +126,7 @@ var _ KnownChecksum = new(execNetArtifact)
 func (a *execNetArtifact) Checksum() Checksum { return a.checksum }
 
 // Kind returns the hardcoded [Kind] constant.
-func (a *execNetArtifact) Kind() Kind { return KindExecNet }
+func (*execNetArtifact) Kind() Kind { return KindExecNet }
 
 // Params is [Checksum] concatenated with [KindExec] params.
 func (a *execNetArtifact) Params(ctx *IContext) {
@@ -157,13 +160,14 @@ func (a *execNetArtifact) Cure(f *FContext) error {
 // negative timeout value is equivalent tp [ExecTimeoutDefault], a timeout value
 // greater than [ExecTimeoutMax] is equivalent to [ExecTimeoutMax].
 //
-// The user-facing name is not accessible from the container and does not
-// affect curing outcome. Because of this, it is omitted from parameter data
-// for computing identifier.
+// The user-facing name and exclusivity value are not accessible from the
+// container and does not affect curing outcome. Because of this, it is omitted
+// from parameter data for computing identifier.
 func NewExec(
 	name string,
 	checksum *Checksum,
 	timeout time.Duration,
+	exclusive bool,
 
 	dir *check.Absolute,
 	env []string,
@@ -181,7 +185,7 @@ func NewExec(
 	if timeout > ExecTimeoutMax {
 		timeout = ExecTimeoutMax
 	}
-	a := execArtifact{name, paths, dir, env, pathname, args, timeout}
+	a := execArtifact{name, paths, dir, env, pathname, args, timeout, exclusive}
 	if checksum == nil {
 		return &a
 	}
@@ -189,7 +193,7 @@ func NewExec(
 }
 
 // Kind returns the hardcoded [Kind] constant.
-func (a *execArtifact) Kind() Kind { return KindExec }
+func (*execArtifact) Kind() Kind { return KindExec }
 
 // Params writes paths, executable pathname and args.
 func (a *execArtifact) Params(ctx *IContext) {
@@ -237,6 +241,9 @@ func (a *execArtifact) Dependencies() []Artifact {
 	return slices.Concat(artifacts...)
 }
 
+// IsExclusive returns the caller-supplied exclusivity value.
+func (a *execArtifact) IsExclusive() bool { return a.exclusive }
+
 // String returns the caller-supplied reporting name.
 func (a *execArtifact) String() string { return a.name }
 
diff --git a/internal/pkg/exec_test.go b/internal/pkg/exec_test.go
index 862a28ca..1c6f8e51 100644
--- a/internal/pkg/exec_test.go
+++ b/internal/pkg/exec_test.go
@@ -39,7 +39,7 @@ func TestExec(t *testing.T) {
 
 			cureMany(t, c, []cureStep{
 				{"container", pkg.NewExec(
-					"exec-offline", nil, 0,
+					"exec-offline", nil, 0, false,
 					pkg.AbsWork,
 					[]string{"HAKUREI_TEST=1"},
 					check.MustAbs("/opt/bin/testtool"),
@@ -62,7 +62,7 @@ func TestExec(t *testing.T) {
 				), ignorePathname, wantChecksumOffline, nil},
 
 				{"error passthrough", pkg.NewExec(
-					"", nil, 0,
+					"", nil, 0, true,
 					pkg.AbsWork,
 					[]string{"HAKUREI_TEST=1"},
 					check.MustAbs("/opt/bin/testtool"),
@@ -85,7 +85,7 @@ func TestExec(t *testing.T) {
 				}},
 
 				{"invalid paths", pkg.NewExec(
-					"", nil, 0,
+					"", nil, 0, false,
 					pkg.AbsWork,
 					[]string{"HAKUREI_TEST=1"},
 					check.MustAbs("/opt/bin/testtool"),
@@ -98,7 +98,7 @@ func TestExec(t *testing.T) {
 			// check init failure passthrough
 			var exitError *exec.ExitError
 			if _, _, err := c.Cure(pkg.NewExec(
-				"", nil, 0,
+				"", nil, 0, false,
 				pkg.AbsWork,
 				nil,
 				check.MustAbs("/opt/bin/testtool"),
@@ -120,7 +120,7 @@ func TestExec(t *testing.T) {
 			)
 			cureMany(t, c, []cureStep{
 				{"container", pkg.NewExec(
-					"exec-net", &wantChecksum, 0,
+					"exec-net", &wantChecksum, 0, false,
 					pkg.AbsWork,
 					[]string{"HAKUREI_TEST=1"},
 					check.MustAbs("/opt/bin/testtool"),
@@ -152,7 +152,7 @@ func TestExec(t *testing.T) {
 
 			cureMany(t, c, []cureStep{
 				{"container", pkg.NewExec(
-					"exec-overlay-root", nil, 0,
+					"exec-overlay-root", nil, 0, false,
 					pkg.AbsWork,
 					[]string{"HAKUREI_TEST=1", "HAKUREI_ROOT=1"},
 					check.MustAbs("/opt/bin/testtool"),
@@ -178,7 +178,7 @@ func TestExec(t *testing.T) {
 
 			cureMany(t, c, []cureStep{
 				{"container", pkg.NewExec(
-					"exec-overlay-work", nil, 0,
+					"exec-overlay-work", nil, 0, false,
 					pkg.AbsWork,
 					[]string{"HAKUREI_TEST=1", "HAKUREI_ROOT=1"},
 					check.MustAbs("/work/bin/testtool"),
@@ -209,7 +209,7 @@ func TestExec(t *testing.T) {
 
 			cureMany(t, c, []cureStep{
 				{"container", pkg.NewExec(
-					"exec-multiple-layers", nil, 0,
+					"exec-multiple-layers", nil, 0, false,
 					pkg.AbsWork,
 					[]string{"HAKUREI_TEST=1", "HAKUREI_ROOT=1"},
 					check.MustAbs("/opt/bin/testtool"),
@@ -262,7 +262,7 @@ func TestExec(t *testing.T) {
 
 			cureMany(t, c, []cureStep{
 				{"container", pkg.NewExec(
-					"exec-layer-promotion", nil, 0,
+					"exec-layer-promotion", nil, 0, true,
 					pkg.AbsWork,
 					[]string{"HAKUREI_TEST=1", "HAKUREI_ROOT=1"},
 					check.MustAbs("/opt/bin/testtool"),
diff --git a/internal/pkg/file.go b/internal/pkg/file.go
index 9e075cd5..c2093d6b 100644
--- a/internal/pkg/file.go
+++ b/internal/pkg/file.go
@@ -37,13 +37,16 @@ func NewFile(name string, data []byte) FileArtifact {
 }
 
 // Kind returns the hardcoded [Kind] constant.
-func (a *fileArtifact) Kind() Kind { return KindFile }
+func (*fileArtifact) Kind() Kind { return KindFile }
 
 // Params writes the result of Cure.
 func (a *fileArtifact) Params(ctx *IContext) { ctx.GetHash().Write(*a) }
 
 // Dependencies returns a nil slice.
-func (a *fileArtifact) Dependencies() []Artifact { return nil }
+func (*fileArtifact) Dependencies() []Artifact { return nil }
+
+// IsExclusive returns false: Cure returns a prepopulated buffer.
+func (*fileArtifact) IsExclusive() bool { return false }
 
 // Checksum computes and returns the checksum of caller-supplied data.
 func (a *fileArtifact) Checksum() Checksum {
diff --git a/internal/pkg/net.go b/internal/pkg/net.go
index 00c2bc61..7e35cd8e 100644
--- a/internal/pkg/net.go
+++ b/internal/pkg/net.go
@@ -40,7 +40,7 @@ func NewHTTPGet(
 }
 
 // Kind returns the hardcoded [Kind] constant.
-func (a *httpArtifact) Kind() Kind { return KindHTTPGet }
+func (*httpArtifact) Kind() Kind { return KindHTTPGet }
 
 // Params writes the backing url string. Client is not represented as it does
 // not affect [Cache.Cure] outcome.
@@ -49,7 +49,10 @@ func (a *httpArtifact) Params(ctx *IContext) {
 }
 
 // Dependencies returns a nil slice.
-func (a *httpArtifact) Dependencies() []Artifact { return nil }
+func (*httpArtifact) Dependencies() []Artifact { return nil }
+
+// IsExclusive returns false: Cure returns as soon as a response is received.
+func (*httpArtifact) IsExclusive() bool { return false }
 
 // Checksum returns the caller-supplied checksum.
 func (a *httpArtifact) Checksum() Checksum { return a.checksum.Value() }
diff --git a/internal/pkg/pkg.go b/internal/pkg/pkg.go
index b3b08510..15114fd7 100644
--- a/internal/pkg/pkg.go
+++ b/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
 		}
diff --git a/internal/pkg/pkg_test.go b/internal/pkg/pkg_test.go
index fe618a6a..d791f107 100644
--- a/internal/pkg/pkg_test.go
+++ b/internal/pkg/pkg_test.go
@@ -96,12 +96,14 @@ func (a *stubArtifact) Kind() pkg.Kind               { return a.kind }
 func (a *stubArtifact) Params(ctx *pkg.IContext)     { ctx.GetHash().Write(a.params) }
 func (a *stubArtifact) Dependencies() []pkg.Artifact { return a.deps }
 func (a *stubArtifact) Cure(t *pkg.TContext) error   { return a.cure(t) }
+func (*stubArtifact) IsExclusive() bool              { return false }
 
 // A stubArtifactF implements [FloodArtifact] with hardcoded behaviour.
 type stubArtifactF struct {
 	kind   pkg.Kind
 	params []byte
 	deps   []pkg.Artifact
+	excl   bool
 
 	cure func(f *pkg.FContext) error
 }
@@ -110,6 +112,7 @@ func (a *stubArtifactF) Kind() pkg.Kind               { return a.kind }
 func (a *stubArtifactF) Params(ctx *pkg.IContext)     { ctx.GetHash().Write(a.params) }
 func (a *stubArtifactF) Dependencies() []pkg.Artifact { return a.deps }
 func (a *stubArtifactF) Cure(f *pkg.FContext) error   { return a.cure(f) }
+func (a *stubArtifactF) IsExclusive() bool            { return a.excl }
 
 // A stubFile implements [FileArtifact] with hardcoded behaviour.
 type stubFile struct {
diff --git a/internal/pkg/tar.go b/internal/pkg/tar.go
index c56176b1..474df034 100644
--- a/internal/pkg/tar.go
+++ b/internal/pkg/tar.go
@@ -80,6 +80,9 @@ func (a *tarArtifact) Dependencies() []Artifact {
 	return []Artifact{a.f}
 }
 
+// IsExclusive returns false: decompressor and tar reader are fully sequential.
+func (a *tarArtifact) IsExclusive() bool { return false }
+
 // A DisallowedTypeflagError describes a disallowed typeflag encountered while
 // unpacking a tarball.
 type DisallowedTypeflagError byte