internal/pkg: optionally exempt implementations from cures counter

This avoids holding up many slots with a long pipeline.

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

internal/pkg/archive.go

@@ -278,6 +278,8 @@ type archiveArtifact struct {
 	f Artifact
 }
 
+var _ CuresExempt = archiveArtifact{}
+
 // NewArchive returns a new [Artifact] backed by the supplied [Artifact]. The
 // source [Artifact] must be a [FileArtifact] and produce a stream compatible
 // with [Reader].
@@ -403,3 +405,7 @@ func (a archiveArtifact) Cure(t *TContext) (err error) {
 	}
 	return
 }
+
+// CuresExempt exempts the cheap [KindArchive] implementation often found at
+// the end of a [FileArtifact] pipeline.
+func (archiveArtifact) CuresExempt() {}

internal/pkg/compress.go

@@ -25,6 +25,7 @@ type decompressArtifact struct {
 }
 
 var _ FileArtifact = new(decompressArtifact)
+var _ CuresExempt = new(decompressArtifact)
 
 // decompressArtifactNamed embeds decompressArtifact for a [fmt.Stringer] stream.
 type decompressArtifactNamed struct {
@@ -117,3 +118,7 @@ func (a *decompressArtifact) Cure(r *RContext) (io.ReadCloser, error) {
 		return nil, os.ErrInvalid
 	}
 }
+
+// CuresExempt exempts the cheap [KindDecompress] implementation often part of
+// a [FileArtifact] pipeline.
+func (*decompressArtifact) CuresExempt() {}

internal/pkg/file.go

@@ -11,6 +11,7 @@ import (
 type fileArtifact []byte
 
 var _ KnownChecksum = new(fileArtifact)
+var _ CuresExempt = new(fileArtifact)
 
 // fileArtifactNamed embeds fileArtifact alongside a caller-supplied name.
 type fileArtifactNamed struct {
@@ -79,3 +80,6 @@ func (a *fileArtifact) Checksum() Checksum {
 func (a *fileArtifact) Cure(*RContext) (io.ReadCloser, error) {
 	return io.NopCloser(bytes.NewReader(*a)), nil
 }
+
+// CuresExempt exempts the cheap [KindFile] implementation.
+func (*fileArtifact) CuresExempt() {}

internal/pkg/pkg.go

@@ -485,6 +485,16 @@ type KnownChecksum interface {
 	Checksum() Checksum
 }
 
+// CuresExempt is optionally implemented for an artifact exempt to the
+// cache-wide cures counter and limit.
+type CuresExempt interface {
+	Artifact
+
+	// CuresExempt is a no-op function but serves to distinguish implementations
+	// that are cures-exempt.
+	CuresExempt()
+}
+
 // FileArtifact refers to an [Artifact] backed by a single file.
 //
 // FileArtifact does not support fine-grained cancellation. Its context is
@@ -1892,6 +1902,10 @@ func (c *Cache) cure(a Artifact, curesExempt bool) (
 		}
 	}()
 
+	if _, ok := a.(CuresExempt); ok {
+		curesExempt = true
+	}
+
 	var (
 		ctx  context.Context
 		done chan<- struct{}

internal/pkg/tar.go

@@ -16,6 +16,8 @@ type tarArtifact struct {
 	f Artifact
 }
 
+var _ CuresExempt = new(tarArtifact)
+
 // tarArtifactNamed embeds tarArtifact for a [fmt.Stringer] tarball.
 type tarArtifactNamed struct {
 	tarArtifact
@@ -211,3 +213,7 @@ func (a *tarArtifact) Cure(t *TContext) (err error) {
 	}
 	return
 }
+
+// CuresExempt exempts the cheap [KindTar] implementation often at the end of a
+// [FileArtifact] pipeline.
+func (*tarArtifact) CuresExempt() {}