internal/pkg: optional executable files

This enables a FileArtifact implementation to specify whether its resulting file should be executable. This change also implements shebang and ELF magic checks in the file artifact.

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

internal/pkg/pkg.go

@@ -478,6 +478,8 @@ type KnownIdent interface {
 	// [Artifact]. This value must be known ahead of time and guaranteed to be
 	// unique without having obtained the full contents of the [Artifact].
 	ID() ID
+
+	Artifact
 }
 
 // KnownChecksum is optionally implemented by [Artifact] for an artifact with
@@ -489,6 +491,8 @@ type KnownChecksum interface {
 	//
 	// Result must remain identical across multiple invocations.
 	Checksum() Checksum
+
+	Artifact
 }
 
 // CuresExempt is optionally implemented for an artifact exempt to the
@@ -506,6 +510,12 @@ type CuresExempt interface {
 // FileArtifact does not support fine-grained cancellation. Its context is
 // inherited from the first [TrivialArtifact] or [FloodArtifact] that opens it.
 type FileArtifact interface {
+	// IsExecutable returns whether the resulting filesystem entry should be made
+	// executable, if the [FileArtifact] is cured to the on-disk cache.
+	//
+	// Result must remain identical across multiple invocations.
+	IsExecutable() bool
+
 	// Cure returns [io.ReadCloser] of the full contents of [FileArtifact]. If
 	// [FileArtifact] implements [KnownChecksum], Cure is responsible for
 	// validating any data it produces and must return [ChecksumMismatchError]
@@ -515,7 +525,8 @@ type FileArtifact interface {
 	//
 	// Callers are responsible for closing the resulting [io.ReadCloser].
 	//
-	// Result must remain identical across multiple invocations.
+	// The resulting [io.ReadCloser] across multiple invocations must have
+	// identical behaviour.
 	Cure(r *RContext) (io.ReadCloser, error)
 
 	Artifact
@@ -2208,12 +2219,17 @@ func (c *Cache) cure(a Artifact, curesExempt, rebuild bool) (
 			return
 		}
 
+		perm := os.FileMode(0400)
+		if f.IsExecutable() {
+			perm = 0500
+		}
+
 		work := c.base.Append(dirWork, ids)
 		var w *os.File
 		if w, err = os.OpenFile(
 			work.String(),
 			os.O_CREATE|os.O_EXCL|os.O_WRONLY,
-			0400,
+			perm,
 		); err != nil {
 			return
 		}