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/file.go

@@ -12,6 +12,7 @@ type fileArtifact []byte
 
 var _ KnownChecksum = new(fileArtifact)
 var _ CuresExempt = new(fileArtifact)
+var _ RevisionArtifact = new(fileArtifact)
 
 // fileArtifactNamed embeds fileArtifact alongside a caller-supplied name.
 type fileArtifactNamed struct {
@@ -76,6 +77,20 @@ func (a *fileArtifact) Checksum() Checksum {
 	return Checksum(h.Sum(nil))
 }
 
+// Revision satisfies [RevisionArtifact] for incompatible IsExecutable behaviour.
+func (*fileArtifact) Revision() uint64 { return 0 }
+
+// IsExecutable returns whether the contents begin with shebang or the ELF
+// magic number.
+func (a *fileArtifact) IsExecutable() bool {
+	return a != nil &&
+		(bytes.HasPrefix(
+			*a, []byte{'#', '!'},
+		) || bytes.HasPrefix(
+			*a, []byte{0x7f, 'E', 'L', 'F'},
+		))
+}
+
 // Cure returns the caller-supplied data.
 func (a *fileArtifact) Cure(*RContext) (io.ReadCloser, error) {
 	return io.NopCloser(bytes.NewReader(*a)), nil