internal/pkg: override "." for directory checksum

This makes the checksum consistent with the final resting state of artifact directories without incurring the cost of an extra pair of chown syscalls.

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

internal/pkg/pkg.go

@@ -376,6 +376,63 @@ func removeAll(pathname *check.Absolute) (chmodErr, removeErr error) {
 	return
 }
 
+// overrideFileInfo overrides the permission bits of [fs.FileInfo] to 0500 and
+// is the concrete type returned by overrideFile.Stat.
+type overrideFileInfo struct{ fs.FileInfo }
+
+// Mode returns [fs.FileMode] with its permission bits set to 0500.
+func (fi overrideFileInfo) Mode() fs.FileMode {
+	return fi.FileInfo.Mode()&(^fs.FileMode(0777)) | 0500
+}
+
+// Sys returns nil to avoid passing the original permission bits.
+func (fi overrideFileInfo) Sys() any { return nil }
+
+// overrideFile overrides the permission bits of [fs.File] to 0500 and is the
+// concrete type returned by dotOverrideFS for calls with "." passed as name.
+type overrideFile struct{ fs.File }
+
+func (f overrideFile) Stat() (fi fs.FileInfo, err error) {
+	fi, err = f.File.Stat()
+	if err != nil {
+		return
+	}
+	fi = overrideFileInfo{fi}
+	return
+}
+
+// dirFS is implemented by the concrete type of the return value of [os.DirFS].
+type dirFS interface {
+	fs.StatFS
+	fs.ReadFileFS
+	fs.ReadDirFS
+	fs.ReadLinkFS
+}
+
+// dotOverrideFS overrides the permission bits of "." to 0500 to avoid the extra
+// system calls to add and remove write bit from the target directory.
+type dotOverrideFS struct{ dirFS }
+
+// Open wraps the underlying [fs.FS] with "." special case.
+func (fsys dotOverrideFS) Open(name string) (f fs.File, err error) {
+	f, err = fsys.dirFS.Open(name)
+	if err != nil || name != "." {
+		return
+	}
+	f = overrideFile{f}
+	return
+}
+
+// Stat wraps the underlying [fs.FS] with "." special case.
+func (fsys dotOverrideFS) Stat(name string) (fi fs.FileInfo, err error) {
+	fi, err = fsys.dirFS.Stat(name)
+	if err != nil || name != "." {
+		return
+	}
+	fi = overrideFileInfo{fi}
+	return
+}
+
 // Cure cures the [Artifact] and returns its pathname and [Checksum].
 func (c *Cache) Cure(a Artifact) (
 	pathname *check.Absolute,
@@ -556,13 +613,11 @@ func (c *Cache) Cure(a Artifact) (
 			return
 		}
 
-		// override this before hashing since it will be made read-only after
-		// the rename anyway so do not let perm bits affect the checksum
-		if err = os.Chmod(workPathname.String(), 0700); err != nil {
-			return
-		}
 		var gotChecksum Checksum
-		if gotChecksum, err = HashDir(workPathname); err != nil {
+		if gotChecksum, err = HashFS(
+			dotOverrideFS{os.DirFS(workPathname.String()).(dirFS)},
+			".",
+		); err != nil {
 			return
 		}
 
@@ -583,6 +638,9 @@ func (c *Cache) Cure(a Artifact) (
 			}
 		}
 
+		if err = os.Chmod(workPathname.String(), 0700); err != nil {
+			return
+		}
 		c.checksumMu.Lock()
 		if err = os.Rename(
 			workPathname.String(),