internal/pkg: temporary scratch space for cure

This allows for more flexibility during implementation. The use case that required this was for expanding single directory tarballs.

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

internal/pkg/pkg.go

@@ -89,7 +89,11 @@ type Artifact interface {
 	// If the implementation produces a single file, it must implement [File]
 	// as well. In that case, Cure must produce a single regular file with
 	// contents identical to that returned by [File.Data].
-	Cure(work *check.Absolute, loadData CacheDataFunc) (err error)
+	//
+	// Implementations may use temp as scratch space. The caller is not required
+	// to create a directory here, implementations are expected to create it if
+	// they wish to use it, using [os.MkdirAll].
+	Cure(work, temp *check.Absolute, loadData CacheDataFunc) (err error)
 }
 
 // KnownIdent is optionally implemented by [Artifact] and is used instead of
@@ -183,6 +187,9 @@ const (
 	// dirWork is the directory name appended to Cache.base for working
 	// pathnames set up during [Cache.Cure].
 	dirWork = "work"
+	// dirTemp is the directory name appended to Cache.base for scratch space
+	// pathnames allocated during [Cache.Cure].
+	dirTemp = "temp"
 
 	// checksumLinknamePrefix is prepended to the encoded [Checksum] value
 	// of an [Artifact] when creating a symbolic link to dirChecksum.
@@ -346,6 +353,28 @@ func (NoOutputError) Error() string {
 	return "artifact cured successfully but did not produce any output"
 }
 
+// removeAll is similar to [os.RemoveAll] but is robust against any permissions.
+func removeAll(pathname *check.Absolute) (chmodErr, removeErr error) {
+	chmodErr = filepath.WalkDir(pathname.String(), func(
+		path string,
+		d fs.DirEntry,
+		err error,
+	) error {
+		if err != nil {
+			return err
+		}
+		if d.IsDir() {
+			return os.Chmod(path, 0700)
+		}
+		return nil
+	})
+	if errors.Is(chmodErr, os.ErrNotExist) {
+		chmodErr = nil
+	}
+	removeErr = os.RemoveAll(pathname.String())
+	return
+}
+
 // Cure cures the [Artifact] and returns its pathname and [Checksum].
 func (c *Cache) Cure(a Artifact) (
 	pathname *check.Absolute,
@@ -489,27 +518,8 @@ func (c *Cache) Cure(a Artifact) (
 
 		workPathname := c.base.Append(dirWork, ids)
 		defer func() {
-			// must not use the value of checksum string as it might be zeroed
-			// to cancel the deferred symlink operation
-
 			if err != nil {
-				chmodErr := filepath.WalkDir(workPathname.String(), func(
-					path string,
-					d fs.DirEntry,
-					err error,
-				) error {
-					if err != nil {
-						return err
-					}
-					if d.IsDir() {
-						return os.Chmod(path, 0700)
-					}
-					return nil
-				})
-				if errors.Is(chmodErr, os.ErrNotExist) {
-					chmodErr = nil
-				}
-				removeErr := os.RemoveAll(workPathname.String())
+				chmodErr, removeErr := removeAll(workPathname)
 				if chmodErr != nil || removeErr != nil {
 					err = errors.Join(err, chmodErr, removeErr)
 				} else if errors.Is(err, os.ErrExist) {
@@ -519,7 +529,12 @@ func (c *Cache) Cure(a Artifact) (
 			}
 		}()
 
-		if err = a.Cure(workPathname, c.loadData); err != nil {
+		tempPathname := c.base.Append(dirTemp, ids)
+		if err = a.Cure(workPathname, tempPathname, c.loadData); err != nil {
+			return
+		}
+		if chmodErr, removeErr := removeAll(tempPathname); chmodErr != nil || removeErr != nil {
+			err = errors.Join(err, chmodErr, removeErr)
 			return
 		}