internal/pkg: check for unclean shutdown

This avoids running into nasty surprises opening a cache that suffered unclean shutdown due to power loss. All other parts of the cache are not prone to inconsistent state.

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

internal/pkg/pkg.go

@@ -2443,6 +2443,37 @@ func open(
 		c.unlock = func() {}
 	}
 
+	for _, name := range []string{
+		dirWork,
+		dirTemp,
+	} {
+		dents, err := os.ReadDir(base.Append(name).String())
+		if err != nil {
+			if errors.Is(err, os.ErrNotExist) {
+				continue
+			}
+			c.unlock()
+			return nil, err
+		}
+		if len(dents) != 0 {
+			c.unlock()
+			return nil, fmt.Errorf(
+				"%s is not empty, scrub likely required",
+				name,
+			)
+		}
+	}
+
+	if _, err := os.ReadDir(base.Append(
+		dirExecScratch,
+	).String()); !errors.Is(err, os.ErrNotExist) {
+		c.unlock()
+		if err != nil {
+			return nil, err
+		}
+		return nil, errors.New(dirExecScratch + " is present, scrub likely required")
+	}
+
 	variantPath := base.Append(fileVariant).String()
 	if p, err := os.ReadFile(variantPath); err != nil {
 		if !errors.Is(err, os.ErrNotExist) {