Answer architecture round 10: the paired ruling at fb2c809 is better; a mounted module names its manifest from its files' root, and the mount wrapper returns nothing beside an error

internal/datafiles/manifest.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"io/fs"
+	"path"
 	"sort"
 	"strings"
 
@@ -49,7 +50,12 @@ func (s Source) Manifest() (Manifest, error) {
 }
 
 // ManifestPath is the manifest's path as errors name it.
-func (s Source) ManifestPath() string { return s.labelled(ManifestFile) }
+func (s Source) ManifestPath() string {
+	if m, isMounted := s.fsys.(mounted); isMounted {
+		return s.labelled(path.Join(m.at, ManifestFile))
+	}
+	return s.labelled(ManifestFile)
+}
 
 // decodeManifest decodes a manifest, refusing any key or shape it does not define, and
 // reports every mistake: first each entry's own, in path order, then each parent naming

internal/datafiles/mount.go

@@ -22,7 +22,10 @@ func (m mounted) Open(name string) (fs.File, error) {
 	}
 	if ok {
 		f, err := m.fsys.Open(".")
-		return mountRoot{File: f, name: path.Base(name)}, err
+		if err != nil {
+			return nil, err
+		}
+		return mountRoot{File: f, name: path.Base(name)}, nil
 	}
 	if next, ok := m.towards(name); ok {
 		return &mountDir{name: path.Base(name), next: next}, nil
@@ -70,7 +73,10 @@ type mountRoot struct {
 
 func (r mountRoot) Stat() (fs.FileInfo, error) {
 	info, err := r.File.Stat()
-	return renamed{FileInfo: info, name: r.name}, err
+	if err != nil {
+		return nil, err
+	}
+	return renamed{FileInfo: info, name: r.name}, nil
 }
 
 func (r mountRoot) ReadDir(n int) ([]fs.DirEntry, error) {