Fail structure validation if any directory is empty

Added logic to the validate SIP structure activity to detect empty
directories and fail validation if they exist.

internal/activities/validate_structure.go

@@ -3,6 +3,7 @@ package activities
 import (
 	"context"
 	"fmt"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"slices"
@@ -34,6 +35,42 @@ func (a *ValidateStructure) Execute(
 ) (*ValidateStructureResult, error) {
 	var failures []string
 
+	// Check for empty directories.
+	paths := make(map[string]int)
+
+	err := filepath.WalkDir(params.SIP.Path, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+
+		if path != params.SIP.Path {
+			relativePath, err := filepath.Rel(params.SIP.Path, path)
+			if err != nil {
+				return err
+			}
+
+			// Initialize this directory's total number of immediate children.
+			if d.IsDir() {
+				paths[relativePath] = 0
+			}
+
+			// Add to parent's total number of immediate children.
+			paths[filepath.Dir(relativePath)] = paths[filepath.Dir(relativePath)] + 1
+		}
+
+		return nil
+	})
+	if err != nil {
+		return nil, fmt.Errorf("ValidateStructure: check for empty directories: %v", err)
+	}
+
+	// Report any empty subdirectories.
+	for path, children := range paths {
+		if children == 0 {
+			failures = append(failures, fmt.Sprintf("SIP subdirectory %q is empty", path))
+		}
+	}
+
 	// Check existence of the content directory.
 	hasContentDir := true
 	if !fsutil.FileExists(params.SIP.ContentPath) {

internal/activities/validate_structure_test.go

@@ -63,7 +63,9 @@ func TestValidateStructure(t *testing.T) {
 
 	unexpectedPiecesSIP, err := sip.New(fs.NewDir(t, "",
 		fs.WithDir("content",
-			fs.WithDir("d_0000001"),
+			fs.WithDir("d_0000001",
+				fs.WithFile("content.txt", ""),
+			),
 			fs.WithFile("unexpected.txt", ""),
 		),
 		fs.WithDir("header",
@@ -72,7 +74,9 @@ func TestValidateStructure(t *testing.T) {
 				fs.WithFile("arelda.xsd", ""),
 			),
 		),
-		fs.WithDir("unexpected"),
+		fs.WithDir("unexpected",
+			fs.WithFile("data.txt", ""),
+		),
 	).Path())
 	assert.NilError(t, err)
 
@@ -82,7 +86,9 @@ func TestValidateStructure(t *testing.T) {
 	missingPiecesAIP, err := sip.New(
 		fs.NewDir(t, "",
 			fs.WithDir("AIP-1234",
-				fs.WithDir("additional"),
+				fs.WithDir("additional",
+					fs.WithFile("content.txt", ""),
+				),
 				fs.WithFile("Prozess_Digitalisierung_PREMIS.xml", ""),
 			),
 		).Join("AIP-1234"),