get cards with GetStores and GetStore, read them through fs.FS (closes #4)

GetStores replaces GetDCFStores: it takes a context, has no count
argument and returns a plain slice. GetStore returns the one mounted
card, or ErrNoStore or ErrMultipleStores.

Each Object keeps the fs.FS its card was read through, and Hash reads
through it, so tests can use fstest.MapFS. A cancelled context stops
the read before the next directory. Library errors wrap with %w.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

README.md

@@ -29,6 +29,7 @@ Then list the DCF stores on the mounted filesystems:
 package main
 
 import (
+	"context"
 	"fmt"
 	"log"
 
@@ -36,17 +37,21 @@ import (
 )
 
 func main() {
-	stores, err := dcf.GetDCFStores(0)
+	stores, err := dcf.GetStores(context.Background())
 	if err != nil {
 		log.Fatal(err)
 	}
 
-	for _, store := range *stores {
+	for _, store := range stores {
 		fmt.Println(store.String())
 	}
 }
 ```
 
+Call `dcf.GetStore` instead when you expect exactly one DCF store. It returns
+`dcf.ErrNoStore` when none is mounted, and `dcf.ErrMultipleStores` when more
+than one is.
+
 To work on the library itself, you need docker and git:
 
 ```sh
@@ -105,18 +110,22 @@ overengineering and reusable code.
 
 All the code is in the `pkg/dcf` package. The repo has no commands.
 
-- `dcf.go` holds the public API. `GetDCFStores` finds the DCF stores and reads
+- `dcf.go` holds the public API. `GetStores` finds the DCF stores and reads each
-  each one.
+  one. `GetStore` returns the one DCF store when exactly one is mounted.
 - `helpers.go` finds the mount points that hold a DCF store.
 - `parse.go` reads one DCF store through an `fs.FS` rooted at its mount point.
+- `filegroup.go` sorts the files of one DCF directory into file groups.
 
 The main types are:
 
 - `Store` is one DCF store on one mounted filesystem.
-- `Directory` is one DCF directory. It has a directory number, a name and the
+- `Directory` is one DCF directory. It has a directory number, a name, the
-  objects inside it.
+  objects inside it and their file groups.
 - `Object` is one file in a store. A file in a DCF directory also has a
-  directory number and a file number.
+  directory number and a file number. `Object.Hash` reads the file through the
+  `fs.FS` of its store. An `Object` you build yourself has none, so its `Hash`
+  returns an error.
+- `FileGroup` is one file group in a DCF directory.
 - `Image` and `Video` embed `Object`. The file extension decides which one a
   file becomes. A file with any other extension, such as a `THM` thumbnail, is
   neither.
@@ -127,6 +136,10 @@ filesystem. Media anywhere else on the filesystem is not part of the store.
 
 `dcf` never follows a symlink.
 
+`GetStores` and `GetStore` take a `context.Context`. When it is cancelled, the
+read stops before the next directory. The returned error matches
+`context.Canceled`.
+
 `dcf` applies the naming rules of the DCF specification, CIPA DC-009-2010, under
 `DCIM`:
 
@@ -149,13 +162,37 @@ The naming rules do not apply under `PRIVATE/M4ROOT`. The images and videos
 there are in `Store.Images` and `Store.Videos`, and their DCF numbers are 0.
 `dcf` skips any symlink there.
 
+A file group is made of the DCF files in one DCF directory that share a file
+number. The specification calls it a DCF object. A RAW file and its JPEG form a
+file group, and so do a video and its `THM` thumbnail.
+
+- The first four characters and the extension can differ within a group.
+- A DCF file that shares its number with no other file is a group of one.
+- Files in different DCF directories are never in the same group.
+- Only DCF files are grouped. Files in `Store.NonConforming` and under
+  `PRIVATE/M4ROOT` are in no group.
+
+`Directory.FileGroups` lists the groups in file number order. Each `FileGroup`
+has:
+
+- `DirectoryNumber` and `FileNumber`
+- `Members`, which is every file in the group
+- `Primary`, which is the member that stands for the whole group
+- `MembersCount`, which returns the number of members
+- `TotalSize`, which returns the total size of the members in bytes
+
+The primary member is the first of these that the group holds:
+
+1. a `JPG` file
+2. any other file in `Store.Images`
+3. a file in `Store.Videos`
+4. any other file except a `THM`
+5. a `THM`
+
+`Store.Images` and `Store.Videos` list each file on its own, whatever its group.
+
 ## TODO
 
-- Handle DCF file groups, where multiple objects share the same index number.
-  `dcf` does not handle these yet. If you need this, send me a link to a zip or
-  tar of a memory card that uses them, and I'll see what I can do.
-- Take a `context.Context` to cancel the walk of a store.
-- Return a plain slice from `GetDCFStores`, not a pointer to one.
 - Copy files off a card, and verify each copy against its SHA256 hash.
 
 ## License

pkg/dcf/dcf.go

@@ -2,14 +2,18 @@
 // Camera File system" specification (JEITA CP-3461). It finds the cards among
 // the mounted filesystems and parses the DCF directories and files on each one
 // into their directory numbers and file numbers. It lists the images and videos
-// on each card, and everything under DCIM that is not a DCF directory or file.
+// on each card, and everything under DCIM that is not a DCF directory or file. It
+// groups the files in each DCF directory that share a file number.
 package dcf
 
 import (
+	"context"
 	"crypto/sha256"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"slices"
@@ -18,6 +22,14 @@ import (
 	"github.com/dustin/go-humanize"
 )
 
+var (
+	// ErrNoStore is the error from GetStore when no DCF store is mounted.
+	ErrNoStore = errors.New("no DCF store is mounted")
+	// ErrMultipleStores is the error from GetStore when more than one DCF store
+	// is mounted.
+	ErrMultipleStores = errors.New("more than one DCF store is mounted")
+)
+
 // Store represents a DCF store, which is a directory structure that contains
 // images and videos in the root of a mounted filesystem.
 type Store struct {
@@ -46,6 +58,22 @@ type Directory struct {
 	// Objects holds every DCF file in the directory, including those that are
 	// neither images nor videos.
 	Objects []*Object
+	// FileGroups holds the same files, grouped by file number, in file number
+	// order.
+	FileGroups []*FileGroup
+}
+
+// FileGroup represents one file group: the files in one DCF directory that share
+// a file number. The DCF specification calls this a DCF object. A file that
+// shares its number with no other file is a group of one.
+type FileGroup struct {
+	DirectoryNumber int
+	FileNumber      int
+	// Members holds every file in the group, including Primary, in name order.
+	// Two members can have the same extension.
+	Members []*Object
+	// Primary is the member that stands for the whole group.
+	Primary *Object
 }
 
 // Object represents a file in a DCF store. It is embedded in Image and Video
@@ -59,6 +87,9 @@ type Object struct {
 	// directory. Both are 0 for a file under PRIVATE/M4ROOT.
 	DirectoryNumber int
 	FileNumber      int
+	// fsys is the fs.FS the store was read through. Hash reads the file through
+	// it.
+	fsys fs.FS
 }
 
 // Image represents an image file on a DCF store.
@@ -114,6 +145,21 @@ func (d *Store) TotalSize() int64 {
 	return d.TotalImageSize() + d.TotalVideoSize()
 }
 
+// MembersCount returns the number of files in the file group.
+func (g *FileGroup) MembersCount() int {
+	return len(g.Members)
+}
+
+// TotalSize returns the total size in bytes of the files in the file group.
+func (g *FileGroup) TotalSize() int64 {
+	totalSize := int64(0)
+	for _, member := range g.Members {
+		totalSize += member.Size
+	}
+
+	return totalSize
+}
+
 func (d *Store) String() string {
 	// A caller can set a negative Size, and uint64 would make that huge.
 	totalSize := uint64(max(d.TotalSize(), 0))
@@ -141,15 +187,17 @@ func (i *Image) String() string {
 	)
 }
 
-// Hash returns the SHA256 hash of the file as a hex string.
+// Hash returns the SHA256 hash of the file as a hex string. It reads the file
+// through the fs.FS of its store. An Object built by hand has none, so Hash
+// returns an error that matches fs.ErrInvalid.
 func (d *Object) Hash() (string, error) {
-	return pathToSHA256(d.FullFilePath())
+	if d.fsys == nil {
-}
+		return "", fmt.Errorf("hashing %s: %w", d.FullFilePath(), fs.ErrInvalid)
+	}
 
-func pathToSHA256(path string) (string, error) {
+	file, err := d.fsys.Open(d.Path)
-	file, err := os.Open(filepath.Clean(path))
 	if err != nil {
-		return "", err
+		return "", fmt.Errorf("hashing %s: %w", d.FullFilePath(), err)
 	}
 
 	defer func() { _ = file.Close() }()
@@ -158,44 +206,64 @@ func pathToSHA256(path string) (string, error) {
 
 	_, err = io.Copy(hash, file)
 	if err != nil {
-		return "", err
+		return "", fmt.Errorf("hashing %s: %w", d.FullFilePath(), err)
 	}
 
 	return hex.EncodeToString(hash.Sum(nil)), nil
 }
 
-// GetDCFStores returns a list of DCF stores found on the system in the root of
+// GetStores finds the DCF stores in the root of the mounted filesystems and reads
-// any mounted filesystems. The integer argument is the number of DCF stores to
+// each one. It does not mount filesystems or search outside of filesystem root
-// return. If the argument is 0, all DCF stores are returned. It does not mount
+// directories. It reads the DCIM and PRIVATE/M4ROOT directories of each
-// filesystems or search outside of filesystem root directories. It does,
+// filesystem to populate the returned Stores. It never follows a symlink. If a
-// however, read the DCIM and PRIVATE/M4ROOT directories of each filesystem to
+// card cannot be read, it returns an error that names the card's mount point.
-// populate the returned Stores. It never follows a symlink. If a card cannot be
+// When ctx is cancelled, the read stops before the next directory, and the error
-// read, it returns an error that names the card's mount point.
+// matches ctx.Err().
-func GetDCFStores(requestedCount int) (*[]Store, error) {
+func GetStores(ctx context.Context) ([]*Store, error) {
-	dcfStorePaths, err := findDCFMountPoints(requestedCount)
+	dcfStorePaths, err := findDCFMountPoints(ctx)
 	if err != nil {
 		return nil, err
 	}
 
-	dcfStores, err := parseStores(dcfStorePaths)
+	return parseStores(ctx, dcfStorePaths)
+}
+
+// GetStore returns the one DCF store when exactly one is mounted. It returns
+// ErrNoStore when none is mounted, and ErrMultipleStores when more than one is.
+// It reads every card first, the way GetStores does, so a card that cannot be
+// read returns that error instead.
+func GetStore(ctx context.Context) (*Store, error) {
+	stores, err := GetStores(ctx)
 	if err != nil {
 		return nil, err
 	}
 
-	return &dcfStores, nil
+	return onlyStore(stores)
+}
+
+// onlyStore returns the store when stores holds exactly one.
+func onlyStore(stores []*Store) (*Store, error) {
+	switch len(stores) {
+	case 0:
+		return nil, ErrNoStore
+	case 1:
+		return stores[0], nil
+	default:
+		return nil, ErrMultipleStores
+	}
 }
 
 // parseStores reads the card at each of the given mount points.
-func parseStores(mountpoints []string) ([]Store, error) {
+func parseStores(ctx context.Context, mountpoints []string) ([]*Store, error) {
-	dcfStores := []Store{}
+	dcfStores := []*Store{}
 
 	for _, mountpoint := range mountpoints {
-		dcfStore, err := parseStore(os.DirFS(mountpoint), mountpoint)
+		dcfStore, err := parseStore(ctx, os.DirFS(mountpoint), mountpoint)
 		if err != nil {
 			return nil, err
 		}
 
-		dcfStores = append(dcfStores, *dcfStore)
+		dcfStores = append(dcfStores, dcfStore)
 	}
 
 	return dcfStores, nil
@@ -203,7 +271,7 @@ func parseStores(mountpoints []string) ([]Store, error) {
 
 func isImageFile(path string) bool {
 	imageExtensions := []string{
-		".jpg", ".jpeg", ".png", ".gif", ".bmp", ".tiff", ".raw", ".arw",
+		".jpg", ".jpeg", ".png", ".gif", ".bmp", ".tif", ".tiff", ".raw", ".arw",
 	}
 
 	return slices.Contains(imageExtensions, strings.ToLower(filepath.Ext(path)))

pkg/dcf/dcf_internal_test.go

@@ -1,9 +1,13 @@
 package dcf
 
 import (
+	"context"
+	"errors"
+	"io/fs"
 	"path/filepath"
 	"strings"
 	"testing"
+	"testing/fstest"
 )
 
 func TestParseStoresReadsEachMountPoint(t *testing.T) {
@@ -12,7 +16,7 @@ func TestParseStoresReadsEachMountPoint(t *testing.T) {
 	first := newCardOnDisk(t, "DCIM/100ABCDE/ABCD0001.JPG")
 	second := newCardOnDisk(t, "PRIVATE/M4ROOT/CLIP/C0001.MP4")
 
-	stores, err := parseStores([]string{first, second})
+	stores, err := parseStores(t.Context(), []string{first, second})
 	if err != nil {
 		t.Fatalf("parseStores: %v", err)
 	}
@@ -50,18 +54,100 @@ func TestParseStoresReturnsErrorNamingTheCard(t *testing.T) {
 	// Every lookup under a plain file fails.
 	mountpoint := filepath.Join(newCardOnDisk(t, "FILE"), "FILE")
 
-	_, err := parseStores([]string{mountpoint})
+	_, err := parseStores(t.Context(), []string{mountpoint})
 	if err == nil || !strings.Contains(err.Error(), mountpoint) {
 		t.Fatalf("parseStores error = %v, want one naming %s", err, mountpoint)
 	}
 }
 
+func TestParseStoresReturnsErrorWhenCancelled(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	mountpoint := newCardOnDisk(t, "DCIM/100ABCDE/ABCD0001.JPG")
+
+	_, err := parseStores(ctx, []string{mountpoint})
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("parseStores error = %v, want %v", err, context.Canceled)
+	}
+}
+
+func TestOnlyStore(t *testing.T) {
+	t.Parallel()
+
+	first := newMountPoint(t, dcimDirectory)
+	second := newMountPoint(t, m4RootDirectory)
+
+	stores, err := parseStores(t.Context(), []string{first, second})
+	if err != nil {
+		t.Fatalf("parseStores: %v", err)
+	}
+
+	_, err = onlyStore(stores[:0])
+	if !errors.Is(err, ErrNoStore) {
+		t.Errorf("onlyStore of no cards: error = %v, want %v", err, ErrNoStore)
+	}
+
+	store, err := onlyStore(stores[:1])
+	if err != nil {
+		t.Fatalf("onlyStore of one card: %v", err)
+	}
+
+	if store.RootDirectory != first {
+		t.Errorf("onlyStore returned the card at %s, want %s",
+			store.RootDirectory, first)
+	}
+
+	_, err = onlyStore(stores)
+	if !errors.Is(err, ErrMultipleStores) {
+		t.Errorf("onlyStore of two cards: error = %v, want %v",
+			err, ErrMultipleStores)
+	}
+}
+
+func TestObjectHashReadsThroughTheCard(t *testing.T) {
+	t.Parallel()
+
+	card := fstest.MapFS{"DCIM/100ABCDE/ABCD0001.JPG": {Data: []byte("abc")}}
+
+	got, err := mustParseStore(t, card).Images[0].Hash()
+	if err != nil {
+		t.Fatalf("Hash() returned error: %v", err)
+	}
+
+	// The published SHA-256 test vector for "abc".
+	want := "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
+	if got != want {
+		t.Fatalf("Hash() = %q, want %q", got, want)
+	}
+}
+
+func TestObjectHashOfMissingFileReturnsError(t *testing.T) {
+	t.Parallel()
+
+	const filePath = "DCIM/100ABCDE/ABCD0001.JPG"
+
+	card := newCard(filePath)
+	image := mustParseStore(t, card).Images[0]
+
+	delete(card, filePath)
+
+	_, err := image.Hash()
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("Hash() error = %v, want %v", err, fs.ErrNotExist)
+	}
+}
+
 func TestIsImageFile(t *testing.T) {
 	t.Parallel()
 
 	cases := map[string]bool{
 		"DCIM/100MSDCF/DSC00001.jpg":    true,
 		"DCIM/100MSDCF/DSC00001.ARW":    true,
+		"DCIM/100MSDCF/DSC00001.TIF":    true,
+		"DCIM/100MSDCF/DSC00001.tif":    true,
 		"PRIVATE/M4ROOT/CLIP/C0001.MP4": false,
 		"DCIM/100MSDCF/notes.txt":       false,
 		"DCIM/100MSDCF/noextension":     false,

pkg/dcf/dcf_test.go

@@ -1,8 +1,9 @@
 package dcf_test
 
 import (
-	"os"
+	"context"
-	"path/filepath"
+	"errors"
+	"io/fs"
 	"testing"
 
 	"sneak.berlin/go/dcf/pkg/dcf"
@@ -125,37 +126,30 @@ func TestVideoString(t *testing.T) {
 	}
 }
 
-func TestObjectHashIsSHA256OfFile(t *testing.T) {
+func TestObjectHashOfObjectBuiltByHandReturnsError(t *testing.T) {
 	t.Parallel()
 
-	root := t.TempDir()
+	object := dcf.Object{DCFStoreRoot: cardRoot, Path: "a.jpg"}
-
-	err := os.WriteFile(filepath.Join(root, "a.jpg"), []byte("abc"), 0o600)
-	if err != nil {
-		t.Fatalf("WriteFile: %v", err)
-	}
-
-	object := dcf.Object{DCFStoreRoot: root, Path: "a.jpg"}
-
-	got, err := object.Hash()
-	if err != nil {
-		t.Fatalf("Hash() returned error: %v", err)
-	}
-
-	// The published SHA-256 test vector for "abc".
-	want := "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
-	if got != want {
-		t.Fatalf("Hash() = %q, want %q", got, want)
-	}
-}
-
-func TestObjectHashOfMissingFileReturnsError(t *testing.T) {
-	t.Parallel()
-
-	object := dcf.Object{DCFStoreRoot: t.TempDir(), Path: "absent.jpg"}
 
 	_, err := object.Hash()
-	if err == nil {
+	if !errors.Is(err, fs.ErrInvalid) {
-		t.Fatal("Hash() of a missing file returned no error")
+		t.Fatalf("Hash() error = %v, want %v", err, fs.ErrInvalid)
+	}
+}
+
+func TestGetStoresAndGetStoreReturnCancellation(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	_, err := dcf.GetStores(ctx)
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("GetStores error = %v, want %v", err, context.Canceled)
+	}
+
+	_, err = dcf.GetStore(ctx)
+	if !errors.Is(err, context.Canceled) {
+		t.Errorf("GetStore error = %v, want %v", err, context.Canceled)
 	}
 }

pkg/dcf/filegroup.go

@@ -0,0 +1,67 @@
+package dcf
+
+import (
+	"cmp"
+	"path"
+	"slices"
+	"strings"
+)
+
+// newFileGroups sorts the objects of one DCF directory into file groups, in file
+// number order. Section 4.3.2.2 of the specification says the files in a DCF
+// directory that share a file number are one group. Their first four characters
+// and their extensions can differ.
+func newFileGroups(objects []*Object) []*FileGroup {
+	var groups []*FileGroup
+
+	groupWithNumber := map[int]*FileGroup{}
+
+	for _, object := range objects {
+		group, found := groupWithNumber[object.FileNumber]
+		if !found {
+			group = &FileGroup{
+				DirectoryNumber: object.DirectoryNumber,
+				FileNumber:      object.FileNumber,
+			}
+			groupWithNumber[object.FileNumber] = group
+			groups = append(groups, group)
+		}
+
+		group.Members = append(group.Members, object)
+	}
+
+	for _, group := range groups {
+		group.Primary = primaryMember(group.Members)
+	}
+
+	slices.SortFunc(groups, func(a, b *FileGroup) int {
+		return cmp.Compare(a.FileNumber, b.FileNumber)
+	})
+
+	return groups
+}
+
+// primaryMember returns the member that stands for the whole group. Section
+// 6.2.1.3 of the specification gives a DCF basic file priority over a DCF extended
+// image file. It says nothing about a DCF optional file, the other kind of JPG, so
+// ranking every JPG first is a choice. Section 4.6 makes a THM only the thumbnail
+// of another file, so it comes last. The specification leaves the rest open, so an
+// image comes before a video, and a video before any other file. Among equals, the
+// first in name order wins.
+func primaryMember(members []*Object) *Object {
+	choices := []func(string) bool{isJPG, isImageFile, isVideoFile, isNotTHM}
+
+	for _, choice := range choices {
+		for _, member := range members {
+			if choice(member.Path) {
+				return member
+			}
+		}
+	}
+
+	return members[0]
+}
+
+func isNotTHM(name string) bool {
+	return !strings.EqualFold(path.Ext(name), ".THM")
+}

pkg/dcf/filegroup_internal_test.go

@@ -0,0 +1,232 @@
+package dcf
+
+import (
+	"slices"
+	"testing"
+	"testing/fstest"
+)
+
+// mustFileGroups returns the file groups of directory, and stops the test unless
+// there are exactly count of them.
+func mustFileGroups(t *testing.T, directory *Directory, count int) []*FileGroup {
+	t.Helper()
+
+	if len(directory.FileGroups) != count {
+		t.Fatalf("%s has %d file groups, want %d",
+			directory.Name, len(directory.FileGroups), count)
+	}
+
+	return directory.FileGroups
+}
+
+// assertFileGroup checks the numbers, the primary member and the members of group.
+func assertFileGroup(
+	t *testing.T,
+	group *FileGroup,
+	directoryNumber, fileNumber int,
+	primary string,
+	members ...string,
+) {
+	t.Helper()
+
+	if group.DirectoryNumber != directoryNumber || group.FileNumber != fileNumber {
+		t.Errorf("group numbers = %d and %d, want %d and %d",
+			group.DirectoryNumber, group.FileNumber, directoryNumber, fileNumber)
+	}
+
+	if group.Primary.Path != primary {
+		t.Errorf("Primary = %q, want %q", group.Primary.Path, primary)
+	}
+
+	paths := make([]string, 0, len(group.Members))
+
+	for _, member := range group.Members {
+		paths = append(paths, member.Path)
+	}
+
+	assertPaths(t, "members", paths, members...)
+
+	if got := group.MembersCount(); got != len(members) {
+		t.Errorf("MembersCount() = %d, want %d", got, len(members))
+	}
+}
+
+// assertSizes checks the total size of group and the size of each member.
+func assertSizes(t *testing.T, group *FileGroup, total int64, sizes ...int64) {
+	t.Helper()
+
+	if got := group.TotalSize(); got != total {
+		t.Errorf("TotalSize() = %d, want %d", got, total)
+	}
+
+	got := make([]int64, 0, len(group.Members))
+
+	for _, member := range group.Members {
+		got = append(got, member.Size)
+	}
+
+	if !slices.Equal(got, sizes) {
+		t.Errorf("member sizes = %v, want %v", got, sizes)
+	}
+}
+
+// assertPrimaryMember checks which of the named members primaryMember picks.
+func assertPrimaryMember(t *testing.T, want string, names ...string) {
+	t.Helper()
+
+	members := make([]*Object, 0, len(names))
+
+	for _, name := range names {
+		members = append(members, &Object{Path: name})
+	}
+
+	if got := primaryMember(members).Path; got != want {
+		t.Errorf("primaryMember of %q = %q, want %q", names, got, want)
+	}
+}
+
+func TestParseStoreGroupsFilesSharingAFileNumber(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, fstest.MapFS{
+		"DCIM/100ABCDE/DSC00001.ARW": {Data: make([]byte, 2000)},
+		"DCIM/100ABCDE/DSC00001.JPG": {Data: make([]byte, 500)},
+		"DCIM/100ABCDE/MVI_0002.MP4": {Data: make([]byte, 9000)},
+		"DCIM/100ABCDE/MVI_0002.THM": {Data: make([]byte, 100)},
+		"DCIM/100ABCDE/DSC00003.JPG": {Data: make([]byte, 700)},
+		"DCIM/101ABCDE/DSC00001.JPG": {Data: make([]byte, 300)},
+	})
+
+	first := mustFileGroups(t, store.Directories[0], 3)
+	second := mustFileGroups(t, store.Directories[1], 1)
+
+	assertFileGroup(t, first[0], 100, 1, "DCIM/100ABCDE/DSC00001.JPG",
+		"DCIM/100ABCDE/DSC00001.ARW",
+		"DCIM/100ABCDE/DSC00001.JPG",
+	)
+	assertSizes(t, first[0], 2500, 2000, 500)
+
+	assertFileGroup(t, first[1], 100, 2, "DCIM/100ABCDE/MVI_0002.MP4",
+		"DCIM/100ABCDE/MVI_0002.MP4",
+		"DCIM/100ABCDE/MVI_0002.THM",
+	)
+	assertSizes(t, first[1], 9100, 9000, 100)
+
+	assertFileGroup(t, first[2], 100, 3, "DCIM/100ABCDE/DSC00003.JPG",
+		"DCIM/100ABCDE/DSC00003.JPG",
+	)
+	assertSizes(t, first[2], 700, 700)
+
+	assertFileGroup(t, second[0], 101, 1, "DCIM/101ABCDE/DSC00001.JPG",
+		"DCIM/101ABCDE/DSC00001.JPG",
+	)
+	assertSizes(t, second[0], 300, 300)
+
+	// The images, the videos and the totals still count each file on its own.
+	if got := store.ImagesCount(); got != 4 {
+		t.Errorf("ImagesCount() = %d, want 4", got)
+	}
+
+	if got := store.VideosCount(); got != 1 {
+		t.Errorf("VideosCount() = %d, want 1", got)
+	}
+
+	if got := store.TotalSize(); got != 12500 {
+		t.Errorf("TotalSize() = %d, want 12500", got)
+	}
+}
+
+func TestParseStoreGroupsFilesWhoseFirstFourCharactersDiffer(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE/ABCD0004.TIF",
+		"DCIM/100ABCDE/THMB0004.THM",
+	))
+
+	groups := mustFileGroups(t, store.Directories[0], 1)
+
+	assertFileGroup(t, groups[0], 100, 4, "DCIM/100ABCDE/ABCD0004.TIF",
+		"DCIM/100ABCDE/ABCD0004.TIF",
+		"DCIM/100ABCDE/THMB0004.THM",
+	)
+}
+
+func TestParseStoreOrdersFileGroupsByFileNumber(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE/ABCD0002.JPG",
+		"DCIM/100ABCDE/WXYZ0001.JPG",
+	))
+
+	groups := mustFileGroups(t, store.Directories[0], 2)
+
+	assertFileGroup(t, groups[0], 100, 1, "DCIM/100ABCDE/WXYZ0001.JPG",
+		"DCIM/100ABCDE/WXYZ0001.JPG",
+	)
+	assertFileGroup(t, groups[1], 100, 2, "DCIM/100ABCDE/ABCD0002.JPG",
+		"DCIM/100ABCDE/ABCD0002.JPG",
+	)
+}
+
+func TestParseStoreLeavesOtherFilesOutOfFileGroups(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard(
+		"DCIM/100ABCDE/ABCD0004.ARW",
+		"DCIM/100ABCDE/ABCD0004.JPG",
+		"DCIM/100ABCDE/PQRS0004.JPG",
+		"DCIM/100ABCDE/README.TXT",
+		"DCIM/MISC/ABCD0005.JPG",
+		"PRIVATE/M4ROOT/CLIP/C0006.MP4",
+	))
+
+	if len(store.Directories) != 1 {
+		t.Fatalf("got %d directories, want 1", len(store.Directories))
+	}
+
+	groups := mustFileGroups(t, store.Directories[0], 1)
+
+	assertFileGroup(t, groups[0], 100, 4, "DCIM/100ABCDE/ABCD0004.ARW",
+		"DCIM/100ABCDE/ABCD0004.ARW",
+	)
+}
+
+func TestParseStoreGivesDirectoryWithoutDCFFilesNoFileGroups(t *testing.T) {
+	t.Parallel()
+
+	store := mustParseStore(t, newCard("DCIM/100ABCDE/README.TXT"))
+
+	mustFileGroups(t, store.Directories[0], 0)
+}
+
+func TestPrimaryMember(t *testing.T) {
+	t.Parallel()
+
+	// A JPG comes first, in either case.
+	assertPrimaryMember(t, "A0010001.JPG", "A0010001.ARW", "A0010001.JPG")
+	assertPrimaryMember(t, "A0010001.JPG", "A0010001.JPG", "A0010001.MOV")
+	assertPrimaryMember(t, "a0010001.jpg", "a0010001.arw", "a0010001.jpg")
+
+	// Then any other image.
+	assertPrimaryMember(t, "Z0010001.ARW", "A0010001.MP4", "Z0010001.ARW")
+	assertPrimaryMember(t, "Z0010001.ARW", "A0010001.WAV", "Z0010001.ARW")
+	assertPrimaryMember(t, "A0010001.TIF", "A0010001.MOV", "A0010001.TIF")
+	assertPrimaryMember(t, "Z0010001.TIF", "A0010001.WAV", "Z0010001.TIF")
+
+	// Then a video.
+	assertPrimaryMember(t, "A0010001.MP4", "A0010001.AAC", "A0010001.MP4")
+	assertPrimaryMember(t, "Z0010001.MP4", "A0010001.THM", "Z0010001.MP4")
+
+	// Then any other file except a THM, in either case.
+	assertPrimaryMember(t, "Z0010001.WAV", "A0010001.THM", "Z0010001.WAV")
+	assertPrimaryMember(t, "a0010001.wav", "a0010001.thm", "a0010001.wav")
+
+	// A THM is primary only when the group holds nothing else.
+	assertPrimaryMember(t, "A0010001.THM", "A0010001.THM")
+
+	// Among equals, the first in name order wins.
+	assertPrimaryMember(t, "A0010001.THM", "A0010001.THM", "Z0010001.THM")
+	assertPrimaryMember(t, "A0010001.WAV", "A0010001.WAV", "Z0010001.XML")
+}

pkg/dcf/helpers.go

@@ -1,6 +1,7 @@
 package dcf
 
 import (
+	"context"
 	"fmt"
 	"log/slog"
 	"os"
@@ -9,12 +10,19 @@ import (
 	"github.com/shirou/gopsutil/disk"
 )
 
-func findAllMountPoints() ([]string, error) {
+func findAllMountPoints(ctx context.Context) ([]string, error) {
 	mountpoints := []string{}
 
-	partitions, err := disk.Partitions(false) // physical devices only, so false
+	// disk.PartitionsWithContext does not check ctx on Linux.
+	err := ctx.Err()
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("listing the mounted filesystems: %w", err)
+	}
+
+	// physical devices only, so false
+	partitions, err := disk.PartitionsWithContext(ctx, false)
+	if err != nil {
+		return nil, fmt.Errorf("listing the mounted filesystems: %w", err)
 	}
 
 	for _, partition := range partitions {
@@ -26,21 +34,18 @@ func findAllMountPoints() ([]string, error) {
 	return mountpoints, nil
 }
 
-// findDCFMountPoints returns a list of strings of the paths of mountpoints
+// findDCFMountPoints returns the paths of the mountpoints that hold a DCF store.
-// that hold a DCF store. Its only argument is an integer of how many mountpoints
+func findDCFMountPoints(ctx context.Context) ([]string, error) {
-// to return. If the argument is 0, all mountpoints are returned.
+	mountpoints, err := findAllMountPoints(ctx)
-func findDCFMountPoints(requestedCount int) ([]string, error) {
-	mountpoints, err := findAllMountPoints()
 	if err != nil {
 		return nil, err
 	}
 
-	return filterDCFMountPoints(mountpoints, requestedCount), nil
+	return filterDCFMountPoints(mountpoints), nil
 }
 
-// filterDCFMountPoints returns the first requestedCount of the mountpoints that
+// filterDCFMountPoints returns the mountpoints that hold a DCF store.
-// hold a DCF store. If requestedCount is 0, it returns all of them.
+func filterDCFMountPoints(mountpoints []string) []string {
-func filterDCFMountPoints(mountpoints []string, requestedCount int) []string {
 	filteredMountpoints := []string{}
 
 	for _, mountpoint := range mountpoints {
@@ -51,9 +56,6 @@ func filterDCFMountPoints(mountpoints []string, requestedCount int) []string {
 		slog.Debug(fmt.Sprintf("Keeping mountpoint %s\n", mountpoint))
 
 		filteredMountpoints = append(filteredMountpoints, mountpoint)
-		if (requestedCount > 0) && (len(filteredMountpoints) >= requestedCount) {
-			break
-		}
 	}
 
 	return filteredMountpoints

pkg/dcf/helpers_internal_test.go

@@ -72,29 +72,16 @@ func TestIsDCFMountPointIgnoresSymlink(t *testing.T) {
 	}
 }
 
-func TestFilterDCFMountPointsReturnsRequestedCount(t *testing.T) {
+func TestFilterDCFMountPointsKeepsThoseHoldingAStore(t *testing.T) {
 	t.Parallel()
 
 	first := newMountPoint(t, "DCIM")
 	second := newMountPoint(t, "PRIVATE/M4ROOT")
-	third := newMountPoint(t, "DCIM")
 
-	// This one holds no DCF store, so it must not use up the requested count.
+	got := filterDCFMountPoints([]string{newMountPoint(t), first, second})
-	mountpoints := []string{newMountPoint(t), first, second, third}
 
-	cases := map[int][]string{
+	want := []string{first, second}
-		0: {first, second, third},
-		1: {first},
-		2: {first, second},
-		3: {first, second, third},
-		4: {first, second, third},
-	}
-
-	for requestedCount, want := range cases {
-		got := filterDCFMountPoints(mountpoints, requestedCount)
 	if !slices.Equal(got, want) {
-			t.Errorf("filterDCFMountPoints(%d) = %v, want %v",
+		t.Errorf("filterDCFMountPoints = %v, want %v", got, want)
-				requestedCount, got, want)
-		}
 	}
 }

pkg/dcf/parse.go

@@ -1,6 +1,7 @@
 package dcf
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"io/fs"
@@ -28,13 +29,13 @@ var (
 // point. The root argument is the path of that mount point. Only DCIM and
 // PRIVATE/M4ROOT are read. A card can lack either one. A symlink is never
 // followed. An error names the mount point, because fsys reports only paths
-// inside it.
+// inside it. When ctx is cancelled, the read stops before the next directory.
-func parseStore(fsys fs.FS, root string) (*Store, error) {
+func parseStore(ctx context.Context, fsys fs.FS, root string) (*Store, error) {
 	store := &Store{RootDirectory: root}
 
-	err := parseDCIM(fsys, store)
+	err := parseDCIM(ctx, fsys, store)
 	if err == nil {
-		err = parseM4Root(fsys, store)
+		err = parseM4Root(ctx, fsys, store)
 	}
 
 	if err != nil {
@@ -68,16 +69,26 @@ func hasDirectory(fsys fs.FS, name string) (bool, error) {
 	return info.IsDir(), nil
 }
 
+// readDir returns the entries of the named directory, unless ctx is cancelled.
+func readDir(ctx context.Context, fsys fs.FS, name string) ([]fs.DirEntry, error) {
+	err := ctx.Err()
+	if err != nil {
+		return nil, err
+	}
+
+	return fs.ReadDir(fsys, name)
+}
+
 // parseDCIM adds the DCF directories under DCIM to store. Everything else there
 // is not conforming. That includes directories that share a directory number,
 // which section 7.1.2 of the specification treats as non-DCF directories.
-func parseDCIM(fsys fs.FS, store *Store) error {
+func parseDCIM(ctx context.Context, fsys fs.FS, store *Store) error {
 	found, err := hasDirectory(fsys, dcimDirectory)
 	if err != nil || !found {
 		return err
 	}
 
-	entries, err := fs.ReadDir(fsys, dcimDirectory)
+	entries, err := readDir(ctx, fsys, dcimDirectory)
 	if err != nil {
 		return err
 	}
@@ -94,9 +105,9 @@ func parseDCIM(fsys fs.FS, store *Store) error {
 		number := parseNumber(dcfDirectoryName, entry.Name())
 
 		if entry.IsDir() && number != 0 && directoriesWithNumber[number] == 1 {
-			err = parseDirectory(fsys, store, entry.Name(), number)
+			err = parseDirectory(ctx, fsys, store, entry.Name(), number)
 		} else {
-			err = addNonConforming(fsys, store, dcimDirectory, entry)
+			err = addNonConforming(ctx, fsys, store, dcimDirectory, entry)
 		}
 
 		if err != nil {
@@ -112,10 +123,16 @@ func parseDCIM(fsys fs.FS, store *Store) error {
 // Everything else there is not conforming. That includes JPG files that share a
 // file number, which section 7.2.2 of the specification says must not show DCF
 // numbers. Other files that share a file number do conform.
-func parseDirectory(fsys fs.FS, store *Store, name string, number int) error {
+func parseDirectory(
+	ctx context.Context,
+	fsys fs.FS,
+	store *Store,
+	name string,
+	number int,
+) error {
 	directoryPath := path.Join(dcimDirectory, name)
 
-	entries, err := fs.ReadDir(fsys, directoryPath)
+	entries, err := readDir(ctx, fsys, directoryPath)
 	if err != nil {
 		return err
 	}
@@ -135,7 +152,7 @@ func parseDirectory(fsys fs.FS, store *Store, name string, number int) error {
 		sharedJPG := isJPG(entry.Name()) && jpgsWithNumber[fileNumber] > 1
 
 		if !entry.Type().IsRegular() || fileNumber == 0 || sharedJPG {
-			err = addNonConforming(fsys, store, directoryPath, entry)
+			err = addNonConforming(ctx, fsys, store, directoryPath, entry)
 			if err != nil {
 				return err
 			}
@@ -145,7 +162,7 @@ func parseDirectory(fsys fs.FS, store *Store, name string, number int) error {
 
 		entryPath := path.Join(directoryPath, entry.Name())
 
-		object, err := newObject(store, entryPath, entry)
+		object, err := newObject(fsys, store, entryPath, entry)
 		if err != nil {
 			return err
 		}
@@ -157,6 +174,7 @@ func parseDirectory(fsys fs.FS, store *Store, name string, number int) error {
 		addMedia(store, object)
 	}
 
+	directory.FileGroups = newFileGroups(directory.Objects)
 	store.Directories = append(store.Directories, directory)
 
 	return nil
@@ -166,7 +184,7 @@ func parseDirectory(fsys fs.FS, store *Store, name string, number int) error {
 // naming rules do not apply there, so these files have no DCF numbers. It skips
 // a symlink. It also skips a name that is not valid UTF-8, because fs.FS refuses
 // to open one.
-func parseM4Root(fsys fs.FS, store *Store) error {
+func parseM4Root(ctx context.Context, fsys fs.FS, store *Store) error {
 	found, err := hasDirectory(fsys, m4RootDirectory)
 	if err != nil || !found {
 		return err
@@ -184,11 +202,16 @@ func parseM4Root(fsys fs.FS, store *Store) error {
 				return fs.SkipDir
 			}
 
+			// fs.WalkDir reads a directory right after this call for it.
+			if entry.IsDir() {
+				return ctx.Err()
+			}
+
 			if !entry.Type().IsRegular() || !validName {
 				return nil
 			}
 
-			object, err := newObject(store, filePath, entry)
+			object, err := newObject(fsys, store, filePath, entry)
 			if err != nil {
 				return err
 			}
@@ -203,7 +226,13 @@ func parseM4Root(fsys fs.FS, store *Store) error {
 // conforming. If the entry is a directory, it lists everything inside it too. It
 // does not follow a symlink. It does not read a directory whose name is not
 // valid UTF-8, because fs.FS refuses to open one.
-func addNonConforming(fsys fs.FS, store *Store, dir string, entry fs.DirEntry) error {
+func addNonConforming(
+	ctx context.Context,
+	fsys fs.FS,
+	store *Store,
+	dir string,
+	entry fs.DirEntry,
+) error {
 	entryPath := path.Join(dir, entry.Name())
 	store.NonConforming = append(store.NonConforming, entryPath)
 
@@ -211,13 +240,13 @@ func addNonConforming(fsys fs.FS, store *Store, dir string, entry fs.DirEntry) e
 		return nil
 	}
 
-	entries, err := fs.ReadDir(fsys, entryPath)
+	entries, err := readDir(ctx, fsys, entryPath)
 	if err != nil {
 		return err
 	}
 
 	for _, inner := range entries {
-		err = addNonConforming(fsys, store, entryPath, inner)
+		err = addNonConforming(ctx, fsys, store, entryPath, inner)
 		if err != nil {
 			return err
 		}
@@ -227,7 +256,12 @@ func addNonConforming(fsys fs.FS, store *Store, dir string, entry fs.DirEntry) e
 }
 
 // newObject returns the Object for the file at filePath, with no DCF numbers.
-func newObject(store *Store, filePath string, entry fs.DirEntry) (Object, error) {
+func newObject(
+	fsys fs.FS,
+	store *Store,
+	filePath string,
+	entry fs.DirEntry,
+) (Object, error) {
 	info, err := entry.Info()
 	if err != nil {
 		return Object{}, err
@@ -238,6 +272,7 @@ func newObject(store *Store, filePath string, entry fs.DirEntry) (Object, error)
 		Path:         filePath,
 		Size:         info.Size(),
 		Extension:    path.Ext(filePath),
+		fsys:         fsys,
 	}, nil
 }
 

pkg/dcf/parse_internal_test.go

@@ -1,6 +1,7 @@
 package dcf
 
 import (
+	"context"
 	"errors"
 	"io/fs"
 	"os"
@@ -65,7 +66,7 @@ func addSymlink(t *testing.T, root, name, target string) {
 func mustParseStore(t *testing.T, fsys fs.FS) *Store {
 	t.Helper()
 
-	store, err := parseStore(fsys, testRoot)
+	store, err := parseStore(t.Context(), fsys, testRoot)
 	if err != nil {
 		t.Fatalf("parseStore: %v", err)
 	}
@@ -113,6 +114,16 @@ func directoryNumbers(store *Store) []int {
 	return numbers
 }
 
+// wantDirectory returns the Directory that parseStore should build for objects.
+func wantDirectory(number int, name string, objects ...*Object) *Directory {
+	return &Directory{
+		Number:     number,
+		Name:       name,
+		Objects:    objects,
+		FileGroups: newFileGroups(objects),
+	}
+}
+
 func assertPaths(t *testing.T, what string, got []string, want ...string) {
 	t.Helper()
 
@@ -147,6 +158,7 @@ func TestParseStorePutsEachFileWhereItBelongs(t *testing.T) {
 		Extension:       ".ARW",
 		DirectoryNumber: 100,
 		FileNumber:      1,
+		fsys:            card,
 	}
 	mp4 := Object{
 		DCFStoreRoot:    testRoot,
@@ -155,6 +167,7 @@ func TestParseStorePutsEachFileWhereItBelongs(t *testing.T) {
 		Extension:       ".MP4",
 		DirectoryNumber: 101,
 		FileNumber:      2,
+		fsys:            card,
 	}
 	thm := Object{
 		DCFStoreRoot:    testRoot,
@@ -163,17 +176,19 @@ func TestParseStorePutsEachFileWhereItBelongs(t *testing.T) {
 		Extension:       ".THM",
 		DirectoryNumber: 101,
 		FileNumber:      2,
+		fsys:            card,
 	}
 	clip := Object{
 		DCFStoreRoot: testRoot,
 		Path:         "PRIVATE/M4ROOT/CLIP/C0002.MP4",
 		Size:         400,
 		Extension:    ".MP4",
+		fsys:         card,
 	}
 
 	wantDirectories := []*Directory{
-		{Number: 100, Name: "100MSDCF", Objects: []*Object{&arw}},
+		wantDirectory(100, "100MSDCF", &arw),
-		{Number: 101, Name: "101CANON", Objects: []*Object{&mp4, &thm}},
+		wantDirectory(101, "101CANON", &mp4, &thm),
 	}
 	if !reflect.DeepEqual(store.Directories, wantDirectories) {
 		t.Errorf("Directories = %+v, want %+v", store.Directories, wantDirectories)
@@ -303,19 +318,18 @@ func TestParseNumberOfFileName(t *testing.T) {
 func TestParseStoreTreatsLowerCaseNamesAsDCFNames(t *testing.T) {
 	t.Parallel()
 
-	store := mustParseStore(t, newCard("DCIM/101abcde/abcd0002.jpg"))
+	card := newCard("DCIM/101abcde/abcd0002.jpg")
 
-	want := []*Directory{{
+	store := mustParseStore(t, card)
-		Number: 101,
+
-		Name:   "101abcde",
+	want := []*Directory{wantDirectory(101, "101abcde", &Object{
-		Objects: []*Object{{
 		DCFStoreRoot:    testRoot,
 		Path:            "DCIM/101abcde/abcd0002.jpg",
 		Extension:       ".jpg",
 		DirectoryNumber: 101,
 		FileNumber:      2,
-		}},
+		fsys:            card,
-	}}
+	})}
 	if !reflect.DeepEqual(store.Directories, want) {
 		t.Errorf("Directories = %+v, want %+v", store.Directories, want)
 	}
@@ -791,7 +805,7 @@ func TestParseStoreReturnsReadErrorsNamingTheCard(t *testing.T) {
 		for _, name := range names {
 			fsys := failingFS{MapFS: card, call: call, name: name}
 
-			_, err := parseStore(fsys, testRoot)
+			_, err := parseStore(t.Context(), fsys, testRoot)
 			if !errors.Is(err, fs.ErrPermission) {
 				t.Errorf("%s of %s: error = %v, want %v",
 					call, name, err, fs.ErrPermission)
@@ -807,3 +821,86 @@ func TestParseStoreReturnsReadErrorsNamingTheCard(t *testing.T) {
 		}
 	}
 }
+
+// cancelingFS is a card that cancels the read when the read reaches the named
+// directory. It records every directory read in read.
+type cancelingFS struct {
+	fstest.MapFS
+
+	name   string
+	cancel context.CancelFunc
+	read   *[]string
+}
+
+func (f cancelingFS) ReadDir(name string) ([]fs.DirEntry, error) {
+	*f.read = append(*f.read, name)
+
+	if name == f.name {
+		f.cancel()
+	}
+
+	return f.MapFS.ReadDir(name)
+}
+
+func TestParseStoreStopsBeforeTheNextDirectoryWhenCancelled(t *testing.T) {
+	t.Parallel()
+
+	card := newCard(
+		"DCIM/100ABCDE/SUB/README.TXT",
+		"DCIM/OTHER/SUB/README.TXT",
+		"PRIVATE/M4ROOT/CLIP/SUB/C0001.MP4",
+	)
+
+	// Each of these directories holds another directory, which must not be read.
+	names := []string{
+		dcimDirectory,
+		"DCIM/100ABCDE",
+		"DCIM/OTHER",
+		m4RootDirectory,
+		"PRIVATE/M4ROOT/CLIP",
+	}
+
+	for _, name := range names {
+		ctx, cancel := context.WithCancel(t.Context())
+
+		var read []string
+
+		fsys := cancelingFS{MapFS: card, name: name, cancel: cancel, read: &read}
+
+		_, err := parseStore(ctx, fsys, testRoot)
+		if !errors.Is(err, context.Canceled) {
+			t.Errorf("cancelled at %s: error = %v, want %v",
+				name, err, context.Canceled)
+		}
+
+		if read[len(read)-1] != name {
+			t.Errorf("cancelled at %s: read %q", name, read)
+		}
+	}
+}
+
+func TestParseStoreReadsNoDirectoryWhenAlreadyCancelled(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	cards := []fstest.MapFS{
+		newCard("DCIM/README.TXT"),
+		newCard("PRIVATE/M4ROOT/C0001.MP4"),
+	}
+
+	for _, card := range cards {
+		var read []string
+
+		// The context is already cancelled, so this needs no name to cancel at.
+		fsys := cancelingFS{MapFS: card, read: &read}
+
+		_, err := parseStore(ctx, fsys, testRoot)
+		if !errors.Is(err, context.Canceled) {
+			t.Errorf("error = %v, want %v", err, context.Canceled)
+		}
+
+		assertPaths(t, "directories read", read)
+	}
+}