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

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
@@ -105,8 +110,8 @@ 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
-  each one.
+- `dcf.go` holds the public API. `GetStores` finds the DCF stores and reads each
+  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.
@@ -117,7 +122,9 @@ The main types are:
 - `Directory` is one DCF directory. It has a directory number, a name, the
   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
@@ -129,6 +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`:
 
@@ -182,8 +193,6 @@ The primary member is the first of these that the group holds:
 
 ## TODO
 
-- 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

@@ -7,10 +7,13 @@
 package dcf
 
 import (
+	"context"
 	"crypto/sha256"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"slices"
@@ -19,6 +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 {
@@ -76,6 +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.
@@ -173,15 +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 := os.Open(filepath.Clean(path))
+	file, err := d.fsys.Open(d.Path)
 	if err != nil {
-		return "", err
+		return "", fmt.Errorf("hashing %s: %w", d.FullFilePath(), err)
 	}
 
 	defer func() { _ = file.Close() }()
@@ -190,44 +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
-// any mounted filesystems. The integer argument is the number of DCF stores to
-// return. If the argument is 0, all DCF stores are returned. It does not mount
-// filesystems or search outside of filesystem root directories. It does,
-// however, read the DCIM and PRIVATE/M4ROOT directories of each filesystem to
-// populate the returned Stores. It never follows a symlink. If a card cannot be
-// read, it returns an error that names the card's mount point.
-func GetDCFStores(requestedCount int) (*[]Store, error) {
-	dcfStorePaths, err := findDCFMountPoints(requestedCount)
+// GetStores finds the DCF stores in the root of the mounted filesystems and reads
+// each one. It does not mount filesystems or search outside of filesystem root
+// directories. It reads the DCIM and PRIVATE/M4ROOT directories of each
+// filesystem to populate the returned Stores. It never follows a symlink. If a
+// card cannot be read, it returns an error that names the card's mount point.
+// When ctx is cancelled, the read stops before the next directory, and the error
+// matches ctx.Err().
+func GetStores(ctx context.Context) ([]*Store, error) {
+	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) {
-	dcfStores := []Store{}
+func parseStores(ctx context.Context, mountpoints []string) ([]*Store, error) {
+	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

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)
 	}
@@ -50,12 +54,92 @@ 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()
 

pkg/dcf/dcf_test.go

@@ -1,8 +1,9 @@
 package dcf_test
 
 import (
-	"os"
-	"path/filepath"
+	"context"
+	"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()
-
-	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"}
+	object := dcf.Object{DCFStoreRoot: cardRoot, Path: "a.jpg"}
 
 	_, err := object.Hash()
-	if err == nil {
-		t.Fatal("Hash() of a missing file returned no error")
+	if !errors.Is(err, fs.ErrInvalid) {
+		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/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
-// that hold a DCF store. Its only argument is an integer of how many mountpoints
-// to return. If the argument is 0, all mountpoints are returned.
-func findDCFMountPoints(requestedCount int) ([]string, error) {
-	mountpoints, err := findAllMountPoints()
+// findDCFMountPoints returns the paths of the mountpoints that hold a DCF store.
+func findDCFMountPoints(ctx context.Context) ([]string, error) {
+	mountpoints, err := findAllMountPoints(ctx)
 	if err != nil {
 		return nil, err
 	}
 
-	return filterDCFMountPoints(mountpoints, requestedCount), nil
+	return filterDCFMountPoints(mountpoints), nil
 }
 
-// filterDCFMountPoints returns the first requestedCount of the mountpoints that
-// hold a DCF store. If requestedCount is 0, it returns all of them.
-func filterDCFMountPoints(mountpoints []string, requestedCount int) []string {
+// filterDCFMountPoints returns the mountpoints that hold a DCF store.
+func filterDCFMountPoints(mountpoints []string) []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.
-	mountpoints := []string{newMountPoint(t), first, second, third}
+	got := filterDCFMountPoints([]string{newMountPoint(t), first, second})
 
-	cases := map[int][]string{
-		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)
+	want := []string{first, second}
 	if !slices.Equal(got, want) {
-			t.Errorf("filterDCFMountPoints(%d) = %v, want %v",
-				requestedCount, got, want)
-		}
+		t.Errorf("filterDCFMountPoints = %v, want %v", 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.
-func parseStore(fsys fs.FS, root string) (*Store, error) {
+// inside it. When ctx is cancelled, the read stops before the next directory.
+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
 		}
@@ -167,7 +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
@@ -185,11 +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
 			}
@@ -204,7 +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)
 
@@ -212,13 +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
 		}
@@ -228,7 +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
@@ -239,6 +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)
 	}
@@ -157,6 +158,7 @@ func TestParseStorePutsEachFileWhereItBelongs(t *testing.T) {
 		Extension:       ".ARW",
 		DirectoryNumber: 100,
 		FileNumber:      1,
+		fsys:            card,
 	}
 	mp4 := Object{
 		DCFStoreRoot:    testRoot,
@@ -165,6 +167,7 @@ func TestParseStorePutsEachFileWhereItBelongs(t *testing.T) {
 		Extension:       ".MP4",
 		DirectoryNumber: 101,
 		FileNumber:      2,
+		fsys:            card,
 	}
 	thm := Object{
 		DCFStoreRoot:    testRoot,
@@ -173,12 +176,14 @@ 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{
@@ -313,7 +318,9 @@ 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")
+
+	store := mustParseStore(t, card)
 
 	want := []*Directory{wantDirectory(101, "101abcde", &Object{
 		DCFStoreRoot:    testRoot,
@@ -321,6 +328,7 @@ func TestParseStoreTreatsLowerCaseNamesAsDCFNames(t *testing.T) {
 		Extension:       ".jpg",
 		DirectoryNumber: 101,
 		FileNumber:      2,
+		fsys:            card,
 	})}
 	if !reflect.DeepEqual(store.Directories, want) {
 		t.Errorf("Directories = %+v, want %+v", store.Directories, want)
@@ -797,7 +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)
@@ -813,3 +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)
+	}
+}