copy files off a card, verified by SHA256 (closes #5)

Store.Import copies every member of every file group and the images
and videos under PRIVATE/M4ROOT to dest/YYYY-MM-DD/<file name>. The day
comes from the new Object.ModTime. A file group stays in the folder of
its primary member.

Each copy is read back and compared by SHA256. A file with the same
hash already there is skipped, and anything else there is a conflict
and left alone. One failed file does not stop the others. A cancelled
context stops at once and removes the partial copy.

Object.Hash now shares its reading code with the import.

Model: opus-5-5

README.md

@@ -3,7 +3,7 @@
 ## Description
 
 `dcf` is a WTFPL-licensed Go library by [@sneak](https://sneak.berlin) that
-reads DCF memory cards from digital cameras.
+reads DCF memory cards from digital cameras and copies the files off them.
 
 DCF is the "Design rule for Camera File system". It is also JEITA specification
 number CP-3461. JEITA is the Japan Electronics and Information Technology
@@ -29,6 +29,7 @@ Then list the DCF stores on the mounted filesystems:
 package main
 
 import (
+	"context"
 	"fmt"
 	"log"
 
@@ -36,17 +37,53 @@ 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 copy the files off a card, call `Import` with a destination directory:
+
+```go
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+
+	"sneak.berlin/go/dcf/pkg/dcf"
+)
+
+func main() {
+	ctx := context.Background()
+
+	store, err := dcf.GetStore(ctx)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	result, err := store.Import(ctx, "/path/to/photos")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Println("copied:", result.Copied)
+	fmt.Println("skipped:", result.Skipped)
+	fmt.Println("conflicts:", result.Conflicts)
+}
+```
+
 To work on the library itself, you need docker and git:
 
 ```sh
@@ -105,11 +142,13 @@ 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 for reading a card. `GetStores` finds the DCF
-  each one.
+  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.
+- `import.go` holds `Store.Import`, which copies files off a card.
 
 The main types are:
 
@@ -117,11 +156,14 @@ 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.ModTime` is its modification time.
+  `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.
+- `ImportResult` lists what `Store.Import` did with each file.
 
 A mounted filesystem is a DCF store when its root holds a `DCIM` or
 `PRIVATE/M4ROOT` directory. It needs only one of them. `dcf` never mounts a
@@ -129,6 +171,10 @@ 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`:
 
@@ -180,11 +226,49 @@ The primary member is the first of these that the group holds:
 
 `Store.Images` and `Store.Videos` list each file on its own, whatever its group.
 
+`Store.Import` copies files off a card to a destination directory. It verifies
+each copy against the SHA256 hash of its source. It only reads the card, so it
+never deletes anything there.
+
+It copies:
+
+- every member of every file group
+- the images and videos under `PRIVATE/M4ROOT`
+
+It does not copy the files in `Store.NonConforming`.
+
+Each file goes to `YYYY-MM-DD/<file name>` under the destination directory. The
+date is the day of the file's modification time, in the computer's time zone.
+Every member of a file group goes to the day of its primary member, so a group
+never splits across two folders. Each copy keeps the modification time of its
+source. Only the user who ran the import can read the copies.
+
+`ImportResult` lists each destination path in one of three lists:
+
+- `Copied` holds the copies this import wrote and verified.
+- `Skipped` holds the paths that already held a file with the same SHA256 hash.
+- `Conflicts` holds the paths that already held anything else. `dcf` leaves that
+  alone and does not copy the file.
+
+A conflict is not an error, so a caller must check `Conflicts`. These are the
+usual causes:
+
+- Two different files have the same name and the same day. Two cards can reuse a
+  file number, and so can two DCF directories on one card. The first file is
+  copied, and the second is a conflict.
+- A partial file remains because the process died during a copy. Delete it and
+  import again.
+
+A copy that fails or does not match the hash of its source is removed. One file
+that fails does not stop the others. The returned error joins the failures, each
+naming its file. The result still lists what was done.
+
+When the context is cancelled, the import stops at once, even inside a file, and
+removes the partial copy. The returned error matches `context.Canceled`.
+
 ## TODO
 
-- Take a `context.Context` to cancel the walk of a store.
+No open items.
-- 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

@@ -3,22 +3,32 @@
 // 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. It
-// groups the files in each DCF directory that share a file number.
+// groups the files in each DCF directory that share a file number. It copies the
+// files off a card, and verifies each copy against its SHA256 hash.
 package dcf
 
 import (
-	"crypto/sha256"
+	"context"
-	"encoding/hex"
+	"errors"
 	"fmt"
-	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"slices"
 	"strings"
+	"time"
 
 	"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 {
@@ -72,10 +82,15 @@ type Object struct {
 	Path         string
 	Size         int64
 	Extension    string
+	// ModTime is the modification time of the file.
+	ModTime time.Time
 	// DirectoryNumber and FileNumber are the DCF numbers of a file in a DCF
 	// 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.
@@ -173,61 +188,80 @@ 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())
+	hash, err := d.hash(context.Background())
-}
-
-func pathToSHA256(path string) (string, error) {
-	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() }()
+	return hash, nil
-
-	hash := sha256.New()
-
-	_, err = io.Copy(hash, file)
-	if err != nil {
-		return "", err
 }
 
-	return hex.EncodeToString(hash.Sum(nil)), nil
+// hash returns the SHA256 hash of the file as a hex string, and stops when ctx is
+// cancelled.
+func (d *Object) hash(ctx context.Context) (string, error) {
+	if d.fsys == nil {
+		return "", fs.ErrInvalid
 	}
 
-// GetDCFStores returns a list of DCF stores found on the system in the root of
+	return hashFile(ctx, d.fsys, d.Path)
-// 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,
+// GetStores finds the DCF stores in the root of the mounted filesystems and reads
-// however, read the DCIM and PRIVATE/M4ROOT directories of each filesystem to
+// each one. It does not mount filesystems or search outside of filesystem root
-// populate the returned Stores. It never follows a symlink. If a card cannot be
+// directories. It reads the DCIM and PRIVATE/M4ROOT directories of each
-// read, it returns an error that names the card's mount point.
+// filesystem to populate the returned Stores. It never follows a symlink. If a
-func GetDCFStores(requestedCount int) (*[]Store, error) {
+// card cannot be read, it returns an error that names the card's mount point.
-	dcfStorePaths, err := findDCFMountPoints(requestedCount)
+// 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) {
+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

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"
+	"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/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/import.go

@@ -0,0 +1,279 @@
+package dcf
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"time"
+)
+
+// The modes of what Import creates. Only the user who ran it can read the copies.
+const (
+	importDirectoryMode = 0o750
+	importFileMode      = 0o600
+)
+
+var errHashMismatch = errors.New("the SHA256 hash of the copy differs from the source")
+
+// ImportResult lists what Store.Import did with each file, by destination path.
+type ImportResult struct {
+	// Copied holds the copies this import wrote and verified.
+	Copied []string
+	// Skipped holds the paths that already held a file with the same SHA256 hash.
+	Skipped []string
+	// Conflicts holds the paths that already held anything else. Import left it
+	// alone and did not copy the file.
+	Conflicts []string
+}
+
+// importFile is one file for Import to copy, and the path to copy it to.
+type importFile struct {
+	source *Object
+	target string
+}
+
+// Import copies the files of the store to dest, and verifies each copy against
+// the SHA256 hash of its source. It copies every member of every file group, and
+// the images and videos under PRIVATE/M4ROOT. It only reads the card.
+//
+// Each file goes to dest/YYYY-MM-DD/<file name>, from its modification time. Every
+// member of a file group goes to the day of its primary member, so a group never
+// splits across two folders.
+//
+// A file with the same hash already at the path is skipped. Anything else there is
+// a conflict, which is not an error. Import never overwrites anything.
+//
+// One file that fails does not stop the others. The returned error joins the
+// failures, each naming its file, and the result still lists what was done. When
+// ctx is cancelled, the import stops at once and removes the partial copy, and the
+// error matches ctx.Err().
+func (d *Store) Import(ctx context.Context, dest string) (ImportResult, error) {
+	var (
+		result   ImportResult
+		failures []error
+	)
+
+	for _, file := range d.importFiles(dest) {
+		err := ctx.Err()
+		if err != nil {
+			failures = append(failures, fmt.Errorf("import stopped: %w", err))
+
+			break
+		}
+
+		err = copyFile(ctx, file, &result)
+		if err != nil {
+			failures = append(failures,
+				fmt.Errorf("importing %s: %w", file.source.FullFilePath(), err))
+		}
+	}
+
+	return result, errors.Join(failures...)
+}
+
+// importFiles lists the files of the store that Import copies to dest.
+func (d *Store) importFiles(dest string) []importFile {
+	var files []importFile
+
+	add := func(source *Object, day time.Time) {
+		name := path.Base(source.Path)
+
+		files = append(files, importFile{
+			source: source,
+			target: filepath.Join(dest, day.Format(time.DateOnly), name),
+		})
+	}
+
+	for _, directory := range d.Directories {
+		for _, group := range directory.FileGroups {
+			for _, member := range group.Members {
+				add(member, group.Primary.ModTime)
+			}
+		}
+	}
+
+	// The files under PRIVATE/M4ROOT are the ones with no DCF numbers.
+	for _, image := range d.Images {
+		if image.DirectoryNumber == 0 {
+			add(&image.Object, image.ModTime)
+		}
+	}
+
+	for _, video := range d.Videos {
+		if video.DirectoryNumber == 0 {
+			add(&video.Object, video.ModTime)
+		}
+	}
+
+	return files
+}
+
+// copyFile copies one file, and adds its target to the list in result that says
+// what happened. It adds the target to no list when it returns an error.
+func copyFile(ctx context.Context, file importFile, result *ImportResult) error {
+	sourceHash, err := file.source.hash(ctx)
+	if err != nil {
+		return err
+	}
+
+	err = os.MkdirAll(filepath.Dir(file.target), importDirectoryMode)
+	if err != nil {
+		return err
+	}
+
+	// O_EXCL fails on anything already at the path, even a symlink, so nothing
+	// there can be overwritten.
+	target, err := os.OpenFile(
+		file.target, os.O_WRONLY|os.O_CREATE|os.O_EXCL, importFileMode,
+	)
+	if errors.Is(err, fs.ErrExist) {
+		return checkExisting(ctx, file.target, sourceHash, result)
+	}
+
+	if err != nil {
+		return err
+	}
+
+	err = writeCopy(ctx, file, target, sourceHash)
+	if err != nil {
+		return errors.Join(err, os.Remove(file.target))
+	}
+
+	result.Copied = append(result.Copied, file.target)
+
+	return nil
+}
+
+// checkExisting adds target to the skipped files if it is a file with the given
+// hash, and to the conflicts if it is anything else.
+func checkExisting(
+	ctx context.Context,
+	target, sourceHash string,
+	result *ImportResult,
+) error {
+	info, err := os.Lstat(target)
+	if err != nil {
+		return err
+	}
+
+	if !info.Mode().IsRegular() {
+		result.Conflicts = append(result.Conflicts, target)
+
+		return nil
+	}
+
+	targetHash, err := hashOnDisk(ctx, target)
+	if err != nil {
+		return err
+	}
+
+	if targetHash == sourceHash {
+		result.Skipped = append(result.Skipped, target)
+	} else {
+		result.Conflicts = append(result.Conflicts, target)
+	}
+
+	return nil
+}
+
+// writeCopy fills the new file target from the source, and closes it. It then
+// reads the written file back to compare its hash with the hash of the source.
+func writeCopy(
+	ctx context.Context,
+	file importFile,
+	target *os.File,
+	sourceHash string,
+) error {
+	err := writeAndClose(ctx, file.source, target)
+	if err != nil {
+		return err
+	}
+
+	targetHash, err := hashOnDisk(ctx, file.target)
+	if err != nil {
+		return err
+	}
+
+	if targetHash != sourceHash {
+		return errHashMismatch
+	}
+
+	// The zero time leaves the access time as it is.
+	return os.Chtimes(file.target, time.Time{}, file.source.ModTime)
+}
+
+// writeAndClose writes the bytes of source to target, syncs target and closes it.
+func writeAndClose(ctx context.Context, source *Object, target *os.File) error {
+	defer func() { _ = target.Close() }()
+
+	sourceFile, err := source.fsys.Open(source.Path)
+	if err != nil {
+		return err
+	}
+
+	defer func() { _ = sourceFile.Close() }()
+
+	_, err = io.Copy(target, contextReader(ctx, sourceFile))
+	if err != nil {
+		return err
+	}
+
+	err = target.Sync()
+	if err != nil {
+		return err
+	}
+
+	return target.Close()
+}
+
+// hashOnDisk returns the SHA256 hash of the file at filePath on the local
+// filesystem.
+func hashOnDisk(ctx context.Context, filePath string) (string, error) {
+	return hashFile(ctx, os.DirFS(filepath.Dir(filePath)), filepath.Base(filePath))
+}
+
+// hashFile returns the SHA256 hash of the named file as a hex string.
+func hashFile(ctx context.Context, fsys fs.FS, name string) (string, error) {
+	file, err := fsys.Open(name)
+	if err != nil {
+		return "", err
+	}
+
+	defer func() { _ = file.Close() }()
+
+	hash := sha256.New()
+
+	_, err = io.Copy(hash, contextReader(ctx, file))
+	if err != nil {
+		return "", err
+	}
+
+	return hex.EncodeToString(hash.Sum(nil)), nil
+}
+
+// readerFunc is a function that works as an io.Reader.
+type readerFunc func([]byte) (int, error)
+
+func (f readerFunc) Read(p []byte) (int, error) {
+	return f(p)
+}
+
+// contextReader returns a reader that fails with ctx.Err() once ctx is cancelled,
+// so a copy stops inside a file.
+func contextReader(ctx context.Context, reader io.Reader) io.Reader {
+	return readerFunc(func(p []byte) (int, error) {
+		err := ctx.Err()
+		if err != nil {
+			return 0, err
+		}
+
+		return reader.Read(p)
+	})
+}

pkg/dcf/import_internal_test.go

@@ -0,0 +1,521 @@
+package dcf
+
+import (
+	"context"
+	"errors"
+	"io/fs"
+	"maps"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"testing/fstest"
+	"time"
+)
+
+// testDay is the folder that a file from cardFile goes to. The other constants
+// are the paths and contents of the files on most cards in these tests.
+const (
+	testDay = "2026-10-03"
+
+	firstJPG      = "DCIM/100ABCDE/DSC00011.JPG"
+	firstJPGData  = "first jpeg"
+	secondJPG     = "DCIM/100ABCDE/DSC00012.JPG"
+	secondJPGData = "second jpeg"
+	testClip      = "PRIVATE/M4ROOT/CLIP/C0011.MP4"
+	testClipData  = "clip"
+)
+
+// cardFile returns a file on a card, modified at noon on testDay.
+func cardFile(contents string) *fstest.MapFile {
+	return &fstest.MapFile{
+		Data:    []byte(contents),
+		ModTime: time.Date(2026, 10, 3, 12, 0, 0, 0, time.UTC),
+	}
+}
+
+// inTestDay returns the paths of the named files in the testDay folder of dest.
+func inTestDay(dest string, names ...string) []string {
+	paths := make([]string, 0, len(names))
+
+	for _, name := range names {
+		paths = append(paths, filepath.Join(dest, testDay, name))
+	}
+
+	return paths
+}
+
+// assertDest checks that dest holds exactly the given files, as a map from each
+// file's path under dest to its contents.
+func assertDest(t *testing.T, dest string, want map[string]string) {
+	t.Helper()
+
+	fsys := os.DirFS(dest)
+	got := map[string]string{}
+
+	err := fs.WalkDir(fsys, ".",
+		func(name string, entry fs.DirEntry, err error) error {
+			if err != nil || !entry.Type().IsRegular() {
+				return err
+			}
+
+			data, err := fs.ReadFile(fsys, name)
+			got[name] = string(data)
+
+			return err
+		})
+	if err != nil {
+		t.Fatalf("reading %s: %v", dest, err)
+	}
+
+	if !maps.Equal(got, want) {
+		t.Errorf("destination holds %q, want %q", got, want)
+	}
+}
+
+func assertEmptyDirectory(t *testing.T, directory string) {
+	t.Helper()
+
+	entries, err := os.ReadDir(directory)
+	if err != nil {
+		t.Fatalf("ReadDir: %v", err)
+	}
+
+	if len(entries) != 0 {
+		t.Errorf("%s holds %v, want nothing", directory, entries)
+	}
+}
+
+// addDestFile writes a file at the named path under dest.
+func addDestFile(t *testing.T, dest, name, contents string) {
+	t.Helper()
+
+	fullPath := filepath.Join(dest, name)
+
+	err := os.MkdirAll(filepath.Dir(fullPath), 0o750)
+	if err != nil {
+		t.Fatalf("MkdirAll: %v", err)
+	}
+
+	err = os.WriteFile(fullPath, []byte(contents), 0o600)
+	if err != nil {
+		t.Fatalf("WriteFile: %v", err)
+	}
+}
+
+func TestImportCopiesEveryFileAndKeepsItsModificationTime(t *testing.T) {
+	t.Parallel()
+
+	dest := t.TempDir()
+
+	store := mustParseStore(t, fstest.MapFS{
+		"DCIM/100ABCDE/DSC00011.ARW":         cardFile("raw"),
+		firstJPG:                             cardFile(firstJPGData),
+		"DCIM/101ABCDE/MVI_0012.MP4":         cardFile("video"),
+		"DCIM/101ABCDE/MVI_0012.THM":         cardFile("thumbnail"),
+		"DCIM/MISC/DSC00013.JPG":             cardFile("not conforming"),
+		testClip:                             cardFile(testClipData),
+		"PRIVATE/M4ROOT/CLIP/C0011M01.XML":   cardFile("neither image nor video"),
+		"PRIVATE/M4ROOT/THMBNL/C0011T01.JPG": cardFile("clip thumbnail"),
+	})
+
+	result, err := store.Import(t.Context(), dest)
+	if err != nil {
+		t.Fatalf("Import: %v", err)
+	}
+
+	assertPaths(t, "Copied", result.Copied, inTestDay(dest,
+		"DSC00011.ARW",
+		"DSC00011.JPG",
+		"MVI_0012.MP4",
+		"MVI_0012.THM",
+		"C0011T01.JPG",
+		"C0011.MP4",
+	)...)
+	assertPaths(t, "Skipped", result.Skipped)
+	assertPaths(t, "Conflicts", result.Conflicts)
+
+	assertDest(t, dest, map[string]string{
+		testDay + "/DSC00011.ARW": "raw",
+		testDay + "/DSC00011.JPG": firstJPGData,
+		testDay + "/MVI_0012.MP4": "video",
+		testDay + "/MVI_0012.THM": "thumbnail",
+		testDay + "/C0011.MP4":    testClipData,
+		testDay + "/C0011T01.JPG": "clip thumbnail",
+	})
+
+	for _, copied := range result.Copied {
+		info, err := os.Stat(copied)
+		if err != nil {
+			t.Fatalf("Stat: %v", err)
+		}
+
+		if want := cardFile("").ModTime; !info.ModTime().Equal(want) {
+			t.Errorf("%s modified at %v, want %v", copied, info.ModTime(), want)
+		}
+	}
+}
+
+func TestImportTwiceSkipsEveryFileTheSecondTime(t *testing.T) {
+	t.Parallel()
+
+	dest := t.TempDir()
+
+	store := mustParseStore(t, fstest.MapFS{
+		firstJPG: cardFile(firstJPGData),
+		testClip: cardFile(testClipData),
+	})
+
+	_, err := store.Import(t.Context(), dest)
+	if err != nil {
+		t.Fatalf("first Import: %v", err)
+	}
+
+	result, err := store.Import(t.Context(), dest)
+	if err != nil {
+		t.Fatalf("second Import: %v", err)
+	}
+
+	assertPaths(t, "Copied", result.Copied)
+	assertPaths(t, "Skipped", result.Skipped,
+		inTestDay(dest, "DSC00011.JPG", "C0011.MP4")...)
+	assertPaths(t, "Conflicts", result.Conflicts)
+
+	assertDest(t, dest, map[string]string{
+		testDay + "/DSC00011.JPG": firstJPGData,
+		testDay + "/C0011.MP4":    testClipData,
+	})
+}
+
+func TestImportLeavesADifferentFileAloneAndReportsAConflict(t *testing.T) {
+	t.Parallel()
+
+	dest := t.TempDir()
+	addDestFile(t, dest, testDay+"/DSC00011.JPG", "different")
+
+	store := mustParseStore(t, fstest.MapFS{
+		firstJPG:  cardFile(firstJPGData),
+		secondJPG: cardFile(secondJPGData),
+	})
+
+	result, err := store.Import(t.Context(), dest)
+	if err != nil {
+		t.Fatalf("Import: %v", err)
+	}
+
+	assertPaths(t, "Copied", result.Copied, inTestDay(dest, "DSC00012.JPG")...)
+	assertPaths(t, "Skipped", result.Skipped)
+	assertPaths(t, "Conflicts", result.Conflicts, inTestDay(dest, "DSC00011.JPG")...)
+
+	assertDest(t, dest, map[string]string{
+		testDay + "/DSC00011.JPG": "different",
+		testDay + "/DSC00012.JPG": secondJPGData,
+	})
+}
+
+func TestImportReportsAConflictBetweenTwoFilesWithOneName(t *testing.T) {
+	t.Parallel()
+
+	dest := t.TempDir()
+
+	store := mustParseStore(t, fstest.MapFS{
+		firstJPG:                     cardFile(firstJPGData),
+		"DCIM/101ABCDE/DSC00011.JPG": cardFile(secondJPGData),
+	})
+
+	result, err := store.Import(t.Context(), dest)
+	if err != nil {
+		t.Fatalf("Import: %v", err)
+	}
+
+	assertPaths(t, "Copied", result.Copied, inTestDay(dest, "DSC00011.JPG")...)
+	assertPaths(t, "Conflicts", result.Conflicts, inTestDay(dest, "DSC00011.JPG")...)
+	assertDest(t, dest, map[string]string{testDay + "/DSC00011.JPG": firstJPGData})
+}
+
+func TestImportReportsAConflictAtADirectoryOrASymlink(t *testing.T) {
+	t.Parallel()
+
+	dest := t.TempDir()
+
+	// The symlink points at a file with the same bytes, which must not count.
+	outside := newCardOnDisk(t, "OUTSIDE.JPG")
+
+	err := os.MkdirAll(filepath.Join(dest, testDay, "DSC00011.JPG"), 0o750)
+	if err != nil {
+		t.Fatalf("MkdirAll: %v", err)
+	}
+
+	addSymlink(t, dest, testDay+"/DSC00012.JPG", filepath.Join(outside, "OUTSIDE.JPG"))
+
+	store := mustParseStore(t, fstest.MapFS{
+		firstJPG:  cardFile("abc"),
+		secondJPG: cardFile("abc"),
+	})
+
+	result, err := store.Import(t.Context(), dest)
+	if err != nil {
+		t.Fatalf("Import: %v", err)
+	}
+
+	assertPaths(t, "Copied", result.Copied)
+	assertPaths(t, "Skipped", result.Skipped)
+	assertPaths(t, "Conflicts", result.Conflicts,
+		inTestDay(dest, "DSC00011.JPG", "DSC00012.JPG")...)
+}
+
+func TestImportPutsAFileGroupInTheFolderOfItsPrimaryMember(t *testing.T) {
+	t.Parallel()
+
+	dest := t.TempDir()
+
+	beforeMidnight := time.Date(2026, 10, 3, 23, 59, 59, 0, time.UTC)
+	afterMidnight := time.Date(2026, 10, 4, 0, 0, 1, 0, time.UTC)
+
+	store := mustParseStore(t, fstest.MapFS{
+		"DCIM/100ABCDE/DSC00011.ARW": {Data: []byte("raw"), ModTime: afterMidnight},
+		firstJPG:                     {Data: []byte("jpeg"), ModTime: beforeMidnight},
+		secondJPG:                    {Data: []byte("next"), ModTime: afterMidnight},
+	})
+
+	_, err := store.Import(t.Context(), dest)
+	if err != nil {
+		t.Fatalf("Import: %v", err)
+	}
+
+	assertDest(t, dest, map[string]string{
+		"2026-10-03/DSC00011.ARW": "raw",
+		"2026-10-03/DSC00011.JPG": "jpeg",
+		"2026-10-04/DSC00012.JPG": "next",
+	})
+
+	// The copy keeps its own modification time, whichever folder it is in.
+	info, err := os.Stat(filepath.Join(dest, "2026-10-03/DSC00011.ARW"))
+	if err != nil {
+		t.Fatalf("Stat: %v", err)
+	}
+
+	if !info.ModTime().Equal(afterMidnight) {
+		t.Errorf("modified at %v, want %v", info.ModTime(), afterMidnight)
+	}
+}
+
+// changingFS is a card whose file at name holds other bytes from its second read
+// on. It counts the reads in opens.
+type changingFS struct {
+	fstest.MapFS
+
+	name  string
+	opens *int
+}
+
+func (f changingFS) Open(name string) (fs.File, error) {
+	if name == f.name {
+		*f.opens++
+		if *f.opens > 1 {
+			return fstest.MapFS{name: cardFile("other bytes")}.Open(name)
+		}
+	}
+
+	return f.MapFS.Open(name)
+}
+
+func TestImportRemovesACopyThatDoesNotMatchItsSource(t *testing.T) {
+	t.Parallel()
+
+	dest := t.TempDir()
+	opens := 0
+
+	store := mustParseStore(t, changingFS{
+		MapFS: fstest.MapFS{
+			firstJPG:  cardFile(firstJPGData),
+			secondJPG: cardFile(secondJPGData),
+		},
+		name:  firstJPG,
+		opens: &opens,
+	})
+
+	result, err := store.Import(t.Context(), dest)
+	if !errors.Is(err, errHashMismatch) {
+		t.Fatalf("Import error = %v, want %v", err, errHashMismatch)
+	}
+
+	if !strings.Contains(err.Error(), firstJPG) {
+		t.Errorf("Import error = %v, want one naming %s", err, firstJPG)
+	}
+
+	assertPaths(t, "Copied", result.Copied, inTestDay(dest, "DSC00012.JPG")...)
+	assertDest(t, dest, map[string]string{testDay + "/DSC00012.JPG": secondJPGData})
+}
+
+func TestImportCopiesTheOtherFilesWhenOneCannotBeRead(t *testing.T) {
+	t.Parallel()
+
+	dest := t.TempDir()
+
+	store := mustParseStore(t, failingFS{
+		MapFS: fstest.MapFS{
+			firstJPG:  cardFile(firstJPGData),
+			secondJPG: cardFile(secondJPGData),
+			testClip:  cardFile(testClipData),
+		},
+		call: "Open",
+		name: firstJPG,
+	})
+
+	result, err := store.Import(t.Context(), dest)
+	if !errors.Is(err, fs.ErrPermission) {
+		t.Fatalf("Import error = %v, want %v", err, fs.ErrPermission)
+	}
+
+	if !strings.Contains(err.Error(), firstJPG) {
+		t.Errorf("Import error = %v, want one naming %s", err, firstJPG)
+	}
+
+	assertPaths(t, "Copied", result.Copied,
+		inTestDay(dest, "DSC00012.JPG", "C0011.MP4")...)
+	assertDest(t, dest, map[string]string{
+		testDay + "/DSC00012.JPG": secondJPGData,
+		testDay + "/C0011.MP4":    testClipData,
+	})
+}
+
+func TestImportNamesEveryFileThatFails(t *testing.T) {
+	t.Parallel()
+
+	dest := t.TempDir()
+
+	// A plain file where the day folder belongs fails every file of that day.
+	addDestFile(t, dest, testDay, "not a folder")
+
+	store := mustParseStore(t, fstest.MapFS{
+		firstJPG:  cardFile(firstJPGData),
+		secondJPG: cardFile(secondJPGData),
+	})
+
+	result, err := store.Import(t.Context(), dest)
+	if err == nil {
+		t.Fatal("Import returned no error")
+	}
+
+	for _, name := range []string{firstJPG, secondJPG} {
+		if !strings.Contains(err.Error(), name) {
+			t.Errorf("Import error = %v, want one naming %s", err, name)
+		}
+	}
+
+	assertPaths(t, "Copied", result.Copied)
+}
+
+func TestImportOfObjectBuiltByHandReturnsError(t *testing.T) {
+	t.Parallel()
+
+	dest := t.TempDir()
+
+	store := &Store{Images: []*Image{{Object: Object{Path: "DSC00011.JPG"}}}}
+
+	_, err := store.Import(t.Context(), dest)
+	if !errors.Is(err, fs.ErrInvalid) {
+		t.Fatalf("Import error = %v, want %v", err, fs.ErrInvalid)
+	}
+
+	assertEmptyDirectory(t, dest)
+}
+
+// cancelingFile is an open file that cancels the import after each read.
+type cancelingFile struct {
+	fs.File
+
+	cancel context.CancelFunc
+}
+
+func (f cancelingFile) Read(p []byte) (int, error) {
+	n, err := f.File.Read(p)
+
+	f.cancel()
+
+	return n, err
+}
+
+// cancelingReadFS is a card that cancels the import during the read of the named
+// file, the time that file is opened for the openCount-th time. It counts the
+// reads in opens.
+type cancelingReadFS struct {
+	fstest.MapFS
+
+	name      string
+	openCount int
+	opens     *int
+	cancel    context.CancelFunc
+}
+
+func (f cancelingReadFS) Open(name string) (fs.File, error) {
+	file, err := f.MapFS.Open(name)
+	if err != nil || name != f.name {
+		return file, err
+	}
+
+	*f.opens++
+	if *f.opens != f.openCount {
+		return file, nil
+	}
+
+	return cancelingFile{File: file, cancel: f.cancel}, nil
+}
+
+func TestImportStopsInsideAFileWhenCancelledAndRemovesThePartialCopy(t *testing.T) {
+	t.Parallel()
+
+	// The first read of a file is hashing it, and the second is copying it.
+	for _, openCount := range []int{1, 2} {
+		ctx, cancel := context.WithCancel(t.Context())
+		dest := t.TempDir()
+		opens := 0
+
+		store := mustParseStore(t, cancelingReadFS{
+			MapFS: fstest.MapFS{
+				// Larger than one read, so the cancellation falls inside the file.
+				firstJPG:  {Data: make([]byte, 1<<20)},
+				secondJPG: cardFile(secondJPGData),
+			},
+			name:      firstJPG,
+			openCount: openCount,
+			opens:     &opens,
+			cancel:    cancel,
+		})
+
+		result, err := store.Import(ctx, dest)
+		if !errors.Is(err, context.Canceled) {
+			t.Errorf("cancelled in read %d: error = %v, want %v",
+				openCount, err, context.Canceled)
+		}
+
+		assertPaths(t, "Copied", result.Copied)
+		assertDest(t, dest, map[string]string{})
+
+		if opens != openCount {
+			t.Errorf("cancelled in read %d: file opened %d times", openCount, opens)
+		}
+
+		cancel()
+	}
+}
+
+func TestImportCopiesNothingWhenAlreadyCancelled(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithCancel(t.Context())
+	cancel()
+
+	dest := t.TempDir()
+
+	store := mustParseStore(t, fstest.MapFS{firstJPG: cardFile(firstJPGData)})
+
+	result, err := store.Import(ctx, dest)
+	if !errors.Is(err, context.Canceled) {
+		t.Fatalf("Import error = %v, want %v", err, context.Canceled)
+	}
+
+	assertPaths(t, "Copied", result.Copied)
+	assertEmptyDirectory(t, dest)
+}

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
 		}
@@ -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 +272,8 @@ func newObject(store *Store, filePath string, entry fs.DirEntry) (Object, error)
 		Path:         filePath,
 		Size:         info.Size(),
 		Extension:    path.Ext(filePath),
+		ModTime:      info.ModTime(),
+		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)
@@ -712,7 +720,7 @@ func TestParseStoreTreatsSymlinksAsMissingDirectories(t *testing.T) {
 }
 
 // failingFS is a card on which one call fails for one path, the way a failing
-// card does. The call is Lstat, ReadDir or Info.
+// card does. The call is Open, Lstat, ReadDir or Info.
 type failingFS struct {
 	fstest.MapFS
 
@@ -720,6 +728,15 @@ type failingFS struct {
 	name string
 }
 
+func (f failingFS) Open(name string) (fs.File, error) {
+	err := f.failure("Open", name)
+	if err != nil {
+		return nil, err
+	}
+
+	return f.MapFS.Open(name)
+}
+
 func (f failingFS) Lstat(name string) (fs.FileInfo, error) {
 	err := f.failure("Lstat", name)
 	if err != nil {
@@ -797,7 +814,7 @@ 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 +830,86 @@ 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)
+	}
+}