Add calendar/address-book sharing backend

Introduce internal/db, a small SQLite-backed store (pure-Go
modernc.org/sqlite, no CGO) at <data_dir>/nidus.db holding
calendar_shares and addressbook_shares grant tables (owner, resource
name, shared-with user, read/write permission). This is the first step
towards user management and a web UI: a real datastore that a future
admin CLI/UI can build on, instead of the static config.yaml.

Wire sharing into the CalDAV/CardDAV backends:
- ListCalendars/ListAddressBooks now also include resources shared with
  the requesting user, exposed under the synthetic local name
  "<owner>~<name>" in the grantee's own home-set — no separate account,
  no data copying, the object still physically lives under the owner's
  store.Store namespace.
- All read paths (Get/List/QueryCalendarObjects, address book
  equivalents) resolve the synthetic name back to (owner, real name) and
  require any share (read or write) to exist.
- All write paths (Put/Delete object, DeleteCalendar/AddressBook)
  additionally require a write-permission share; read-only shares get a
  403 Forbidden.
- CreateCalendar/CreateAddressBook remain scoped to the acting user's own
  namespace — sharing an existing collection is done via ShareCalendar/
  ShareAddressBook, not by creating one directly in someone else's name.

Add internal/db/shares_test.go (grant/lookup/update/unshare/list
semantics) and internal/{caldav,carddav}/backend_test.go (shared
calendar/address book visibility, write permission enforcement,
unauthorized access rejection). Update README (features, new "Sharing
calendars and address books" section, project layout, dependencies) and
copilot-instructions.md to document the new package and sharing model.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/carddav/backend.go

@@ -14,24 +14,32 @@ import (
 	"github.com/emersion/go-webdav/carddav"
 	"github.com/yourusername/caldav-server/internal/auth"
 	"github.com/yourusername/caldav-server/internal/config"
+	"github.com/yourusername/caldav-server/internal/db"
 	"github.com/yourusername/caldav-server/internal/store"
 )
 
+// sharedNameSep separates the owner from the address book name in the
+// synthetic name used for address books shared with another user (see
+// caldav.sharedNameSep, kept identical for consistency).
+const sharedNameSep = "~"
+
 // Backend implements carddav.Backend using a filesystem store.
 type Backend struct {
 	cfg    *config.Config
 	store  *store.Store
+	dbase  *db.DB // may be nil if sharing is not configured
 	logger *slog.Logger
 }
 
-// NewBackend creates a CardDAV backend.
-func NewBackend(cfg *config.Config, st *store.Store, logger *slog.Logger) *Backend {
-	return &Backend{cfg: cfg, store: st, logger: logger}
+// NewBackend creates a CardDAV backend. dbase may be nil, in which case
+// address book sharing is disabled (only a user's own books are visible).
+func NewBackend(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.Logger) *Backend {
+	return &Backend{cfg: cfg, store: st, dbase: dbase, logger: logger}
 }
 
 // NewHandler returns an http.Handler for the /card/ prefix.
-func NewHandler(cfg *config.Config, st *store.Store, logger *slog.Logger) http.Handler {
-	b := NewBackend(cfg, st, logger)
+func NewHandler(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.Logger) http.Handler {
+	b := NewBackend(cfg, st, dbase, logger)
 	return &carddav.Handler{Backend: b}
 }
 
@@ -70,7 +78,7 @@ func (b *Backend) ListAddressBooks(ctx context.Context) ([]carddav.AddressBook,
 			b.logger.Warn("ensuring address book directory", "book", name, "error", err)
 			continue
 		}
-		books = append(books, b.bookMeta(p.Username, name))
+		books = append(books, b.bookMeta(p.Username, name, name))
 	}
 
 	// Also include extra books that exist on disk
@@ -82,7 +90,22 @@ func (b *Backend) ListAddressBooks(ctx context.Context) ([]carddav.AddressBook,
 	for _, dir := range disk {
 		if strings.HasPrefix(dir, "card-") && !configured[dir] {
 			name := strings.TrimPrefix(dir, "card-")
-			books = append(books, b.bookMeta(p.Username, name))
+			books = append(books, b.bookMeta(p.Username, name, name))
+		}
+	}
+
+	// Include address books other users have shared with this one.
+	if b.dbase != nil {
+		shares, err := b.dbase.AddressBooksSharedWith(p.Username)
+		if err != nil {
+			b.logger.Warn("listing shared address books", "error", err)
+		}
+		for _, sh := range shares {
+			if _, err := b.store.GetCollection(sh.Owner, "card-"+sh.AddressBookName); err != nil {
+				continue // owner's address book no longer exists
+			}
+			localName := sharedBookName(sh.Owner, sh.AddressBookName)
+			books = append(books, b.bookMeta(sh.Owner, sh.AddressBookName, localName))
 		}
 	}
 
@@ -90,24 +113,32 @@ func (b *Backend) ListAddressBooks(ctx context.Context) ([]carddav.AddressBook,
 }
 
 func (b *Backend) GetAddressBook(ctx context.Context, bookPath string) (*carddav.AddressBook, error) {
-	user, name, err := b.parseBookPath(ctx, bookPath)
+	requester, localName, err := b.parseBookPath(ctx, bookPath)
 	if err != nil {
 		return nil, err
 	}
-	if _, err := b.store.GetCollection(user, "card-"+name); err != nil {
+	owner, realName, _, err := b.resolveBook(requester, localName, false)
+	if err != nil {
+		return nil, err
+	}
+	if _, err := b.store.GetCollection(owner, "card-"+realName); err != nil {
 		return nil, webdav.NewHTTPError(http.StatusNotFound, err)
 	}
-	book := b.bookMeta(user, name)
+	book := b.bookMeta(owner, realName, localName)
 	return &book, nil
 }
 
 func (b *Backend) GetAddressObject(ctx context.Context, objPath string, req *carddav.AddressDataRequest) (*carddav.AddressObject, error) {
-	user, bookName, objID, err := b.parseObjPath(ctx, objPath)
+	requester, localName, objID, err := b.parseObjPath(ctx, objPath)
+	if err != nil {
+		return nil, err
+	}
+	owner, realName, _, err := b.resolveBook(requester, localName, false)
 	if err != nil {
 		return nil, err
 	}
 
-	data, err := b.store.GetObject(user, "card-"+bookName, objID)
+	data, err := b.store.GetObject(owner, "card-"+realName, objID)
 	if err != nil {
 		return nil, webdav.NewHTTPError(http.StatusNotFound, err)
 	}
@@ -116,23 +147,27 @@ func (b *Backend) GetAddressObject(ctx context.Context, objPath string, req *car
 }
 
 func (b *Backend) ListAddressObjects(ctx context.Context, bookPath string, req *carddav.AddressDataRequest) ([]carddav.AddressObject, error) {
-	user, bookName, err := b.parseBookPath(ctx, bookPath)
+	requester, localName, err := b.parseBookPath(ctx, bookPath)
+	if err != nil {
+		return nil, err
+	}
+	owner, realName, _, err := b.resolveBook(requester, localName, false)
 	if err != nil {
 		return nil, err
 	}
 
-	ids, err := b.store.ListObjects(user, "card-"+bookName)
+	ids, err := b.store.ListObjects(owner, "card-"+realName)
 	if err != nil {
 		return nil, err
 	}
 
 	var objs []carddav.AddressObject
 	for _, id := range ids {
-		data, err := b.store.GetObject(user, "card-"+bookName, id)
+		data, err := b.store.GetObject(owner, "card-"+realName, id)
 		if err != nil {
 			continue
 		}
-		obj, err := b.decodeObject(cardObjectPath(bookName, id), data)
+		obj, err := b.decodeObject(cardObjectPath(localName, id), data)
 		if err != nil {
 			b.logger.Warn("decoding vcard object", "id", id, "error", err)
 			continue
@@ -160,15 +195,23 @@ func (b *Backend) CreateAddressBook(ctx context.Context, book *carddav.AddressBo
 }
 
 func (b *Backend) DeleteAddressBook(ctx context.Context, bookPath string) error {
-	user, name, err := b.parseBookPath(ctx, bookPath)
+	requester, localName, err := b.parseBookPath(ctx, bookPath)
 	if err != nil {
 		return err
 	}
-	return b.store.DeleteCollection(user, "card-"+name)
+	owner, realName, _, err := b.resolveBook(requester, localName, true)
+	if err != nil {
+		return err
+	}
+	return b.store.DeleteCollection(owner, "card-"+realName)
 }
 
 func (b *Backend) PutAddressObject(ctx context.Context, objPath string, card vcard.Card, opts *carddav.PutAddressObjectOptions) (*carddav.AddressObject, error) {
-	user, bookName, objID, err := b.parseObjPath(ctx, objPath)
+	requester, localName, objID, err := b.parseObjPath(ctx, objPath)
+	if err != nil {
+		return nil, err
+	}
+	owner, realName, _, err := b.resolveBook(requester, localName, true)
 	if err != nil {
 		return nil, err
 	}
@@ -180,7 +223,7 @@ func (b *Backend) PutAddressObject(ctx context.Context, objPath string, card vca
 	}
 
 	data := []byte(buf.String())
-	if err := b.store.PutObject(user, "card-"+bookName, objID, data); err != nil {
+	if err := b.store.PutObject(owner, "card-"+realName, objID, data); err != nil {
 		return nil, fmt.Errorf("storing address object: %w", err)
 	}
 
@@ -188,11 +231,15 @@ func (b *Backend) PutAddressObject(ctx context.Context, objPath string, card vca
 }
 
 func (b *Backend) DeleteAddressObject(ctx context.Context, objPath string) error {
-	user, bookName, objID, err := b.parseObjPath(ctx, objPath)
+	requester, localName, objID, err := b.parseObjPath(ctx, objPath)
 	if err != nil {
 		return err
 	}
-	if err := b.store.DeleteObject(user, "card-"+bookName, objID); err != nil {
+	owner, realName, _, err := b.resolveBook(requester, localName, true)
+	if err != nil {
+		return err
+	}
+	if err := b.store.DeleteObject(owner, "card-"+realName, objID); err != nil {
 		return webdav.NewHTTPError(http.StatusNotFound, err)
 	}
 	return nil
@@ -200,11 +247,37 @@ func (b *Backend) DeleteAddressObject(ctx context.Context, objPath string) error
 
 // -------- helpers --------
 
-func (b *Backend) bookMeta(user, name string) carddav.AddressBook {
+// sharedBookName builds the synthetic local name a shared address book is
+// exposed under to the user it was shared with.
+func sharedBookName(owner, bookName string) string {
+	return owner + sharedNameSep + bookName
+}
+
+// resolveBook maps a local address book name (as seen in a URL path by
+// requester) to its real owner and on-disk name, checking permissions
+// along the way. Mirrors caldav.Backend.resolveCalendar.
+func (b *Backend) resolveBook(requester, localName string, requireWrite bool) (owner, realName string, perm db.Permission, err error) {
+	if ownerName, bookName, ok := strings.Cut(localName, sharedNameSep); ok {
+		if b.dbase == nil {
+			return "", "", "", webdav.NewHTTPError(http.StatusNotFound, fmt.Errorf("sharing not enabled"))
+		}
+		share, err := b.dbase.AddressBookShareFor(ownerName, bookName, requester)
+		if err != nil {
+			return "", "", "", webdav.NewHTTPError(http.StatusForbidden, fmt.Errorf("address book not shared with you"))
+		}
+		if requireWrite && share.Permission != db.PermWrite {
+			return "", "", "", webdav.NewHTTPError(http.StatusForbidden, fmt.Errorf("read-only share"))
+		}
+		return ownerName, bookName, share.Permission, nil
+	}
+	return requester, localName, db.PermWrite, nil
+}
+
+func (b *Backend) bookMeta(owner, realName, localName string) carddav.AddressBook {
 	return carddav.AddressBook{
-		Path:            cardHomePath() + name + "/",
-		Name:            name,
-		Description:     fmt.Sprintf("%s's %s address book", user, name),
+		Path:            cardHomePath() + localName + "/",
+		Name:            localName,
+		Description:     fmt.Sprintf("%s's %s address book", owner, realName),
 		MaxResourceSize: 10 * 1024 * 1024,
 	}
 }

internal/carddav/backend_test.go

@@ -0,0 +1,138 @@
+package carddav
+
+import (
+	"context"
+	"io"
+	"log/slog"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	vcard "github.com/emersion/go-vcard"
+	"github.com/yourusername/caldav-server/internal/auth"
+	"github.com/yourusername/caldav-server/internal/config"
+	"github.com/yourusername/caldav-server/internal/db"
+	"github.com/yourusername/caldav-server/internal/store"
+)
+
+func newTestBackend(t *testing.T) (*Backend, *db.DB) {
+	t.Helper()
+	dir := t.TempDir()
+
+	st, err := store.NewStore(filepath.Join(dir, "data"))
+	if err != nil {
+		t.Fatalf("NewStore: %v", err)
+	}
+	dbase, err := db.Open(filepath.Join(dir, "test.db"))
+	if err != nil {
+		t.Fatalf("db.Open: %v", err)
+	}
+	t.Cleanup(func() { dbase.Close() })
+
+	cfg := &config.Config{
+		Users: map[string]config.UserConfig{
+			"alice": {AddressBooks: []string{"contacts"}},
+			"bob":   {AddressBooks: []string{"personal"}},
+		},
+	}
+	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
+	return NewBackend(cfg, st, dbase, logger), dbase
+}
+
+func ctxFor(username string) context.Context {
+	return auth.NewContext(context.Background(), &auth.Principal{Username: username})
+}
+
+// minimalCard returns a minimal, valid vCard for use in tests.
+func minimalCard() vcard.Card {
+	const raw = "BEGIN:VCARD\r\n" +
+		"VERSION:3.0\r\n" +
+		"UID:card1@nidus.test\r\n" +
+		"FN:Test Person\r\n" +
+		"END:VCARD\r\n"
+	card, err := vcard.NewDecoder(strings.NewReader(raw)).Decode()
+	if err != nil {
+		panic("minimalCard: " + err.Error())
+	}
+	return card
+}
+
+func TestListAddressBooksIncludesSharedBook(t *testing.T) {
+	b, dbase := newTestBackend(t)
+
+	if err := dbase.ShareAddressBook("alice", "contacts", "bob", db.PermRead); err != nil {
+		t.Fatalf("ShareAddressBook: %v", err)
+	}
+	// The shared book must exist on disk for it to be listed; normally this
+	// happens when alice's own ListAddressBooks runs and ensures it.
+	if _, err := b.ListAddressBooks(ctxFor("alice")); err != nil {
+		t.Fatalf("ListAddressBooks(alice): %v", err)
+	}
+
+	books, err := b.ListAddressBooks(ctxFor("bob"))
+	if err != nil {
+		t.Fatalf("ListAddressBooks: %v", err)
+	}
+
+	var found bool
+	wantName := sharedBookName("alice", "contacts")
+	for _, book := range books {
+		if book.Name == wantName {
+			found = true
+		}
+	}
+	if !found {
+		t.Errorf("shared address book %q not found in ListAddressBooks result: %+v", wantName, books)
+	}
+}
+
+func TestSharedAddressBookReadOnlyRejectsWrite(t *testing.T) {
+	b, dbase := newTestBackend(t)
+
+	if err := dbase.ShareAddressBook("alice", "contacts", "bob", db.PermRead); err != nil {
+		t.Fatalf("ShareAddressBook: %v", err)
+	}
+
+	localName := sharedBookName("alice", "contacts")
+	objPath := cardObjectPath(localName, "card1.vcf")
+
+	_, err := b.PutAddressObject(ctxFor("bob"), objPath, minimalCard(), nil)
+	if err == nil {
+		t.Fatal("expected error writing to read-only shared address book, got nil")
+	}
+}
+
+func TestSharedAddressBookWriteAllowed(t *testing.T) {
+	b, dbase := newTestBackend(t)
+
+	if err := dbase.ShareAddressBook("alice", "contacts", "bob", db.PermWrite); err != nil {
+		t.Fatalf("ShareAddressBook: %v", err)
+	}
+
+	localName := sharedBookName("alice", "contacts")
+	objPath := cardObjectPath(localName, "card1.vcf")
+
+	if _, err := b.PutAddressObject(ctxFor("bob"), objPath, minimalCard(), nil); err != nil {
+		t.Fatalf("PutAddressObject with write share: %v", err)
+	}
+
+	// The object should now be visible under alice's own address book too,
+	// since it's stored in her namespace.
+	obj, err := b.GetAddressObject(ctxFor("alice"), cardObjectPath("contacts", "card1.vcf"), nil)
+	if err != nil {
+		t.Fatalf("GetAddressObject as owner: %v", err)
+	}
+	if obj == nil {
+		t.Fatal("expected non-nil object")
+	}
+}
+
+func TestUnauthorizedUserCannotAccessUnsharedBook(t *testing.T) {
+	b, _ := newTestBackend(t)
+
+	localName := sharedBookName("alice", "contacts")
+	_, err := b.GetAddressBook(ctxFor("bob"), cardHomePath()+localName+"/")
+	if err == nil {
+		t.Fatal("expected error accessing unshared address book, got nil")
+	}
+}