feat(taildrop): fix DirectFileMode, void callbacks, and empty WaitingFiles

- Add SetStagedFileOps to Extension: sets fileOps without enabling
  DirectFileMode, so WASM clients use staged retrieval (WaitingFiles,
  OpenFile, DeleteFile) instead of direct-write mode.
- Add directFileOps bool field: SetFileOps (Android SAF) sets it true;
  SetStagedFileOps (WASM JS) leaves it false. onChangeProfile now uses
  `fops != nil && e.directFileOps` to determine DirectFileMode.
- Add jsCallVoid to jsFileOps: void ops (openWriter, write, closeWriter,
  remove) now use cb(err?: string) instead of cb(null, err: string).
- Fix waitingFiles() returning JSON null when no files are waiting:
  normalise nil slice to empty slice before marshalling.
- Update wireTaildropFileOps to call SetStagedFileOps.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/tsconnect/wasm/taildrop.go

@@ -0,0 +1,370 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+// This file bridges the Taildrop FileOps interface to JS callbacks,
+// using the same channel+FuncOf pattern as the Go stdlib's WASM HTTP
+// transport (src/net/http/roundtrip_js.go): Go passes a js.FuncOf to JS,
+// then blocks on a channel until JS calls it back — which may be async.
+
+package main
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"net/http"
+	"net/url"
+	"os"
+	"syscall/js"
+	"time"
+
+	"tailscale.com/client/tailscale/apitype"
+	"tailscale.com/feature/taildrop"
+	"tailscale.com/ipn/ipnlocal"
+	"tailscale.com/tailcfg"
+)
+
+// Compile-time check that jsFileOps implements taildrop.FileOps.
+var _ taildrop.FileOps = (*jsFileOps)(nil)
+
+// taildropExt returns the taildrop extension, or an error if unavailable.
+func (i *jsIPN) taildropExt() (*taildrop.Extension, error) {
+	ext, ok := ipnlocal.GetExt[*taildrop.Extension](i.lb)
+	if !ok {
+		return nil, errors.New("taildrop extension not available")
+	}
+	return ext, nil
+}
+
+// listFileTargets returns the peers that can receive Taildrop files as a JSON
+// array of {stableNodeID, name, addresses, os} objects.
+func (i *jsIPN) listFileTargets() js.Value {
+	return makePromise(func() (any, error) {
+		ext, err := i.taildropExt()
+		if err != nil {
+			return nil, err
+		}
+		fts, err := ext.FileTargets()
+		if err != nil {
+			return nil, err
+		}
+		type jsTarget struct {
+			StableNodeID string   `json:"stableNodeID"`
+			Name         string   `json:"name"`
+			Addresses    []string `json:"addresses"`
+			OS           string   `json:"os"`
+		}
+		out := make([]jsTarget, 0, len(fts))
+		for _, ft := range fts {
+			addrs := make([]string, 0, len(ft.Node.Addresses))
+			for _, a := range ft.Node.Addresses {
+				addrs = append(addrs, a.Addr().String())
+			}
+			out = append(out, jsTarget{
+				StableNodeID: string(ft.Node.StableID),
+				Name:         ft.Node.Name,
+				Addresses:    addrs,
+				OS:           ft.Node.Hostinfo.OS(),
+			})
+		}
+		b, err := json.Marshal(out)
+		if err != nil {
+			return nil, err
+		}
+		return string(b), nil
+	})
+}
+
+// sendFile sends data as filename to the peer identified by stableNodeID.
+func (i *jsIPN) sendFile(stableNodeID, filename string, data js.Value) js.Value {
+	return makePromise(func() (any, error) {
+		ext, err := i.taildropExt()
+		if err != nil {
+			return nil, err
+		}
+		fts, err := ext.FileTargets()
+		if err != nil {
+			return nil, err
+		}
+		var ft *apitype.FileTarget
+		for _, x := range fts {
+			if x.Node.StableID == tailcfg.StableNodeID(stableNodeID) {
+				ft = x
+				break
+			}
+		}
+		if ft == nil {
+			return nil, fmt.Errorf("node %q not found or not a file target", stableNodeID)
+		}
+		dstURL, err := url.Parse(ft.PeerAPIURL)
+		if err != nil {
+			return nil, fmt.Errorf("bogus peer URL: %w", err)
+		}
+		b := make([]byte, data.Get("byteLength").Int())
+		js.CopyBytesToGo(b, data)
+		req, err := http.NewRequest("PUT", dstURL.String()+"/v0/put/"+url.PathEscape(filename), bytes.NewReader(b))
+		if err != nil {
+			return nil, err
+		}
+		req.ContentLength = int64(len(b))
+		client := &http.Client{Transport: i.lb.Dialer().PeerAPITransport()}
+		resp, err := client.Do(req)
+		if err != nil {
+			return nil, err
+		}
+		defer resp.Body.Close()
+		if resp.StatusCode != http.StatusOK && resp.StatusCode != http.StatusCreated {
+			body, _ := io.ReadAll(resp.Body)
+			return nil, fmt.Errorf("send file: %s: %s", resp.Status, bytes.TrimSpace(body))
+		}
+		return nil, nil
+	})
+}
+
+// waitingFiles returns received files waiting for pickup as a JSON array of
+// {name, size} objects. Always returns an array (never null).
+func (i *jsIPN) waitingFiles() js.Value {
+	return makePromise(func() (any, error) {
+		ext, err := i.taildropExt()
+		if err != nil {
+			return nil, err
+		}
+		wfs, err := ext.WaitingFiles()
+		if err != nil {
+			return nil, err
+		}
+		if wfs == nil {
+			wfs = []apitype.WaitingFile{}
+		}
+		b, err := json.Marshal(wfs)
+		if err != nil {
+			return nil, err
+		}
+		return string(b), nil
+	})
+}
+
+// openWaitingFile returns the contents of a received file as a Uint8Array.
+func (i *jsIPN) openWaitingFile(name string) js.Value {
+	return makePromise(func() (any, error) {
+		ext, err := i.taildropExt()
+		if err != nil {
+			return nil, err
+		}
+		rc, _, err := ext.OpenFile(name)
+		if err != nil {
+			return nil, err
+		}
+		defer rc.Close()
+		data, err := io.ReadAll(rc)
+		if err != nil {
+			return nil, err
+		}
+		buf := js.Global().Get("Uint8Array").New(len(data))
+		js.CopyBytesToJS(buf, data)
+		return buf, nil
+	})
+}
+
+// deleteWaitingFile deletes a received file by name.
+func (i *jsIPN) deleteWaitingFile(name string) js.Value {
+	return makePromise(func() (any, error) {
+		ext, err := i.taildropExt()
+		if err != nil {
+			return nil, err
+		}
+		return nil, ext.DeleteFile(name)
+	})
+}
+
+// wireTaildropFileOps installs a JS-backed FileOps on the taildrop extension
+// if jsObj is a non-null JS object. It must be called after NewLocalBackend
+// and before lb.Start (i.e. before run() is called by the user), so that the
+// FileOps is in place when the extension's onChangeProfile hook fires on init.
+//
+// SetStagedFileOps is used instead of SetFileOps so that files are staged for
+// explicit retrieval via WaitingFiles/OpenFile rather than delivered directly
+// (DirectFileMode=false). The JS caller fetches them via waitingFiles() et al.
+func wireTaildropFileOps(lb *ipnlocal.LocalBackend, jsObj js.Value) {
+	if jsObj.IsUndefined() || jsObj.IsNull() {
+		return
+	}
+	ext, ok := ipnlocal.GetExt[*taildrop.Extension](lb)
+	if !ok {
+		return
+	}
+	ext.SetStagedFileOps(&jsFileOps{v: jsObj})
+}
+
+// jsFileOps implements [taildrop.FileOps] by delegating to JS callbacks.
+// JS methods use one of two callback conventions:
+//
+// Void ops (openWriter, write, closeWriter, remove): cb(err?: string)
+//
+//	on success: cb()  or  cb("")
+//	on error:   cb("error message")
+//	not found:  cb("ENOENT")
+//
+// Result ops (rename, listFiles, stat, openReader): cb(result: T | null, err?: string)
+//
+//	on success: cb(result)
+//	on error:   cb(null, "error message")
+//	not found:  cb(null, "ENOENT")
+type jsFileOps struct {
+	v js.Value
+}
+
+// jsCallResult invokes method on j.v, appending a Go-owned js.FuncOf as the
+// final argument. It blocks until JS calls back with (result, errStr?), then
+// returns (result, error). An absent or empty errStr means success.
+//
+// JS convention for result ops: cb(result: T | null, err?: string)
+func (j jsFileOps) jsCallResult(method string, args ...any) (js.Value, error) {
+	type result struct {
+		val js.Value
+		err error
+	}
+	ch := make(chan result, 1)
+	cb := js.FuncOf(func(this js.Value, cbArgs []js.Value) any {
+		var r result
+		if len(cbArgs) > 0 {
+			if t := cbArgs[0].Type(); t != js.TypeNull && t != js.TypeUndefined {
+				r.val = cbArgs[0]
+			}
+		}
+		if len(cbArgs) > 1 && cbArgs[1].Type() == js.TypeString {
+			if s := cbArgs[1].String(); s != "" {
+				r.err = errors.New(s)
+			}
+		}
+		ch <- r
+		return nil
+	})
+	defer cb.Release()
+	j.v.Call(method, append(args, cb)...)
+	r := <-ch
+	return r.val, r.err
+}
+
+// jsCallVoid invokes method on j.v for operations that return no result,
+// appending a Go-owned js.FuncOf as the final argument. It blocks until JS
+// calls back with an optional error string, then returns the error or nil.
+//
+// JS convention for void ops: cb(err?: string)
+func (j jsFileOps) jsCallVoid(method string, args ...any) error {
+	ch := make(chan error, 1)
+	cb := js.FuncOf(func(this js.Value, cbArgs []js.Value) any {
+		var err error
+		if len(cbArgs) > 0 && cbArgs[0].Type() == js.TypeString {
+			if s := cbArgs[0].String(); s != "" {
+				err = errors.New(s)
+			}
+		}
+		ch <- err
+		return nil
+	})
+	defer cb.Release()
+	j.v.Call(method, append(args, cb)...)
+	return <-ch
+}
+
+// isJSNotExist reports whether err is the sentinel "ENOENT" from JS.
+func isJSNotExist(err error) bool {
+	return err != nil && err.Error() == "ENOENT"
+}
+
+func (j jsFileOps) OpenWriter(name string, offset int64, _ os.FileMode) (io.WriteCloser, string, error) {
+	if err := j.jsCallVoid("openWriter", name, offset); err != nil {
+		return nil, "", err
+	}
+	return &jsWriteCloser{ops: j, name: name}, name, nil
+}
+
+type jsWriteCloser struct {
+	ops  jsFileOps
+	name string
+}
+
+func (w *jsWriteCloser) Write(p []byte) (int, error) {
+	buf := js.Global().Get("Uint8Array").New(len(p))
+	js.CopyBytesToJS(buf, p)
+	if err := w.ops.jsCallVoid("write", w.name, buf); err != nil {
+		return 0, err
+	}
+	return len(p), nil
+}
+
+func (w *jsWriteCloser) Close() error {
+	return w.ops.jsCallVoid("closeWriter", w.name)
+}
+
+func (j jsFileOps) Remove(name string) error {
+	err := j.jsCallVoid("remove", name)
+	if isJSNotExist(err) {
+		return &fs.PathError{Op: "remove", Path: name, Err: fs.ErrNotExist}
+	}
+	return err
+}
+
+func (j jsFileOps) Rename(oldPath, newName string) (string, error) {
+	val, err := j.jsCallResult("rename", oldPath, newName)
+	if err != nil {
+		return "", err
+	}
+	return val.String(), nil
+}
+
+func (j jsFileOps) ListFiles() ([]string, error) {
+	val, err := j.jsCallResult("listFiles")
+	if err != nil {
+		return nil, err
+	}
+	n := val.Length()
+	names := make([]string, n)
+	for i := 0; i < n; i++ {
+		names[i] = val.Index(i).String()
+	}
+	return names, nil
+}
+
+func (j jsFileOps) Stat(name string) (fs.FileInfo, error) {
+	val, err := j.jsCallResult("stat", name)
+	if isJSNotExist(err) {
+		return nil, &fs.PathError{Op: "stat", Path: name, Err: fs.ErrNotExist}
+	}
+	if err != nil {
+		return nil, err
+	}
+	// Use Float to correctly handle files larger than 2 GiB (int is 32-bit on wasm).
+	return &jsFileInfo{name: name, size: int64(val.Float())}, nil
+}
+
+func (j jsFileOps) OpenReader(name string) (io.ReadCloser, error) {
+	val, err := j.jsCallResult("openReader", name)
+	if isJSNotExist(err) {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+	}
+	if err != nil {
+		return nil, err
+	}
+	b := make([]byte, val.Get("byteLength").Int())
+	js.CopyBytesToGo(b, val)
+	return io.NopCloser(bytes.NewReader(b)), nil
+}
+
+// jsFileInfo is a minimal [fs.FileInfo] backed by a name and a size.
+// Only Size() is used by the taildrop manager; the other fields are stubs.
+type jsFileInfo struct {
+	name string
+	size int64
+}
+
+func (i *jsFileInfo) Name() string       { return i.name }
+func (i *jsFileInfo) Size() int64        { return i.size }
+func (i *jsFileInfo) Mode() fs.FileMode  { return 0o444 }
+func (i *jsFileInfo) ModTime() time.Time { return time.Time{} }
+func (i *jsFileInfo) IsDir() bool        { return false }
+func (i *jsFileInfo) Sys() any           { return nil }

cmd/tsconnect/wasm/wasm_js.go

@@ -160,6 +160,7 @@ func newIPN(jsConfig js.Value) map[string]any {
 	if err := ns.Start(lb); err != nil {
 		log.Fatalf("failed to start netstack: %v", err)
 	}
+	wireTaildropFileOps(lb, jsConfig.Get("fileOps"))
 	srv.SetLocalBackend(lb)
 
 	jsIPN := &jsIPN{
@@ -267,6 +268,33 @@ func newIPN(jsConfig js.Value) map[string]any {
 			}
 			return jsIPN.setExitNodeEnabled(args[0].Bool())
 		}),
+		"listFileTargets": js.FuncOf(func(this js.Value, args []js.Value) any {
+			return jsIPN.listFileTargets()
+		}),
+		"sendFile": js.FuncOf(func(this js.Value, args []js.Value) any {
+			if len(args) != 3 {
+				log.Printf("Usage: sendFile(stableNodeID, filename, data)")
+				return nil
+			}
+			return jsIPN.sendFile(args[0].String(), args[1].String(), args[2])
+		}),
+		"waitingFiles": js.FuncOf(func(this js.Value, args []js.Value) any {
+			return jsIPN.waitingFiles()
+		}),
+		"openWaitingFile": js.FuncOf(func(this js.Value, args []js.Value) any {
+			if len(args) != 1 {
+				log.Printf("Usage: openWaitingFile(name)")
+				return nil
+			}
+			return jsIPN.openWaitingFile(args[0].String())
+		}),
+		"deleteWaitingFile": js.FuncOf(func(this js.Value, args []js.Value) any {
+			if len(args) != 1 {
+				log.Printf("Usage: deleteWaitingFile(name)")
+				return nil
+			}
+			return jsIPN.deleteWaitingFile(args[0].String())
+		}),
 	}
 }
 

feature/taildrop/ext.go

@@ -76,6 +76,12 @@ type Extension struct {
 	// This is currently being used for Android to use the Storage Access Framework.
 	fileOps FileOps
 
+	// directFileOps, when true, means that files received via fileOps should be
+	// delivered directly to the caller (DirectFileMode=true). Set by SetFileOps.
+	// SetStagedFileOps leaves this false so that received files are staged for
+	// explicit retrieval via WaitingFiles/OpenFile (used by the WASM JS bridge).
+	directFileOps bool
+
 	nodeBackendForTest ipnext.NodeBackend // if non-nil, pretend we're this node state for tests
 
 	mu             sync.Mutex // Lock order: lb.mu > e.mu
@@ -155,9 +161,10 @@ func (e *Extension) onChangeProfile(profile ipn.LoginProfileView, _ ipn.PrefsVie
 	// Use the provided [FileOps] implementation (typically for SAF access on Android),
 	// or create an [fsFileOps] instance rooted at fileRoot.
 	//
-	// A non-nil [FileOps] also implies that we are in DirectFileMode.
+	// A non-nil [FileOps] with directFileOps=true implies DirectFileMode (Android SAF).
+	// A non-nil [FileOps] with directFileOps=false uses staged mode (WASM JS bridge).
 	fops := e.fileOps
-	isDirectFileMode := fops != nil
+	isDirectFileMode := fops != nil && e.directFileOps
 	if fops == nil {
 		var fileRoot string
 		if fileRoot, isDirectFileMode = e.fileRoot(uid, activeLogin); fileRoot == "" {

feature/taildrop/fileops_fs.go

@@ -1,6 +1,6 @@
 // Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
-//go:build !android
+//go:build !android && !js
 
 package taildrop
 

feature/taildrop/fileops_js.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build js
+
+package taildrop
+
+import "errors"
+
+func init() {
+	// On WASM there is no real filesystem. newFileOps is only reached when
+	// SetFileOps was not called; return a clear error rather than panicking.
+	newFileOps = func(dir string) (FileOps, error) {
+		return nil, errors.New("taildrop: no filesystem on WASM; provide fileOps in the IPN config")
+	}
+}

feature/taildrop/paths.go

@@ -18,10 +18,21 @@ func (e *Extension) SetDirectFileRoot(root string) {
 	e.directFileRoot = root
 }
 
-// SetFileOps sets the platform specific file operations. This is used
+// SetFileOps sets the platform-specific file operations. This is used
 // to call Android's Storage Access Framework APIs.
+// It implies DirectFileMode, so received files are delivered directly to the
+// caller rather than staged for retrieval via WaitingFiles/OpenFile.
 func (e *Extension) SetFileOps(fileOps FileOps) {
 	e.fileOps = fileOps
+	e.directFileOps = true
+}
+
+// SetStagedFileOps sets the platform-specific file operations without enabling
+// DirectFileMode. Received files are staged for explicit retrieval via
+// WaitingFiles, OpenFile, and DeleteFile. Used by the WASM JS bridge.
+func (e *Extension) SetStagedFileOps(fileOps FileOps) {
+	e.fileOps = fileOps
+	e.directFileOps = false
 }
 
 func (e *Extension) setPlatformDefaultDirectFileRoot() {