feature/taildrop: move rest of Taildrop out of LocalBackend

Updates #12614

Change-Id: If451dec1d796f6a4216fe485975c87f0c62a53e5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Co-authored-by: Nick Khyl <nickk@tailscale.com>

feature/taildrop/paths.go

@@ -0,0 +1,131 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package taildrop
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"tailscale.com/version/distro"
+)
+
+// SetDirectFileRoot sets the directory where received files are written.
+//
+// This must be called before Tailscale is started.
+func (e *Extension) SetDirectFileRoot(root string) {
+	e.directFileRoot = root
+}
+
+func (e *Extension) setPlatformDefaultDirectFileRoot() {
+	dg := distro.Get()
+
+	switch dg {
+	case distro.Synology, distro.TrueNAS, distro.QNAP, distro.Unraid:
+		// See if they have a "Taildrop" share.
+		// See https://github.com/tailscale/tailscale/issues/2179#issuecomment-982821319
+		path, err := findTaildropDir(dg)
+		if err != nil {
+			e.logf("%s Taildrop support: %v", dg, err)
+		} else {
+			e.logf("%s Taildrop: using %v", dg, path)
+			e.directFileRoot = path
+		}
+	}
+}
+
+func findTaildropDir(dg distro.Distro) (string, error) {
+	const name = "Taildrop"
+	switch dg {
+	case distro.Synology:
+		return findSynologyTaildropDir(name)
+	case distro.TrueNAS:
+		return findTrueNASTaildropDir(name)
+	case distro.QNAP:
+		return findQnapTaildropDir(name)
+	case distro.Unraid:
+		return findUnraidTaildropDir(name)
+	}
+	return "", fmt.Errorf("%s is an unsupported distro for Taildrop dir", dg)
+}
+
+// findSynologyTaildropDir looks for the first volume containing a
+// "Taildrop" directory.  We'd run "synoshare --get Taildrop" command
+// but on DSM7 at least, we lack permissions to run that.
+func findSynologyTaildropDir(name string) (dir string, err error) {
+	for i := 1; i <= 16; i++ {
+		dir = fmt.Sprintf("/volume%v/%s", i, name)
+		if fi, err := os.Stat(dir); err == nil && fi.IsDir() {
+			return dir, nil
+		}
+	}
+	return "", fmt.Errorf("shared folder %q not found", name)
+}
+
+// findTrueNASTaildropDir returns the first matching directory of
+// /mnt/{name} or /mnt/*/{name}
+func findTrueNASTaildropDir(name string) (dir string, err error) {
+	// If we're running in a jail, a mount point could just be added at /mnt/Taildrop
+	dir = fmt.Sprintf("/mnt/%s", name)
+	if fi, err := os.Stat(dir); err == nil && fi.IsDir() {
+		return dir, nil
+	}
+
+	// but if running on the host, it may be something like /mnt/Primary/Taildrop
+	fis, err := os.ReadDir("/mnt")
+	if err != nil {
+		return "", fmt.Errorf("error reading /mnt: %w", err)
+	}
+	for _, fi := range fis {
+		dir = fmt.Sprintf("/mnt/%s/%s", fi.Name(), name)
+		if fi, err := os.Stat(dir); err == nil && fi.IsDir() {
+			return dir, nil
+		}
+	}
+	return "", fmt.Errorf("shared folder %q not found", name)
+}
+
+// findQnapTaildropDir checks if a Shared Folder named "Taildrop" exists.
+func findQnapTaildropDir(name string) (string, error) {
+	dir := fmt.Sprintf("/share/%s", name)
+	fi, err := os.Stat(dir)
+	if err != nil {
+		return "", fmt.Errorf("shared folder %q not found", name)
+	}
+	if fi.IsDir() {
+		return dir, nil
+	}
+
+	// share/Taildrop is usually a symlink to CACHEDEV1_DATA/Taildrop/ or some such.
+	fullpath, err := filepath.EvalSymlinks(dir)
+	if err != nil {
+		return "", fmt.Errorf("symlink to shared folder %q not found", name)
+	}
+	if fi, err = os.Stat(fullpath); err == nil && fi.IsDir() {
+		return dir, nil // return the symlink, how QNAP set it up
+	}
+	return "", fmt.Errorf("shared folder %q not found", name)
+}
+
+// findUnraidTaildropDir looks for a directory linked at
+// /var/lib/tailscale/Taildrop. This is a symlink to the
+// path specified by the user in the Unraid Web UI
+func findUnraidTaildropDir(name string) (string, error) {
+	dir := fmt.Sprintf("/var/lib/tailscale/%s", name)
+	_, err := os.Stat(dir)
+	if err != nil {
+		return "", fmt.Errorf("symlink %q not found", name)
+	}
+
+	fullpath, err := filepath.EvalSymlinks(dir)
+	if err != nil {
+		return "", fmt.Errorf("symlink %q to shared folder not valid", name)
+	}
+
+	fi, err := os.Stat(fullpath)
+	if err == nil && fi.IsDir() {
+		return dir, nil // return the symlink
+	}
+	return "", fmt.Errorf("shared folder %q not found", name)
+}