envknob: support disk-based envknobs on the macsys build

Per my investigation just now, the $HOME environment variable is unset
on the macsys (standalone macOS GUI) variant, but the current working
directory is valid. Look for the environment variable file in that
location in addition to inside the home directory.

Updates #3707

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I481ae2e0d19b316244373e06865e3b5c3a9f3b88

envknob/envknob.go

@@ -17,6 +17,7 @@ package envknob
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -503,7 +504,7 @@ func ApplyDiskConfigError() error { return applyDiskConfigErr }
 //
 // On macOS, use one of:
 //
-//   - ~/Library/Containers/io.tailscale.ipn.macsys/Data/tailscaled-env.txt
+//   - /private/var/root/Library/Containers/io.tailscale.ipn.macsys.network-extension/Data/tailscaled-env.txt
 //     for standalone macOS GUI builds
 //   - ~/Library/Containers/io.tailscale.ipn.macos.network-extension/Data/tailscaled-env.txt
 //     for App Store builds
@@ -533,44 +534,73 @@ func ApplyDiskConfig() (err error) {
 		return applyKeyValueEnv(f)
 	}
 
-	name := getPlatformEnvFile()
-	if name == "" {
+	names := getPlatformEnvFiles()
+	if len(names) == 0 {
 		return nil
 	}
+
+	var errs []error
+	for _, name := range names {
 		f, err = os.Open(name)
 		if os.IsNotExist(err) {
-		return nil
+			continue
 		}
 		if err != nil {
-		return err
+			errs = append(errs, err)
+			continue
 		}
 		defer f.Close()
+
 		return applyKeyValueEnv(f)
 	}
 
-// getPlatformEnvFile returns the current platform's path to an optional
-// tailscaled-env.txt file. It returns an empty string if none is defined
-// for the platform.
-func getPlatformEnvFile() string {
+	// If we have any errors, return them; if all errors are such that
+	// os.IsNotExist(err) returns true, then errs is empty and we will
+	// return nil.
+	return errors.Join(errs...)
+}
+
+// getPlatformEnvFiles returns a list of paths to the current platform's
+// optional tailscaled-env.txt file. It returns an empty list if none is
+// defined for the platform.
+func getPlatformEnvFiles() []string {
 	switch runtime.GOOS {
 	case "windows":
-		return filepath.Join(os.Getenv("ProgramData"), "Tailscale", "tailscaled-env.txt")
+		return []string{
+			filepath.Join(os.Getenv("ProgramData"), "Tailscale", "tailscaled-env.txt"),
+		}
 	case "linux":
 		if distro.Get() == distro.Synology {
-			return "/etc/tailscale/tailscaled-env.txt"
+			return []string{"/etc/tailscale/tailscaled-env.txt"}
 		}
 	case "darwin":
 		if version.IsSandboxedMacOS() { // the two GUI variants (App Store or separate download)
-			// This will be user-visible as ~/Library/Containers/$VARIANT/Data/tailscaled-env.txt
-			// where $VARIANT is "io.tailscale.ipn.macsys" for macsys (downloadable mac GUI builds)
-			// or "io.tailscale.ipn.macos.network-extension" for App Store builds.
-			return filepath.Join(os.Getenv("HOME"), "tailscaled-env.txt")
+			// On the App Store variant, the home directory is set
+			// to something like:
+			//	~/Library/Containers/io.tailscale.ipn.macos.network-extension/Data
+			//
+			// On the macsys (downloadable Mac GUI) variant, the
+			// home directory can be unset, but we have a working
+			// directory that looks like:
+			//	/private/var/root/Library/Containers/io.tailscale.ipn.macsys.network-extension/Data
+			//
+			// Try both and see if we can find the file in either
+			// location.
+			var candidates []string
+			if home := os.Getenv("HOME"); home != "" {
+				candidates = append(candidates, filepath.Join(home, "tailscaled-env.txt"))
+			}
+			if wd, err := os.Getwd(); err == nil {
+				candidates = append(candidates, filepath.Join(wd, "tailscaled-env.txt"))
+			}
+
+			return candidates
 		} else {
 			// Open source / homebrew variable, running tailscaled-on-macOS.
-			return "/etc/tailscale/tailscaled-env.txt"
+			return []string{"/etc/tailscale/tailscaled-env.txt"}
 		}
 	}
-	return ""
+	return nil
 }
 
 // applyKeyValueEnv reads key=value lines r and calls Setenv for each.