os/user: use os.UserHomeDir for user.HomeDir

Using os.UserHomeDir for user.HomeDir helps us deduplicate the
logic and keep the behavior consistent.

Also make os.UserHomeDir return "/sdcard" in android.

See: https://go-review.googlesource.com/c/go/+/37960/1/src/os/user/lookup_stubs.go#48

Fixes #31070

Change-Id: I521bad050bc5761ecc5c0085501374d2cf8e6897
Reviewed-on: https://go-review.googlesource.com/c/go/+/169540
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/file.go

@@ -468,8 +468,10 @@ func UserHomeDir() (string, error) {
 	}
 	// On some geese the home directory is not always defined.
 	switch runtime.GOOS {
-	case "nacl", "android":
+	case "nacl":
 		return "/", nil
+	case "android":
+		return "/sdcard", nil
 	case "darwin":
 		if runtime.GOARCH == "arm" || runtime.GOARCH == "arm64" {
 			return "/", nil

src/os/user/lookup_stubs.go

@@ -26,12 +26,14 @@ func current() (*User, error) {
 	if err == nil {
 		return u, nil
 	}
+
+	homeDir, _ := os.UserHomeDir()
 	u = &User{
 		Uid:      uid,
 		Gid:      currentGID(),
 		Username: os.Getenv("USER"),
 		Name:     "", // ignored
-		HomeDir:  os.Getenv("HOME"),
+		HomeDir:  homeDir,
 	}
 	// On NaCL and Android, return a dummy user instead of failing.
 	switch runtime.GOOS {
@@ -42,9 +44,6 @@ func current() (*User, error) {
 		if u.Username == "" {
 			u.Username = "nacl"
 		}
-		if u.HomeDir == "" {
-			u.HomeDir = "/"
-		}
 	case "android":
 		if u.Uid == "" {
 			u.Uid = "1"
@@ -52,9 +51,6 @@ func current() (*User, error) {
 		if u.Username == "" {
 			u.Username = "android"
 		}
-		if u.HomeDir == "" {
-			u.HomeDir = "/sdcard"
-		}
 	}
 	// cgo isn't available, but if we found the minimum information
 	// without it, use it: