os: have UserCacheDir return an error on failure

Previously, it would return an empty string if it
could not determine the user's cache directory.
Return an error instead.

Change-Id: I74f00b1ad3858efa3fe2700c599271ebfe5764b6
Reviewed-on: https://go-review.googlesource.com/120757
Run-TryBot: Andrew Bonventre <[email protected]>
Reviewed-by: Josh Bleecher Snyder <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: andybons

Diff for: api/go1.11.txt

@@ -451,7 +451,7 @@ pkg net/http/httptrace, type ClientTrace struct, Got1xxResponse func(int, textpr
 pkg os, const ModeIrregular = 524288
 pkg os, const ModeIrregular FileMode
 pkg os, const ModeType = 2399666176
-pkg os, func UserCacheDir() string
+pkg os, func UserCacheDir() (string, error)
 pkg os/signal, func Ignored(os.Signal) bool
 pkg regexp/syntax, method (Op) String() string
 pkg runtime/trace, func IsEnabled() bool

Diff for: src/os/file.go

@@ -342,25 +342,28 @@ func TempDir() string {
 // On Plan 9, it returns $home/lib/cache.
 //
 // If the location cannot be determined (for example, $HOME is not defined),
-// then it will return an empty string.
-func UserCacheDir() string {
+// then it will return an error.
+func UserCacheDir() (string, error) {
 	var dir string
 
 	switch runtime.GOOS {
 	case "windows":
 		dir = Getenv("LocalAppData")
+		if dir == "" {
+			return "", errors.New("%LocalAppData% is not defined")
+		}
 
 	case "darwin":
 		dir = Getenv("HOME")
 		if dir == "" {
-			return ""
+			return "", errors.New("$HOME is not defined")
 		}
 		dir += "/Library/Caches"
 
 	case "plan9":
 		dir = Getenv("home")
 		if dir == "" {
-			return ""
+			return "", errors.New("$home is not defined")
 		}
 		dir += "/lib/cache"
 
@@ -369,13 +372,13 @@ func UserCacheDir() string {
 		if dir == "" {
 			dir = Getenv("HOME")
 			if dir == "" {
-				return ""
+				return "", errors.New("neither $XDG_CACHE_HOME nor $HOME are defined")
 			}
 			dir += "/.cache"
 		}
 	}
 
-	return dir
+	return dir, nil
 }
 
 // Chmod changes the mode of the named file to mode.