os: allow $HOME to not exist in TestUserHomeDir

This seems like a cleaner fix for the situation Debian has patched
around with
https://sources.debian.org/patches/golang-1.19/1.19.3-1/0001-Disable-test-for-UserHomeDir.patch/

Tested with 'GOCACHE=$(go env GOCACHE) HOME=/home/nobody go test os'.

Change-Id: I9fd00da38b86df2edfcd8cb87629e2875573903e
Reviewed-on: https://go-review.googlesource.com/c/go/+/456126
Run-TryBot: Bryan Mills <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
Reviewed-by: Rob Pike <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: Bryan C. Mills

src/os/file.go

@@ -486,6 +486,9 @@ func UserConfigDir() (string, error) {
 // On Unix, including macOS, it returns the $HOME environment variable.
 // On Windows, it returns %USERPROFILE%.
 // On Plan 9, it returns the $home environment variable.
+//
+// If the expected variable is not set in the environment, UserHomeDir
+// returns either a platform-specific default value or a non-nil error.
 func UserHomeDir() (string, error) {
 	env, enverr := "HOME", "$HOME"
 	switch runtime.GOOS {

src/os/os_test.go

@@ -2639,10 +2639,20 @@ func TestUserHomeDir(t *testing.T) {
 		t.Fatal("UserHomeDir returned an empty string but no error")
 	}
 	if err != nil {
-		t.Skipf("UserHomeDir failed: %v", err)
+		// UserHomeDir may return a non-nil error if the environment variable
+		// for the home directory is empty or unset in the environment.
+		t.Skipf("skipping: %v", err)
 	}
+
 	fi, err := Stat(dir)
 	if err != nil {
+		if os.IsNotExist(err) {
+			// The user's home directory has a well-defined location, but does not
+			// exist. (Maybe nothing has written to it yet? That could happen, for
+			// example, on minimal VM images used for CI testing.)
+			t.Log(err)
+			return
+		}
 		t.Fatal(err)
 	}
 	if !fi.IsDir() {