unix: rework TestGetwd to handle test dirs whose names contain symlinks

TestGetwd can fail on systems that have unexpected directory/partition configurations
using symlinks.

Fixes golang/go#26678

Change-Id: I44dba94c5e8b6a8e1e7e112cd7d0541d22b65d84
GitHub-Last-Rev: 727a6cc6a718969828a15d1016c89ea9be8a12e0
GitHub-Pull-Request: golang/sys#22
Reviewed-on: https://go-review.googlesource.com/c/148537
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_unix_test.go

@@ -453,9 +453,9 @@ func TestGetwd(t *testing.T) {
 		t.Fatalf("Open .: %s", err)
 	}
 	defer fd.Close()
-	// These are chosen carefully not to be symlinks on a Mac
-	// (unlike, say, /var, /etc)
-	dirs := []string{"/", "/usr/bin"}
+	// Directory list for test. Do not worry if any are symlinks or do not
+	// exist on some common unix desktop environments. That will be checked.
+	dirs := []string{"/", "/usr/bin", "/etc", "/var", "/opt"}
 	switch runtime.GOOS {
 	case "android":
 		dirs = []string{"/", "/system/bin"}
@@ -475,6 +475,17 @@ func TestGetwd(t *testing.T) {
 	}
 	oldwd := os.Getenv("PWD")
 	for _, d := range dirs {
+		// Check whether d exists, is a dir and that d's path does not contain a symlink
+		fi, err := os.Stat(d)
+		if err != nil || !fi.IsDir() {
+			t.Logf("Test dir %s stat error (%v) or not a directory, skipping", d, err)
+			continue
+		}
+		check, err := filepath.EvalSymlinks(d)
+		if err != nil || check != d {
+			t.Logf("Test dir %s (%s) is symlink or other error (%v), skipping", d, check, err)
+			continue
+		}
 		err = os.Chdir(d)
 		if err != nil {
 			t.Fatalf("Chdir: %v", err)