os,time: fix tests on iOS

When fixing tests for for self-hosted iOS builds, I
broke hosted builds.

Updates #31722

Change-Id: Id4e7d234fbd86cb2d29d320d75f4441efd663d12
Reviewed-on: https://go-review.googlesource.com/c/go/+/174698
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/os_test.go

@@ -1204,6 +1204,7 @@ func TestChdirAndGetwd(t *testing.T) {
 	case "darwin":
 		switch runtime.GOARCH {
 		case "arm", "arm64":
+			dirs = nil
 			for _, d := range []string{"d1", "d2"} {
 				dir, err := ioutil.TempDir("", d)
 				if err != nil {

src/time/zoneinfo_ios.go

@@ -13,20 +13,31 @@ import (
 )
 
 var zoneSources = []string{
-	getZipParent() + "/zoneinfo.zip",
-	runtime.GOROOT() + "/lib/time/zoneinfo.zip",
+	getZoneRoot() + "/zoneinfo.zip",
 }
 
-func getZipParent() string {
-	wd, err := syscall.Getwd()
-	if err != nil {
-		return "/XXXNOEXIST"
-	}
-
+func getZoneRoot() string {
 	// The working directory at initialization is the root of the
 	// app bundle: "/private/.../bundlename.app". That's where we
-	// keep zoneinfo.zip.
-	return wd
+	// keep zoneinfo.zip for tethered iOS builds.
+	// For self-hosted iOS builds, the zoneinfo.zip is in GOROOT.
+	roots := []string{runtime.GOROOT() + "/lib/time"}
+	wd, err := syscall.Getwd()
+	if err == nil {
+		roots = append(roots, wd)
+	}
+	for _, r := range roots {
+		var st syscall.Stat_t
+		fd, err := syscall.Open(r, syscall.O_RDONLY, 0)
+		if err != nil {
+			continue
+		}
+		defer syscall.Close(fd)
+		if err := syscall.Fstat(fd, &st); err == nil {
+			return r
+		}
+	}
+	return "/XXXNOEXIST"
 }
 
 func initLocal() {