os.Stat: returned error on nonexistent path did not satisfy IsNotExist

Add direct test for the problem to make the fix commit clearer.

Noticed while implementing MkdirTemp on mac; the upstream tests for MkdirTemp fail without this.

src/os/os_anyos_test.go

@@ -4,6 +4,7 @@ package os_test
 
 import (
 	. "os"
+	"path/filepath"
 	"strconv"
 	"testing"
 	"time"
@@ -31,6 +32,16 @@ func TestMkdir(t *testing.T) {
 	}
 }
 
+func TestStatBadDir(t *testing.T) {
+	dir := TempDir()
+	badDir := filepath.Join(dir, "not-exist/really-not-exist")
+	_, err := Stat(badDir)
+	// TODO: PathError moved to io/fs in go 1.16; fix next line once we drop go 1.15 support.
+	if pe, ok := err.(*PathError); !ok || !IsNotExist(err) || pe.Path != badDir {
+		t.Errorf("Mkdir error = %#v; want PathError for path %q satisifying IsNotExist", err, badDir)
+	}
+}
+
 func writeFile(t *testing.T, fname string, flag int, text string) string {
 	f, err := OpenFile(fname, flag, 0666)
 	if err != nil {

src/os/stat_unix.go

@@ -19,7 +19,7 @@ func (f *File) Sync() error {
 func statNolog(name string) (FileInfo, error) {
 	var fs fileStat
 	err := ignoringEINTR(func() error {
-		return syscall.Stat(name, &fs.sys)
+		return handleSyscallError(syscall.Stat(name, &fs.sys))
 	})
 	if err != nil {
 		return nil, &PathError{Op: "stat", Path: name, Err: err}
@@ -32,7 +32,7 @@ func statNolog(name string) (FileInfo, error) {
 func lstatNolog(name string) (FileInfo, error) {
 	var fs fileStat
 	err := ignoringEINTR(func() error {
-		return syscall.Lstat(name, &fs.sys)
+		return handleSyscallError(syscall.Lstat(name, &fs.sys))
 	})
 	if err != nil {
 		return nil, &PathError{Op: "lstat", Path: name, Err: err}