os: implement readdir for darwin and linux

readdir is disabled on linux for 386 and arm until syscall.seek is implemented there.

windows is hard, so leaving that for later.

File src/os/dir_other_go115.go can be deleted when we drop support for go 1.15.

Also adds TestReadNonDir, which was helpful while debugging.

src/os/dir_test.go

@@ -0,0 +1,181 @@
+// +build darwin linux,!baremetal,!js,!wasi,!386,!arm
+
+package os_test
+
+import (
+	. "os"
+	"testing"
+)
+
+func testReaddirnames(dir string, contents []string, t *testing.T) {
+	file, err := Open(dir)
+	if err != nil {
+		t.Fatalf("open %q failed: %v", dir, err)
+	}
+	defer file.Close()
+	s, err2 := file.Readdirnames(-1)
+	if err2 != nil {
+		t.Fatalf("Readdirnames %q failed: %v", dir, err2)
+	}
+	for _, m := range contents {
+		found := false
+		for _, n := range s {
+			if n == "." || n == ".." {
+				t.Errorf("got %q in directory", n)
+			}
+			if !equal(m, n) {
+				continue
+			}
+			if found {
+				t.Error("present twice:", m)
+			}
+			found = true
+		}
+		if !found {
+			t.Error("could not find", m)
+		}
+	}
+	if s == nil {
+		t.Error("Readdirnames returned nil instead of empty slice")
+	}
+}
+
+func testReaddir(dir string, contents []string, t *testing.T) {
+	file, err := Open(dir)
+	if err != nil {
+		t.Fatalf("open %q failed: %v", dir, err)
+	}
+	defer file.Close()
+	s, err2 := file.Readdir(-1)
+	if err2 != nil {
+		t.Fatalf("Readdir %q failed: %v", dir, err2)
+	}
+	for _, m := range contents {
+		found := false
+		for _, n := range s {
+			if n.Name() == "." || n.Name() == ".." {
+				t.Errorf("got %q in directory", n.Name())
+			}
+			if !equal(m, n.Name()) {
+				continue
+			}
+			if found {
+				t.Error("present twice:", m)
+			}
+			found = true
+		}
+		if !found {
+			t.Error("could not find", m)
+		}
+	}
+	if s == nil {
+		t.Error("Readdir returned nil instead of empty slice")
+	}
+}
+
+func testReadDir(dir string, contents []string, t *testing.T) {
+	file, err := Open(dir)
+	if err != nil {
+		t.Fatalf("open %q failed: %v", dir, err)
+	}
+	defer file.Close()
+	s, err2 := file.ReadDir(-1)
+	if err2 != nil {
+		t.Fatalf("ReadDir %q failed: %v", dir, err2)
+	}
+	for _, m := range contents {
+		found := false
+		for _, n := range s {
+			if n.Name() == "." || n.Name() == ".." {
+				t.Errorf("got %q in directory", n)
+			}
+			if !equal(m, n.Name()) {
+				continue
+			}
+			if found {
+				t.Error("present twice:", m)
+			}
+			found = true
+			lstat, err := Lstat(dir + "/" + m)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if n.IsDir() != lstat.IsDir() {
+				t.Errorf("%s: IsDir=%v, want %v", m, n.IsDir(), lstat.IsDir())
+			}
+			if n.Type() != lstat.Mode().Type() {
+				t.Errorf("%s: IsDir=%v, want %v", m, n.Type(), lstat.Mode().Type())
+			}
+			info, err := n.Info()
+			if err != nil {
+				t.Errorf("%s: Info: %v", m, err)
+				continue
+			}
+			if !SameFile(info, lstat) {
+				t.Errorf("%s: Info: SameFile(info, lstat) = false", m)
+			}
+		}
+		if !found {
+			t.Error("could not find", m)
+		}
+	}
+	if s == nil {
+		t.Error("ReadDir returned nil instead of empty slice")
+	}
+}
+
+func TestFileReaddirnames(t *testing.T) {
+	testReaddirnames(".", dot, t)
+	testReaddirnames(TempDir(), nil, t)
+}
+
+func TestFileReaddir(t *testing.T) {
+	testReaddir(".", dot, t)
+	testReaddir(TempDir(), nil, t)
+}
+
+func TestFileReadDir(t *testing.T) {
+	testReadDir(".", dot, t)
+	testReadDir(TempDir(), nil, t)
+}
+
+func TestReadDir(t *testing.T) {
+	dirname := "rumpelstilzchen"
+	_, err := ReadDir(dirname)
+	if err == nil {
+		t.Fatalf("ReadDir %s: error expected, none found", dirname)
+	}
+
+	dirname = "testdata"
+	list, err := ReadDir(dirname)
+	if err != nil {
+		t.Fatalf("ReadDir %s: %v", dirname, err)
+	}
+
+	foundFile := false
+	foundSubDir := false
+	for _, dir := range list {
+		switch {
+		case !dir.IsDir() && dir.Name() == "hello":
+			foundFile = true
+		case dir.IsDir() && dir.Name() == "issue37161":
+			foundSubDir = true
+		}
+	}
+	if !foundFile {
+		t.Fatalf("ReadDir %s: hello file not found", dirname)
+	}
+	if !foundSubDir {
+		t.Fatalf("ReadDir %s: testdata directory not found", dirname)
+	}
+}
+
+// TestReadNonDir just verifies that opening a non-directory returns an error.
+func TestReadNonDir(t *testing.T) {
+	// Use filename of this source file; it is known to exist, and go tests run in source tree.
+	dirname := "dir_test.go"
+	_, err := ReadDir(dirname)
+	if err == nil {
+		t.Fatalf("ReadDir %s: error on non-dir expected, none found", dirname)
+	}
+}