os: clean up tests

- Use testenv.Command instead of exec.Command to try to get more
  useful timeout behavior.

- Parallelize tests that appear not to require global state.
  (And add explanatory comments for a few that are not
  parallelizable for subtle reasons.)

- Consolidate some “Helper” tests with their parent tests.

- Use t.TempDir instead of os.MkdirTemp when appropriate.

- Factor out subtests for repeated test helpers.

For #36107.
Updates #22315.

Change-Id: Ic24b6957094dcd40908a59f48e44c8993729222b
Reviewed-on: https://go-review.googlesource.com/c/go/+/458015
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>

16 files changed, 450 insertions, 250 deletions

diff --git a/src/os/env_test.go b/src/os/env_test.go
index f8d56ef8e0..1b9e26594c 100644
--- a/src/os/env_test.go
+++ b/src/os/env_test.go
@@ -171,6 +171,8 @@ func TestLookupEnv(t *testing.T) {
 // Check that they are properly reported by LookupEnv and can be set by SetEnv.
 // See https://golang.org/issue/49886.
 func TestEnvironConsistency(t *testing.T) {
+	t.Parallel()
+
 	for _, kv := range Environ() {
 		i := strings.Index(kv, "=")
 		if i == 0 {
diff --git a/src/os/error_test.go b/src/os/error_test.go
index 4ab6246d2e..4fa6146194 100644
--- a/src/os/error_test.go
+++ b/src/os/error_test.go
@@ -14,6 +14,8 @@ import (
 )
 
 func TestErrIsExist(t *testing.T) {
+	t.Parallel()
+
 	f, err := os.CreateTemp("", "_Go_ErrIsExist")
 	if err != nil {
 		t.Fatalf("open ErrIsExist tempfile: %s", err)
@@ -148,6 +150,8 @@ func TestIsPermission(t *testing.T) {
 }
 
 func TestErrPathNUL(t *testing.T) {
+	t.Parallel()
+
 	f, err := os.CreateTemp("", "_Go_ErrPathNUL\x00")
 	if err == nil {
 		f.Close()
diff --git a/src/os/exec_unix_test.go b/src/os/exec_unix_test.go
index 332ffe9041..82c072a746 100644
--- a/src/os/exec_unix_test.go
+++ b/src/os/exec_unix_test.go
@@ -14,11 +14,9 @@ import (
 
 func TestErrProcessDone(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
-	path, err := testenv.GoTool()
-	if err != nil {
-		t.Errorf("finding go tool: %v", err)
-	}
-	p, err := StartProcess(path, []string{"go"}, &ProcAttr{})
+	t.Parallel()
+
+	p, err := StartProcess(testenv.GoToolPath(t), []string{"go"}, &ProcAttr{})
 	if err != nil {
 		t.Errorf("starting test process: %v", err)
 	}
diff --git a/src/os/executable_test.go b/src/os/executable_test.go
index b69fe41ea3..8d1794d340 100644
--- a/src/os/executable_test.go
+++ b/src/os/executable_test.go
@@ -8,7 +8,6 @@ import (
 	"fmt"
 	"internal/testenv"
 	"os"
-	osexec "os/exec"
 	"path/filepath"
 	"runtime"
 	"testing"
@@ -18,6 +17,8 @@ const executable_EnvVar = "OSTEST_OUTPUT_EXECPATH"
 
 func TestExecutable(t *testing.T) {
 	testenv.MustHaveExec(t)
+	t.Parallel()
+
 	ep, err := os.Executable()
 	if err != nil {
 		t.Fatalf("Executable failed: %v", err)
@@ -29,18 +30,18 @@ func TestExecutable(t *testing.T) {
 		t.Fatalf("filepath.Rel: %v", err)
 	}
 
-	cmd := &osexec.Cmd{}
+	cmd := testenv.Command(t, fn, "-test.run=XXXX")
 	// make child start with a relative program path
 	cmd.Dir = dir
 	cmd.Path = fn
-	// forge argv[0] for child, so that we can verify we could correctly
-	// get real path of the executable without influenced by argv[0].
-	cmd.Args = []string{"-", "-test.run=XXXX"}
 	if runtime.GOOS == "openbsd" || runtime.GOOS == "aix" {
 		// OpenBSD and AIX rely on argv[0]
-		cmd.Args[0] = fn
+	} else {
+		// forge argv[0] for child, so that we can verify we could correctly
+		// get real path of the executable without influenced by argv[0].
+		cmd.Args[0] = "-"
 	}
-	cmd.Env = append(os.Environ(), fmt.Sprintf("%s=1", executable_EnvVar))
+	cmd.Env = append(cmd.Environ(), fmt.Sprintf("%s=1", executable_EnvVar))
 	out, err := cmd.CombinedOutput()
 	if err != nil {
 		t.Fatalf("exec(self) failed: %v", err)
@@ -95,6 +96,7 @@ func TestExecutableDeleted(t *testing.T) {
 	case "openbsd", "freebsd", "aix":
 		t.Skipf("%v does not support reading deleted binary name", runtime.GOOS)
 	}
+	t.Parallel()
 
 	dir := t.TempDir()
 
diff --git a/src/os/os_test.go b/src/os/os_test.go
index 277b2455e6..7d39eb3e02 100644
--- a/src/os/os_test.go
+++ b/src/os/os_test.go
@@ -13,7 +13,7 @@ import (
 	"io/fs"
 	"os"
 	. "os"
-	osexec "os/exec"
+	"os/exec"
 	"path/filepath"
 	"reflect"
 	"runtime"
@@ -172,6 +172,8 @@ var sfdir = sysdir.name
 var sfname = sysdir.files[0]
 
 func TestStat(t *testing.T) {
+	t.Parallel()
+
 	path := sfdir + "/" + sfname
 	dir, err := Stat(path)
 	if err != nil {
@@ -246,6 +248,8 @@ func TestStatSymlinkLoop(t *testing.T) {
 }
 
 func TestFstat(t *testing.T) {
+	t.Parallel()
+
 	path := sfdir + "/" + sfname
 	file, err1 := Open(path)
 	if err1 != nil {
@@ -266,6 +270,8 @@ func TestFstat(t *testing.T) {
 }
 
 func TestLstat(t *testing.T) {
+	t.Parallel()
+
 	path := sfdir + "/" + sfname
 	dir, err := Lstat(path)
 	if err != nil {
@@ -284,6 +290,8 @@ func TestLstat(t *testing.T) {
 
 // Read with length 0 should not return EOF.
 func TestRead0(t *testing.T) {
+	t.Parallel()
+
 	path := sfdir + "/" + sfname
 	f, err := Open(path)
 	if err != nil {
@@ -305,6 +313,8 @@ func TestRead0(t *testing.T) {
 
 // Reading a closed file should return ErrClosed error
 func TestReadClosed(t *testing.T) {
+	t.Parallel()
+
 	path := sfdir + "/" + sfname
 	file, err := Open(path)
 	if err != nil {
@@ -321,139 +331,157 @@ func TestReadClosed(t *testing.T) {
 	}
 }
 
-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
+func testReaddirnames(dir string, contents []string) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Parallel()
+
+		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("present twice:", m)
+			if !found {
+				t.Error("could not find", m)
 			}
-			found = true
 		}
-		if !found {
-			t.Error("could not find", m)
+		if s == nil {
+			t.Error("Readdirnames returned nil instead of empty slice")
 		}
 	}
-	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
+func testReaddir(dir string, contents []string) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Parallel()
+
+		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("present twice:", m)
+			if !found {
+				t.Error("could not find", m)
 			}
-			found = true
 		}
-		if !found {
-			t.Error("could not find", m)
+		if s == nil {
+			t.Error("Readdir returned nil instead of empty slice")
 		}
 	}
-	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
+func testReadDir(dir string, contents []string) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Parallel()
+
+		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 !SameFile(info, lstat) {
-				t.Errorf("%s: Info: SameFile(info, lstat) = false", m)
+			if !found {
+				t.Error("could not find", m)
 			}
 		}
-		if !found {
-			t.Error("could not find", m)
+		if s == nil {
+			t.Error("ReadDir returned nil instead of empty slice")
 		}
 	}
-	if s == nil {
-		t.Error("ReadDir returned nil instead of empty slice")
-	}
 }
 
 func TestFileReaddirnames(t *testing.T) {
-	testReaddirnames(".", dot, t)
-	testReaddirnames(sysdir.name, sysdir.files, t)
-	testReaddirnames(t.TempDir(), nil, t)
+	t.Parallel()
+
+	t.Run(".", testReaddirnames(".", dot))
+	t.Run("sysdir", testReaddirnames(sysdir.name, sysdir.files))
+	t.Run("TempDir", testReaddirnames(t.TempDir(), nil))
 }
 
 func TestFileReaddir(t *testing.T) {
-	testReaddir(".", dot, t)
-	testReaddir(sysdir.name, sysdir.files, t)
-	testReaddir(t.TempDir(), nil, t)
+	t.Parallel()
+
+	t.Run(".", testReaddir(".", dot))
+	t.Run("sysdir", testReaddir(sysdir.name, sysdir.files))
+	t.Run("TempDir", testReaddir(t.TempDir(), nil))
 }
 
 func TestFileReadDir(t *testing.T) {
-	testReadDir(".", dot, t)
-	testReadDir(sysdir.name, sysdir.files, t)
-	testReadDir(t.TempDir(), nil, t)
+	t.Parallel()
+
+	t.Run(".", testReadDir(".", dot))
+	t.Run("sysdir", testReadDir(sysdir.name, sysdir.files))
+	t.Run("TempDir", testReadDir(t.TempDir(), nil))
 }
 
 func benchmarkReaddirname(path string, b *testing.B) {
@@ -587,6 +615,8 @@ func smallReaddirnames(file *File, length int, t *testing.T) []string {
 // Check that reading a directory one entry at a time gives the same result
 // as reading it all at once.
 func TestReaddirnamesOneAtATime(t *testing.T) {
+	t.Parallel()
+
 	// big directory that doesn't change often.
 	dir := "/usr/bin"
 	switch runtime.GOOS {
@@ -632,6 +662,8 @@ func TestReaddirNValues(t *testing.T) {
 	if testing.Short() {
 		t.Skip("test.short; skipping")
 	}
+	t.Parallel()
+
 	dir := t.TempDir()
 	for i := 1; i <= 105; i++ {
 		f, err := Create(filepath.Join(dir, fmt.Sprintf("%d", i)))
@@ -727,13 +759,7 @@ func TestReaddirStatFailures(t *testing.T) {
 		// testing it wouldn't work.
 		t.Skipf("skipping test on %v", runtime.GOOS)
 	}
-	dir := t.TempDir()
-	touch(t, filepath.Join(dir, "good1"))
-	touch(t, filepath.Join(dir, "x")) // will disappear or have an error
-	touch(t, filepath.Join(dir, "good2"))
-	defer func() {
-		*LstatP = Lstat
-	}()
+
 	var xerr error // error to return for x
 	*LstatP = func(path string) (FileInfo, error) {
 		if xerr != nil && strings.HasSuffix(path, "x") {
@@ -741,6 +767,12 @@ func TestReaddirStatFailures(t *testing.T) {
 		}
 		return Lstat(path)
 	}
+	defer func() { *LstatP = Lstat }()
+
+	dir := t.TempDir()
+	touch(t, filepath.Join(dir, "good1"))
+	touch(t, filepath.Join(dir, "x")) // will disappear or have an error
+	touch(t, filepath.Join(dir, "good2"))
 	readDir := func() ([]FileInfo, error) {
 		d, err := Open(dir)
 		if err != nil {
@@ -784,11 +816,12 @@ func TestReaddirStatFailures(t *testing.T) {
 
 // Readdir on a regular file should fail.
 func TestReaddirOfFile(t *testing.T) {
-	f, err := os.CreateTemp("", "_Go_ReaddirOfFile")
+	t.Parallel()
+
+	f, err := os.CreateTemp(t.TempDir(), "_Go_ReaddirOfFile")
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer Remove(f.Name())
 	f.Write([]byte("foo"))
 	f.Close()
 	reg, err := Open(f.Name())
@@ -1151,34 +1184,39 @@ func TestRenameCaseDifference(pt *testing.T) {
 	}
 }
 
-func exec(t *testing.T, dir, cmd string, args []string, expect string) {
-	r, w, err := Pipe()
-	if err != nil {
-		t.Fatalf("Pipe: %v", err)
-	}
-	defer r.Close()
-	attr := &ProcAttr{Dir: dir, Files: []*File{nil, w, Stderr}}
-	p, err := StartProcess(cmd, args, attr)
-	if err != nil {
-		t.Fatalf("StartProcess: %v", err)
-	}
-	w.Close()
+func testStartProcess(dir, cmd string, args []string, expect string) func(t *testing.T) {
+	return func(t *testing.T) {
+		t.Parallel()
 
-	var b strings.Builder
-	io.Copy(&b, r)
-	output := b.String()
+		r, w, err := Pipe()
+		if err != nil {
+			t.Fatalf("Pipe: %v", err)
+		}
+		defer r.Close()
+		attr := &ProcAttr{Dir: dir, Files: []*File{nil, w, Stderr}}
+		p, err := StartProcess(cmd, args, attr)
+		if err != nil {
+			t.Fatalf("StartProcess: %v", err)
+		}
+		w.Close()
+
+		var b strings.Builder
+		io.Copy(&b, r)
+		output := b.String()
 
-	fi1, _ := Stat(strings.TrimSpace(output))
-	fi2, _ := Stat(expect)
-	if !SameFile(fi1, fi2) {
-		t.Errorf("exec %q returned %q wanted %q",
-			strings.Join(append([]string{cmd}, args...), " "), output, expect)
+		fi1, _ := Stat(strings.TrimSpace(output))
+		fi2, _ := Stat(expect)
+		if !SameFile(fi1, fi2) {
+			t.Errorf("exec %q returned %q wanted %q",
+				strings.Join(append([]string{cmd}, args...), " "), output, expect)
+		}
+		p.Wait()
 	}
-	p.Wait()
 }
 
 func TestStartProcess(t *testing.T) {
 	testenv.MustHaveExec(t)
+	t.Parallel()
 
 	var dir, cmd string
 	var args []string
@@ -1191,7 +1229,7 @@ func TestStartProcess(t *testing.T) {
 		args = []string{"/c", "cd"}
 	default:
 		var err error
-		cmd, err = osexec.LookPath("pwd")
+		cmd, err = exec.LookPath("pwd")
 		if err != nil {
 			t.Fatalf("Can't find pwd: %v", err)
 		}
@@ -1201,10 +1239,8 @@ func TestStartProcess(t *testing.T) {
 	}
 	cmddir, cmdbase := filepath.Split(cmd)
 	args = append([]string{cmdbase}, args...)
-	// Test absolute executable path.
-	exec(t, dir, cmd, args, dir)
-	// Test relative executable path.
-	exec(t, cmddir, cmdbase, args, cmddir)
+	t.Run("absolute", testStartProcess(dir, cmd, args, dir))
+	t.Run("relative", testStartProcess(cmddir, cmdbase, args, cmddir))
 }
 
 func checkMode(t *testing.T, path string, mode FileMode) {
@@ -1218,6 +1254,8 @@ func checkMode(t *testing.T, path string, mode FileMode) {
 }
 
 func TestChmod(t *testing.T) {
+	t.Parallel()
+
 	f := newFile("TestChmod", t)
 	defer Remove(f.Name())
 	defer f.Close()
@@ -1254,6 +1292,8 @@ func checkSize(t *testing.T, f *File, size int64) {
 }
 
 func TestFTruncate(t *testing.T) {
+	t.Parallel()
+
 	f := newFile("TestFTruncate", t)
 	defer Remove(f.Name())
 	defer f.Close()
@@ -1274,6 +1314,8 @@ func TestFTruncate(t *testing.T) {
 }
 
 func TestTruncate(t *testing.T) {
+	t.Parallel()
+
 	f := newFile("TestTruncate", t)
 	defer Remove(f.Name())
 	defer f.Close()
@@ -1298,6 +1340,8 @@ func TestTruncate(t *testing.T) {
 // On NFS, timings can be off due to caching of meta-data on
 // NFS servers (Issue 848).
 func TestChtimes(t *testing.T) {
+	t.Parallel()
+
 	f := newFile("TestChtimes", t)
 	defer Remove(f.Name())
 
@@ -1312,6 +1356,8 @@ func TestChtimes(t *testing.T) {
 // On NFS, timings can be off due to caching of meta-data on
 // NFS servers (Issue 848).
 func TestChtimesDir(t *testing.T) {
+	t.Parallel()
+
 	name := newDir("TestChtimes", t)
 	defer RemoveAll(name)
 
@@ -1549,6 +1595,8 @@ func TestProgWideChdir(t *testing.T) {
 }
 
 func TestSeek(t *testing.T) {
+	t.Parallel()
+
 	f := newFile("TestSeek", t)
 	defer Remove(f.Name())
 	defer f.Close()
@@ -1597,6 +1645,7 @@ func TestSeekError(t *testing.T) {
 	case "js", "plan9":
 		t.Skipf("skipping test on %v", runtime.GOOS)
 	}
+	t.Parallel()
 
 	r, w, err := Pipe()
 	if err != nil {
@@ -1643,6 +1692,8 @@ var openErrorTests = []openErrorTest{
 }
 
 func TestOpenError(t *testing.T) {
+	t.Parallel()
+
 	for _, tt := range openErrorTests {
 		f, err := OpenFile(tt.path, tt.mode, 0)
 		if err == nil {
@@ -1697,9 +1748,9 @@ func runBinHostname(t *testing.T) string {
 	}
 	defer r.Close()
 
-	path, err := osexec.LookPath("hostname")
+	path, err := exec.LookPath("hostname")
 	if err != nil {
-		if errors.Is(err, osexec.ErrNotFound) {
+		if errors.Is(err, exec.ErrNotFound) {
 			t.Skip("skipping test; test requires hostname but it does not exist")
 		}
 		t.Fatal(err)
@@ -1749,6 +1800,8 @@ func testWindowsHostname(t *testing.T, hostname string) {
 }
 
 func TestHostname(t *testing.T) {
+	t.Parallel()
+
 	hostname, err := Hostname()
 	if err != nil {
 		t.Fatal(err)
@@ -1786,6 +1839,8 @@ func TestHostname(t *testing.T) {
 }
 
 func TestReadAt(t *testing.T) {
+	t.Parallel()
+
 	f := newFile("TestReadAt", t)
 	defer Remove(f.Name())
 	defer f.Close()
@@ -1808,6 +1863,8 @@ func TestReadAt(t *testing.T) {
 // the pread syscall, where the channel offset was erroneously updated after
 // calling pread on a file.
 func TestReadAtOffset(t *testing.T) {
+	t.Parallel()
+
 	f := newFile("TestReadAtOffset", t)
 	defer Remove(f.Name())
 	defer f.Close()
@@ -1837,6 +1894,8 @@ func TestReadAtOffset(t *testing.T) {
 
 // Verify that ReadAt doesn't allow negative offset.
 func TestReadAtNegativeOffset(t *testing.T) {
+	t.Parallel()
+
 	f := newFile("TestReadAtNegativeOffset", t)
 	defer Remove(f.Name())
 	defer f.Close()
@@ -1856,6 +1915,8 @@ func TestReadAtNegativeOffset(t *testing.T) {
 }
 
 func TestWriteAt(t *testing.T) {
+	t.Parallel()
+
 	f := newFile("TestWriteAt", t)
 	defer Remove(f.Name())
 	defer f.Close()
@@ -1879,6 +1940,8 @@ func TestWriteAt(t *testing.T) {
 
 // Verify that WriteAt doesn't allow negative offset.
 func TestWriteAtNegativeOffset(t *testing.T) {
+	t.Parallel()
+
 	f := newFile("TestWriteAtNegativeOffset", t)
 	defer Remove(f.Name())
 	defer f.Close()
@@ -1957,6 +2020,8 @@ func TestAppend(t *testing.T) {
 }
 
 func TestStatDirWithTrailingSlash(t *testing.T) {
+	t.Parallel()
+
 	// Create new temporary directory and arrange to clean it up.
 	path := t.TempDir()
 
@@ -2051,6 +2116,8 @@ func testDevNullFile(t *testing.T, devNullName string) {
 }
 
 func TestDevNullFile(t *testing.T) {
+	t.Parallel()
+
 	testDevNullFile(t, DevNull)
 	if runtime.GOOS == "windows" {
 		testDevNullFile(t, "./nul")
@@ -2086,6 +2153,8 @@ func TestLargeWriteToConsole(t *testing.T) {
 }
 
 func TestStatDirModeExec(t *testing.T) {
+	t.Parallel()
+
 	const mode = 0111
 
 	path := t.TempDir()
@@ -2108,8 +2177,6 @@ func TestStatStdin(t *testing.T) {
 		t.Skipf("%s doesn't have /bin/sh", runtime.GOOS)
 	}
 
-	testenv.MustHaveExec(t)
-
 	if Getenv("GO_WANT_HELPER_PROCESS") == "1" {
 		st, err := Stdin.Stat()
 		if err != nil {
@@ -2119,6 +2186,9 @@ func TestStatStdin(t *testing.T) {
 		Exit(0)
 	}
 
+	testenv.MustHaveExec(t)
+	t.Parallel()
+
 	fi, err := Stdin.Stat()
 	if err != nil {
 		t.Fatal(err)
@@ -2130,7 +2200,7 @@ func TestStatStdin(t *testing.T) {
 		t.Fatalf("unexpected Stdin mode (%v), want ModeCharDevice or ModeNamedPipe", mode)
 	}
 
-	var cmd *osexec.Cmd
+	var cmd *exec.Cmd
 	if runtime.GOOS == "windows" {
 		cmd = testenv.Command(t, "cmd", "/c", "echo output | "+Args[0]+" -test.run=TestStatStdin")
 	} else {
@@ -2151,6 +2221,7 @@ func TestStatStdin(t *testing.T) {
 
 func TestStatRelativeSymlink(t *testing.T) {
 	testenv.MustHaveSymlink(t)
+	t.Parallel()
 
 	tmpdir := t.TempDir()
 	target := filepath.Join(tmpdir, "target")
@@ -2199,6 +2270,8 @@ func TestStatRelativeSymlink(t *testing.T) {
 }
 
 func TestReadAtEOF(t *testing.T) {
+	t.Parallel()
+
 	f := newFile("TestReadAtEOF", t)
 	defer Remove(f.Name())
 	defer f.Close()
@@ -2215,6 +2288,8 @@ func TestReadAtEOF(t *testing.T) {
 }
 
 func TestLongPath(t *testing.T) {
+	t.Parallel()
+
 	tmpdir := newDir("TestLongPath", t)
 	defer func(d string) {
 		if err := RemoveAll(d); err != nil {
@@ -2290,7 +2365,7 @@ func testKillProcess(t *testing.T, processKiller func(p *Process)) {
 
 	// Re-exec the test binary to start a process that hangs until stdin is closed.
 	cmd := testenv.Command(t, Args[0])
-	cmd.Env = append(os.Environ(), "GO_OS_TEST_DRAIN_STDIN=1")
+	cmd.Env = append(cmd.Environ(), "GO_OS_TEST_DRAIN_STDIN=1")
 	stdout, err := cmd.StdoutPipe()
 	if err != nil {
 		t.Fatal(err)
@@ -2333,13 +2408,14 @@ func TestGetppid(t *testing.T) {
 		t.Skipf("skipping test on plan9; see issue 8206")
 	}
 
-	testenv.MustHaveExec(t)
-
 	if Getenv("GO_WANT_HELPER_PROCESS") == "1" {
 		fmt.Print(Getppid())
 		Exit(0)
 	}
 
+	testenv.MustHaveExec(t)
+	t.Parallel()
+
 	cmd := testenv.Command(t, Args[0], "-test.run=TestGetppid")
 	cmd.Env = append(Environ(), "GO_WANT_HELPER_PROCESS=1")
 
@@ -2392,6 +2468,8 @@ var nilFileMethodTests = []struct {
 
 // Test that all File methods give ErrInvalid if the receiver is nil.
 func TestNilFileMethods(t *testing.T) {
+	t.Parallel()
+
 	for _, tt := range nilFileMethodTests {
 		var file *File
 		got := tt.f(file)
@@ -2524,31 +2602,38 @@ func TestPipeThreads(t *testing.T) {
 	}
 }
 
-func testDoubleCloseError(t *testing.T, path string) {
-	file, err := Open(path)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if err := file.Close(); err != nil {
-		t.Fatalf("unexpected error from Close: %v", err)
-	}
-	if err := file.Close(); err == nil {
-		t.Error("second Close did not fail")
-	} else if pe, ok := err.(*PathError); !ok {
-		t.Errorf("second Close: got %T, want %T", err, pe)
-	} else if pe.Err != ErrClosed {
-		t.Errorf("second Close: got %q, want %q", pe.Err, ErrClosed)
-	} else {
-		t.Logf("second close returned expected error %q", err)
+func testDoubleCloseError(path string) func(*testing.T) {
+	return func(t *testing.T) {
+		t.Parallel()
+
+		file, err := Open(path)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := file.Close(); err != nil {
+			t.Fatalf("unexpected error from Close: %v", err)
+		}
+		if err := file.Close(); err == nil {
+			t.Error("second Close did not fail")
+		} else if pe, ok := err.(*PathError); !ok {
+			t.Errorf("second Close: got %T, want %T", err, pe)
+		} else if pe.Err != ErrClosed {
+			t.Errorf("second Close: got %q, want %q", pe.Err, ErrClosed)
+		} else {
+			t.Logf("second close returned expected error %q", err)
+		}
 	}
 }
 
 func TestDoubleCloseError(t *testing.T) {
-	testDoubleCloseError(t, filepath.Join(sfdir, sfname))
-	testDoubleCloseError(t, sfdir)
+	t.Parallel()
+	t.Run("file", testDoubleCloseError(filepath.Join(sfdir, sfname)))
+	t.Run("dir", testDoubleCloseError(sfdir))
 }
 
 func TestUserHomeDir(t *testing.T) {
+	t.Parallel()
+
 	dir, err := UserHomeDir()
 	if dir == "" && err == nil {
 		t.Fatal("UserHomeDir returned an empty string but no error")
@@ -2566,6 +2651,8 @@ func TestUserHomeDir(t *testing.T) {
 }
 
 func TestDirSeek(t *testing.T) {
+	t.Parallel()
+
 	wd, err := Getwd()
 	if err != nil {
 		t.Fatal(err)
@@ -2608,6 +2695,8 @@ func TestReaddirSmallSeek(t *testing.T) {
 	// See issue 37161. Read only one entry from a directory,
 	// seek to the beginning, and read again. We should not see
 	// duplicate entries.
+	t.Parallel()
+
 	wd, err := Getwd()
 	if err != nil {
 		t.Fatal(err)
@@ -2647,6 +2736,7 @@ func isDeadlineExceeded(err error) bool {
 // Test that opening a file does not change its permissions.  Issue 38225.
 func TestOpenFileKeepsPermissions(t *testing.T) {
 	t.Parallel()
+
 	dir := t.TempDir()
 	name := filepath.Join(dir, "x")
 	f, err := Create(name)
@@ -2676,6 +2766,8 @@ func TestOpenFileKeepsPermissions(t *testing.T) {
 }
 
 func TestDirFS(t *testing.T) {
+	t.Parallel()
+
 	// On Windows, we force the MFT to update by reading the actual metadata from GetFileInformationByHandle and then
 	// explicitly setting that. Otherwise it might get out of sync with FindFirstFile. See golang.org/issues/42637.
 	if runtime.GOOS == "windows" {
@@ -2737,6 +2829,8 @@ func TestDirFS(t *testing.T) {
 }
 
 func TestDirFSRootDir(t *testing.T) {
+	t.Parallel()
+
 	cwd, err := os.Getwd()
 	if err != nil {
 		t.Fatal(err)
@@ -2755,6 +2849,8 @@ func TestDirFSRootDir(t *testing.T) {
 }
 
 func TestDirFSEmptyDir(t *testing.T) {
+	t.Parallel()
+
 	d := DirFS("")
 	cwd, _ := os.Getwd()
 	for _, path := range []string{
@@ -2772,6 +2868,7 @@ func TestDirFSPathsValid(t *testing.T) {
 	if runtime.GOOS == "windows" {
 		t.Skipf("skipping on Windows")
 	}
+	t.Parallel()
 
 	d := t.TempDir()
 	if err := os.WriteFile(filepath.Join(d, "control.txt"), []byte(string("Hello, world!")), 0644); err != nil {
@@ -2796,6 +2893,8 @@ func TestDirFSPathsValid(t *testing.T) {
 }
 
 func TestReadFileProc(t *testing.T) {
+	t.Parallel()
+
 	// Linux files in /proc report 0 size,
 	// but then if ReadFile reads just a single byte at offset 0,
 	// the read at offset 1 returns EOF instead of more data.
@@ -2838,6 +2937,7 @@ func TestPipeIOCloseRace(t *testing.T) {
 	if runtime.GOOS == "js" {
 		t.Skip("skipping on js: no pipes")
 	}
+	t.Parallel()
 
 	r, w, err := Pipe()
 	if err != nil {
@@ -2915,6 +3015,7 @@ func TestPipeCloseRace(t *testing.T) {
 	if runtime.GOOS == "js" {
 		t.Skip("skipping on js: no pipes")
 	}
+	t.Parallel()
 
 	r, w, err := Pipe()
 	if err != nil {
diff --git a/src/os/os_unix_test.go b/src/os/os_unix_test.go
index c3c703f860..47c7ca66f1 100644
--- a/src/os/os_unix_test.go
+++ b/src/os/os_unix_test.go
@@ -40,6 +40,8 @@ func checkUidGid(t *testing.T, path string, uid, gid int) {
 }
 
 func TestChown(t *testing.T) {
+	t.Parallel()
+
 	// Use TempDir() to make sure we're on a local file system,
 	// so that the group ids returned by Getgroups will be allowed
 	// on the file. On NFS, the Getgroups groups are
@@ -83,6 +85,8 @@ func TestChown(t *testing.T) {
 }
 
 func TestFileChown(t *testing.T) {
+	t.Parallel()
+
 	// Use TempDir() to make sure we're on a local file system,
 	// so that the group ids returned by Getgroups will be allowed
 	// on the file. On NFS, the Getgroups groups are
@@ -126,6 +130,8 @@ func TestFileChown(t *testing.T) {
 }
 
 func TestLchown(t *testing.T) {
+	t.Parallel()
+
 	// Use TempDir() to make sure we're on a local file system,
 	// so that the group ids returned by Getgroups will be allowed
 	// on the file. On NFS, the Getgroups groups are
@@ -214,6 +220,8 @@ func TestReaddirRemoveRace(t *testing.T) {
 
 // Issue 23120: respect umask when doing Mkdir with the sticky bit
 func TestMkdirStickyUmask(t *testing.T) {
+	t.Parallel()
+
 	const umask = 0077
 	dir := newDir("TestMkdirStickyUmask", t)
 	defer RemoveAll(dir)
diff --git a/src/os/os_windows_test.go b/src/os/os_windows_test.go
index b9fad71bfd..17b03e9508 100644
--- a/src/os/os_windows_test.go
+++ b/src/os/os_windows_test.go
@@ -14,7 +14,6 @@ import (
 	"io"
 	"io/fs"
 	"os"
-	osexec "os/exec"
 	"path/filepath"
 	"reflect"
 	"runtime"
@@ -49,11 +48,7 @@ func chdir(t *testing.T, dir string) {
 }
 
 func TestSameWindowsFile(t *testing.T) {
-	temp, err := os.MkdirTemp("", "TestSameWindowsFile")
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(temp)
+	temp := t.TempDir()
 	chdir(t, temp)
 
 	f, err := os.Create("a")
@@ -99,15 +94,11 @@ type dirLinkTest struct {
 }
 
 func testDirLinks(t *testing.T, tests []dirLinkTest) {
-	tmpdir, err := os.MkdirTemp("", "testDirLinks")
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(tmpdir)
+	tmpdir := t.TempDir()
 	chdir(t, tmpdir)
 
 	dir := filepath.Join(tmpdir, "dir")
-	err = os.Mkdir(dir, 0777)
+	err := os.Mkdir(dir, 0777)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -298,14 +289,14 @@ func TestDirectoryJunction(t *testing.T) {
 			},
 		},
 	}
-	output, _ := osexec.Command("cmd", "/c", "mklink", "/?").Output()
+	output, _ := testenv.Command(t, "cmd", "/c", "mklink", "/?").Output()
 	mklinkSupportsJunctionLinks := strings.Contains(string(output), " /J ")
 	if mklinkSupportsJunctionLinks {
 		tests = append(tests,
 			dirLinkTest{
 				name: "use_mklink_cmd",
 				mklink: func(link, target string) error {
-					output, err := osexec.Command("cmd", "/c", "mklink", "/J", link, target).CombinedOutput()
+					output, err := testenv.Command(t, "cmd", "/c", "mklink", "/J", link, target).CombinedOutput()
 					if err != nil {
 						t.Errorf("failed to run mklink %v %v: %v %q", link, target, err, output)
 					}
@@ -371,14 +362,14 @@ func createSymbolicLink(link string, target *reparseData, isrelative bool) error
 
 func TestDirectorySymbolicLink(t *testing.T) {
 	var tests []dirLinkTest
-	output, _ := osexec.Command("cmd", "/c", "mklink", "/?").Output()
+	output, _ := testenv.Command(t, "cmd", "/c", "mklink", "/?").Output()
 	mklinkSupportsDirectorySymbolicLinks := strings.Contains(string(output), " /D ")
 	if mklinkSupportsDirectorySymbolicLinks {
 		tests = append(tests,
 			dirLinkTest{
 				name: "use_mklink_cmd",
 				mklink: func(link, target string) error {
-					output, err := osexec.Command("cmd", "/c", "mklink", "/D", link, target).CombinedOutput()
+					output, err := testenv.Command(t, "cmd", "/c", "mklink", "/D", link, target).CombinedOutput()
 					if err != nil {
 						t.Errorf("failed to run mklink %v %v: %v %q", link, target, err, output)
 					}
@@ -439,19 +430,14 @@ func TestNetworkSymbolicLink(t *testing.T) {
 
 	const _NERR_ServerNotStarted = syscall.Errno(2114)
 
-	dir, err := os.MkdirTemp("", "TestNetworkSymbolicLink")
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(dir)
-
+	dir := t.TempDir()
 	chdir(t, dir)
 
 	shareName := "GoSymbolicLinkTestShare" // hope no conflictions
 	sharePath := filepath.Join(dir, shareName)
 	testDir := "TestDir"
 
-	err = os.MkdirAll(filepath.Join(sharePath, testDir), 0777)
+	err := os.MkdirAll(filepath.Join(sharePath, testDir), 0777)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -534,6 +520,8 @@ func TestNetworkSymbolicLink(t *testing.T) {
 }
 
 func TestStartProcessAttr(t *testing.T) {
+	t.Parallel()
+
 	p, err := os.StartProcess(os.Getenv("COMSPEC"), []string{"/c", "cd"}, new(os.ProcAttr))
 	if err != nil {
 		return
@@ -546,6 +534,8 @@ func TestShareNotExistError(t *testing.T) {
 	if testing.Short() {
 		t.Skip("slow test that uses network; skipping")
 	}
+	t.Parallel()
+
 	_, err := os.Stat(`\\no_such_server\no_such_share\no_such_file`)
 	if err == nil {
 		t.Fatal("stat succeeded, but expected to fail")
@@ -592,11 +582,7 @@ func TestStatDir(t *testing.T) {
 }
 
 func TestOpenVolumeName(t *testing.T) {
-	tmpdir, err := os.MkdirTemp("", "TestOpenVolumeName")
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(tmpdir)
+	tmpdir := t.TempDir()
 	chdir(t, tmpdir)
 
 	want := []string{"file1", "file2", "file3", "gopher.txt"}
@@ -626,6 +612,8 @@ func TestOpenVolumeName(t *testing.T) {
 }
 
 func TestDeleteReadOnly(t *testing.T) {
+	t.Parallel()
+
 	tmpdir := t.TempDir()
 	p := filepath.Join(tmpdir, "a")
 	// This sets FILE_ATTRIBUTE_READONLY.
@@ -723,6 +711,8 @@ func TestReadStdin(t *testing.T) {
 }
 
 func TestStatPagefile(t *testing.T) {
+	t.Parallel()
+
 	fi, err := os.Stat(`c:\pagefile.sys`)
 	if err == nil {
 		if fi.Name() == "" {
@@ -769,6 +759,11 @@ func compareCommandLineToArgvWithSyscall(t *testing.T, cmd string) {
 }
 
 func TestCmdArgs(t *testing.T) {
+	if testing.Short() {
+		t.Skipf("in short mode; skipping test that builds a binary")
+	}
+	t.Parallel()
+
 	tmpdir := t.TempDir()
 
 	const prog = `
@@ -789,7 +784,7 @@ func main() {
 	}
 
 	exe := filepath.Join(tmpdir, "main.exe")
-	cmd := osexec.Command(testenv.GoToolPath(t), "build", "-o", exe, src)
+	cmd := testenv.Command(t, testenv.GoToolPath(t), "build", "-o", exe, src)
 	cmd.Dir = tmpdir
 	out, err := cmd.CombinedOutput()
 	if err != nil {
@@ -854,7 +849,7 @@ func main() {
 
 		// test both syscall.EscapeArg and os.commandLineToArgv
 		args := os.CommandLineToArgv(exe + cmd)
-		out, err := osexec.Command(args[0], args[1:]...).CombinedOutput()
+		out, err := testenv.Command(t, args[0], args[1:]...).CombinedOutput()
 		if err != nil {
 			t.Fatalf("running %q failed: %v\n%v", args, err, string(out))
 		}
@@ -883,6 +878,8 @@ func findOneDriveDir() (string, error) {
 
 // TestOneDrive verifies that OneDrive folder is a directory and not a symlink.
 func TestOneDrive(t *testing.T) {
+	t.Parallel()
+
 	dir, err := findOneDriveDir()
 	if err != nil {
 		t.Skipf("Skipping, because we did not find OneDrive directory: %v", err)
@@ -891,6 +888,8 @@ func TestOneDrive(t *testing.T) {
 }
 
 func TestWindowsDevNullFile(t *testing.T) {
+	t.Parallel()
+
 	f1, err := os.Open("NUL")
 	if err != nil {
 		t.Fatal(err)
@@ -919,6 +918,8 @@ func TestWindowsDevNullFile(t *testing.T) {
 }
 
 func TestFileStatNUL(t *testing.T) {
+	t.Parallel()
+
 	f, err := os.Open("NUL")
 	if err != nil {
 		t.Fatal(err)
@@ -933,6 +934,8 @@ func TestFileStatNUL(t *testing.T) {
 }
 
 func TestStatNUL(t *testing.T) {
+	t.Parallel()
+
 	fi, err := os.Stat("NUL")
 	if err != nil {
 		t.Fatal(err)
@@ -1098,6 +1101,8 @@ func TestWorkingDirectoryRelativeSymlink(t *testing.T) {
 
 // TestStatOfInvalidName is regression test for issue #24999.
 func TestStatOfInvalidName(t *testing.T) {
+	t.Parallel()
+
 	_, err := os.Stat("*.go")
 	if err == nil {
 		t.Fatal(`os.Stat("*.go") unexpectedly succeeded`)
@@ -1121,20 +1126,26 @@ func findUnusedDriveLetter() (string, error) {
 }
 
 func TestRootDirAsTemp(t *testing.T) {
-	testenv.MustHaveExec(t)
-
 	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
 		fmt.Print(os.TempDir())
 		os.Exit(0)
 	}
 
-	newtmp, err := findUnusedDriveLetter()
+	testenv.MustHaveExec(t)
+	t.Parallel()
+
+	exe, err := os.Executable()
 	if err != nil {
 		t.Fatal(err)
 	}
 
-	cmd := osexec.Command(os.Args[0], "-test.run=TestRootDirAsTemp")
-	cmd.Env = os.Environ()
+	newtmp, err := findUnusedDriveLetter()
+	if err != nil {
+		t.Skip(err)
+	}
+
+	cmd := testenv.Command(t, exe, "-test.run=TestRootDirAsTemp")
+	cmd.Env = cmd.Environ()
 	cmd.Env = append(cmd.Env, "GO_WANT_HELPER_PROCESS=1")
 	cmd.Env = append(cmd.Env, "TMP="+newtmp)
 	cmd.Env = append(cmd.Env, "TEMP="+newtmp)
@@ -1159,21 +1170,21 @@ func testReadlink(t *testing.T, path, want string) {
 }
 
 func mklink(t *testing.T, link, target string) {
-	output, err := osexec.Command("cmd", "/c", "mklink", link, target).CombinedOutput()
+	output, err := testenv.Command(t, "cmd", "/c", "mklink", link, target).CombinedOutput()
 	if err != nil {
 		t.Fatalf("failed to run mklink %v %v: %v %q", link, target, err, output)
 	}
 }
 
 func mklinkj(t *testing.T, link, target string) {
-	output, err := osexec.Command("cmd", "/c", "mklink", "/J", link, target).CombinedOutput()
+	output, err := testenv.Command(t, "cmd", "/c", "mklink", "/J", link, target).CombinedOutput()
 	if err != nil {
 		t.Fatalf("failed to run mklink %v %v: %v %q", link, target, err, output)
 	}
 }
 
 func mklinkd(t *testing.T, link, target string) {
-	output, err := osexec.Command("cmd", "/c", "mklink", "/D", link, target).CombinedOutput()
+	output, err := testenv.Command(t, "cmd", "/c", "mklink", "/D", link, target).CombinedOutput()
 	if err != nil {
 		t.Fatalf("failed to run mklink %v %v: %v %q", link, target, err, output)
 	}
@@ -1194,7 +1205,7 @@ func TestWindowsReadlink(t *testing.T) {
 	chdir(t, tmpdir)
 
 	vol := filepath.VolumeName(tmpdir)
-	output, err := osexec.Command("cmd", "/c", "mountvol", vol, "/L").CombinedOutput()
+	output, err := testenv.Command(t, "cmd", "/c", "mountvol", vol, "/L").CombinedOutput()
 	if err != nil {
 		t.Fatalf("failed to run mountvol %v /L: %v %q", vol, err, output)
 	}
@@ -1254,6 +1265,8 @@ func TestWindowsReadlink(t *testing.T) {
 }
 
 func TestOpenDirTOCTOU(t *testing.T) {
+	t.Parallel()
+
 	// Check opened directories can't be renamed until the handle is closed.
 	// See issue 52747.
 	tmpdir := t.TempDir()
diff --git a/src/os/path_test.go b/src/os/path_test.go
index 59f7283485..2a4e9565dc 100644
--- a/src/os/path_test.go
+++ b/src/os/path_test.go
@@ -16,6 +16,8 @@ import (
 var isReadonlyError = func(error) bool { return false }
 
 func TestMkdirAll(t *testing.T) {
+	t.Parallel()
+
 	tmpDir := TempDir()
 	path := tmpDir + "/_TestMkdirAll_/dir/./dir2"
 	err := MkdirAll(path, 0777)
@@ -76,6 +78,7 @@ func TestMkdirAll(t *testing.T) {
 
 func TestMkdirAllWithSymlink(t *testing.T) {
 	testenv.MustHaveSymlink(t)
+	t.Parallel()
 
 	tmpDir := t.TempDir()
 	dir := tmpDir + "/dir"
@@ -99,6 +102,10 @@ func TestMkdirAllAtSlash(t *testing.T) {
 	case "android", "ios", "plan9", "windows":
 		t.Skipf("skipping on %s", runtime.GOOS)
 	}
+	if testenv.Builder() == "" {
+		t.Skipf("skipping non-hermetic test outside of Go builders")
+	}
+
 	RemoveAll("/_go_os_test")
 	const dir = "/_go_os_test/dir"
 	err := MkdirAll(dir, 0777)
diff --git a/src/os/path_windows_test.go b/src/os/path_windows_test.go
index e960bcb19c..2506b4f0d8 100644
--- a/src/os/path_windows_test.go
+++ b/src/os/path_windows_test.go
@@ -15,6 +15,8 @@ func TestFixLongPath(t *testing.T) {
 	if os.CanUseLongPaths {
 		return
 	}
+	t.Parallel()
+
 	// 248 is long enough to trigger the longer-than-248 checks in
 	// fixLongPath, but short enough not to make a path component
 	// longer than 255, which is illegal on Windows. (which
@@ -50,6 +52,8 @@ func TestFixLongPath(t *testing.T) {
 }
 
 func TestMkdirAllLongPath(t *testing.T) {
+	t.Parallel()
+
 	tmpDir := t.TempDir()
 	path := tmpDir
 	for i := 0; i < 100; i++ {
@@ -64,6 +68,7 @@ func TestMkdirAllLongPath(t *testing.T) {
 }
 
 func TestMkdirAllExtendedLength(t *testing.T) {
+	t.Parallel()
 	tmpDir := t.TempDir()
 
 	const prefix = `\\?\`
@@ -86,6 +91,8 @@ func TestMkdirAllExtendedLength(t *testing.T) {
 }
 
 func TestOpenRootSlash(t *testing.T) {
+	t.Parallel()
+
 	tests := []string{
 		`/`,
 		`\`,
diff --git a/src/os/pipe_test.go b/src/os/pipe_test.go
index a20a12aac4..27a96ad586 100644
--- a/src/os/pipe_test.go
+++ b/src/os/pipe_test.go
@@ -16,7 +16,7 @@ import (
 	"io"
 	"io/fs"
 	"os"
-	osexec "os/exec"
+	"os/exec"
 	"os/signal"
 	"runtime"
 	"strconv"
@@ -28,6 +28,16 @@ import (
 )
 
 func TestEPIPE(t *testing.T) {
+	// This test cannot be run in parallel because of a race similar
+	// to the one reported in https://go.dev/issue/22315.
+	//
+	// Even though the pipe is opened with O_CLOEXEC, if another test forks in
+	// between the call to os.Pipe and the call to r.Close, that child process can
+	// retain an open copy of r's file descriptor until it execs. If one of our
+	// Write calls occurs during that interval it can spuriously succeed,
+	// buffering the write to the child's copy of the pipe (even though the child
+	// will not actually read the buffered bytes).
+
 	r, w, err := os.Pipe()
 	if err != nil {
 		t.Fatal(err)
@@ -64,7 +74,35 @@ func TestStdPipe(t *testing.T) {
 	case "windows":
 		t.Skip("Windows doesn't support SIGPIPE")
 	}
+
+	if os.Getenv("GO_TEST_STD_PIPE_HELPER") != "" {
+		if os.Getenv("GO_TEST_STD_PIPE_HELPER_SIGNAL") != "" {
+			signal.Notify(make(chan os.Signal, 1), syscall.SIGPIPE)
+		}
+		switch os.Getenv("GO_TEST_STD_PIPE_HELPER") {
+		case "1":
+			os.Stdout.Write([]byte("stdout"))
+		case "2":
+			os.Stderr.Write([]byte("stderr"))
+		case "3":
+			if _, err := os.NewFile(3, "3").Write([]byte("3")); err == nil {
+				os.Exit(3)
+			}
+		default:
+			panic("unrecognized value for GO_TEST_STD_PIPE_HELPER")
+		}
+		// For stdout/stderr, we should have crashed with a broken pipe error.
+		// The caller will be looking for that exit status,
+		// so just exit normally here to cause a failure in the caller.
+		// For descriptor 3, a normal exit is expected.
+		os.Exit(0)
+	}
+
 	testenv.MustHaveExec(t)
+	// This test cannot be run in parallel due to the same race as for TestEPIPE.
+	// (We expect a write to a closed pipe can fail, but a concurrent fork of a
+	// child process can cause the pipe to unexpectedly remain open.)
+
 	r, w, err := os.Pipe()
 	if err != nil {
 		t.Fatal(err)
@@ -80,7 +118,7 @@ func TestStdPipe(t *testing.T) {
 	// all writes should fail with EPIPE and then exit 0.
 	for _, sig := range []bool{false, true} {
 		for dest := 1; dest < 4; dest++ {
-			cmd := osexec.Command(os.Args[0], "-test.run", "TestStdPipeHelper")
+			cmd := testenv.Command(t, os.Args[0], "-test.run", "TestStdPipe")
 			cmd.Stdout = w
 			cmd.Stderr = w
 			cmd.ExtraFiles = []*os.File{w}
@@ -92,7 +130,7 @@ func TestStdPipe(t *testing.T) {
 				if !sig && dest < 3 {
 					t.Errorf("unexpected success of write to closed pipe %d sig %t in child", dest, sig)
 				}
-			} else if ee, ok := err.(*osexec.ExitError); !ok {
+			} else if ee, ok := err.(*exec.ExitError); !ok {
 				t.Errorf("unexpected exec error type %T: %v", err, err)
 			} else if ws, ok := ee.Sys().(syscall.WaitStatus); !ok {
 				t.Errorf("unexpected wait status type %T: %v", ee.Sys(), ee.Sys())
@@ -107,14 +145,14 @@ func TestStdPipe(t *testing.T) {
 	}
 
 	// Test redirecting stdout but not stderr.  Issue 40076.
-	cmd := osexec.Command(os.Args[0], "-test.run", "TestStdPipeHelper")
+	cmd := testenv.Command(t, os.Args[0], "-test.run", "TestStdPipe")
 	cmd.Stdout = w
 	var stderr bytes.Buffer
 	cmd.Stderr = &stderr
-	cmd.Env = append(os.Environ(), "GO_TEST_STD_PIPE_HELPER=1")
+	cmd.Env = append(cmd.Environ(), "GO_TEST_STD_PIPE_HELPER=1")
 	if err := cmd.Run(); err == nil {
 		t.Errorf("unexpected success of write to closed stdout")
-	} else if ee, ok := err.(*osexec.ExitError); !ok {
+	} else if ee, ok := err.(*exec.ExitError); !ok {
 		t.Errorf("unexpected exec error type %T: %v", err, err)
 	} else if ws, ok := ee.Sys().(syscall.WaitStatus); !ok {
 		t.Errorf("unexpected wait status type %T: %v", ee.Sys(), ee.Sys())
@@ -126,31 +164,11 @@ func TestStdPipe(t *testing.T) {
 	}
 }
 
-// This is a helper for TestStdPipe. It's not a test in itself.
-func TestStdPipeHelper(t *testing.T) {
-	if os.Getenv("GO_TEST_STD_PIPE_HELPER_SIGNAL") != "" {
-		signal.Notify(make(chan os.Signal, 1), syscall.SIGPIPE)
-	}
-	switch os.Getenv("GO_TEST_STD_PIPE_HELPER") {
-	case "1":
-		os.Stdout.Write([]byte("stdout"))
-	case "2":
-		os.Stderr.Write([]byte("stderr"))
-	case "3":
-		if _, err := os.NewFile(3, "3").Write([]byte("3")); err == nil {
-			os.Exit(3)
-		}
-	default:
-		t.Skip("skipping test helper")
-	}
-	// For stdout/stderr, we should have crashed with a broken pipe error.
-	// The caller will be looking for that exit status,
-	// so just exit normally here to cause a failure in the caller.
-	// For descriptor 3, a normal exit is expected.
-	os.Exit(0)
-}
-
 func testClosedPipeRace(t *testing.T, read bool) {
+	// This test cannot be run in parallel due to the same race as for TestEPIPE.
+	// (We expect a write to a closed pipe can fail, but a concurrent fork of a
+	// child process can cause the pipe to unexpectedly remain open.)
+
 	limit := 1
 	if !read {
 		// Get the amount we have to write to overload a pipe
@@ -237,14 +255,16 @@ func TestReadNonblockingFd(t *testing.T) {
 	}
 
 	testenv.MustHaveExec(t)
+	t.Parallel()
+
 	r, w, err := os.Pipe()
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer r.Close()
 	defer w.Close()
-	cmd := osexec.Command(os.Args[0], "-test.run="+t.Name())
-	cmd.Env = append(os.Environ(), "GO_WANT_READ_NONBLOCKING_FD=1")
+	cmd := testenv.Command(t, os.Args[0], "-test.run="+t.Name())
+	cmd.Env = append(cmd.Environ(), "GO_WANT_READ_NONBLOCKING_FD=1")
 	cmd.Stdin = r
 	output, err := cmd.CombinedOutput()
 	t.Logf("%s", output)
@@ -254,6 +274,8 @@ func TestReadNonblockingFd(t *testing.T) {
 }
 
 func TestCloseWithBlockingReadByNewFile(t *testing.T) {
+	t.Parallel()
+
 	var p [2]syscallDescriptor
 	err := syscall.Pipe(p[:])
 	if err != nil {
@@ -264,6 +286,8 @@ func TestCloseWithBlockingReadByNewFile(t *testing.T) {
 }
 
 func TestCloseWithBlockingReadByFd(t *testing.T) {
+	t.Parallel()
+
 	r, w, err := os.Pipe()
 	if err != nil {
 		t.Fatal(err)
@@ -410,6 +434,11 @@ func testPipeEOF(t *testing.T, r io.ReadCloser, w io.WriteCloser) {
 
 // Issue 24481.
 func TestFdRace(t *testing.T) {
+	// This test starts 100 simultaneous goroutines, which could bury a more
+	// interesting stack if this or some other test happens to panic. It is also
+	// nearly instantaneous, so any latency benefit from running it in parallel
+	// would be minimal.
+
 	r, w, err := os.Pipe()
 	if err != nil {
 		t.Fatal(err)
diff --git a/src/os/read_test.go b/src/os/read_test.go
index 5c58d7d7df..5511dad948 100644
--- a/src/os/read_test.go
+++ b/src/os/read_test.go
@@ -22,6 +22,8 @@ func checkNamedSize(t *testing.T, path string, size int64) {
 }
 
 func TestReadFile(t *testing.T) {
+	t.Parallel()
+
 	filename := "rumpelstilzchen"
 	contents, err := ReadFile(filename)
 	if err == nil {
@@ -38,6 +40,8 @@ func TestReadFile(t *testing.T) {
 }
 
 func TestWriteFile(t *testing.T) {
+	t.Parallel()
+
 	f, err := CreateTemp("", "ioutil-test")
 	if err != nil {
 		t.Fatal(err)
@@ -67,6 +71,7 @@ func TestReadOnlyWriteFile(t *testing.T) {
 	if Getuid() == 0 {
 		t.Skipf("Root can write to read-only files anyway, so skip the read-only test.")
 	}
+	t.Parallel()
 
 	// We don't want to use CreateTemp directly, since that opens a file for us as 0600.
 	tempDir, err := MkdirTemp("", t.Name())
@@ -96,6 +101,8 @@ func TestReadOnlyWriteFile(t *testing.T) {
 }
 
 func TestReadDir(t *testing.T) {
+	t.Parallel()
+
 	dirname := "rumpelstilzchen"
 	_, err := ReadDir(dirname)
 	if err == nil {
diff --git a/src/os/readfrom_linux_test.go b/src/os/readfrom_linux_test.go
index 982a2b6330..20408a887c 100644
--- a/src/os/readfrom_linux_test.go
+++ b/src/os/readfrom_linux_test.go
@@ -420,6 +420,8 @@ func (h *copyFileRangeHook) uninstall() {
 
 // On some kernels copy_file_range fails on files in /proc.
 func TestProcCopy(t *testing.T) {
+	t.Parallel()
+
 	const cmdlineFile = "/proc/self/cmdline"
 	cmdline, err := os.ReadFile(cmdlineFile)
 	if err != nil {
diff --git a/src/os/removeall_test.go b/src/os/removeall_test.go
index 08dfdd8ec0..aa1c04325d 100644
--- a/src/os/removeall_test.go
+++ b/src/os/removeall_test.go
@@ -15,6 +15,8 @@ import (
 )
 
 func TestRemoveAll(t *testing.T) {
+	t.Parallel()
+
 	tmpDir := t.TempDir()
 	if err := RemoveAll(""); err != nil {
 		t.Errorf("RemoveAll(\"\"): %v; want nil", err)
@@ -122,6 +124,7 @@ func TestRemoveAllLarge(t *testing.T) {
 	if testing.Short() {
 		t.Skip("skipping in short mode")
 	}
+	t.Parallel()
 
 	tmpDir := t.TempDir()
 	path := filepath.Join(tmpDir, "_TestRemoveAllLarge_")
@@ -382,6 +385,7 @@ func TestRemoveAllWithMoreErrorThanReqSize(t *testing.T) {
 	if testing.Short() {
 		t.Skip("skipping in short mode")
 	}
+	t.Parallel()
 
 	tmpDir := t.TempDir()
 	path := filepath.Join(tmpDir, "_TestRemoveAllWithMoreErrorThanReqSize_")
diff --git a/src/os/stat_test.go b/src/os/stat_test.go
index 8d99f64609..c876d434fd 100644
--- a/src/os/stat_test.go
+++ b/src/os/stat_test.go
@@ -184,6 +184,7 @@ func testSymlinkSameFile(t *testing.T, path, link string) {
 
 func TestDirAndSymlinkStats(t *testing.T) {
 	testenv.MustHaveSymlink(t)
+	t.Parallel()
 
 	tmpdir := t.TempDir()
 	dir := filepath.Join(tmpdir, "dir")
@@ -209,6 +210,7 @@ func TestDirAndSymlinkStats(t *testing.T) {
 
 func TestFileAndSymlinkStats(t *testing.T) {
 	testenv.MustHaveSymlink(t)
+	t.Parallel()
 
 	tmpdir := t.TempDir()
 	file := filepath.Join(tmpdir, "file")
@@ -235,6 +237,7 @@ func TestFileAndSymlinkStats(t *testing.T) {
 // see issue 27225 for details
 func TestSymlinkWithTrailingSlash(t *testing.T) {
 	testenv.MustHaveSymlink(t)
+	t.Parallel()
 
 	tmpdir := t.TempDir()
 	dir := filepath.Join(tmpdir, "dir")
diff --git a/src/os/tempfile_test.go b/src/os/tempfile_test.go
index e5b74bc21f..82f0aabda0 100644
--- a/src/os/tempfile_test.go
+++ b/src/os/tempfile_test.go
@@ -15,6 +15,8 @@ import (
 )
 
 func TestCreateTemp(t *testing.T) {
+	t.Parallel()
+
 	dir, err := MkdirTemp("", "TestCreateTempBadDir")
 	if err != nil {
 		t.Fatal(err)
@@ -29,6 +31,8 @@ func TestCreateTemp(t *testing.T) {
 }
 
 func TestCreateTempPattern(t *testing.T) {
+	t.Parallel()
+
 	tests := []struct{ pattern, prefix, suffix string }{
 		{"tempfile_test", "tempfile_test", ""},
 		{"tempfile_test*", "tempfile_test", ""},
@@ -51,6 +55,8 @@ func TestCreateTempPattern(t *testing.T) {
 }
 
 func TestCreateTempBadPattern(t *testing.T) {
+	t.Parallel()
+
 	tmpDir, err := MkdirTemp("", t.Name())
 	if err != nil {
 		t.Fatal(err)
@@ -91,6 +97,8 @@ func TestCreateTempBadPattern(t *testing.T) {
 }
 
 func TestMkdirTemp(t *testing.T) {
+	t.Parallel()
+
 	name, err := MkdirTemp("/_not_exists_", "foo")
 	if name != "" || err == nil {
 		t.Errorf("MkdirTemp(`/_not_exists_`, `foo`) = %v, %v", name, err)
@@ -142,6 +150,8 @@ func TestMkdirTemp(t *testing.T) {
 // test that we return a nice error message if the dir argument to TempDir doesn't
 // exist (or that it's empty and TempDir doesn't exist)
 func TestMkdirTempBadDir(t *testing.T) {
+	t.Parallel()
+
 	dir, err := MkdirTemp("", "MkdirTempBadDir")
 	if err != nil {
 		t.Fatal(err)
@@ -156,6 +166,8 @@ func TestMkdirTempBadDir(t *testing.T) {
 }
 
 func TestMkdirTempBadPattern(t *testing.T) {
+	t.Parallel()
+
 	tmpDir, err := MkdirTemp("", t.Name())
 	if err != nil {
 		t.Fatal(err)
diff --git a/src/os/timeout_test.go b/src/os/timeout_test.go
index ff0d77a413..110b914e13 100644
--- a/src/os/timeout_test.go
+++ b/src/os/timeout_test.go
@@ -25,6 +25,7 @@ func TestNonpollableDeadline(t *testing.T) {
 	if runtime.GOOS != "linux" {
 		t.Skipf("skipping on %s", runtime.GOOS)
 	}
+	t.Parallel()
 
 	f, err := os.CreateTemp("", "ostest")
 	if err != nil {