os: use an actual RemoveAll failure in TestRemoveAllWithMoreErrorThanReqSize

Previously we injected an error, and the injection points were
(empirically) not realistic on some platforms.

Instead, we now make the directory read-only, which (on most
platforms) suffices to prevent the removal of its files.

Fixes #35117
Updates #29921

Change-Id: Ica4e2818566f8c14df3eed7c3b8de5c0abeb6963
Reviewed-on: https://go-review.googlesource.com/c/go/+/203502Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/export_test.go

@@ -9,4 +9,3 @@ package os
 var Atime = atime
 var LstatP = &lstat
 var ErrWriteAtInAppendMode = errWriteAtInAppendMode
-var RemoveAllTestHook = &removeAllTestHook

src/os/removeall_at.go

@@ -153,7 +153,6 @@ func removeAllFrom(parent *File, base string) error {
 
 	// Remove the directory itself.
 	unlinkError := unix.Unlinkat(parentFd, base, unix.AT_REMOVEDIR)
-	unlinkError = removeAllTestHook(unlinkError)
 	if unlinkError == nil || IsNotExist(unlinkError) {
 		return nil
 	}

src/os/removeall_noat.go

@@ -124,7 +124,6 @@ func removeAll(path string) error {
 
 	// Remove directory.
 	err1 := Remove(path)
-	err1 = removeAllTestHook(err1)
 	if err1 == nil || IsNotExist(err1) {
 		return nil
 	}

src/os/removeall_test.go

@@ -5,7 +5,6 @@
 package os_test
 
 import (
-	"errors"
 	"fmt"
 	"io/ioutil"
 	. "os"
@@ -413,14 +412,6 @@ func TestRemoveAllWithMoreErrorThanReqSize(t *testing.T) {
 		t.Skip("skipping in short mode")
 	}
 
-	defer func(oldHook func(error) error) {
-		*RemoveAllTestHook = oldHook
-	}(*RemoveAllTestHook)
-
-	*RemoveAllTestHook = func(err error) error {
-		return errors.New("error from RemoveAllTestHook")
-	}
-
 	tmpDir, err := ioutil.TempDir("", "TestRemoveAll-")
 	if err != nil {
 		t.Fatal(err)
@@ -429,7 +420,7 @@ func TestRemoveAllWithMoreErrorThanReqSize(t *testing.T) {
 
 	path := filepath.Join(tmpDir, "_TestRemoveAllWithMoreErrorThanReqSize_")
 
-	// Make directory with 1025 files and remove.
+	// Make directory with 1025 read-only files.
 	if err := MkdirAll(path, 0777); err != nil {
 		t.Fatalf("MkdirAll %q: %s", path, err)
 	}
@@ -442,13 +433,40 @@ func TestRemoveAllWithMoreErrorThanReqSize(t *testing.T) {
 		fd.Close()
 	}
 
-	// This call should not hang
-	if err := RemoveAll(path); err == nil {
-		t.Fatal("Want error from RemoveAllTestHook, got nil")
+	// Make the parent directory read-only. On some platforms, this is what
+	// prevents os.Remove from removing the files within that directory.
+	if err := Chmod(path, 0555); err != nil {
+		t.Fatal(err)
 	}
+	defer Chmod(path, 0755)
 
-	// We hook to inject error, but the actual files must be deleted
-	if _, err := Lstat(path); err == nil {
-		t.Fatal("directory must be deleted even with removeAllTetHook run")
+	// This call should not hang, even on a platform that disallows file deletion
+	// from read-only directories.
+	err = RemoveAll(path)
+
+	if Getuid() == 0 {
+		// On many platforms, root can remove files from read-only directories.
+		return
+	}
+	if err == nil {
+		t.Fatal("RemoveAll(<read-only directory>) = nil; want error")
+	}
+
+	dir, err := Open(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer dir.Close()
+
+	if runtime.GOOS == "windows" {
+		// Marking a directory in Windows does not prevent the os package from
+		// creating or removing files within it.
+		// (See https://golang.org/issue/35042.)
+		return
+	}
+
+	names, _ := dir.Readdirnames(1025)
+	if len(names) < 1025 {
+		t.Fatalf("RemoveAll(<read-only directory>) unexpectedly removed %d read-only files from that directory", 1025-len(names))
 	}
 }