tests/root_test: add TestConcurrentUserOps and TestAsUserSleep

Both work with

	golang.org/x/sys v0.0.0-20220520151302-bc2c85ada10a

but break with

	golang.org/x/sys v0.30.0

#893
#892

tests/root_test/issue893_test.go

@@ -0,0 +1,99 @@
+//go:build linux
+
+package root_test
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"sync"
+	"syscall"
+	"testing"
+	"time"
+
+	"github.com/rfjakob/gocryptfs/v2/tests/test_helpers"
+)
+
+// gocryptfs v2.5.0 upgraded x/sys/unix and we found out that, since
+// https://github.com/golang/sys/commit/d0df966e6959f00dc1c74363e537872647352d51 ,
+// unix.Setreuid() and friends now affect the whole process instead of only the
+// current thread, breaking allow_other: https://github.com/rfjakob/gocryptfs/issues/893
+//
+// Let's not have this happen again by testing it here.
+func TestConcurrentUserOps(t *testing.T) {
+	if os.Getuid() != 0 {
+		t.Skip("must run as root")
+	}
+
+	var wg sync.WaitGroup
+
+	oneStressor := func(tid int) {
+		defer wg.Done()
+		err := asUser(10000+tid, 20000+tid, nil, func() (err error) {
+			for i := 0; i < 100; i++ {
+				d := fmt.Sprintf("%s/tid%d.i%d/foo/bar/baz", test_helpers.DefaultPlainDir, tid, i)
+				if err = os.MkdirAll(d, 0700); err != nil {
+					return
+				}
+				if err = ioutil.WriteFile(d+"/foo", nil, 0400); err != nil {
+					return
+				}
+				if err = ioutil.WriteFile(d+"/bar", []byte("aaaaaaaaaaaaaaaaaaaaa"), 0400); err != nil {
+					return
+				}
+				if err = syscall.Unlink(d + "/foo"); err != nil {
+					return
+				}
+				if err = os.Mkdir(d+"/foo", 0700); err != nil {
+					return
+				}
+			}
+			return nil
+		})
+		if err != nil {
+			t.Error(err)
+		}
+	}
+
+	threads := 4
+	wg.Add(threads)
+	for tid := 0; tid < threads; tid++ {
+		go oneStressor(tid)
+	}
+	wg.Wait()
+}
+
+// Test that our root_test.asUser function works as expected under concurrency by
+// similating a long-runnig operation with sleep(10ms).
+// https://github.com/rfjakob/gocryptfs/issues/893
+func TestAsUserSleep(t *testing.T) {
+	if os.Getuid() != 0 {
+		t.Skip("must run as root")
+	}
+
+	var wg sync.WaitGroup
+	f := func(euid_want int) error {
+		euid_have := syscall.Geteuid()
+		if euid_want != euid_have {
+			return fmt.Errorf("wrong euid: want=%d have=%d", euid_want, euid_have)
+		}
+		time.Sleep(10 * time.Millisecond)
+		euid_have2 := syscall.Geteuid()
+		if euid_want != euid_have2 {
+			return fmt.Errorf("wrong euid: want=%d have2=%d", euid_want, euid_have2)
+		}
+		return nil
+	}
+	threads := 100
+	wg.Add(threads)
+	for i := 0; i < threads; i++ {
+		go func(i int) {
+			defer wg.Done()
+			err := asUser(10000+i, 20000+i, nil, func() error { return f(10000 + i) })
+			if err != nil {
+				t.Error(err)
+			}
+		}(i)
+	}
+	wg.Wait()
+}

tests/root_test/main_test.go

@@ -1,3 +1,5 @@
+//go:build linux
+
 package root_test
 
 import (
@@ -9,7 +11,7 @@ import (
 
 func TestMain(m *testing.M) {
 	test_helpers.ResetTmpDir(true)
-	os.Chmod(test_helpers.DefaultCipherDir, 0755)
+	os.Chmod(test_helpers.DefaultCipherDir, 0777)
 	test_helpers.MountOrExit(test_helpers.DefaultCipherDir, test_helpers.DefaultPlainDir, "-zerokey", "-allow_other")
 	r := m.Run()
 	test_helpers.UnmountPanic(test_helpers.DefaultPlainDir)

tests/root_test/root_test.go

@@ -1,5 +1,4 @@
 //go:build linux
-// +build linux
 
 // Package root_test contains tests that need root
 // permissions to run