syscall: use runtime.KeepAlive for ProcThreadAttributeList arguments

It turns out that if you write Go pointers to Go memory, the Go compiler
must be involved so that it generates various calls to the GC in the
process. Letting Windows write Go pointers to Go memory violated this.
So, we replace that with just a boring call to runtime.KeepAlive. That's
not a great API, but this is all internal code anyway. We fix it up
more elegantly for external consumption in x/sys/windows with CL 300369.

Fixes #44900.

Change-Id: Id6599a793af9c4815f6c9387b00796923f32cb97
Reviewed-on: https://go-review.googlesource.com/c/go/+/300349
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/syscall/exec_windows.go

@@ -7,6 +7,7 @@
 package syscall
 
 import (
+	"runtime"
 	"sync"
 	"unicode/utf16"
 	"unsafe"
@@ -368,6 +369,8 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 		return 0, 0, err
 	}
 	defer CloseHandle(Handle(pi.Thread))
+	runtime.KeepAlive(fd)
+	runtime.KeepAlive(sys)
 
 	return int(pi.ProcessId), uintptr(pi.Process), nil
 }

src/syscall/syscall_windows.go

@@ -1257,7 +1257,7 @@ func newProcThreadAttributeList(maxAttrCount uint32) (*_PROC_THREAD_ATTRIBUTE_LI
 		return nil, err
 	}
 	// size is guaranteed to be ≥1 by initializeProcThreadAttributeList.
-	al := (*_PROC_THREAD_ATTRIBUTE_LIST)(unsafe.Pointer(&make([]unsafe.Pointer, (size+ptrSize-1)/ptrSize)[0]))
+	al := (*_PROC_THREAD_ATTRIBUTE_LIST)(unsafe.Pointer(&make([]byte, size)[0]))
 	err = initializeProcThreadAttributeList(al, maxAttrCount, 0, &size)
 	if err != nil {
 		return nil, err

src/syscall/syscall_windows_test.go

@@ -7,11 +7,8 @@ package syscall_test
 import (
 	"os"
 	"path/filepath"
-	"runtime"
 	"syscall"
 	"testing"
-	"time"
-	"unsafe"
 )
 
 func TestWin32finddata(t *testing.T) {
@@ -78,36 +75,3 @@ func TestTOKEN_ALL_ACCESS(t *testing.T) {
 		t.Errorf("TOKEN_ALL_ACCESS = %x, want 0xF01FF", syscall.TOKEN_ALL_ACCESS)
 	}
 }
-
-func TestProcThreadAttributeListPointers(t *testing.T) {
-	list, err := syscall.NewProcThreadAttributeList(1)
-	if err != nil {
-		t.Errorf("unable to create ProcThreadAttributeList: %v", err)
-	}
-	done := make(chan struct{})
-	fds := make([]syscall.Handle, 20)
-	runtime.SetFinalizer(&fds[0], func(*syscall.Handle) {
-		close(done)
-	})
-	err = syscall.UpdateProcThreadAttribute(list, 0, syscall.PROC_THREAD_ATTRIBUTE_HANDLE_LIST, unsafe.Pointer(&fds[0]), uintptr(len(fds))*unsafe.Sizeof(fds[0]), nil, nil)
-	if err != nil {
-		syscall.DeleteProcThreadAttributeList(list)
-		t.Errorf("unable to update ProcThreadAttributeList: %v", err)
-		return
-	}
-	runtime.GC()
-	runtime.GC()
-	select {
-	case <-done:
-		t.Error("ProcThreadAttributeList was garbage collected unexpectedly")
-	default:
-	}
-	syscall.DeleteProcThreadAttributeList(list)
-	runtime.GC()
-	runtime.GC()
-	select {
-	case <-done:
-	case <-time.After(time.Second):
-		t.Error("ProcThreadAttributeList was not garbage collected after a second")
-	}
-}

src/syscall/types_windows.go

@@ -4,8 +4,6 @@
 
 package syscall
 
-import "unsafe"
-
 const (
 	// Windows errors.
 	ERROR_FILE_NOT_FOUND      Errno = 2
@@ -493,11 +491,7 @@ type StartupInfo struct {
 }
 
 type _PROC_THREAD_ATTRIBUTE_LIST struct {
-	// This is of type unsafe.Pointer, not of type byte or uintptr, because
-	// the contents of it is mostly a list of pointers, and in most cases,
-	// that's a list of pointers to Go-allocated objects. In order to keep
-	// the GC from collecting these objects, we declare this as unsafe.Pointer.
-	_ [1]unsafe.Pointer
+	_ [1]byte
 }
 
 const (