windows: fix EnumProcesses to pass the correct array size

Implementation generated directly with mkwinsyscall has a wrong
assumption about the expected value for PIDs buffer size.

This change adds some small manual code that converts the input
slice length to the number of bytes of the array backing the slice.

A test is also added. It fails with the previous implementation.

Fixes golang/go#60223

Change-Id: I5e2414acb29c6c949e5e6acd328043f8a8883887

windows/syscall_windows.go

@@ -405,7 +405,7 @@ func NewCallbackCDecl(fn interface{}) uintptr {
 //sys	VerQueryValue(block unsafe.Pointer, subBlock string, pointerToBufferPointer unsafe.Pointer, bufSize *uint32) (err error) = version.VerQueryValueW
 
 // Process Status API (PSAPI)
-//sys	EnumProcesses(processIds []uint32, bytesReturned *uint32) (err error) = psapi.EnumProcesses
+//sys	enumProcesses(processIds *uint32, nSize uint32, bytesReturned *uint32) (err error) = psapi.EnumProcesses
 //sys	EnumProcessModules(process Handle, module *Handle, cb uint32, cbNeeded *uint32) (err error) = psapi.EnumProcessModules
 //sys	EnumProcessModulesEx(process Handle, module *Handle, cb uint32, cbNeeded *uint32, filterFlag uint32) (err error) = psapi.EnumProcessModulesEx
 //sys	GetModuleInformation(process Handle, module Handle, modinfo *ModuleInfo, cb uint32) (err error) = psapi.GetModuleInformation
@@ -1354,6 +1354,15 @@ func SetsockoptIPv6Mreq(fd Handle, level, opt int, mreq *IPv6Mreq) (err error) {
 	return syscall.EWINDOWS
 }
 
+func EnumProcesses(processIds []uint32, bytesReturned *uint32) error {
+	var p *uint32
+	if len(processIds) > 0 {
+		p = &processIds[0]
+	}
+	size := uint32(len(processIds) * 4)
+	return enumProcesses(p, size, bytesReturned)
+}
+
 func Getpid() (pid int) { return int(GetCurrentProcessId()) }
 
 func FindFirstFile(name *uint16, data *Win32finddata) (handle Handle, err error) {

windows/syscall_windows_test.go

@@ -717,6 +717,25 @@ func TestWinVerifyTrust(t *testing.T) {
 
 }
 
+func TestEnumProcesses(t *testing.T) {
+	var (
+		pids    [1]uint32
+		outSize uint32
+	)
+	err := windows.EnumProcesses(pids[:], &outSize)
+	if err != nil {
+		t.Fatalf("unable to enumerate processes: %v", err)
+	}
+
+	// Regression check for https://github.com/golang/go/issues/60223
+	if outSize != 4 {
+		t.Errorf("unexpected bytes returned: %d", outSize)
+	}
+	if pids[0] == 0 {
+		t.Error("got PID=0")
+	}
+}
+
 func TestProcessModules(t *testing.T) {
 	process, err := windows.GetCurrentProcess()
 	if err != nil {