Fix poor detection of a dupe instance

Old FindWindow() function could mistake an Explorer folder 'UI for ETW'
for another instance of 'UI for ETW' (since window title is identical)
New custom function registers a unique window message that allows to
uniquely identify another instance of 'UI for ETW'
Bonus: restore the minimized window of 'UI for ETW'

Resolves: google#147

UIforETW/UIforETW.cpp

@@ -170,6 +170,25 @@ CUIforETWApp::CUIforETWApp() noexcept
 
 CUIforETWApp theApp;
 
+BOOL CALLBACK CUIforETWApp::findDupeWindow(HWND hWnd, LPARAM lParam)
+{
+	DWORD_PTR result;
+	LRESULT msgRet = SendMessageTimeout(hWnd, uwmAreYouMe, 0, 0,
+		SMTO_BLOCK |      // Don't process any other requests until the function returns
+		SMTO_ABORTIFHUNG, // No waiting if the thread appears to not respond
+		200, &result);    // Timeout in ms
+		if(msgRet == 0)           // failed to send message to this window, ignore it
+		{
+			return TRUE;
+		}
+		if(result == uwmAreYouMe) // found a dupe window
+		{
+			HWND * target = (HWND *)lParam;
+			*target = hWnd;
+			return FALSE;           // stop window enumeration
+		}
+		return TRUE;              // not a dupe window, continue window enumeration
+}
 
 // CUIforETWApp initialization
 
@@ -191,18 +210,22 @@ BOOL CUIforETWApp::InitInstance()
 
 	SetRegistryKey(L"RandomASCII");
 
-	constexpr wchar_t identifier[] = L"{B7D2F8B8-2F28-4366-9D7A-691019D89185}";
+	constexpr wchar_t identifier[] = L"Global\\{B7D2F8B8-2F28-4366-9D7A-691019D89185}";
 	HANDLE mutex = CreateMutexW(nullptr, FALSE, identifier);
 	// Only allow one copy to be running at a time.
-	if (mutex && GetLastError() == ERROR_ALREADY_EXISTS) {
-		// Activate the previous window if possible. Note that if you have another
-		// window with this title (an explorer window for a UI for ETW folder for
-		// instance) then the wrong window may be activated. See
-		// https://github.com/google/UIforETW/issues/147 for details.
-		HWND prevWindow = FindWindow(NULL, L"UI for ETW");
-		if (prevWindow)
+	if (mutex && GetLastError() == ERROR_ALREADY_EXISTS ||
+	             GetLastError() == ERROR_ACCESS_DENIED) // created in another session
+	{
+		// Activate the previous window
+		HWND prevWindow = NULL;
+		EnumWindows(findDupeWindow,(LPARAM)&prevWindow);
+		if (prevWindow != NULL)
 		{
 			SetForegroundWindow(prevWindow);
+			if (IsIconic(prevWindow)) // restore minimized window
+			{
+				ShowWindow(prevWindow, SW_RESTORE);
+			}
 		}
 
 		// Already running.

UIforETW/UIforETW.h

@@ -37,9 +37,11 @@ class CUIforETWApp : public CWinApp
 	virtual BOOL InitInstance() override;
 	afx_msg void OnHelp() noexcept;
 
+	static BOOL CALLBACK findDupeWindow(HWND hWnd, LPARAM lParam);
 // Implementation
 
 	DECLARE_MESSAGE_MAP()
 };
 
+inline const UINT uwmAreYouMe = RegisterWindowMessage(L"678fd291e05741edb987c0a4f5650866");
 extern CUIforETWApp theApp;

UIforETW/UIforETWDlg.cpp

@@ -260,6 +260,7 @@ BEGIN_MESSAGE_MAP(CUIforETWDlg, CDialog)
 	ON_WM_ACTIVATE()
 	ON_WM_TIMER()
 	ON_BN_CLICKED(IDC_CLRTRACING, &CUIforETWDlg::OnBnClickedClrtracing)
+	ON_REGISTERED_MESSAGE(uwmAreYouMe, &CUIforETWDlg::OnAreYouMe)
 END_MESSAGE_MAP()
 
 
@@ -2538,3 +2539,8 @@ void CUIforETWDlg::OnActivate(UINT nState, CWnd* pWndOther, BOOL bMinimized)
 	}
 	CDialog::OnActivate(nState, pWndOther, bMinimized);
 }
+
+LRESULT CUIforETWDlg::OnAreYouMe(WPARAM, LPARAM) noexcept
+{
+	return uwmAreYouMe;
+}

UIforETW/UIforETWDlg.h

@@ -258,6 +258,7 @@ class CUIforETWDlg : public CDialog
 	bool bShutdownCompleted_ = false;
 
 	// Generated message map functions
+	afx_msg LRESULT OnAreYouMe(WPARAM, LPARAM) noexcept;
 	virtual BOOL OnInitDialog() override;
 	afx_msg void OnSysCommand(UINT nID, LPARAM lParam);
 	afx_msg void OnPaint();