unit PM.SingleInstance;

{
  Single-instance guard.

  AcquireOrSignal:
    - First instance: creates a named mutex and returns True. Caller proceeds.
    - Subsequent instance: detects the mutex, broadcasts WM_PMSHOW so the
      running instance restores from tray, returns False. Caller exits.

  WM_PMSHOW is a RegisterWindowMessage('PMServer_ShowExisting') — system-
  unique, all processes that register the same string get the same ID.
  PM.Bridge listens for it on its message-only window.
}

interface

uses
  Winapi.Windows, Winapi.Messages;

const
  // Mutex name lives in the Local\ namespace → per-user-session, so a
  // second user on the same machine (RDP, Switch User) can still launch
  // their own instance. The Global\ namespace would block them.
  PMSERVER_MUTEX_NAME = 'Local\PMServer.SingleInstance.Mutex';

// System-wide unique message ID, computed once. Bridge + .dpr both call
// this to get the same UINT.
function WM_PMShowMessage: UINT;

// Try to become the single instance. True = we are first; False = another
// instance was already running (we have signalled it and the caller must
// exit immediately).
function AcquireOrSignal: Boolean;

implementation

var
  _Mutex: THandle = 0;
  _WmShow: UINT  = 0;

function WM_PMShowMessage: UINT;
begin
  if _WmShow = 0 then
    _WmShow := RegisterWindowMessage('PMServer_ShowExisting');
  Result := _WmShow;
end;

function AcquireOrSignal: Boolean;
var
  LErr: DWORD;
begin
  _Mutex := CreateMutex(nil, True, PMSERVER_MUTEX_NAME);
  LErr := GetLastError;

  if (_Mutex <> 0) and (LErr <> ERROR_ALREADY_EXISTS) then
  begin
    // We are the first instance. Keep the mutex alive for the process
    // lifetime — Windows releases it automatically on exit.
    Result := True;
    Exit;
  end;

  // Another instance is already running. Close our handle (it isn't ours)
  // and broadcast the show-message to all top-level windows. The running
  // bridge picks it up on its message-only window.
  if _Mutex <> 0 then
  begin
    CloseHandle(_Mutex);
    _Mutex := 0;
  end;
  PostMessage(HWND_BROADCAST, WM_PMShowMessage, 0, 0);
  Result := False;
end;

end.