ProcessHelper: Fix setChildProcessModifier()

The original change that introduced the usage of
QProcess::UnixProcessFlag introduced also a regression.
Since ProcessHelper::setLowPriority() and setUnixTerminalDisabled()
are part of public API, we can't use m_lowPriority and
m_unixTerminalDisabled inside c'tor for early return before setting the
setChildProcessModifier(), because these values may be changed later,
before starting the process.

Amends e8ca1877de

Fixes: QTCREATORBUG-29457
Change-Id: Ief360c123f6284c7df105ef530c4602555b5b069
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/utils/processutils.cpp

@@ -105,19 +105,35 @@ BOOL CALLBACK sendInterruptMessageToAllWindowsOfProcess_enumWnd(HWND hwnd, LPARA
 
 ProcessHelper::ProcessHelper(QObject *parent)
     : QProcess(parent), m_processStartHandler(this)
+{}
+
+void ProcessHelper::setLowPriority()
+{
+    m_lowPriority = true;
+    enableChildProcessModifier();
+}
+
+void ProcessHelper::setUnixTerminalDisabled()
 {
 #if defined(Q_OS_UNIX)
-    bool needSetsid = m_unixTerminalDisabled;
-#  if QT_VERSION >= QT_VERSION_CHECK(6, 7, 0)
-    if (needSetsid) {
+#  if QT_VERSION < QT_VERSION_CHECK(6, 7, 0)
+    m_unixTerminalDisabled = true;
+    enableChildProcessModifier();
+#  else
     setUnixProcessParameters(QProcess::UnixProcessFlag::CreateNewSession);
-        needSetsid = false;
-    }
 #  endif
+#endif
+}
 
-    if (!m_lowPriority && !needSetsid)
-        return;
-    setChildProcessModifier([=, this] {
+void ProcessHelper::setUseCtrlCStub(bool enabled)
+{
+    m_useCtrlCStub = enabled;
+}
+
+void ProcessHelper::enableChildProcessModifier()
+{
+#if defined(Q_OS_UNIX)
+    setChildProcessModifier([this] {
         // nice value range is -20 to +19 where -20 is highest, 0 default and +19 is lowest
         if (m_lowPriority) {
             errno = 0;
@@ -125,18 +141,15 @@ ProcessHelper::ProcessHelper(QObject *parent)
                 perror("Failed to set nice value");
         }
 
+#  if QT_VERSION < QT_VERSION_CHECK(6, 7, 0)
         // Disable terminal by becoming a session leader.
-        if (needSetsid)
+        if (m_unixTerminalDisabled)
             setsid();
+#  endif
     });
 #endif
 }
 
-void ProcessHelper::setUseCtrlCStub(bool enabled)
-{
-    m_useCtrlCStub = enabled;
-}
-
 void ProcessHelper::terminateProcess()
 {
 #ifdef Q_OS_WIN

src/libs/utils/processutils.h

@@ -40,8 +40,8 @@ public:
 
     using QProcess::setErrorString;
 
-    void setLowPriority() { m_lowPriority = true; }
-    void setUnixTerminalDisabled() { m_unixTerminalDisabled = true; }
+    void setLowPriority();
+    void setUnixTerminalDisabled();
     void setUseCtrlCStub(bool enabled); // release only
 
     static void terminateProcess(QProcess *process);
@@ -49,6 +49,7 @@ public:
     static void interruptPid(qint64 pid);
 
 private:
+    void enableChildProcessModifier();
     void terminateProcess();
 
     bool m_lowPriority = false;