Do not overwrite detailed error message if process fails to launch

On Unix we get a detailed error message when a process fails to start,
but later on we overwrite it with a generic "Process fails to start".
Fix this by keeping the original error message (if one is available).

This fixes a regression introduced in commit 5147f73ac3.

Task-number: QTBUG-49286
Change-Id: Idd0f0fed9773d39f2947fc3e532b51e670952caf
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qprocess.cpp

@@ -1179,14 +1179,15 @@ bool QProcessPrivate::_q_startupNotification()
 
     if (startupSocketNotifier)
         startupSocketNotifier->setEnabled(false);
-    if (processStarted()) {
+    QString errorMessage;
+    if (processStarted(&errorMessage)) {
         q->setProcessState(QProcess::Running);
         emit q->started(QProcess::QPrivateSignal());
         return true;
     }
 
     q->setProcessState(QProcess::NotRunning);
-    setErrorAndEmit(QProcess::FailedToStart);
+    setErrorAndEmit(QProcess::FailedToStart, errorMessage);
 #ifdef Q_OS_UNIX
     // make sure the process manager removes this entry
     waitForDeadChild();

src/corelib/io/qprocess_p.h

@@ -352,7 +352,7 @@ public:
 #elif defined(QPROCESS_USE_SPAWN)
     pid_t spawnChild(pid_t *ppid, const char *workingDirectory, char **argv, char **envp);
 #endif
-    bool processStarted();
+    bool processStarted(QString *errorMessage = Q_NULLPTR);
     void terminateProcess();
     void killProcess();
     void findExitCode();

src/corelib/io/qprocess_unix.cpp

@@ -714,7 +714,7 @@ report_errno:
 }
 #endif
 
-bool QProcessPrivate::processStarted()
+bool QProcessPrivate::processStarted(QString *errorMessage)
 {
     ushort buf[errorBufferMax];
     int i = qt_safe_read(childStartedPipe[0], &buf, sizeof buf);
@@ -731,8 +731,8 @@ bool QProcessPrivate::processStarted()
 #endif
 
     // did we read an error message?
-    if (i > 0)
-        q_func()->setErrorString(QString((const QChar *)buf, i / sizeof(QChar)));
+    if ((i > 0) && errorMessage)
+        *errorMessage = QString((const QChar *)buf, i / sizeof(QChar));
 
     return i <= 0;
 }

src/corelib/io/qprocess_win.cpp

@@ -543,7 +543,7 @@ void QProcessPrivate::startProcess()
     _q_startupNotification();
 }
 
-bool QProcessPrivate::processStarted()
+bool QProcessPrivate::processStarted(QString * /*errorMessage*/)
 {
     return processState == QProcess::Running;
 }

src/corelib/io/qprocess_wince.cpp

@@ -160,7 +160,7 @@ void QProcessPrivate::startProcess()
     _q_startupNotification();
 }
 
-bool QProcessPrivate::processStarted()
+bool QProcessPrivate::processStarted(QString * /*errorMessage*/)
 {
     return processState == QProcess::Running;
 }