FTP: Workaround for servers without HELP command support

Ignores errors produced by the HELP command. It will continue
executing the commands sent to QFtp when a server does not have a
HELP command implemented. Commands SIZE, MDTM, and PWD are not going
to be used if the HELP command failed.

[ChangeLog][QtNetwork][QNetworkAccessManager] Don't fail when FTP does
not implement the HELP command.

Task-number: QTBUG-69477
Change-Id: I0ebd51b134535730c6bef83de1abf1a427b8d2ce
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/network/access/qftp.cpp

@@ -2122,6 +2122,17 @@ void QFtp::abort()
     d_func()->pi.abort();
 }
 
+/*!
+    \internal
+    Clears the last error.
+
+    \sa currentCommand()
+*/
+void QFtp::clearError()
+{
+    d_func()->error = NoError;
+}
+
 /*!
     \internal
     Returns the identifier of the FTP command that is being executed

src/network/access/qftp_p.h

@@ -157,6 +157,9 @@ Q_SIGNALS:
     void commandFinished(int, bool);
     void done(bool);
 
+protected:
+    void clearError();
+
 private:
     Q_DISABLE_COPY_MOVE(QFtp)
     Q_DECLARE_PRIVATE(QFtp)

src/network/access/qnetworkaccessftpbackend.cpp

@@ -99,6 +99,8 @@ public:
         connect(this, SIGNAL(done(bool)), this, SLOT(deleteLater()));
         close();
     }
+
+    using QFtp::clearError;
 };
 
 QNetworkAccessFtpBackend::QNetworkAccessFtpBackend()
@@ -282,7 +284,10 @@ void QNetworkAccessFtpBackend::ftpDone()
     }
 
     // check for errors:
-    if (ftp->error() != QFtp::NoError) {
+    if (state == CheckingFeatures && ftp->error() == QFtp::UnknownError) {
+        qWarning("QNetworkAccessFtpBackend: HELP command failed, ignoring it");
+        ftp->clearError();
+    } else if (ftp->error() != QFtp::NoError) {
         QString msg;
         if (operation() == QNetworkAccessManager::GetOperation)
             msg = tr("Error while downloading %1: %2");