#2147 Could not connect to existing gnuplot_qt. Starting a new one. on Cygwin

[Tatsuro MATSUOKA]

This is related to
https://sourceforge.net/p/gnuplot/mailman/message/36603526/

On Cygwin, plot does not start unless I attach the patch below to qt5 source.

--- a/qtbase-opensource-src-5.9.4/src/network/socket/qlocalsocket_unix.cpp
+++ b/qtbase-opensource-src-5.9.4/src/network/socket/qlocalsocket_unix.cpp
@@ -239,7 +239,7 @@ void QLocalSocket::connectToServer(OpenM
    }

    // create the socket
-    if (-1 == (d->connectingSocket = qt_safe_socket(PF_UNIX, SOCK_STREAM, 0, O_NONBLOCK))) {
+    if (-1 == (d->connectingSocket = qt_safe_socket(PF_UNIX, SOCK_STREAM, 0))) {
        d->errorOccurred(UnsupportedSocketOperationError,
                        QLatin1String("QLocalSocket::connectToServer"));
        return;

gnuplot> pl x
Could not connect to existing gnuplot_qt. Starting a new one.

Warning: slow font initializationgnuplot>

[Tatsuro MATSUOKA]

I forgot to mention the details.
gnuplot 5.3 (commit [74cf67])
Qt version on Cygwin64 (5.9.4)

[Ethan Merritt]

That commit identifier is the current tip. Did you mean to point to an earlier change?

Question: You show a warning message "slot font initialization". Does that warning come from the program with your Qt patch or without your Qt patch?

[Tatsuro MATSUOKA]

That commit identifier is the current tip. Did you mean to point to an earlier change?
First pacth for Cygwin appeared at just 5.0 released.
http://gnuplot.10905.n7.nabble.com/Gnuplot-version-5-0-is-now-available-td19185.html#a19202

After that I privately got a patch infomation on 2018/3/12, Mon 17:17

> > You should simply avoid using O_NONBLOCK. Please, try the following patch:
>
> >
> > --- a/qtbase-everywhere-src-5.10.1/src/network/socket/qlocalsocket_unix.cpp
> > +++ b/qtbase-everywhere-src-5.10.1/src/network/socket/qlocalsocket_unix.cpp
> > @@ -239,7 +239,7 @@ void QLocalSocket::connectToServer(OpenM
> >      }
> >
> >      // create the socket
> > -    if (-1 == (d->connectingSocket = qt_safe_socket(PF_UNIX, SOCK_STREAM, 0,
> > O_NONBLOCK))) {
> > +    if (-1 == (d->connectingSocket = qt_safe_socket(PF_UNIX, SOCK_STREAM,
> > 0))) {
> >          d->errorOccurred(UnsupportedSocketOperationError,
> >                        
> > QLatin1String("QLocalSocket::connectToServer"));
> >          return;
> >
> > --
> > Enrico

At least between 5.0 and 5.2 situation changed in both gnuplot and qt on Cygwin.

[Tatsuro MATSUOKA]

"slot font initialization" appears for with or without patch for qt.

[Ethan Merritt]

Then I do not understand.
What is the bug? What is different in gnuplot after you apply that patch to Qt?

[Tatsuro MATSUOKA]

Sorry for confusing description.

Before patch
gnuplot> pl x
Could not connect to existing gnuplot_qt. Starting a new one.

Warning: slow font initializationgnuplot>
(plot never appears)

After patch
gnuplot> pl x

Warning: slow font initializationgnuplot>
(plot appears)

Before patch gnuplot_qt is executed but gnuplot fails to connect to it.

[Ethan Merritt]

Does the patch below help any?

diff --git a/src/qtterminal/qt_term.cpp b/src/qtterminal/qt_term.cpp
index d9f675e..38177a9 100644
--- a/src/qtterminal/qt_term.cpp
+++ b/src/qtterminal/qt_term.cpp
@@ -307,7 +307,7 @@ void qt_connectToServer(const QString& server, bool retry = true)
        QDateTime timeout = QDateTime::currentDateTime().addMSecs(30000);
        do
        {
-         qt->socket.connectToServer(server);
+         qt->socket.connectToServer(server, QIODevice::ReadWrite);
                if (!qt->socket.waitForConnected(-1)) {
                        // qDebug() << qt->socket.errorString();
                        GP_SLEEP(0.2);  // yield CPU for 0.2 seconds

[Tatsuro MATSUOKA]

Unfortunately, the result is the same as before. (gnuplot cannot connect with gnuplot_qt)

gnuplot> pl x
Could not connect to existing gnuplot_qt. Starting a new one.

[Ethan Merritt]

My summary from searching through the history shown in the links above:

• gnuplot calls QLocalSocket::connectToServer("name")
• Qt tries to open a named pipe for the socket using O_NONBLOCK
• This fails because Cygwin has problems with non-blocking named pipes. Back in 2010 Enrico Forestieri posted some simple examples of this (standalone tests, no involvement of gnuplot code) but this apparently did not lead to a fix.
• the Qt patch at the head of this bug report would make gnuplot work, but needs to be wrapped in a conditional test for CYGWIN. I can understand why upstream Qt would not want to deal with this.

Conclusion: We probably can't fix it in the gnuplot code. There is a trivial work-around that requires modifying the Qt source code, but this would only make sense for the purpose of making a Qt package specifically for Cygwin.

[Tatsuro MATSUOKA]

Thank for your summary.
I understand this is issue of Cygwin. (Not Qt for Cygwin but issue of Cygwin itself).
I report your summary to Cygwin ML.

[Tatsuro MATSUOKA]

Please tell me the code receive part of
qt->socket.connectToServer(server);
in gnuplot_qt code.

[Ethan Merritt]

I may not understand the question.

If the program works correctly, the connectToServer operation creates a pipe from which the program can read. On your Cygwin system the connectToServer operation apparently never succeeds so there is no socket to read from.

When the program functions correctly, the routine that reads from the pipe is qt_waitforinput() at qt_term.cpp:989

[Tatsuro MATSUOKA]

OK. I have mislead. That is the pipe connection. Cygwin emilates the Unixy pipe using Windows nemaed pipe. In the emulateing process, Cygwin seems fails O_NONBLOCK connection.

[Ethan Merritt]

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