QtCreator 2.8.1 Not Debugging Locals in Ubuntu 13.10

Date: 20-Oct-2013/16:59

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UPDATE 12-Dec-2013 The problem described in this post is now reportedly fixed in Qt 3.0. So the easier thing than this procedure is to upgrade to that, if you can!
Every now and again I build a new Qt development virtual machine, to try the "bleeding edge". This weekend I tried moving some Qt 4.8 projects to Qt 5.1. And I decided to do this under a new Kubuntu 13.10 installation, with QtCreator 2.8.1.
It got started pretty painlessly. I bumped around and visited several blogs where people had gone through similar steps. There were some hassles with updating the CMake files (which I consider on par with voodoo), and a few reasonably well-documented changes that needed to be made the source code.
But the problem that took down pretty much a whole day of investigation turned out to be an interaction between the gdb that ships with Ubuntu 13.10 not playing well with Qt Creator. The result was that no variables could be inspected or viewed in the debugger (a clearly untenable situation). The appearance was similar to this StackOverflow question:
But that person's problem was running an outdated version of GDB, while mine was the latest. It turns out the problem can also occur if your GDB is too new... for instance, compiled against Python3 instead of Python2. :-/
Assuming your GDB is built with any Python abilities at all, you can check the version by running gdb and then at the prompt:
(gdb) python print sys.version
If that gives you an "invalid syntax" error, it's because Python3 requires parentheses around the print. So try doing:
(gdb) python print(sys.version)
If you get a 3.x.x back from that, then you're living proof that what the Qt docs say about supported gdb versions is no longer true:
The minimal supported version in this case is FSF GDB 7.4.1, using Python version 2.6 or 2.7. Note that Python 3.x is not supported by GDB. (wrong!)
What is true is that at this time of writing, the QtCreator dumper scripts won't run under Python3. Someone recently reported this to Ubuntu, and submitted a bug report to Qt along with some preliminary steps to adapt the dumpers.
At this time it looks like getting a version of GDB built against Python2 and using that instead is the easiest thing to do. Here's how... (and thanks to all the comments I've received, although integrating and testing them has made this into a bit more time than I wanted to spend! But glad people are finding it useful...)

Building GDB with embedded Python2

Note I did this on Kubuntu 13.10
Note If you happen to be building for ARM, commentator Juha points out that you can add –target=arm-fsl-linux-gnueabi to the configure command line and get an ARM version of GDB.
Once you've got the build, do a sanity check on the version by running /usr/local/gdb-python2/bin/gdb and testing with python print sys.version. (If you try to run gdb from the directory, remember to invoke it with ./gdb or else you'll get the default installation) If you didn't get an embedded Python 2.x, don't bother continuing until you figure out why not.
The fact that I got Python2 by default raises the question of what flags or switches the Ubuntu people used to get Python3. Knowing the answer to that is beyond the scope of this post—and involves dealing more with autotools (another voodoo-oriented build environment). But if someone wants to leave a comment explaining that, I guess I'll add it. :-)

Get QtCreator to use the new GDB

Though it would seem that there are settings dialogs for making QtCreator use a different GDB than the one in /usr/bin/gdb (in Tools->Options->Build & Run->Kits), that wouldn't work for me.
UPDATE Commentator Simon reports that the feature of setting the debugger in Qt Creator worked in 2.8.1, if he ran aptitude purge gdb. Commentator Matthias said it just worked for him. Commentator Vlad reports that there is a different setting in the beta of Qt Creator 3.0, under (Tools > Options > Build & Run > Debuggers, for this path which overrides it correctly. He says it works. So if you're even more bleeding edge than I am, you might try that.)
So YMMV. But I couldn't override the GDB setting in the automatic "Kit". If I added a new kit and poked a debugger path into the slot where it seems you can set it... it seems to be ignored and just runs the old one anyway. It didn't matter what kit I would "Make Default".
To prove that it wasn't paying attention to my settings, I did sudo mv /usr/bin/gdb /usr/bin/gdb-python3 so it couldn't be found. The debugger declared "Could Not Run: No debugger set up.", even though I had specified /usr/local/gdb-python2/bin/gdb in another configuration I'd chosen to be the default. The simple solution I used was sudo ln -s /usr/local/gdb-python2/bin/gdb /usr/bin/gdb.

Additional notes

If you're having problems that this doesn't solve, then Window->Views->Debugger Log gives a lot of debug spew where you might find a smoking gun. It guided me toward knowing at least what to look at; I started snooping around Python linkage due to seeing this:
&"Traceback (most recent call last):\n"
&"  File \"<string>\", line 1, in <module>\n"
&"NameError: name 'execfile' is not defined\n"
&"Error while executing Python code.\n"
151^error,msg="Error while executing Python code."
This caused me to go read up a bit on how the Qt Creator debugger uses Python. It suggested that the so-called "Python Helpers" don't work in newer versions of Linux (my case was running Kubuntu 13.10):
"There are two approaches to displaying complex data types. The first and original one is to use debugging helpers based on C++. While it has been superseded on most platforms by the more robust and more flexible second approch that uses Python scripting, it is the only feasible one on Windows/MSVC, Mac OS, and old Linux distributions. Moreover, this approach is automatically chosen as fallback if the Python based approach fails."
But even when I disabled the Python Helpers, I still had problems. So it turns out that Python is used for other purposes in the debugger, even when C++ helpers are in effect.
And the moral of the story is: You can't trust the system. :-)

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