[Development] New (optional?) dependency: xcb-util-errors

[Lind Jorgen]

Its a small library, so I think it is sensible to compile it in the plugin (even with the error data). I don't think it makes much sense at current time to check if there is a system version available and then use that. However, what is bothering me is that this is code that we don't want to be relevant for 99.99% of people. We already use plugins in plugins in the xcb backend. We could lazily load this on the first error?

Br,
Jørgen

________________________________________
From: development-bounces+jorgen.lind=theqtcompany.com at qt-project.org <development-bounces+jorgen.lind=theqtcompany.com at qt-project.org> on behalf of Uli Schlachter <psychon at znc.in>
Sent: 26 April 2015 21:39
To: development at qt-project.org
Subject: [Development] New (optional?) dependency: xcb-util-errors

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Hi everyone,

here are some 'recent' examples of error messages printed for X11 protocol errors:


  QXcbConnection::handleXcbError - QXcbConnection: XCB error: 170 (Unknown),
sequence: 162, resource id: 90, major code: 146 (Unknown), minor code: 20

https://codereview.qt-project.org/#/c/108209/
(See my comment from Mar 14 8:52 AM)


  ~ > qtcreator -noload Welcome
  QXcbConnection: Failed to get the primary output of the screen
  Starting process:  "/usr/bin/cmake" "--help"
  QXcbConnection: XCB error: 172 (Unknown), sequence: 158, resource id: 150,
major code: 149 (Unknown), minor code: 20

https://bugreports.qt.io/browse/QTBUG-31389
(And lots of other comments in there)


  QXcbConnection: XCB error: 159 (Unknown), sequence: 156, resource id: 0,
major code: 149 (Unknown), minor code: 20

https://codereview.qt-project.org/74133


These error messages are not really helpful. Here is a (bad[0]) example for a
better error message:

  QXcbConnection: XCB error: 3 (Window), sequence: 7, resource id: 42, major
code: 4 (DestroyWindow), minor code: 0 (Non-extension request / Unknown)


The first of my three examples is what made me work on a library for figuring
out more useful names than "Unknown" for these error messages. The result is
xcb-util-errors:

http://cgit.freedesktop.org/xcb/util-errors/
http://lists.freedesktop.org/archives/xcb/2015-April/010264.html

Now I want to make Qt use this library for better error messages:

https://codereview.qt-project.org/109873

The actual question that I want to ask with this mail: This is a new external
dependency. Is it ok to add this dependency to Qt?

I have some options to offer for this which only differ in their behavior when
./configure is called with -system-xcb (which is the default):

(A) Handle this library like other xcb libraries: Require an installed copy
with -system-xcb (the default) and use a bundled version with -qt-xcb.
(B) Use an installed copy with -system-xcb if available, else use the bundled
version. Always use the bundled version with -qt-xcb.
(C) Use an installed copy with -system-xcb if available, else just print the
numeric values without a human readable description. Use the bundled version
with -qt-xcb.

With (A) we have a new hard dependency which might be hard to satisfy for some
people.

For (B) I'm not sure about the source code structure that should be used.
AFAIK the code in src/3rdparty/xcb is only used with -qt-xcb and ignored
otherwise. Thus it would be inconsistent to put xcb-util-errors there.

Note that (C) is worse than the current state where Qt at least contains some
lookup tables for core (=not coming from some extension) X11 errors and events.

What do you think and prefer? Does adding this dependency make sense at all?

Cheers,
Uli

P.S.: Thanks to Shawn Rutledge und Alejandro Exojo for talking to me about
this in #qt-labs. They came up with options (B) and (C).

[0]: This is a bad example, because the current code would also generate such
an error message for *this* example, since no X11 extensions are involved.
However, this library works for all X11 protocol error and response numbers
and also for all major and minor numbers (at least those for which a XCB XML
protocol description is available, which only excludes some deprecated things).
- --
- - Captain, I think I should tell you I've never
  actually landed a starship before.
- - That's all right, Lieutenant, neither have I.
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