[Pyobjc-dev] Bug fix in exception bridging code

[Bill B. <bb...@co...>]

The whole exercise with figuring out how to use gdb w/my app (that is 
composed of multiple frameworks that are dynaloaded) was to track down 
a bug in bridge.  Specifically, the bridge was crashing if I tried to 
invoke a selector on an Python subclass of an ObjC object where neither 
the ObjC or Python subclass implemented the method (I'm in the middle 
of porting code from ObjC to Python [and not just as a mental exercise] 
and haven't finished implementing all methods) -- the classic forward:: 
/ does not recognize selector situation.

According to the documentation, the PyErr_Fetch() function can return 
NULL for the exc_value and exc_traceback arguments.  This appears to be 
exactly what is happening -- see the gdb session below -- in that 
exc_traceback is NULL.   Calling Py_DECREF() with a NULL pointer causes 
a bus error / bad access error.

Patched.  Fixed.  Committed.  Ronald:  Is it correct? :-)

Program received signal EXC_BAD_ACCESS, Could not access memory.
0x004168c8 in ObjCErr_ToObjC () at Modules/objc/objc_util.m:222
222             Py_DECREF(exc_traceback);
(gdb) l
217                     userInfo:userInfo];
218
219             Py_DECREF(repr);
220             Py_DECREF(exc_type);
221             Py_DECREF(exc_value);
222             Py_DECREF(exc_traceback);
223
224             [val raise];
225     }
226
Current language:  auto; currently objective-c
(gdb) bt
#0  0x004168c8 in ObjCErr_ToObjC () at Modules/objc/objc_util.m:222
#1  0x0041caf0 in object_method_methodSignatureForSelector 
(self=0x1ee6d50, selector=0x907e2470, aSelector=0xdd32bc) at 
Modules/objc/class-builder.m:869
#2  0x90935664 in -[NSObject(NSForwardInvocation) forward::] ()
#3  0x907ac130 in _objc_msgForward ()
#4  0x00dcf8d4 in -[ViewTableDataSource issueUrlButtonAction:] 
(self=0x6c8b80, _cmd=0xdd2bf8, sender=0xe60b20) at 
Intent-Application/ViewTableDataSource.m:468
#5  0x9426fe2c in -[NSApplication sendAction:to:from:] ()
#6  0x942fbff0 in -[NSControl sendAction:to:] ()
#7  0x942b3a38 in -[NSCell _sendActionFrom:] ()
#8  0x942b3fc4 in -[NSCell trackMouse:inRect:ofView:untilMouseUp:] ()
#9  0x9429ef78 in -[NSButtonCell 
trackMouse:inRect:ofView:untilMouseUp:] ()
#10 0x944763b0 in -[NSTableView mouseDown:] ()
#11 0x9450dfd4 in -[NSWindow sendEvent:] ()
#12 0x9426e328 in -[NSApplication sendEvent:] ()
#13 0x9426a524 in -[NSApplication run] ()
#14 0x94272598 in NSApplicationMain ()
#15 0x00c23f40 in objc_NSApplicationMain (self=0x229d2c0, 
args=0x907c85c8, kwds=0x0) at Modules/Cocoa/_AppKit.m:80
#16 0x00045930 in PyObject_Call ()
#17 0x0005df64 in PyEval_GetFuncDesc ()
#18 0x0005b1f8 in PyEval_EvalCode ()
#19 0x0005c634 in PyEval_EvalCodeEx ()
#20 0x00058a80 in PyEval_EvalCode ()
#21 0x00027e90 in PyRun_FileExFlags ()
#22 0x00026ef4 in PyRun_SimpleFileExFlags ()
#23 0x000069f0 in Py_Main ()
#24 0x00002970 in start ()
#25 0x000027f0 in start ()
(gdb) p exc_traceback
$1 = (PyObject *) 0x0
(gdb)
(gdb) p exc_value
$2 = (PyObject *) 0x1ec6fe0