Обсуждение: Issues with C++ exception handling in an FDW

Hello,

I've run into a very odd issue calling C++ code that uses exceptions from within our PostgreSQL FDW.  Specifically, we
havebroken our FDW into two components, a C layer that looks quite similar to the FDW for text files and a C++ layer
thatis called into by the C layer to interface with our storage file format. 

We compile these two components into separate shared libraries, thus we have:

c-fdw.so
c++-fdw.so

and the c-fdw.so is compiled using -Wl,-rpath to allow it to find the c++-fdw.so at load time.

When there are no errors everything works flawlessly, however, we noticed that even throwing an exception in the C++
layerwas causing an immediate segmentation fault.  Even when encapsulated in a try { } catch(...) { } block. 

If anyone has seen anything like this, any pointers or suggestions would be much appreciated.  I have followed all of
therecommendations in the PostgreSQL documentation, with no luck.  I am not overloading the _init() functions in either
sharedlibrary (another potential source of errors I have read about). 

Thanks!
Craig Soules

On Mon, Jan 30, 2012 at 6:04 PM, Soules, Craig <craig.soules@hp.com> wrote:
> When there are no errors everything works flawlessly, however, we noticed that even throwing an exception in the C++
layerwas causing an immediate segmentation fault.  Even when encapsulated in a try { } catch(...) { } block. 

Stack backtrace?

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On 30 January 2012 23:04, Soules, Craig <craig.soules@hp.com> wrote:
> When there are no errors everything works flawlessly, however, we noticed that even throwing an exception in the C++
layerwas causing an immediate segmentation fault.  Even when encapsulated in a try { } catch(...) { } block. 
>
> If anyone has seen anything like this, any pointers or suggestions would be much appreciated.  I have followed all of
therecommendations in the PostgreSQL documentation, with no luck.  I am not overloading the _init() functions in either
sharedlibrary (another potential source of errors I have read about). 

I suggest that you generalise from the example of PLV8. The basic
problem is that the effect of longjmp()ing over an area of the stack
with a C++ non-POD type is undefined. I don't think you can even use
structs, as they have implicit destructors in C++.

--
Peter Geoghegan       http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services

On Tuesday, January 31, 2012 07:52:52 PM Peter Geoghegan wrote:
> On 30 January 2012 23:04, Soules, Craig <craig.soules@hp.com> wrote:
> > When there are no errors everything works flawlessly, however, we noticed
> > that even throwing an exception in the C++ layer was causing an
> > immediate segmentation fault.  Even when encapsulated in a try { }
> > catch(...) { } block.
> > 
> > If anyone has seen anything like this, any pointers or suggestions would
> > be much appreciated.  I have followed all of the recommendations in the
> > PostgreSQL documentation, with no luck.  I am not overloading the
> > _init() functions in either shared library (another potential source of
> > errors I have read about).
> 
> I suggest that you generalise from the example of PLV8. The basic
> problem is that the effect of longjmp()ing over an area of the stack
> with a C++ non-POD type is undefined. I don't think you can even use
> structs, as they have implicit destructors in C++.
The PODness of a struct depends on its contents.

Andres

On 31 January 2012 19:01, Andres Freund <andres@anarazel.de> wrote:
>> I suggest that you generalise from the example of PLV8. The basic
>> problem is that the effect of longjmp()ing over an area of the stack
>> with a C++ non-POD type is undefined. I don't think you can even use
>> structs, as they have implicit destructors in C++.
> The PODness of a struct depends on its contents.

Right. If I was going to invest much effort in this sort of thing, I
might even write a static assertion that verified a given type's
POD-ness over time, by declaring it within a union...which would
work....unless you were using C++11. Or, just use std::is_pod to build
a static assertion.

--
Peter Geoghegan       http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services

> I suggest that you generalise from the example of PLV8. The basic
> problem is that the effect of longjmp()ing over an area of the stack
> with a C++ non-POD type is undefined. I don't think you can even use
> structs, as they have implicit destructors in C++.

I had thought that this was only an issue if you tried to longjmp() over a section of C++ code starting from a postgres
backendC function?  From the PostgreSQL documentation: 

" If calling backend functions from C++ code, be sure that the C++ call stack contains only plain old data structures
(POD).This is necessary because backend errors generate a distant longjmp() that does not properly unroll a C++ call
stackwith non-POD objects." 

But this is not what our code is doing.  Our code is a C++ function that only does the following:

try {   throw 1;
} catch (int e) {
} catch (...) {
}

which causes an immediate segmentation fault.  To answer another responders question, the stack trace looks as follows:

#0  0x00002b3ce8f40fa5 in __cxa_allocate_exception ()  from /usr/lib64/libstdc++.so.6
#1  0x00002b3ce77b6256 in initMBSource (state=0x1ab87a80)   at
/data/soules/metaboxA-bugfix/Metabox/debug_build/src/lib/query/dsFdwShim.cpp:16791
#2  0x00002b3ce6c0b0aa in dsBeginForeignScan (node=0x1ab872d0,    eflags=<value optimized out>) at dataseries_fdw.c:819
#3  0x000000000057606c in ExecInitForeignScan ()
#4  0x000000000055c715 in ExecInitNode ()
#5  0x000000000056874c in ExecInitAgg ()
#6  0x000000000055c6a5 in ExecInitNode ()
#7  0x000000000055b944 in standard_ExecutorStart ()
#8  0x0000000000621b96 in PortalStart ()
#9  0x000000000061edad in exec_simple_query ()
#10 0x000000000061f624 in PostgresMain ()
#11 0x00000000005e4c5c in ServerLoop ()
#12 0x00000000005e595c in PostmasterMain ()
#13 0x000000000058a77e in main ()

Note: #2 is the entry into our C library and #1 is the entry into our C++ library

This appears to be some kind of allocation error, but the machine on which I'm running has plenty of free ram:

[~]$ free            total       used       free     shared    buffers     cached
Mem:      24682888   10505920   14176968          0    1220496    7412352
-/+ buffers/cache:    1873072   22809816
Swap:      2096472          0    2096472

I also don't understand how it could truly be an allocation issue since we new/delete plenty of memory during a
successfulrun (as well as using plenty of C++ containers which do internal allocation). 

Hopefully this helps jog thoughts on my issue!

Thanks again!
Craig