Re: Patch for pjsua2

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Hello Ming and all,
I now have an idea how we can keep the exception specifications working and avoid API incompatibility for the SWIG Java bindings. The idea is to still get rid of the exception specifications (throws ...) in the PJSUA2 header files because "throws" is now deprecated syntax. As a consequence, we would have to add information to the SWIG input files which functions can throw an exception ("pj::error") and which not. As a default, I think we should assume that a function throws "pj::error" and we should tell SWIG about all functions that cannot possibly throw "pj::error". So a sort of "whitelist" approach. Here is an example how this could look like in SWIG syntax:
%handlePjError; // this installs a default exception handler that handles "pj::Error" 

// list of methods that cannot throw "pj::Error"
%noexception pj::Call::Call;
%noexception pj::Call::xyz;
%noexception pj::Call::~Call;
// ... and more methods to follow
The reason for the SWIG default behaviour (to assume that a function will throw "pj::Error) is that if someone adds a new C++ method to PJSUA2 and forgets to adopt the exception specification, marking the method as "noexception" afterwards is easy and does not break the generated Java API. Assuming that the default was to assume that the function does not throw "pj::Error", we would have to break the compatibility if we forget to add the SWIG exception specification for a new method and need to add it later.
A disadvantage of this approach is that the "%noexception" list is going to be quite long with >200 entries for sure.
Of course we rely on a correct "%noexception" list to maintain API compatibility on the Java side. I already have an idea in mind how we could automatically generate this list based on the existing "throws" specification.
What do you think about this idea? I have prototyped it already and it seems to work. If you are not satisfied with this solution, I could investigate the "PJ_THROWS" again. 


Best regards,

   Christian


On 17/10/18 09:30, Christian Hoff wrote:

Hello Ming,
thanks for your response. You made me understand the problems with my patch and that my patch breaks the Java language binding.
I will do some brainstorming and see if a good solution idea comes to my mind. Then I will align the idea with you before I start with the implementation. If anyone on this mailing list has a good idea how to solve this issue, he or she can also let me know. 

Thanks for your support on this!


Best regards,

   Christian


On 17/10/18 08:42, Ming wrote:

Hi Christian,

There's quite a number of them actually. Counting from the Call class
only in Call.hpp, I find 12 methods that do not throw(Error) while 21
do, but the biggest difference is of course the constructor and
destructor. It will surely break a lot of applications and cause them
to fail to build. I tried it here and even our sample Java app
couldn't compile if all functions throw exceptions.

We appreciate your hard work, unfortunately Java and Android is a
major platform for PJSIP, and the exception specification is important
for this language/platform. In fact, due to the popularity of Android
alone, I will not be surprised if there are more users using PJSUA2
Java binding as compared to the native C++.

The PJ_THROW() is just an idea, it may not even have to be that way.
The necessary requirement is to make exception specifications and
runtime exceptions still work for languages and platforms that do
support them.

Best regards,
Ming
On Wed, Oct 17, 2018 at 2:15 PM, Christian Hoff <christian_hoff@xxxxxxx> wrote: 
Hello Ming,
I see. Well, of course I can rework the patch to use this "PJ_THROW(..)" as you suggest - even though this would be quite frustrating because I spent so much time on eliminating the "throw" specification altogether, which is the much more complex option compared to a PJ_THROW() macro. So I would have to throw quite some code away. But I could still do it if you are convinced 
that this is the right way to go. Personally I am not really convinced
because using "throw(..)" is deprecated in C++, so I think it might be
better to get rid of this deprecated syntax altogether.
But what I am wondering about is this: are there actually any C++ methods in 
PJSIP that currently do not have a "throw (Error)" in the throws
specification? And how many are they? Is this actually a frequent case? From 
my experience it seems that almost all methods are declared as "throw
(Error)". So the incompatibility might be minimal, but I am not sure.

Best regards,

    Christian



On 17/10/18 04:03, Ming wrote:

Hi Christian,

Unfortunately both options are still not ideal :)

To make all generated Java methods to throw exceptions may require a
lot of changes in existing PJSIP Java apps, which is of course
undesirable.

We still prefer the idea of replacing throw() with PJ_THROW(), which
will translate to no-op for C/C++ and throw() for SWIG. This way,
backward incompatibility is minimized while full feature is
maintained.

Best regards,
Ming
On Wed, Oct 17, 2018 at 4:46 AM, Christian Hoff <christian_hoff@xxxxxxx> 
wrote:

I am fordwarding this mail to the mailing list as well.


On 16/10/18 22:42, Christian Hoff wrote:

Hello Ming,
you are absolutely right and I am sorry for the confusion. Of course the 
SWIG Java bindings throw "java.lang.Exception" and not the
"RuntimeException" as I assumed.
You actually discovered a bug in my patch. The problem with my patch is 
that
I am throwing a java.lang.Exception from the Java JNI code, but this
Exception is now no longer mentioned in the "throws" specification of the Java methods generated by SWIG - even though it is a checked exception. 
In
fact my change (unintentionally) removed the "throw" specification also 
on
the Java side! For this reason you no longer get compile errors when
omitting the "catch" blocks in your sample code. This is a bug. Because 
the
Exception is thrown from JNI C++ code, Java cannot find out that the
generated C++ JNI code is actually violating the Java exception
specification of the generated method (which declares that no checked
exceptions should be thrown).
After some reading of the SWIG documentation, I found out that there is a 
relatively simple way to fix this issue and to restore the "throw"
specification on the Java side. To do this, please replace the line no. 
50
in file "pjsip-apps/src/swig/pj_error.i" by

    %javaexception("java.lang.Exception") {
This should restore the "throw" specification for all PJSIP Java methods. 
I
tested this briefly and it works. One possible downside of this change is however that this change will add a "throw Exception" specification to 
all
generated Java methods inside PJSIP, so every PJSIP Java generated method 
now explicitly throws java.lang.Exception. This could be an issue if
there
are some PJSIP methods that cannot throw "pj::Error". In this case the 
user
has to handle an exception that can never occur. However I am not even 
sure
if such methods exist.

Another option would be throw non-checked exceptions - such as
RuntimeException - from the PJSIP Java bindings. But I think you like the 
idea of exception specifications and checked exceptions and therefore
probably the first option will be the better one. As I said, the fix
works
by just changing a single line.


Best regards,

     Christian


On 16/10/18 11:24, Ming wrote:

Hi Christian,

Thanks for your detailed explanation, which I'm sure help us
(especially me) improve our knowledge about Java.

After checking the code (in pjsua2.i), our current exception actually
uses java.lang.Exception, which, according to the reference below,
belongs to checked exceptions. I verified this by trying to compile
our sample code and remove a try-catch block and it did fail to
compile.

Ref:
https://www.geeksforgeeks.org/checked-vs-unchecked-exceptions-in-java/ 

Best regards,
Ming
On Tue, Oct 16, 2018 at 2:45 PM, Christian Hoff <christian_hoff@xxxxxxx> 
wrote:

Hello Ming,
thank you very much for your quick feedback and I am happy to hear that 
the
propagation of exceptions works well with my patch!

Regarding your point about exceptions and thr "throw (...)"
specification:
You are right in saying that an exception specification is still an
important in programming languages such as Java. However in Java two
kinds
of exceptions exist and only one kind of them (the checked exception)
works
with exception specifications:
Checked exceptions. Checked exceptions are the kind of exceptions that 
you
are talking about, where the programmer is forced to somehow handle the 
exception. Handling the exception can mean either installing a catch
handler
or adding this exception type to the "throws" declaration of your method. 
If
the exception is not handled, a compile error will be generated - as you 
wrote.
Unchecked exceptions. Unchecked exceptions are the kind of exceptions
that
the programmer is not forced to handle. For this kind of exception, the 
programmer is not forced to install a "catch" handler, so no compile
error
will be generated if the exception is not handled at all. Such exceptions also do not appear in the "throws" declaration of the methods. Unchecked exceptions are all exceptions that derive from the RuntimeException and 
Error classes.

For PJSIP a "pj::Error" was mapped to a RuntimeException up to now
(already
before my changes), so a "pj::Error" from the C++ PJSIP library raises an unchecked exception. This is true before and after my changes. This means 
that - as of now - a Java programmer is not forced to handle a
"pj::Error"
from the C++ code. This is because the "pj::Error" will be mapped to an unchecked exception. The consequence is that there is no compile error if the user forgets to install a catch handler on the Java side for errors 
from
PJSIP. The exception for the "pj::Error" also does not appear in the
"throws" declaration of the Java methods generated by SWIG as only
checked
exceptions should appear there, but not unchecked exceptions such as
"RuntimeException".
What I want to say in short is that my patch does not change anything in 
terms of exception handling on the Java side. The old behaviour was
already
to throw an unchecked exception that the user was not required to catch 
and
the new approach is the same. And already before my changes we were not 
using exception specifications on the Java side as we were using
unchecked
exceptions. Or have I missed or overlooked something?
What we might want to consider for the PJSIP Java bindings is to change 
the
exception handling there to throw checked exceptions if a "pj::Error" is 
thrown from the C++ code, in order to force users to handle a
"pj::Error".
But this is more or less a separate change that does not really have
anything to do with my changes.

Thanks again for the quick testing and the quick feedback!


Have a nice day and best regards,

      Christian


On 16/10/18 06:31, Ming wrote:

Hi Christian,
I did some testing here and the exception does work great during runtime 
now!
However, in my opinion, the "throw()" specification is still important 
for SWIG. While it's true that throw() is deprecated for C++, it
remains a valid (and important) part of other languages such as Java.
And having throw() in the declaration of the function will help the
programmers during the development process.

For example, if you try to call a function which throws an exception
in Java but forget to put try and catch, the compiler will give an
error such as:
sample.java:155: error: unreported exception Exception; must be caught 
or declared to be thrown

But it's now gone, so the developer must now carefully check each
function in order to know whether it can throw an exception.

Best regards,
Ming

On Mon, Oct 15, 2018 at 10:18 AM, Ming <ming@xxxxxxxxx> wrote:

Hi Christian,

Thank you for the hard work. Let us review and test this. Given the
scale of the platforms and language binding affected, this may take
some time. We will update you again.

Best regards,
Ming
On Mon, Oct 15, 2018 at 4:19 AM, Christian Hoff <christian_hoff@xxxxxxx> 
wrote:

Hello all,
during the last weeks, I worked hard on the patch for PJSUA2 and C++ 17 compatibility again. For those who have forgotten: Currently the PJSUA2 headers cannot be compiled with C++ 17 because C++ 17 removes support for 
exception specifications. But unfortunately PJSUA2 makes have use of
exception specifications.
The open issue with my last patch was that SWIG no longer generates catch handlers that catch "pj::Error", as Ming pointed out. I have now updated 
the
patch so that SWIG also generates the required catch handlers. Doing so 
has
been a quite difficult, tedious and time-consuming task. But finally I am 
done and I am happy to share the resulting patch for inclusion into
pjsip.
The updated patch is attached to this mail.
The magic on the SWIG side is done with the "%exception" directive, which installs a default global exception handler. In my updated patch, I am 
now
using this "%exception" directive to generate the catch handler for
"pj::Error" (see the new file "pjsip-apps/src/swig/pj_error.i"). The
rationale behind this was that I really wanted to remove all the "throw 
(pj::Error)" from the header files, so I did not want to have a
"PJ_THROW"
that would expand to nothing during the compile and expand to the old
"throw
(...)" when using SWIG. "throw (...)" is now deprecated syntax and we
should
not keep it around for the sake of SWIG in my opinion.

Some basic testing was also done:
To verify the C++ 17 compatibility, I compiled my application with the 
installed adopted pjsua2 header files.
To verify the generated SWIG language bindings and catch handlers:

For the Python language binding, I verified that the compile works.
Afterwards I installed the generated language binding and ran the test 
file
"test.py". "test.py" also verifies the exception handling. I also
inspected
the generated catch handlers for pj::Error visually.
For the Java and C# bindings, I verified that the compile works and
inspected the generated catch handlers visually. Unfortunately I did not find a suitable unit test for the exception handling, but I am optimistic that it works nevertheless. I could not run the generated code for the C# bindings because I do not have Xamarin installed on my machine. But as I 
said, at least I can compile the C++ wrapper code for C#.
I hope that this patch can now be considered for inclusion into PJSIP. 


Best regards,

      Christian


On 25/09/18 10:26, Max Truxa wrote:

Hello Christian,
how about replacing the throw specifications with a macro that expands 
to nothing when compiling as C++ but `throw(Error)` when being parsed
by SWIG?

Something like this:

#ifdef SWIG // defined if SWIG is parsing the file
#   define PJ_CAN_THROW     throw(Error)
#else
#   define PJ_CAN_THROW
#endif

struct Foo
{
      void bar() PJ_CAN_THROW;
};

Regards,

- Max
On Tue, Sep 25, 2018 at 8:30 AM Christian Hoff <christian_hoff@xxxxxxx> 
wrote:

Hello Ming,

this is also an interesting idea. But will this actually work..? I am
wondering because SWIG only parses the header files of PJSUA2. However the actual function implementation is in the ".cpp" source files and in 
these files you can find the code that throws the exceptions and that
could use "PJ_THROW()". But to my knowledge SWIG does not parse the
source files and would thus not know which functions may call
"PJ_THROW". Or am I wrong about this?


Best regards,

      Christian


On 25/09/18 03:08, Ming wrote:

Hi Christian,

We haven't found a good way either. Our idea is to put PJ_THROW()
macro to replace throw(), which can somehow be used by SWIG to
generate the exception specs.

Regards,
Ming
On Tue, Sep 25, 2018 at 4:21 AM Christian Hoff <christian_hoff@xxxxxxx> 
wrote:

Hello Ming and all,

I have now done some research into SWIG and exceptions. After reading
the SWIG documentation, I found out that SWIG indeed parses the C++
exception specification to figure out for what types of exceptions
"catch(..)" should be generated (see
http://www.swig.org/Doc3.0/SWIGDocumentation.html#SWIGPlus_exception_specifications ). Exception types that are specified in the exception specification are 
automatically propagated to the higher language (Java, C# etc.). Now
that we need to remove the exception specification, we lose that
capability. Thanks to Ming for the hint. However there is the
possibility to adapt the SWIG interface file
("pjsip-apps/src/swig/pjsua2.i") and to add explicit "%catches"
statements for each method that describe which types of exceptions the method could possibility throw. So I guess I need to rework my patch for this. I just hope that I will find some way to avoid having to annotate every method with "%catches".... I'll see. Or if someone knows how to do 
it, then I would be very grateful for a reply.


Best regards,

        Christian


On 24/09/18 12:43, Christian Hoff wrote:

Hello Ming,

thanks for your reply! I will have a look into the SWIG topic this
evening and will let you know my findings.


Best regards,

      Christian


On 24/09/18 09:03, Ming wrote:

Hi Christian,

We apologize for the delay in response. We're currently reviewing it
and have also managed to do some testing. While for C/C++, this seems
to be okay, we're still considering how to best handle this for our
Android and Java (via SWIG binding), which may still need the "throw"
specifications in order for the developers to know that the functions
may throw some exceptions.

If you have any feedback regarding this, please let us know.

Best regards,
Ming

On Mon, Sep 24, 2018 at 2:32 PM Christian Hoff
<christian_hoff@xxxxxxx> wrote:

Hello all,

some feedback for this patch is still appreciated. In general I am
getting somewhat frustrated about the collaboration with people who
want
to contribute to the project. Already in the past other people have
posted patches here to this list, but did not get any response - not
even a response what needs to be changed for the patch to be
accepted. I
think this is not a good mode of collaboration. Everyone who posts a
patch here to this mailing list has done this with the intention to
help
the project PJSIP to move along (and has spent much time developing the patch), so you should at least return the favour and review the patch. 

This particular issue that I am trying to address with my patch (that
the PJSUA2 headers don't compile against C++17) is an issue that you
will eventually need to fix anyhow, so why not fix it right now? As of now, modern versions of GCC use C++ 14 by default, it is only a matter 
of time until C++ 17 will become the default and people will start
complaining that there are compile errors from the PJSUA2 headers.

Besides this, my own app uses PJSUA2 with C++ 17 features, so I would
have to revert back to C++14 if this patch is not accepted upstream.
This would be really unnecessary. So I would really appreciate a patch 
review.

Best regards,

       Christian


On 18/09/18 20:57, Christian Hoff wrote:

Hello all,
Hello Ming & Riza,
do you have any feedback regarding this patch for C++ 17 support? I am 
also willing to rework it if required, but some feedback would be
nice.


Thanks in advance and kind regards,

        Christian


On 12/09/18 18:23, Christian Hoff wrote:

Hello all,

currently it is not possible to compile a pjsua2 application with
C++17. The reason is that C++17 removes support for dynamic exception
specifications (that is "throws ..." declarations at the end of a
function). Unfortunately PJSUA2 makes heavy use of exception
specifications and most PJSUA2 functions are declared as "throw
(Error)" - meaning that they can throw an exception of type "Error"
that the programmer is supposed to catch.

The consequence is that a lot of compile errors can be observed when
compiling a pjsua2 application with C++17. The compile errors look as
follows:

/usr/local/include/pjsua2/json.hpp:80:33: error: ISO C++17 does not
allow dynamic exception specifications
         virtual string saveString() throw(Error);
                                     ^~~~~

The compile of the same application worked fine when using the C++14
standard, but it got broken when I switched from C++14 to the C++17
standard.

See
https://www.bfilipek.com/2017/05/cpp17-details-fixes-deprecation.html#removing-deprecated-exception-specifications-from-c17 

and
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0003r0.html
for more background about the removal of exception specifications.

To make the application compile again, I had to remove "throw(Error)"
in the whole PJSUA2 source code and include files. This seems to be
the only viable and good fix for this issue. I wrote my own "sed"
script to do that and made some cosmetic corrections afterwards. Then
I generated the attached patch file. Could you please apply this
patch to the code base or give me feedback what I still need to
improve? I verified that pjsip compiles fine with the patch applied.

Thank you very much!


Best regards,

       Christian

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