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Re: More info on Swig 1.31 vs 1.33

From: Peter Samuelson <peter_at_p12n.org>
Date: Tue, 29 Jan 2008 19:04:57 -0600

Roderich, thanks for all your hard work!

Kouhei, Joe, * - can anyone have a look at this compatibility issue
with svn + ruby + swig 1.3.33? Roderich Schupp has figured out exactly
what the problem is, but I have no idea how to address it.

His analysis applies to 1.4.x. Apologies if this has already been
fixed on trunk - I can't easily check, as 'make check-swig-rb' has been
failing for some time for unrelated reasons, like
"subversion/bindings/swig/ruby/svn/error.rb:68: uninitialized constant
Svn::Error::WcMismatchedChangelist (NameError)".


[Roderich Schupp writes...]


I got my shovel and dug some more into these
"TypeError: Expected argument 1 of type ..., but got Array []" errors.
Here's what I found (warning lengthy explanation ahead).

(1) In general, Swig tries to deliver output parameters (as viewed from
the C level) as multiple return values in the wrapper (for target languages
that support multiple return values, like Python, Perl or Ruby). If the
C function also has a return value, this counts as the zeroeth output

(2) The means to map an output parameter from C to the target
language is an argout typemap. Now almost all argout typemaps
for Ruby look like this:

 %typemap(ruby, argout, fragment="output_helper") some_type
    /* marshal $1 (some_type) into Ruby as variable foo */

    $result = output_helper($result, foo);

where the final line

    $result = output_helper($result, foo);

does the following (note that $return is a Swig meta variable
and stands for the actual name of the variable that will
finally be returned from the generated function wrapper):

   if $esult == Nil then
      # the current output parameter is the first output parameter
      $result = foo
   elsif !($result is a Ruby array)
      # the current output parameter is the second output parameter
      $result = new_ruby_array($result, foo)
      # the current output parameter is the third or higher output parameter
      push $result, foo

(3) There's one problem with this: it doesn't work if foo is already
a Ruby array AND foo is the first output parameter AND there are
other output parameters. In that case the wrapper should return
a Ruby array with foo as its first element and the other output parameters
as the rest of the elements. But it will actually return foo itself,
with the other output parameters appended to foo.

The following change tries to address this problem:
It counts the number of output parameters in a first pass and if this
is > 1 inserts the line

  vresult = rb_ary_new(); # ***

just before the code rom the argout typemaps is emitted.
This fix is correct, Swig is not to blame. Now over to Subversion.

(4) If you look at the Ruby wrapper generated by Swig
for a Subversion function like

void svn_opt_format_option(const char **string,
                      const apr_getopt_option_t *opt,
                      svn_boolean_t doc,
                      apr_pool_t *pool);

(and compare code generated with Swig 1.31 and 1.33)
you see that Swig 1.33 just adds the above line ***. This will
result in an error "expected string, but got Array". But why
does Swig think that this function has more than one output parameter -
obviously there's only one (string). But Swig just does what
it's been told because Subversion (line 350 in
specifies an argout typemap for parameters of type apr_pool_t*

%typemap(ruby, argout) apr_pool_t *pool
   svn_swig_rb_set_pool($result, _global_svn_swig_rb_pool);

This isn't really output parameter processing (hint: it doesn't
assign to $result), it is merely abusing an argout typemap
to apply some final processing AFTER all "real"
output parameters have been processed (and accumulated
in $result), but BEFORE the final "return $result".
This works since apr_pool_t* parameters
are generally the last parameter in SVN and APR functions.

Note that this approach is rather specific to Ruby. Python does
the equivalent processing in a freearg typemap (line 319).
(freearg typemap code is emitted at roughly the same place
as the code for the final argout parameter.)
But Ruby probably can't just switch to a freearg typemap,
because it requires access to $result which isn't available
in freearg typemaps (though I don't understand what
svn_swig_rb_set_pool does with $result).

Cheers, Roderich

Received on 2008-01-30 02:05:11 CET

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