Make GILReleaser exception-safe

[gleichdick]

Description

If an exception occurs while the GIL is released,
it won't be reacquired and the stack unwinding will take place
without the GIL. Releasing and Reacquiring wil be removed.

For example,

bp::object wrap_long_computation(bp::object arg) {
   auto parsed_arg = parse_arg(arg);
   GILReleaser gr;
   auto res1 = do_long_computation_1(parsed_arg);
   gr.reacquire();
   bp::object res2 = do_more_python_stuff();
   gr.release();
   Result3 res3 = do_long_computation_2(res1);
   gr.reacquire();
   return encode_result(res2, res3);
}

is not exception safe, because if do_long_computation_2() throws an exception, the Python object res2 will be destroyed without the GIL.
Instead,

bp::object wrap_long_computation(bp::object arg) {
   auto parsed_arg = parse_arg(arg);
   Result1 res1;
   {
      GILReleaser gr;
      res1 = do_long_computation_1(parsed_arg);
   }
   bp::object res2 = do_more_python_stuff();
   Result3 res3;
   {
      GILReleaser gr;
      res3 = do_long_computation_2(res1);
   }
   return encode_result(res2, res3);
}

will be safe.

An IIFE would also work

...
  Result3 res3 = [&](){
     GILReleaser gr;
     return do_long_computation_2(res1);
  }();
...

Checklist

• Required by CI: Code is auto formatted using clang-format
• Extend the tutorials / documentation reference
• Document API changes relevant to the user in the MIGRATION.md notes
• Create tests, which fail without this PR reference
• Include a screenshot if changing a GUI
• While waiting for someone to review your request, please help review another open pull request to support the maintainers

[codecov]

Codecov Report

Merging #2363 into master will decrease coverage by 0.01%.
The diff coverage is 21.06%.

@@            Coverage Diff             @@
##           master    #2363      +/-   ##
==========================================
- Coverage   56.35%   56.34%   -0.00%     
==========================================
  Files         285      285              
  Lines       24242    24242              
==========================================
- Hits        13658    13657       -1     
- Misses      10584    10585       +1     

Impacted Files	Coverage Δ
...ove_group_interface/src/wrap_python_move_group.cpp	41.72% <6.25%> (ø)
...ls/include/moveit/py_bindings_tools/gil_releaser.h	100.00% <100.00%> (ø)
...ipulation/pick_place/src/manipulation_pipeline.cpp	72.35% <0.00%> (-1.06%)	⬇️

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[v4hn]

Thank you for the detailed analysis and the patch! 🏅

Both make sense to me and should be merged soon.

[rhaschke]

LGTM. Thanks.