Re: Python bindings API confusion

On 2020/07/29 1:02, Yasuhito FUTATSUKI wrote:
> On 2020/07/28 23:34, C. Michael Pilato wrote:
>> Hey, all.
>>
>> I'm writing some code that performs commits via the Subversion Python
>> bindings, and I'm struggling to understand some things I see there.
>>
>> In the svn_fs.i interface file, there's this block of code:
>>
>> /* -----------------------------------------------------------------------
>> Fix the return value for svn_fs_commit_txn(). If the conflict result is
>> NULL, then %append_output() is passed Py_None, but that goofs up
>> because that is *also* the marker for "I haven't started assembling a
>> multi-valued return yet" which means the second return value (new_rev)
>> will not cause a 2-tuple to be manufactured.
>>
>> The answer is to explicitly create a 2-tuple return value.
>>
>> FIXME: Do the Perl and Ruby bindings need to do something similar?
>> */
>> #ifdef SWIGPYTHON
>> %typemap(argout) (const char **conflict_p, svn_revnum_t *new_rev) {
>> /* this is always Py_None */
>> Py_DECREF($result);
>> /* build the result tuple */
>> $result = Py_BuildValue("zi", *$1, (long)*$2);
>> }
>> #endif
>
> Ah, it returns a tuple of str and int, not a tuple of bytes and int.

I made a patch addressing to it, as attached
swig-py-fix-svn_fs_commit-patch.txt.
(Only affect on Python 3, no function change on Python 2.7)

>> This reads and claims to behave exactly as I'd expect, given the dual
>> return values of svn_fs_commit_txn (and svn_repos_fs_commit_txn which wraps
>> it). And since this interface file is included from svn_repos.i, I would
>> expect those typemaps to apply also to svn_repos_fs_commit_txn, which has
>> matching parameter types and names.
>
> It seems it isn't match because svn_repos_fs_commit_txn have extra
> argument "svn_repos_t *repos" between them.
>
>> But this isn't how the code appears to work in practice. A successful
>> commit gets back from svn.repos.fs_commit_txn not a 2-tuple, but just the
>> newly created revision number. Moreover, if my commit succeeds but the
>> post-commit hook fails, svn.repos.fs_commit_txn() raises an Exception,
>> masking the return of the newly created revision number altogether. Now,
>> the masked revision number thing I can understand -- it's hard to do in
>> Python what we promise to do in C (which is to return an svn_error_t but
>> still set the new_rev return value). But the 2-tuple thing I have no
>> explanation for. Any ideas?
>
> I think it is need one more typemap for it in svn_repos.i like
>
> [[[
> #ifdef SWIGPYTHON
> %typemap(argout) (const char **conflict_p, svn_repos_t *repos, svn_revnum_t *new_rev) {
> /* this is always Py_None */
> Py_DECREF($result);
> /* build the result tuple */
> $result = Py_BuildValue("zi", *$1, (long)*$3);
> }
> #endif
> ]]]

Also I made a patch for it, based on fixed typemap for svn_fs_commit_txn,
as attached swig-py-fix-svn_repos_fs_commit-patch.txt

Difference on SWIG generated subversion/bindings/swig/python/svn_repos.c
between before and after this patch is below. (SWIG 3.0.12, for Python 3)
[[[
--- subversion/bindings/swig/python/svn_repos.c.before_patch 2020-07-29 05:45:35.626521000 +0900
+++ subversion/bindings/swig/python/svn_repos.c 2020-07-29 06:41:51.220682000 +0900
@@ -12126,23 +12126,16 @@
     resultobj = Py_None;
   }
   {
- PyObject *s;
- if (*arg1 == NULL) {
- Py_INCREF(Py_None);
- s = Py_None;
- }
- else {
- s = PyBytes_FromString(*arg1);
- if (s == NULL)
- SWIG_fail;
- }
- resultobj = SWIG_Python_AppendOutput(resultobj, s);
- }
- if (SWIG_IsTmpObj(res3)) {
- resultobj = SWIG_Python_AppendOutput(resultobj, SWIG_From_long((*arg3)));
- } else {
- int new_flags = SWIG_IsNewObj(res3) ? (SWIG_POINTER_OWN | 0 ) : 0 ;
- resultobj = SWIG_Python_AppendOutput(resultobj, SWIG_NewPointerObj((void*)(arg3), SWIGTYPE_p_long, new_flags));
+ /* this is always Py_None */
+ Py_DECREF(resultobj);
+ /* build the result tuple */
+ resultobj = Py_BuildValue(
+ #if PY_VERSION_HEX >= 0x03000000
+ "yi",
+ #else
+ "zi",
+ #endif
+ *arg1, (long)*arg3);
   }
   {
     Py_XDECREF(_global_py_pool);
]]]

I couldn't make test cases for them, so I didn't test, yet.

Cheers,

-- 
Yasuhito FUTATSUKI <futatuki_at_yf.bsclub.org>