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Re: Python bindings API confusion

From: Yasuhito FUTATSUKI <futatuki_at_yf.bsdclub.org>
Date: Wed, 29 Jul 2020 01:02:46 +0900

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?
> */
> %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.

> 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

%typemap(argout) (const char **conflict_p, svn_repos_t *repos, svn_revnum_t *new_rev) {
    /* this is always Py_None */
    /* build the result tuple */
    $result = Py_BuildValue("zi", *$1, (long)*$3);

but I didn't tried yet.


Yasuhito FUTATSUKI <futatuki_at_yf.bsclub.org>
Received on 2020-07-28 18:05:45 CEST

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