ikiwiki/doc/rcs/cvs/discussion.mdwn

I've started reviewing this, and the main thing I don't like is the
post-commit wrapper wrapper that ikiwiki-makerepo is patched to set up.
That just seems unnecessarily complicated. Why can't ikiwiki itself detect
the "cvs add <directory>" call and avoid doing anything in that case?
--[[Joey]] 

> The wrapper wrapper does three things:
>
> 7. It ignores `cvs add <directory>`, since this is a weird CVS
> behavior that ikiwiki gets confused by and doesn't need to act on.
> 7. It prevents `cvs` locking against itself: `cvs commit` takes a
> write lock and runs the post-commit hook, which runs `cvs update`,
> which wants a read lock and sleeps forever -- unless the post-commit
> hook runs in the background so the commit can "finish".
> 7. It fails silently if the ikiwiki post-commit hook is missing.
> CVS doesn't have any magic post-commit filenames, so hooks have to
> be configured explicitly. I don't think a commit will actually fail
> if a configured post-commit hook is missing (though I can't test
> this at the moment).
>
> Thing 1 can probably be handled within ikiwiki, if that seems less
> gross to you.

>> It seems like it might be. You can use a `getopt` hook to check
>> `@ARGV` to see how it was called. --[[Joey]] 

>>> This does the trick iff the post-commit wrapper passes its args
>>> along. Committed on my branch. This seems potentially dangerous,
>>> since the args passed to ikiwiki are influenced by web commits.
>>> I don't see an exploit, but for paranoia's sake, maybe the wrapper
>>> should only be built with execv() if the cvs plugin is loaded?
>>> --[[schmonz]]

>>>> Hadn't considered that. While in wrapper mode the normal getopt is not
>>>> done, plugin getopt still runs, and so any unsafe options that 
>>>> other plugins support could be a problem if another user runs
>>>> the setuid wrapper and passes those options through. --[[Joey]]

>>>>> I've tried compiling the argument check into the wrapper as
>>>>> the first thing main() does, and was surprised to find that
>>>>> this doesn't prevent the `cvs add <dir>` deadlock in a web
>>>>> commit. I was convinced this'd be a reasonable solution,
>>>>> especially if conditionalized on the cvs plugin being loaded,
>>>>> but it doesn't work. And I stuck debug printfs at the beginning
>>>>> of all the rcs_foo() subs, and whatever `cvs add <dir>` is
>>>>> doing to ikiwiki isn't visible to my plugin, because none of
>>>>> those subs are getting called. Nuts. Can you think of anything
>>>>> else that might solve the problem, or should I go back to
>>>>> generating a minimal wrapper wrapper that checks for just
>>>>> this one thing? --[[schmonz]]

>>>>>> I don't see how there could possibly be a difference between
>>>>>> ikiwiki's C wrapper and your shell wrapper wrapper here. --[[Joey]]

>>>>>>> I was comparing strings overly precisely. Fixed on my branch.
>>>>>>> I've also knocked off the two most pressing to-do items. I
>>>>>>> think the plugin's ready for prime time. --[[schmonz]]

> Thing 2 I'm less sure of. (I'd like to see the web UI return
> immediately on save anyway, to a temporary "rebuilding, please wait
> if you feel like knowing when it's done" page, but this problem
> with CVS happens with any kind of commit, and could conceivably
> happen with some other VCS.)

>> None of the other VCSes let a write lock block a read lock, apparently.
>> 
>> Anyway, re the backgrounding, when committing via the web, the
>> post-commit hook doesn't run anyway; the rendering is done via the
>> ikiwiki CGI. It would certianly be nice if it popped up a quick "working"
>> page and replaced it with the updated page when done, but that's
>> unrelated;  the post-commit
>> hook only does rendering when committing using the VCS directly. The
>> backgrounding you do actually seems safe enough -- but tacking
>> on a " &" to the ikiwiki wrapper call doesn't need a wrapper script,
>> does it? --[[Joey]]

>>> Nope, it works fine to append it to the `CVSROOT/loginfo` line.
>>> Fixed on my branch. --[[schmonz]]

> Thing 3 I think I did in order to squelch the error messages that
> were bollixing up the CGI. It was easy to do this in the wrapper
> wrapper, but if that's going away, it can be done just as easily
> with output redirection in `CVSROOT/loginfo`.
>
> --[[schmonz]]

>> If the error messages screw up the CGI they must go to stdout.
>> I thought we had stderr even in the the CVS dark ages. ;-) --[[Joey]] 

>>> Some messages go to stderr, but definitely not all. That's why
>>> I wound up reaching for IPC::Cmd, to execute the command line
>>> safely while shutting CVS up. Anyway, I've tested what happens
>>> if a configured post-commit hook is missing, and it seems fine,
>>> probably also thanks to IPC::Cmd.
>>> --[[schmonz]]

----


Further review.. --[[Joey]] 

I don't understand what `cvs_shquote_commit` is
trying to do with the test message, but it seems
highly likely to be insecure; I never trust anything 
that relies on safely quoting user input passed to the shell. 

(As an aside, `shell_quote` can die on certian inputs.)

Seems to me that, if `IPC::Cmd` exposes input to the shell
(which I have not verified but its docs don't specify; a bad sign)
you chose the wrong tool and ended up doing down the wrong
route, dragging in shell quoting problems and fixes. Since you
chose to use `IPC::Cmd`	just because you wanted to shut
up CVS stderr, my suggestion would be to use plain `system`
to run the command, with stderr temporarily sent to /dev/null:

	open(my $savederr, ">&STDERR");
	open(STDERR, ">", "/dev/null");
	my $ret=system("cvs", "-Q", @_);
	open(STDERR, ">$savederr");

`cvs_runcvs` should not take an array reference. It's
usual for this type of function to take a list of parameters
to pass to the command.

> Thanks for reading carefully. I've tested your suggestions and
> applied them on my branch. --[[schmonz]]

----

I've abstracted out CVS's involvement in the wrapper, adding a new
"wrapperargcheck" hook to examine `argc/argv` and return success or
failure (failure causes the wrapper to terminate) and implementing
this hook in the plugin. In the non-CVS case, the check immediately
returns success, so the added overhead is just a function call.

Given how rarely anything should need to reach in and modify the
wrapper -- I'd go so far as to say we shouldn't make it too easy
-- I don't think it's worth the effort to try and design a more
general-purpose way to do so. If and when some other problem thinks
it wants to be solved by a new wrapper hook, it's easy enough to add
one. Until then, I'd say it's more important to keep the wrapper as
short and clear as possible. --[[schmonz]]

> I've committed a slightly different hook, which should be general enough
> that `IkiWiki::Receive` can also use it, so please adapt your code to
> that. --[[Joey]]

>> Done. --[[schmonz]].