HandlerDispatch doesn't treat OK/DECLINED result properly for all phases.

         Key: MODPYTHON-129
         URL: http://issues.apache.org/jira/browse/MODPYTHON-129
     Project: mod_python
        Type: Bug
  Components: core  
    Versions: 3.2    
    Reporter: Graham Dumpleton

Todays daily bug report, or is it? ;-)

The Python*Handler documentation says:

"""Multiple handlers can be specified on a single line, in which case they will 
be called sequentially, from left to right. Same handler directives can be 
specified multiple times as well, with the same result - all handlers listed 
will be executed sequentially, from first to last. If any handler in the 
sequence returns a value other than apache.OK, then execution of all subsequent 
handlers is aborted."""

That is, no matter which phase is being processed, mod_python will stop 
processing them if a value other than OK is returned.

Problem is that this isn't how Apache itself treats the result from handlers. 
Apache actually implements two different ways for dealing with the result from 
the handlers. Which is used depends on which processing phase is occuring. This 
is all specified by the Apache magic macro code:

                            (request_rec *r), (r), DECLINED)

                            (request_rec *r), (r), DECLINED)

                            (request_rec *r), (r), DECLINED)

                            (request_rec *r), (r), DECLINED)

                          (request_rec *r), (r), OK, DECLINED)

                            (request_rec *r), (r), DECLINED)

                          (request_rec *r), (r), OK, DECLINED)

What this gobblegook expands to are loops which will stop processing handlers 
based on the result.

For the AP_IMPLEMENT_HOOK_RUN_ALL macro, all handlers in the phase will be run 
unless one returns something other than OK or DECLINED. Returning OK means that 
it did something and it worked okay. Returing DECLINED means that it didn't do 
anything at all. In both these cases, it still goes onto the next handler in 
that phase. After that it will go onto the next phase.

Returning an error will cause appropriate error response to go back to client 
with any other handlers in the phase, as well as later phases being skipped. 
Returning DONE is much like returning an error but Apache interprets it as 
meaning a complete response was constructed and that it doesn't have to 
generate any response.

For the AP_IMPLEMENT_HOOK_RUN_FIRST macro, all handlers will be run only if 
they all return DECLINED. In other words, if a handler returns OK it will skip 
the following handlers in that phase and then move onto the next phase. 
Returning an error or DONE is like above.

In the case of mod_python, what it does doesn't fit into either. It is closer 
to behaving like the AP_IMPLEMENT_HOOK_RUN_ALL macro except that it stops 
processing further handlers in the phase if DECLINED is returned.

As to what problems this causes, imagine you had registered multiple 
authentication handlers which supported different authentication mechanisms. 
This is the case where AP_IMPLEMENT_HOOK_RUN_FIRST  macro is used. The idea is 
that each authentication handler would check the value associated with the 
AuthType directive to determine if it should do anything. If it was the 
AuthType it implements, if it were a C based handler module, it would returned 
DECLINED to indicate it hadn't done anything and that the next handler should 
instead be tried. Each handler would thus be called until one handler says that 
is for me, says the user is valid and returns OK or returns an error rejecting 

If you wanted to write these multiple authentication handlers in Python you 
can't do it. This is because the way mod_python works, if you return DECLINED 
it would actually skip the remainder of the mod_python declared handlers 
whereas you still want them to be executed. Apache would still execute any 
other C based handlers in the phase though. The only way to get mod_python to 
execute later mod_python handlers in the phase is to return OK, but if you do 
that and it happens to be the last handler in the mod_python list of handlers, 
it will return OK to Apache and Apache will then think a handler successfully 
handled it and not then execute any subsequent C based handlers in that phase.

There are going to be other sorts of problems with phases implemented using 
AP_IMPLEMENT_HOOK_RUN_ALL as well, as a handler that validly returns DECLINED 
to say it didn't do anything will cause mod_python to skip later mod_python 
handlers as well. If it were only C based handlers, that wouldn't be the case.

In summary, it doesn't work how it probably should. 

Note that the above relates to phases other than content handler. Still have to 
work out what Apache does for content handler phase when there are multiple 
handlers for the phase.

No one has probably noticed these problems as no one seems to use mod_python 
for implementing these other phases, simply using mod_python as a jumping off 
point for content handlers.

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