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From "Gregory (Grisha) Trubetskoy" <gri...@apache.org>
Subject Re: Python Server Pages
Date Sat, 05 Apr 2003 05:17:02 GMT

On 4 Apr 2003, Sterling Hughes wrote:

> PSP supports
> modifying request and response parameters, however it also provides
> reasonable defaults.  So for example, the content-type of an application
> starts with text/html.  Headers are automatically sent when the
> application first produces output, etc.

If I understood it correctly, this is made available via a global request
object - it might be a better option if this was the mod_python's request
object, which I think has more features. I don't know much about the
mod_psp request, but mod_python's is modeled heavily after apache's
request_rec. The details are here:

http://www.modpython.org/live/current/doc-html/pyapi-mprequest.html

BTW, the latest mod_python uses Python's new class implementation, whereas
the 2.x version uses the old "built-in" object style. The implication is
that new objects require Python higher version than 2.2.1 (I think).

> 1) mod_python gains two new MIME handlers, application/x-httpd-python
> and application/x-httpd-psp.  These MIME handlers behave similairly to
> how they did in mod_psp.

In mod_python I didn't use MIME types, it is mapped by use of AddHandler,
don't know if this is good or bad. So the association of a file with
mod_python typically looks like this:

AddHandler python-program .py

(As a side note, I think I saw that mod_perl's new handler name is
mod_perl, so their config looks like "AddHandler mod_perl .pl", which I
think is an idea worth borrowing).

Anyway - if someone could explain the advantages/disadvantages of using
mime types, I'd appreciate since I don't seem to grasp the idea at the
moment.

In any event, the handling of handlers is different between httpd 1.3 and
2.0. In 1.3 you passed a list of handlers/mime-types as part of module
config, and then httpd took care of calling your handler when needed. In
httpd 2.0 the handlers are called for every request and the responsibility
for examining the mime type now belongs to the individual module, so
handler code typically begins with something like:

    if (!req->handler || strcmp(req->handler, "python-program"))
        return DECLINED;

So there is a (probably insignificant) performance hit to adding new
types.

The mod_python reqeust processing looks something like this:

    python_handler() in mod_python.c
        o  check to see whther there is a mod_python handler (not to
             be confused with apache handler - mod_python handler is
             the name of a python module which will handle the request and
             it is set via PythonHandler directive). It's possible to
             specify multiple handlers, in which case they will be called
             sequentially. There is a special C structure called hstack
             which contains this list of handlers. Handlers can also be
             added onto the stack dynamically from inside Python.
        o  select subinterpreter name based on virtual host name
	     and possibly directory/file name
        o  if this subinterpeter exists, switch to it, otherwise create
             it, then switch to it. This is done in get_interpreter().
             get_interpreter() actually imports the mod_python.apache
             module (written in Python) and instantiates a CallBack obj.
             (the instantiation is only done once when subinterepreter is
             created, from there on the object is reused)
        o  create the mod_python request object
        o  call into Python (specically call CallBack.HandlerDispatch())
	   --- from this point we're running Python code --
           1. HandlerDispatch() looks at the hstack object to find out
               which python handler (i.e. a normal Python module) to
               import
           2. The module is imported (and possibly re-imported if it
               changed on disk since last import)
           3. The name of the module is removed from hstack
           4. Control is passed to a method inside this module.
              o This is where content is generated, and written
                  to the client using req.write()
           5. Loop back to step 1. and repeat until hstack is empty

> 2) The Python Server Pages functionality gets exported in a traditional
> mod_python module, and therefore you could do something like:
>
> from mod_python import psp
>
> def handler(req):
> 	code = psp.parse(filename)
>
> You can also use the psp.include() function to parse and execute psp
> code from within mod_python handlers and application/x-httpd-python
> python scripts.  mod_psp already has this functionality.

So as a starting point, given that there'd exist a mod_python._psp module,
(which would be _psp.so I guess) a mod_python handler could be implemented
without any changes to mod_python and look something like:

from mod_python import apache
from mod_python import _psp

def handler(req):

    try:
        # the request object is passed in because somehow it would
        # be made available to the inlined Python, or is this something
        # that can be done just prior to exec?
        code = _psp.parse(req, filename)
        exec code
    except SomeErrors:

        # do something...
        return apache.SOME_ERROR

    return apache.OK

If the above code snippet lived in a file callde psp.py, then a typical
httpd.conf would look like:

AddHandler python-program .psp
PythonHandler mod_python.psp

OK, now that I just typed all this I realized I don't know how mod_psp
sends its output to the client - in mod_python it's done via
req.write()...

Now, if we want a special mime-type for psp, then I think the above is
still good, we just need to insert a snippet of code at the beginning of
python_handler() C function that adds mod_python.psp to hstack if it sees
that particular mime type.

Feel free to comment, the above was typed straight off the top of my head
and probably contains errors/ommissions, but I think we have a starting
point of a discussion.

Grisha


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