[ naviserver-Feature Requests-1162223 ] Tcl Callbacks

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Feature Requests item #1162223, was opened at 2005-03-13 00:22
Message generated for change (Comment added) made by seryakov
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Category: None
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Stephen Deasey (sdeasey)
Assigned to: Stephen Deasey (sdeasey)
Summary: Tcl Callbacks

Initial Comment:

The server offers a dual C/Tcl interface.  There are a
number of of places where Tcl code needs to be executed
by the C core, or the core doesn't know whether a
callback is C or Tcl.

I've attached the patch
naviserver-4.1-tclcallbacks.patch.  It extracts the Tcl
callback infrastructure from tclsched.c into
tclcallbacks.c and makes it a public interface.  Here
it is:

Ns_TclNewCallback()
Ns_TclNewCallbackObj()
Ns_TclEvalCallback()
Ns_TclCallbackProc()
Ns_TclFreeCallback()
Ns_TclCallbackArgProc()

It provides a standard interface for Tcl callbacks
throughout the server, handling interp allocation,
introspection, evaluation and cleanup.

Also, tclcallbacks.c is somewhat analogous to
callbacks.c, and so the generic ns_atexit etc.
callbacks which were in tclsched.c have been moved to
the tclcallbacks file.

The first set of code to be converted to the new
interface is tclsched.c itself, and I've converted to
using Ns_ParseObjv() while I was in there.  File size
has dropped ~250 lines.


One thing I'm wondering about is whether the
Ns_TclEvalCallback() routine should take extra args to
send to the Tcl proc.  There seems to be no standard
scheme for ordering the args so there still needs to be
a way to manually construct the command, which is why
the members of the Ns_TclCallback struct are public. 
But for this very reason, it might be worth providing a
standard way to do this so all future callbacks are
consistent.


It would be nice in the future to come up with some way
of registereing these callback points such that you
don't have to create an ns_info subcommand for example
manually.  It should be possible for loadable modules
to register their callback points and have them show up
in the right place.



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>Comment By: Vlad Seryakov (seryakov)
Date: 2005-03-17 14:16

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Yes, let's start slowly, we know the direction we are going,
let 's do it step by step starting with simple things. We
can have generic Tcl callbacks already and all new callbacks
will use it and we will see how flexible this mechanism is.

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Comment By: Stephen Deasey (sdeasey)
Date: 2005-03-17 11:28

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Yes, I forgot to mention that Tcl callbacks are a little
more general than those for C in callbacks.c.  Those take a
single arg which is the context you registered with the
proc, and return void.   Tcl callbacks on the other hand can
return a result which at the C level gets appended to a
dstring if you pass that in.

The members of the Ns_TclCallback structure are public so
that you can construct a command and args in any format. 
This is the question I was asking originally: should there
be a default recomended way to do this?

So for example Ns_TclEvalCallback() might take varargs
cstrings, which if passed would be added to the tcl command
to execute:

  command ?cbarg1? ?cbarg2? ?context?

Where cbarg2 and cbarg2 are specific to the type of callback
and context is what was registered with the proc by the Tcl
programmer.

Unfortunately that wouldn't cover all the bases as some of
the existing APIs do weird things like place callback
specific args after the context or if no context was passed,
always append an empty string.  For new callbacks, something
standard like the above scheme should be used.


If you look at the patch for virtual hosting you'll see that
two new Tcl callbacks were created and they contain simillar
boiler plate.  This makes me think that maybe there should
be something like an [ns_addcallback serverroot myrootproc
mycontext] interface.

It's a little more complicated than that because some
callbacks are singular, like the serverroot proc, and some
are plural, like filters.  So maybe the thing to do is get
some experience with this simple version and then expand it
later...





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Comment By: Vlad Seryakov (seryakov)
Date: 2005-03-13 00:46

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It looks good, unified callback interface would be better
but Tcl filters. traces, requestprocs uses different
arguments/parameters and currently the only way to get info
about them to use ns_info interface and Ns_ArgProc
interface. Tcl_Callback are for simple proc/args callbacks,
filters is special case, see if you can convert them.
But i am for this change.

----------------------------------------------------------------------

Comment By: Stephen Deasey (sdeasey)
Date: 2005-03-13 00:30

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I've attached a patch which converts the tclrequest.c file
to use the new callback API as an example.  It also uses the
Ns_ParseObjv() API, and I've removed support for the legacy
conn variable.

It removes ~200 lines and all utility functions.  The
callback API provides default Ns_ArgProc introspection, but
this will be much nicer when combined with Vlad's RFE
1161597  :-)



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