Jon Steinhart's (jon@fourwinds.com) External Program Hooks
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This blurb describes a changes to nmh that implement an interface
to external programs.  This interface is different than the limited
interface provided by things like the rmmproc context entry.


Why Did I Do This?
~~~~~~~~~~~~~~~~~~

I'm working on a project (grokmail) that will get released via GPL sometime
soon.  This project keeps a database synchronized with the messages in the
mail system.  New functionality is built on top of this database.  This
functionality allows fast searching, and searching based on interest
criteria.  The latter can be used for spam filtering.

The changes to nmh allow external programs to be run whenever a message is
added to a folder, removed from a folder, or refiled.  The changes are
implemented in a general way so that it can be used for other purposes than
mine.

What Are The Changes?
~~~~~~~~~~~~~~~~~~~~~

The changes add four new profile components:

add-hook:	This is the full pathname of a program that is invoked
		whenever a message is added to a folder.  The program
		is passed one argument which is the full pathname of the
		message file.  The program is executed after the message
		is written so that it can act upon that message.

del-hook:	This is the full pathname of a program that is invoked
		whenever a message is deleted from a folder.  The program
		is passed one argument which is the full pathname of the
		message file.  The program is executed before the message
		is written so that it can act upon that message.

ref-hook:	This is the full pathname of a program that is invoked
		whenever a message is refiled.  The program is passed two
		arguments:  the first is the full pathname of the original
		message file, the second is the full pathname of the final
		message file.  The program is executed after the message
		is written.
	
msg-hook:	This is a text message that is output if the execution of
		one of the external hook programs fails.  There is a built-in
		default message if none is specified.

The definition of refiling is a bit tricky.  The refile hook is executed if a
message is moved from one place to another.  So, for example, the command

	refile -link

causes the add hook to be executed, not the refile hook, because a new message
is created, the old one isn't moved.

These changes affect the following commands:

burst:		The add hook is executed for messages burst from a digest, and
		for the table of contents if -inplace is specified.  The delete
		hook is executed for the original message if -inplace is
		specified.  The refile hook is executed for messages that are
		moved.

folder:		The refile hook is executed for -pack.

inc:		The add hook is executed when messages are incorporated.

refile:		Either the add or refile hooks are executed.

rmf:		The delete hook is executed when messages are deleted.

rmm:		The delete hook is executed when messages are deleted.

sortm:		The refile hook is executed for each message moved.  Note that
		a magic temporary message number of 2147483647 is used to hold
		messages as they are being shuffled.



Did I Do This Correctly?
~~~~~~~~~~~~~~~~~~~~~~~~

Well, sort of.  This all works, but I'm not really happy with it.  The issue
is that an examination of the nmh code shows that message handling is scattered
all over the place.  Although there are library routines such as folder_addmsg
and folder_delmsgs, they are not used consistently.  Long term, I think that it
would be better to make all message handling go through the same choke points.

Also, I added a function to run the external programs.  This sort of stuff is
also scattered around the nmh code, for example in the code to run the rmmproc.
Again, I'd like to make this more consistent in the long term.

What Files Did I Change?
~~~~~~~~~~~~~~~~~~~~~~~~
uip/
	burst.c
	inc.c
	refile.c
	rmf.c
	sortm.c

sbr/
	folder_addmsg.c
	folder_delmsgs.c
	folder_pack.c
	ext_hook.c	(new file)