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Simple Dillo plugins

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[see also (related and less ancient)]

dpi1-v1.0 (draft 10, with implementation)
by: Jorge Arellano, Eric Gaudet
Jul 26, 2003
  This  document  is an hybrid between a draft and a doc. This is
because  the full dpi1 spec isn't completely finished yet, but we
already have a working framework to develop with.

                     Simple plugins (dpi1)


   Hopefully,  dillo  will  have  two  types of plugins: one of a
simple  nature  (described  in this document), that operates in a
similar  way  of  CGIs  (with  some extensions), and another that
integrates interaction with the rendering engine and its widgets.

   The  fact  is  that  our  current  rendering  system,  and its
associated  internal  layers  is  still under construction, makes
almost  impossible  to  begin  working  on dpi2, and on the other
hand, gives an excellent oportunity to focus on dpi1 (that's also
expected to be easier to program with).

   We  expect  dpi1 to be incrementally extensible, but also very
simple in its concept at the same time.

   Please bear the simplicity concept in mind while studying this
document. That's the basis for a powerful and clean dpi spec.


   Dillo aims to be a small, fast and efficient web browser. This
kind of PI is designed to push out of the main code features that
usually  are optional, and also some others that are required (as
bookmarks for instance).

   The  design  idea  is  to keep the main code-base small and to
simplify  the  development of new features, by not requiring a PI
programer to know the whole internals of dillo.

   This  way,  extensions  can  be easily coded, and there's also
place  for  developing unrelated programs that can use dillo as a


   Technically, the plugin program is a coprocess (an independent
program  spawned  by  the  browser)  that  communicates  with the
browser through e certain channel.

   The current implementation uses unix domain sockets:

    ---------    ------------->    ------
   | Browser |     | socket |     |  PI  |
    ---------    <-------------    ------

   Note:  actually,  there's a daemon manager process involved in
the  communication between dillo and its PIs. It is documented in
"doc/Dpid.txt",  but in the end, things work as the above diagram
shows. PI developers should read both this spec and Dpid.txt.

   The  main idea is to have a bidirectional channel on which the
browser  and  the  plugin  can  communicate.  The  data  flow  is
encapsulated in a small but flexible protocol (dpi protocol) that
resembles HTML tags.

   For instance, for sending an status message from the plugin to
the browser:

   <dpi cmd='send_status_message' msg='Hello browser!'>

Note that with this scheme:

   *  PI-programs  can  be  one-demand/one-response,  and there's
space  for  multiple  negotiations  between  the  browser and the
plugin too.

   *  Status/progress messages can be passed back to the browser.

   *  Browser and plugin can request information from each other,
      and they can also set data!

   *  It  is possible to handle persistent dpi1 processes, though
not a requirement.

   *  It's  easy  to extend the protocol by adding new operations
without affecting backward compatibility.

   *  There's plenty of choice for PI languages.

   *  Plugin  programs  are  managed with an intermediate daemon.
This makes possible having several dillo instances using the same

How does it work?

   In general terms, the browser starts (or contacts) the PI when
it finds something that belongs to it (as an URI), and handles it
to  the PI. The PI receives the request, processes it and answers
back  in  the  form  of an HTML page, or status message, or a dpi

   A simple session (one-demand/one-response) can look like this:

   1.- The browser finds an URI that belongs to a certain PI.

   2.- The browser prepares to send the URI to the PI.

   3.- The PI starts.

   4.- The browser handles the URI to the PI with a dpi command.

   5.- The PI  makes/composes its answer (an HTML page) and sends
it back within dpi protocol datagrams.

   6.- The PI acknowledges the browser when done,  and optionally

Some examples

   Before getting into the protocol itself, it may be ilustrative
to  look  at  some functionality that could be handled using this
kind  of  PIs.  For  those willing to participate in the protocol
developing process, it's certainly adviced to keep these examples
in mind for testing design ideas.

   1) Downloads
      * current cache allocates them in memory, so handling them
      to an external program (PI) has several advantages.
      *  the PI chooses which agent (wget, curl, ...) to use, and
      sends feedback to dillo's download interface (HTML).
      * Downloads don't stop if dillo crashes.
   2) FTP
      * to browse FTP directories (PI outputs HTML).
      * Downloads using 1).
      * progress messages (as loggin in, etc) can go to the
      status bar.
   3) Bookmarks
      * The BM PI maintains the BM database (a single file!).
      * The PI builds an HTML page for dillo.
      *  Needs  coomunication  with the browser (URI, titles, add
      bookmark, etc).
      * Some of the functions can be handled with FORMS.
      *  Browser  may  clean  BM-pages  out  of the stack, for
      them not to show in history.
      * Exporting BM becomes a matter of Save-page-as!

      NOTE: All of this has been already implemented (BM).

   4) Preferences (AKA configuration)
      * PI requests information from the browser (current prefs).
      * PI commands the browser to change its "state"
      * PI maintains dillorc
      * As 3), may require stack-cleanning of page.
   5) Info reader.
      * Just a handy info to html filter (very simple).
      * one-demand/one-response.
   6) Doc viewer (?)
      * uses an external program to convert doc to html
   7) Help browser
      * A PI that knows of local documentation, presents an index
      and probably a search feature!
   8) A handler PI for "mailto:".

  Well, that's the idea of simple PI. A way of extending dillo to
suit end-user needs, without requiring extensive knowledge of its
internals.  Note  also  that these extensions don't make into the
core of the browser.

                     Developing a dillo plugin

   I  think  the  best  way to get started will be to explain the
basics  of  "dillo <=> PI" communication, then review the already
implemented tags of the dpi protocol, give some tips, and finally
let you examine the code of the already developed plugins.

   * Don't forget to read: "doc/Dpid.txt" (inside the tarball).

Dpi communication

   dillo => PI:

   There're three ways:
      1.- dpi tag
      2.- static URL
      3.- dynamic URL (with form data)

   Note  that  all  of them are in fact dpi tags, the division is
made to separate some concepts.

   1.-  The  first  one is for any specific dpi command sent from
the browser to a PI. For example:

   <dpi cmd='add_bookmark' url='' title='Here!'>

   2.-  The  second  one  usually refers an URL "inside" the PI's
scope. It commands the PI to do something (usually to send back a
page). For intance:

   <dpi cmd='open_url' url='dpi:/bm/'>

   In  this  case the PI must react sending the main page for the
user's bookmarks.

   3.- The third one is the most versatile. It sends back (to the
PI)  all  the  data  associated with a FORM that was most usually
sent by the PI itself before. For instance:

   <dpi cmd='open_url'

   (all in a single line)

  That's  how  the  PI  knows we want to modify the first section
name of the bookmarks file.

   PI => dillo:

   There're two ways:

      1.- dpi tag
      2.- HTML page

   1.-  The first one is the generic mechanism and usually serves
to command the browser. For instance:

   <dpi cmd='send_status_message' msg='Bookmark added!'>

   2.- The second one tells the browser we'll send an HTML page.

   <dpi cmd='start_send_page' url='dpi:/bm/'>

                            The protocol

   Amazingly  (if you've already tried the bookmarks PI), all its
functionallity is carried on with these few dpi tags:

   dillo => PI:
    <dpi cmd='open_url' url='%s'>
    <dpi cmd='add_bookmark' url='%s' title='%s'>
    <dpi cmd='chat' msg='%s'>    

   PI => dillo:
    <dpi cmd='start_send_page' url='%s'>
    <dpi cmd='send_status_message' msg='%s'>
    <dpi cmd='reload_request' url='%s'>
    <dpi cmd='chat' msg='%s'>

   So it is NOT that hard!

Tips for developing a new PI

   First,  examine  the  above mentioned protocol tags, then read
the  code  of hello.c ("Hello world" PI) and figure out its work.
You should also have read Dpid.txt at this point.

   After that, it'll be clear what the basic skeleton is, and how
to send back a page to dillo.

   The  next step is to make a FORM, get some data from the user,
and  to send back a new page that manipulates the input data. For

   1.- Design an HTML page with the form
   2.- Design an HTML page with the desired answer skeleton
   3.- Make your PI send the form as the main page
       (as an answer to "dpi:/mydpi/", for instance)

   Once you have that, you'll need to process the input data:

   4.- Make your PI print what it receives so it becomes clear
       what you're dealing with.
   5.- Either try to process it right away, or study how it's
       done in the 'Bmsrv_parse_buf' function of the bookmarks

   Now you have the parsed data:

   5.- Manipulate it somehow
   6.- Build the answer page and send it back

   If  you  made  this  far,  the  rest will be clear enough from
studying the commented bookmarks' code!

   If you don't want to make a form-based PI, but one that spawns
another process you may start looking in the ftp PI.

What's missing in this implementation

   1.- Some dpi commands
       (This is good in the sense of a protocol being finished
       when the last uneeded commands are removed! :)
   2.- PI start/initialization.
   3.- All the menu handling (from PI to browser) is to be

   (there're some ideas about these near the end)

   1.- Dpi commands

   From the set of dpi commands, I want to comment two:


     <dpi cmd='start_send_page' length='%d'>

   This  was  meant to encapsulate HTML chunks of certain length.
The  final  chunk  should  be marked with length='0' or something
akin.  The main purpose of it is to allow communication after the
HTML  page  is sent. That way, for instance, the bookmarks plugin
could  send  a  staus  message  adding  more  information about a
performed operation (as "Deleted 3 bookmarks").

   The  only reason I didn't code it from the start was to get to
a working framework as fast a s possible. It's not hard though.

   Currently   the  dpi  framework  uses  'start_send_page'  wich
assumes  that  all  the following data, up to EOF, is part of the
HTML page.


   This  will  be a generic way of exchanging information. Useful
for sending/setting preferences for instance. Very similar to the
previous command, and much like the current 'chat' command.

   BTW,  the 'chat' command is not necessary (just gave a look to
the  funny  and irrelevant talk between dillo and the bm PI!). It
was just a way of testing bidirectional data exchange.

   If you need to send/request some data, feel free to use 'chat'
until 'send_data' is implemented.

   2.- PI start/initialization

   There's  a  lot  more  info about it in the following pages. A
polished interface for registering/launching plugins is yet to be

   We  want  dillo and dpid to be able to add, remove and upgrade
plugins in a clear and transparent way. We're working on it...

   3.- Menu handling

   Just read the following section!.

                               SECTION II
                        (this is the draft part)

   This  section  is mainly a gathering of ideas about what's not
yet implemented:

   1.- Dpi commands
   2.- Menus
   3.- PI initialization
   4.- Plugins options
   5.- Plan
   6.- Future extensions
   7.- Final notes

Dpi commands

  NOTE:  this  commands  list  is just a set of ideas. It doesn't
mean  they  will be implemented. We gather them here just to keep
track of the interesting ideas they bring.

dillo -> PI

CMD   : DpiAbort
Sent  by dillo to abort an "in-progress" plugin. Dillo might send
a "kill -9" command to the PI if it doesn't answer.
(most probably dillo will ask the dpid to kill a PI, and the dpid
itself will send a kill signal to the PI).

CMD   : DpiBye
The plugin is commanded to quit (after it's done).
It  may  be  used  to  end  a "resident" PI that's doing nothing.
Plugins can end and exit when finished or keep sleeping, but they
MUST acknowledge dillo when they're done on a per client basis.

CMD   : DpiError
Sent by dillo when a command is not understood.

PI -> dillo

CMD   : DpiRequestInfo
Sent to the browser to request information. 
For instance:
  - Dillo version
  - title of current page
  - URL of current page
  - preferences (the whole preferences as text)
  - All active colors (background, text, link, visited link)

CMD   : DpiSetData
Sent to the browser to change its "state". 
Example: preferences, menus.

CMD   : DpiDone
Sent by the PI when finished servicing a client.


   Menus  need to be defined thinking on the needs of current and
future PIs. The may not require menu entries (as the FTP PI), may
require a single menu entry, or a whole menu, or a button.

   The key point here is to provide simple support for menus, and
not to complicate the protocol with seldom required features.

<< simplified menu registration protocol --EG
  Main goals:
  -  a  plugin  may or may not want to be registered in the menus
  when initializing: menus are optionnal.
  -  a plugin may want to change some or all its menus entries at
  - the plugins menu looks like this: there's a "Plugins" menu in 
  dillo's menubar, where all plugins are shown, provided they 
  asked  for it during the init stage. Then it's up to the plugin
  to ask this entry to be an url or a submenu.

CMD   : DpiMenu

Menu commands:
- insert: add an entry named <string> just after the entry ID. Dillo 
must answer with the new entry ID.
- delete: delete an entry ID and all dependent sub-menus.
- modify: modity the <string> of an entry.
- submenu: make an entry a submenu.
- url: give <string> as the url to be called when the menu entry is 

PI initialization

  The main problem with running each PI everytime dillo starts is
the  added  startup  time. If, for instance, there's a PERL and a
Python PI, the time increase is too much.

  We're  working on a scheme that avoids running each plugin when
dillo  starts. It has a "pluginsrc" kind of file with information
about  each PI. The whole work is done by dpid, but there'll be a
control program for the user interface. Something like:

  dpidc [register | upgrade | stop | ...]

  dpidc  communicates  with  dpid  using the same service request
socket  that  dillo  currently  uses.  All the operations will be
defined with dpip tags.

  What  I like from this approach is that it is very flexible and
can  be  implemented incrementally ("dpidc register" is enough to
  It  also  avoids the burden of having to periodically check the
dpis directory structure for changes or updates.
  It  also  lets shell scripts an easy way to do the "dirty" work
of  installing  dpis;  this may help building a dillo package for
certain distros.

  For instance, these commands may be added to the protocol:

dillo -> dpi:
CMD   : DpiInit                      
It should include some challenge data to assert it's a dpi.
and from dpi->dillo:                 
CMD   : DpiInitAnswer                
Data is composed of a list of name=value pairs separated with ';'
just like in an URL (it must answer the challenge).
  Name="bookmarks";prefix="bm";MenuEntry="View Bookmarks"

Plugins options

 There're  two  different  things  here, the PI registration data
(managed  by  dpid in the 'pluginsrc' file for every PI), and the
specific  options  a  specific PI-program may provide for tunning
its  behaviour. The later one should be managed by the PI program


   Currently  the  dpi framework has working dpis for: bookmarks,
ftp, downloads and hello.

  The  plan  is  to  polish&stabilize  both,  the  dpis  and  the
framwork, with a view to 0.8.0 release.

Future extensions (needs a lot more thought)

1.- Postfixed call to a Dillo-plugin

   Dillo plugins  registered as such will be called if the loaded
page's  <extension> in "<url>.<extension>" matches a Dillo-plugin
registration,  or  if the PI has registered the same MIME type of
that content, and thus the PI acts as a file-processing filter.

   Dillo  will  call  the  Dillo-plugin  sending  to it the whole
loaded  file, and expecting some data back. This can be used as a
file-to-html filter.
   Example:   ""  will
render  the  rtf  file  as  html if a Dillo-plugin has registered
itself as dealing with "rtf" files.

Final notes

   The draft part of this material is not "written on stone", but
provided  as a structural basis on which to develop the simple PI
protocol. Please take it with a pinch of salt, in the best spirit
of a RFC.

   As   you  can  see,  there's  plenty  of  material  here,  and
discussing  threads can get very large if posts are not carefully
thought  of.  Please take your time to study and fully understand
the material, and only submit well backed/tested ideas.

   Thanks  a  lot to all of those that have contributed in making
dpi1 possible!