#+TITLE:Nyxt WebExtensions API support library

This is a WebKitWebExtension library to support the necessary
[[https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Browser_support_for_JavaScript_APIs][JavaScript APIs]]. Most of the communication with the browser happens
via WebKitUserMessages. To hook into the library in a WebKit-based
browser, connect to the [[https://webkitgtk.org/reference/webkit2gtk/stable/WebKitWebView.html#WebKitWebView-user-message-received]["user-message-received"]] signal and dispatch it
based on message name -- it will be the same as the JS function name
(e.g. "runtime.sendMessage").
* Extension initialization and extension descriptions
The extension should be initialized with user data. User data should
be a string GVariant containing JSON descriptions for extensions the
library should handle. The schema is:
#+begin_src js
  [
      {name : // Extension name as string
       [id, // ID as a string
        manifest, // full text of the extension manifest as a string
        is_privileged, // integer: 0 if non-privileged, 1 if privileged.

        // "Privileged" means that this extension can run the code in
        // the default JS world/environment/context

        {file_name : // string with a file name relative to the extension root
         url, // string URL to load this extension with. Right now should be a data: URL
         ... // same for all the files extension is allowed to access
        },
        tab_id // ID of the tab that this library is housed in.
        // We imply one WebKitWebContext per tab here.
       ]
      },
      ... // more extension descriptions
  ]
#+end_src

Initialization without user_data is not an option and crashes the view
it loads into. There used to be an option of user_data-less loading
(see
https://github.com/atlas-engineer/nyxt/commit/38b3b84a0e48e9a81ea1eb393c66c48531a0dc8c
and
https://github.com/atlas-engineer/nyxt/commit/dc51d55d478f1fb155aa7db99ae20e093b712e86),
but it's not there anymore.

* Implementing an API endpoint (example of browser.foo.bar())
** (if there are no files for the browser.foo API yet) Create a foo.h and foo.c files.
  Header is usually boring and contains something like
  #+begin_src C
      #ifndef __FOO_H__
    #define __FOO_H__

    void inject_foo_api (char* extension_name);

    #endif /* __FOO_H__ */
  #+end_src
  where =inject_foo_api= is a conventional name for API-injecting function.
  The foo.c file should be something like
  #+begin_src C
    #include "globals.h"
    #include "foo.h"

    void inject_foo_api (char* extension_name)
    {
            /* This is to get the extension-specific context to inject values into. */
            JSCContext *context = get_extension_context(IS_PRIVILEGED ? NULL : extension_name);
            /* Create the foo object with an empty constructor. */
            MAKE_CLASS(context, Foo, "foo");

            /* Method/property/event definitions go there... */

            /* Bind foo to browser.foo. */
            jsc_value_object_set_property(
                    jsc_context_evaluate(context, "browser", -1), "foo",
                    jsc_context_evaluate(context, "foo", -1));
    }

#+end_src

and then go to nyxt.c and add
#+begin_src C
  inject_foo_api((char *) extension_name);
#+end_src
to =inject_apis= and =#include "foo.h"= to the top of the file. Your API should load automatically after you recompile and load the library.

** Write a JS method
*** No need in browser communication (easy)
Say, foo.bar doesn't need to communicate with the browser and always
returns the same result. The callback could then be as simple as
#+begin_src C
  static JSCValue *
  /* Translate "foo.bar callback" into underscored name. */
  foo_bar_callback ()
  {
          /* The JS context callback runs in. A convenient way to get
           ,* the context to return/inject things into. */
          JSCContext *context = jsc_context_get_current();
          return jsc_value_new_number(context, (double) 42);
  }
#+end_src

and you could bind it in =inject_foo_api= with
#+begin_src C
  /* The number and list of args is obviously adjustable. */
  MAKE_FN(context, "runtime", "getManifest", runtime_get_manifest_callback, extension_name, JSC_TYPE_VALUE, 0, G_TYPE_NONE);
#+end_src
And after the API is injected you'll have a simple =browser.foo.bar=
method to get your answers anytime you like.

The examples of such simple functions are =browser.runtime.getManifest()=
and =browser.runtime.getURL()= (both can be found in runtime.c.)

*** Need a response from the browser (hard)
In case you need to communicate with the browser, it gets a bit more
convoluted. The sequence of actions that happens there is:
- JS: Call a method.
- C: Enter the callback and send the message to the browser there.
  - The message should be indexed by the unique ID of the request so
    that there are no race conditions.
- C: construct the Promise waiting for the browser response (via =make_promise=).
- C/JS: Run =browser.drain= on the ID of the message (see
  browser.c/globals.c for the exact mechanism).
- BROWSER: Respond to the message in the "user-message-received" signal.
- C (=message_reply_and_save_callback=): Put the result into global data storage (=DATA=).
- C (implicit call in =browser.drain=): Check the global data storage
  for the result being returned (see =get_result= in globals.c).
- C: If there's a result, fetch it from =DATA= (still inside the =browser.drain= inside the Promise initializer) and set it as the Promise result.
  - If the result starts with =ERROR_MESSAGE_PREFIX=, return the Error object with the description of the error.

Now to the code. The C callback is something like
#+begin_src C
  static unsigned long int
  foo_bar_callback ()
  {
          WebKitUserMessage *message = webkit_user_message_new("foo.bar", NULL);
          SEND_MESSAGE_RETURN_PROMISE(message, jsc_context_get_current(), i);
          /* OR, macro-expanded version of the above */
          unsigned long int i = get_next_data_counter(); /* globals.c */
          webkit_web_page_send_message_to_view(
                  PAGE, Message, NULL, message_reply_and_save_callback,
                  (void*) i);
          return make_promise(Context, i); /* globals.c */
  }
#+end_src
And the way to bind it in =inject_foo_api= would be
#+begin_src C
  MAKE_FN(context, "foo", "bar", runtime_get_platform_info_callback, NULL, JSC_TYPE_VALUE, 0, G_TYPE_NONE);
#+end_src

With the possible expansion of the macro left as an exercise for you
if you need to break the abstraction there.

Almost any WebExtension JS function return a Promise, thus this is the
template for most of them.
*** Need a browser-mediated communication (insane)
These cases are rare and most complex. Examples are
=browser.runtime.sendMessage()= and =browser.tabs.sendMessage()=. The
solutions are usually function specific, but let's walk through the
=browser.runtime.sendMessage()= just in case it can come in handy.

First, a callback.
#+begin_src C

  static JSCValue *
  runtime_send_message_callback (GPtrArray *args, void* extension_name)
  {
          /* Get context to evaluate things in. */
          JSCContext *context = jsc_context_get_current();
          JSCValue *extension_id;
          /* Create a wrapper object to embed the data into. */
          JSCValue *payload;
          /* Sanitize args per specification. */
          if (args->len == 1) {
                  payload = args->pdata[0];
                  extension_id = jsc_value_new_string(
                          context, get_extension_id((char *) extension_name));
          } else if (args->len == 2 &&
                     jsc_value_object_has_property(
                             args->pdata[1], "includeTlsChannelId") &&
                     jsc_value_is_undefined(
                             jsc_value_object_get_property_at_index(args->pdata[1], 1))) {
                  payload = args->pdata[0];
                  extension_id = jsc_value_new_string(
                          context, get_extension_id((char *) extension_name));
          } else if (args->len == 2 || args->len == 3) {
                  extension_id = args->pdata[0];
                  payload = args->pdata[1];
          }
          /* Create a wrapper object to embed the data into. */
          JSCValue *wrapper = jsc_value_new_object(context, NULL, NULL);
          jsc_value_object_set_property(wrapper, "extensionId", extension_id);
          jsc_value_object_set_property(wrapper, "message", payload);
          /* Translate this object to JSON for easier serialization. */
          char *json = jsc_value_to_json(wrapper, 0);
          /* Wrap the JSON into GVariant, as messages require GVariants as contents. */
          GVariant *variant = g_variant_new("ms", json);
          /* Create message with the meaningful name and contents. */
          WebKitUserMessage *message = webkit_user_message_new("runtime.sendMessage", variant);
          /* Send the message and return the Promise. */
          SEND_MESSAGE_RETURN_PROMISE(message, context, id);
  }
#+end_src

That's mostly familiar, with maybe JSCValue juggling that's a bit more
complicated than in trivial functions.

What should happen at the browser side is a call to the
=browser.runtime.onMessage= of another part of the extension. So this is
yet another piece of asynchronous communication, and it's the one that
the browser handles. Because of the possible lag between browser and
extension, we need the browser to send another message to certify that
there's a response. That's why there's the "message" clause in
=user_message_received= (nyxt.c):
#+begin_src C
  if (!strcmp("message", name) && contents){
          /* Temporarily parse data. */
          JSCValue *meta = jsc_value_new_from_json(jsc_context_new(), contents);
          /* Get the extension context based on the received data. */
          char *extension_name = jsc_value_to_string(jsc_value_object_get_property(meta, "extensionName"));
          JSCContext *context = get_extension_context(extension_name);
          /* Re-create object in the proper context. */
          meta = jsc_value_new_from_json(context, contents);
          /* Get properties of the object */
          JSCValue *sender = jsc_value_object_get_property(meta, "sender");
          JSCValue *object = jsc_value_object_get_property(meta, "message");
          unsigned long int id = get_next_data_counter();
          /* Call a closure with browser.runtime.onMessage of the
           ,* extension and respond with browser.replyMessage once the
           ,* Promise is resolved. */
          JSCValue *tmp = jsc_value_function_call(
                  jsc_context_evaluate(
                          context, "var run  = (object, sender, id) => {\
  var p = browser.runtime.onMessage.run(object, sender);                  \
  if (p && p !== undefined)                                               \
          p.then((result) => browser.replyMessage(id, result));    \
  return p;                                                               \
  };                                                                      \
                                                                          \
  run", -1),
                  JSC_TYPE_VALUE, object, JSC_TYPE_VALUE, sender,
                  G_TYPE_ULONG, id,
                  G_TYPE_NONE);
          /* Check whether it's false/undefined as the evaluation result. */
          if (tmp &&
              JSC_IS_VALUE(tmp) &&
              !(jsc_value_is_boolean(tmp) && !jsc_value_to_boolean(tmp)) &&
              !(jsc_value_is_undefined(tmp))) {
                  /* If it's not false/undefined and thus there's a
                   ,* response, set it. */
                  g_object_ref(message);
                  unsigned long int *key = g_malloc(sizeof(unsigned long int));
                  ,*key = id;
                  g_hash_table_insert(DATA, (void *) key, message);
          }
          else {
                  /* If there's no result, reply with NULL. */
                  webkit_user_message_send_reply(
                          message, webkit_user_message_new(name, NULL));
          }
  }
#+end_src
yet another layer of callbacks here, but we're close to unwrapping
it. Here's what =browser.replyMessage= does, as its C callback:
#+begin_src C
  static void
  browser_reply_message_callback (unsigned long int message_id, JSCValue *result)
  {
          /* Wrap the result into GVariant. */
          GVariant *reply_contents = g_variant_new_string(
                  jsc_value_to_json(result, 0));
          /* Create a message reply. */
          WebKitUserMessage *reply = webkit_user_message_new("message", reply_contents);
          /* Send the reply to the initial message. */
          webkit_user_message_send_reply(g_hash_table_lookup(DATA, &message_id), reply);
  }
#+end_src

So, to drive the browser-extension communication, there seems to be no
better way than to do nested user messages and JS Promises. Altering
the list from the previous heading:
- JS: Call a method.
- C: Enter the callback and send the message to the browser there.
  - The message should be indexed by the unique ID of the request so
    that there are no race conditions.
- C: construct the Promise waiting for the browser response (via =make_promise=).
- C/JS: Run =browser.drain= on the ID of the message (see
  browser.c/globals.c for the exact mechanism).
- BROWSER: Send the message to another extension instance and wait for the result.
- BROWSER: Send a separate message to the extension.
- C (=user_message_received=): Process the message received from the
  browser and set the global data to the result of the initial
  message.
- C (implicit call in =browser.drain=): Check the global data storage
  for the result being returned (see =get_result= in globals.c).
- C: If there's a result, fetch it from =DATA= (still inside the =browser.drain= inside the Promise initializer) and set it as the Promise result.
  - If the result starts with =ERROR_MESSAGE_PREFIX=, return the Error object with the description of the error.
** Add a property
Usually it's as simple as
#+begin_src C
  jsc_value_object_set_property(
          JSCEVAL(context, "foo"), "bar",
          jsc_value_new_number(context, (double) 42));
#+end_src

If it's something more complex, I'd try tinkering with [[https://webkitgtk.org/reference/jsc-glib/stable/JSCClass.html#JSCClassVTable][JSCClassVTable]].
It looks scary, though.
** Add an event
WebExtensions events are special -- they have an unusual API with a
list of listeners that can be added/deleted at will and that are ran
in sequence with the result returned as the result of the event.

For these, there's an extevent.c sub-library. You usually don't need
to even know what's there, except that every event has a =run()= method
to run all the listeners with arbitrary arguments and that
=MAKE_EVENT()= C macro creates a fully usable event.

So, the general way to create event would be
#+begin_src C
    MAKE_EVENT(context, "runtime", "onMessage");
    /* OR */
  jsc_value_object_set_property(
                  JSCEVAL(context, "runtime"), "onMessage",
                  jsc_value_constructor_call(
                          /* "Event" is already taken by JS-native class. */
                          jsc_context_get_value(context, "ExtEvent"),
                          JSC_TYPE_VALUE, jsc_value_new_null(context),
                          G_TYPE_NONE));
#+end_src

And if you need to invoke it with all the listeners the extension could've added to it, simply do =browser.runtime.onMessage.run(some, args, there)=.

* APIs and messages
Every message matching the JS API call passes certain parameters
(always a string GVariant, mostly containing JSON) that you can use to
process the call. Those are usually function arguments augmented by
some meta-information. Here are all the functions implemented so far
with the example parameters passed with the messages:
** tabs.query
The only parameter is the JSON-stringified =query-object= [[https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/query][passed to the function]].
** tabs.create
The only parameter is the JSON-stringified =create-properties= [[https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/create][passed to it]].
** tabs.getCurrent
Has no parameters (=NULL= instead of GVariant).
** tabs.get
Parameters are a integer ID of the tab passed as string.
** tabs.print
No parameters.
** tabs.sendMessage & runtime.sendMessage
Params are a JSON string object looking like
#+begin_src js
  {
      "extensionId" : string, // The ID of the extension sending the message
      "tabId" : number, // The ID of the tab receiving the message
      "message" : any // The actual message
  }
#+end_src
for tabs.sendMessage and
#+begin_src js
  {
      "extensionId" : string, // The ID of the extension sending the message
      "message" : any // The actual message
  }
#+end_src
for runtime.sendMessage.

The tabs.sendMessage/runtime.sendMessage is a complex piece of API
requiring a communication of several parts of the extension. So, what
you need to do is to:
- Get the message from one part of the extension.
- Parse it according to the schemas above.
- Find the matching recipient.
- Send a new WebKitUser message to this tab
  - Type should be "message".
  - Params should conform to the scheme that [[https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/onMessage#parameters][runtime.onMessage requires]]:
#+begin_src js
  {
      "sender" : object, // The description of the tab that sent the message
      "message" : any // The actual message
  }
#+end_src

Once this message is replied to by this library, you can reply to
original message with the reply contents (reply params will be a
JSON-serialized result of listeners of onMessage or undefined in case
nothing ran/there was an error).
** tabs.insertCSS
Params are:
#+begin_src js
  {
      "extensionId" : string, // The ID of the extension adding CSS
      "tabId" : number, // The ID of the tab to insert it to
      "css" : any // The actual CSS description
  }
#+end_src

FIXME: Right now this function doesn't wait for reply and simply
resolves after 10 milliseconds.
** tabs.removeCSS
Params are:
#+begin_src js
  {
      "extensionId" : string, // The ID of the extension removing CSS
      "tabId" : number, // The ID of the tab to remove it from
      "css" : any // The actual CSS description
  }
#+end_src

FIXME: Right now this function doesn't wait for reply and simply
resolves after 10 milliseconds.
** tabs.executeScript
Params are:
#+begin_src js
  {
      "extensionId" : string, // The ID of the extension executing a script
      "tabId" : number, // The ID of the tab to execute it in
      "script" : any // The actual script description
  }
#+end_src

FIXME: Right now this function doesn't wait for reply and simply
resolves after 10 milliseconds.
** management.getSelf
The parameters are an extension name as a string.
** runtime.getPlatformInfo
There are no parameters, but the response should be a valid [[https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/PlatformInfo][runtime.PlatformInfo]].
** runtime.getBrowserInfo
No params either, but response should pass valid [[https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/runtime/getBrowserInfo#return_value][runtime.BrowserInfo]].
** storage.<storageArea>.get, storage.<storageArea>.set, storage.<storageArea>.remove
Params are:
#+begin_src js
  {
      "extensionId" : string, // The ID of the extension requesting a storagae acces
      "keys" : string, array of string, or object // The keys to get values of
  }
#+end_src
** storage.<storageArea>.clear
The only parameter is the extension ID as a string.
