Add rudimentary plugin guide

Relates to #1103

docs/guides/trinity/architecture.rst

@@ -109,8 +109,8 @@ that all database reads and writes are done by a single process.
 Networking Process
 ------------------
 
-The networking process is what kicks of the peer to peer communication, starts the syncing
-process and also serves the JSON-RPC API. It does so by running an instance of
+The networking process is what kicks of the peer to peer communication and starts the syncing
+process. It does so by running an instance of
 :func:`~trinity.nodes.base.Node` in an event loop.
 
 Notice that the instance of :func:`~trinity.nodes.base.Node` has access to the APIs that the
@@ -119,3 +119,14 @@ connections to other peers, starts of the syncing process but will call APIs tha
 the database processes when it comes to actual importing of blocks or reading and writing of other
 things from the database.
 
+The networking process also host an instance of the
+:class:`~trinity.extensibility.plugin_manager.PluginManager` to run plugins that need to deeply
+integrate with the networking process (Further reading:
+:doc:`Writing Plugins</guides/trinity/writing_plugins>`).
+
+Plugin Processes
+----------------
+
+Apart from running these three core processes, there may be additional processes for plugins that
+run in isolated processes. Isolated plugins are explained in depth in the
+:doc:`Writing Plugins</guides/trinity/writing_plugins>` guide.

docs/guides/trinity/index.rst

@@ -9,4 +9,5 @@ This section aims to provide hands-on guides to demonstrate how to use Trinity.
    :caption: Guides
 
    quickstart
-   architecture
+   architecture
+   writing_plugins

docs/guides/trinity/writing_plugins.rst

@@ -0,0 +1,263 @@
+Writing Plugins
+===============
+
+Trinity aims to be a highly flexible Ethereum node to support lots of different use cases
+beyond just participating in the regular networking traffic.
+
+To support this goal, Trinity allows developers to create plugins that hook into the system to
+extend its functionality. In fact, Trinity dogfoods its Plugin API in the sense that several
+built-in features are written as plugins that just happen to be shipped among the rest of the core
+modules. For instance, the JSON-RPC API, the Transaction Pool as well as the ``trinity attach``
+command that provides an interactive REPL with `Web3` integration are all built as plugins.
+
+Trinity tries to follow the practice: If something can be written as a plugin, it should be written
+as a plugin.
+
+
+What can plugins do?
+~~~~~~~~~~~~~~~~~~~~
+
+Plugin support in Trinity is still very new and the API hasn't stabilized yet. That said, plugins
+are already pretty powerful and are only becoming more so as the APIs of the underlying services
+improve over time.
+
+Here's a list of functionality that is currently provided by plugins:
+
+- JSON-RPC API
+- Transaction Pool
+- EthStats Reporting
+- Interactive REPL with Web3 integration
+- Crash Recovery Command
+
+
+Understanding the different plugin categories
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are currently three different types of plugins that we'll all cover in this guide.
+
+- Plugins that overtake and redefine the entire ``trinity`` command
+- Plugins that spawn their own new isolated process
+- Plugins that run in the shared `networking` process
+
+
+
+Plugins that redefine the Trinity process
+-----------------------------------------
+
+This is the simplest category of plugins as it doesn't really *hook* into the Trinity process but 
+hijacks it entirely instead. We may be left wonderering: Why would one want to do that?
+
+The only reason to write such a plugin is to execute some code that we want to group under the
+``trinity`` command. A great example for such a plugin is the ``trinity attach`` command that gives
+us a REPL attached to a running Trinity instance. This plugin could have easily be written as a
+standalone program and associated with a command such as ``trinity-attach``. However, using a
+subcommand ``attach`` is the more idiomatic approach and this type of plugin gives us simple way
+to develop exactly that.
+
+We build this kind of plugin by subclassing from
+:class:`~trinity.extensibility.plugin.BaseMainProcessPlugin`. A detailed example will follow soon.
+
+
+Plugins that spawn their own new isolated process
+-------------------------------------------------
+
+Of course, if all what plugins could do is to hijack the `trinity` command, there wouldn't be
+much room to actually extend the *runtime functionality* of Trinity. If we want to create plugins
+that boot with and run alongside the main node activity, we need to write a different kind of
+plugin. These type of plugins can respond to events such as a peers connecting/disconnecting and
+can access information that is only available within the running application.
+
+The JSON-RPC API is a great example as it exposes information such as the current count
+of connected peers which is live information that can only be accessed by talking to other parts
+of the application at runtime.
+
+This is the default type of plugin we want to build if:
+
+- we want to execute logic **together** with the command that boots Trinity (as opposed
+  to executing it in a separate command)
+- we want to execute logic that integrates with parts of Trinity that can only be accessed at
+  runtime (as opposed to e.g. just reading things from the database)
+
+We build this kind of plugin subclassing from
+:class:`~trinity.extensibility.plugin.BaseIsolatedPlugin`.  A detailed example will follow soon.
+
+
+Plugins that run inside the networking process
+----------------------------------------------
+
+If the previous category sounded as if it could handle every possible use case, it's because it's
+actually meant to. In reality though, not all internal APIs yet work well across process
+boundaries. In practice, this means that sometimes we want to make sure that a plugin runs in the
+same process as the rest of the networking code.
+
+.. warning::
+  The need to run plugins in the networking process is declining as the internals of Trinity become
+  more and more multi-process friendly over time. While it isn't entirely clear yet, there's a fair
+  chance this type of plugin will become obsolete at some point and may eventually be removed.
+
+  We should only choose this type of plugin category if what we are trying to build cannot be built
+  with a :class:`~trinity.extensibility.plugin.BaseIsolatedPlugin`.
+
+We build this kind of plugin subclassing from
+:class:`~trinity.extensibility.plugin.BaseAsyncStopPlugin`.  A detailed example will follow soon.
+
+
+The plugin lifecycle
+~~~~~~~~~~~~~~~~~~~~
+
+Plugins can be in one of the following status at a time:
+
+- ``NOT_READY``
+- ``READY``
+- ``STARTED``
+- ``STOPPED``
+
+The current status of a plugin is also reflected in the
+:meth:`~trinity.extensibility.plugin.BasePlugin.status` property.
+
+.. note::
+
+  Strictly speaking, there's also a special state that only applies to the
+  :class:`~trinity.extensibility.plugin.BaseMainProcessPlugin` which comes into effect when such a
+  plugin hijacks the Trinity process entirely. That being said, in that case, the resulting process
+  is in fact something entirely different than Trinity and the whole plugin infrastruture doesn't
+  even continue to exist from the moment on where that plugin takes over the Trinity process. This
+  is why we do not list it as an actual state of the regular plugin lifecycle.
+
+Plugin state: ``NOT_READY``
+---------------------------
+
+Every plugin starts out being in the ``NOT_READY`` state. This state begins with the instantiation
+of the plugin and lasts until the :meth:`~trinity.extensibility.plugin.BasePlugin.on_ready` hook
+was called which happens as soon the core infrastructure of Trinity is ready.
+
+Plugin state: ``READY``
+-----------------------
+
+After Trinity has finished setting up the core infrastructure, every plugin has its
+:class:`~trinity.extensibility.plugin.PluginContext` set and
+:meth:`~trinity.extensibility.plugin.BasePlugin.on_ready` is called. At this point the plugin has
+access to important information such as the parsed arguments or the 
+:class:`~trinity.config.TrinityConfig`. It also has access to the central event bus via its
+:meth:`~trinity.extensibility.plugin.BasePlugin.event_bus` property which enables the plugin to
+communicate with other parts of the application including other plugins.
+
+Plugin state: ``STARTED``
+-------------------------
+
+A plugin is in the ``STARTED`` state after the
+:meth:`~trinity.extensibility.plugin.BasePlugin.start` method was called. Plugins call this method
+themselves whenever they want to start which may be based on some condition like Trinity being
+started with certain parameters or some event being propagated on the central event bus.
+
+.. note::
+  Calling :meth:`~trinity.extensibility.plugin.BasePlugin.start` while the plugin is in the
+  ``NOT_READY`` state or when it is already in ``STARTED`` will cause an exception to be raised.
+
+
+Plugin state: ``STOPPED``
+-------------------------
+
+A plugin is in the ``STOPPED`` state after the 
+:meth:`~trinity.extensibility.plugin.BasePlugin.stop` method was called and finished any tear down
+work.
+
+Defining plugins
+~~~~~~~~~~~~~~~~
+
+We define a plugin by deriving from either
+:class:`~trinity.extensibility.plugin.BaseMainProcessPlugin`,
+:class:`~trinity.extensibility.plugin.BaseIsolatedPlugin` or 
+:class:`~trinity.extensibility.plugin.BaseAsyncStopPlugin` depending on the kind of plugin that we
+intend to write. For now, we'll stick to :class:`~trinity.extensibility.plugin.BaseIsolatedPlugin`
+which is the most commonly used plugin category.
+
+Every plugin needs to overwrite ``name`` so voilà, here's our first plugin!
+
+.. literalinclude:: ../../../trinity/plugins/examples/peer_count_reporter/plugin.py
+   :language: python
+   :start-after: --START CLASS--
+   :end-before: def configure_parser
+
+Of course that doesn't do anything useful yet, bear with us.
+
+Configuring Command Line Arguments
+----------------------------------
+
+More often than not we want to have control over if or when a plugin should start. Adding
+command-line arguments that are specific to such a plugin, which we then check, validate, and act
+on, is a good way to deal with that. Implementing
+:meth:`~trinity.extensibility.plugin.BasePlugin.configure_parser` enables us to do exactly that.
+
+This method is called when Trinity starts and bootstraps the plugin system, in other words,
+**before** the start of any plugin. It is passed a :class:`~argparse.ArgumentParser` as well as a
+:class:`~argparse._SubParsersAction` which allows it to amend the configuration of Trinity's
+command line arguments in many different ways.
+
+For example, here we are adding a boolean flag ``--report-peer-count`` to Trinity.
+
+.. literalinclude:: ../../../trinity/plugins/examples/peer_count_reporter/plugin.py
+   :language: python
+   :pyobject: PeerCountReporterPlugin.configure_parser
+
+To be clear, this does not yet cause our plugin to automatically start if ``--report-peer-count``
+is passed, it simply changes the parser to be aware of such flag and hence allows us to check for
+its existence later.
+
+.. note::
+
+  For a more advanced example, that also configures a subcommand, checkout the ``trinity attach``
+  plugin.
+
+
+Defining a plugins starting point
+---------------------------------
+
+Every plugin needs to have a well defined starting point. The exact mechanics slightly differ
+in case of a :class:`~trinity.extensibility.plugin.BaseMainProcessPlugin` but remain fairly similar
+for the other types of plugins which we'll be focussing on for now.
+
+Plugins need to implement the :meth:`~trinity.extensibility.plugin.BasePlugin.do_start` method
+to define their own bootstrapping logic. This logic may involve setting up event listeners, running
+code in a loop or any other kind of action.
+
+.. warning::
+
+  Technically, there's nothing preventing a plugin from performing starting logic in the
+  :meth:`~trinity.extensibility.plugin.BasePlugin.on_ready` hook. However, doing that is an anti
+  pattern as the plugin infrastructure won't know about the running plugin, can't propagate the
+  :class:`~trinity.extensibility.events.PluginStartedEvent` and the plugin won't be properly shut
+  down with Trinity if the node closes.
+
+Causing a plugin to start
+-------------------------
+
+As we've read in the previous section not all plugins should run at any point in time. In fact, the
+circumstances under which we want a plugin to begin its work may vary from plugin to plugin.
+
+We may want a plugin to only start running if:
+
+- a certain (combination) of command line arguments was given
+- another plugin or group of plugins started
+- a certain number of connected peers was exceeded / undershot
+- a certain block number was reached
+- ...
+
+Hence, to actually start a plugin, the plugin needs to invoke the
+:meth:`~trinity.extensibility.plugin.BasePlugin.start` method at any moment when it is in its
+``READY`` state.
+
+Communication pattern
+~~~~~~~~~~~~~~~~~~~~~
+
+Coming soon: Spoiler: Plugins can communicate with other parts of the application or even other
+plugins via the central event bus.
+
+Making plugins discoverable
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Coming soon.
+
+.. warning::
+  **Wait?! This is it? No! This is draft version of the plugin guide as small DEVCON IV gitft.
+  This will turn into a much more detailed guide shortly after the devcon craze is over.**

tests/trinity/plugins/test_examples.py

@@ -0,0 +1,8 @@
+from trinity.plugins.examples import (
+    PeerCountReporterPlugin,
+)
+
+
+def test_can_instantiate_examples():
+    plugin = PeerCountReporterPlugin()
+    assert plugin.name == "Peer Count Reporter"

trinity/plugins/examples/__init__.py

@@ -0,0 +1 @@
+from .peer_count_reporter.plugin import PeerCountReporterPlugin  # noqa: F401

trinity/plugins/examples/peer_count_reporter/plugin.py

@@ -0,0 +1,20 @@
+# This might end up as a temporary place for this. The following code is
+# included in the documentation (literalinclude!) and uses a more concise
+# form of imports.
+
+from argparse import ArgumentParser, _SubParsersAction
+
+from trinity.extensibility import BaseIsolatedPlugin
+
+
+# --START CLASS--
+class PeerCountReporterPlugin(BaseIsolatedPlugin):
+
+    @property
+    def name(self) -> str:
+        return "Peer Count Reporter"
+
+    def configure_parser(self,
+                         arg_parser: ArgumentParser,
+                         subparser: _SubParsersAction) -> None:
+        arg_parser.add_argument("--report-peer-count", type=bool, required=False)