Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+# Remote Agent overall architecture
+
+This document will cover the Remote Agent architecture by following the sequence of steps needed to start the agent, connect a client and debug a target.
+
+## Remote Agent startup
+
+Everything starts with the `RemoteAgent` implementation, which handles command line
+arguments (--remote-debugging-port) to eventually
+start a server listening on the TCP port 9222 (or the one specified by the command line).
+The browser target websocket URL will be printed to stderr.
+To do that this component glue together three main high level components:
+
+* `server/HTTPD`
+  This is a copy of httpd.js, from the `/netwerk/` folder. This is a JS
+  implementation of an HTTP server.  This will be used to implement the various
+  HTTP endpoints of CDP.  There is a few static URL implemented by `JSONHandler`
+  and one dynamic URL per target.
+
+* `cdp/JSONHandler`
+  This implements the following three static HTTP endpoints:
+  * `/json/version`:
+      Returns information about the runtime as well as the url of the browser target websocket url.
+  * `/json/list`:
+      Returns a list of all debuggable targets with, for each, their dynamic websocket URL.
+      For now it only reports tabs, but will report workers and addons as soon as we
+      support them.  The main browser target is the only one target not listed here.
+  * `/json/protocol`:
+      Returns a big dictionary describing the supported protocol.
+      This is currently hard coded and returns the full CDP protocol schema, including APIs we don’t support.
+      We have a future intention to fix this and report only what Firefox implements.
+    You can connect to these websocket URL in order to debug things.
+
+  * `cdp/targets/TargetList`:
+    This component is responsible of maintaining the list of all debuggable targets.
+    For now it can be either:
+    * The main browser target
+      A special target which allows to inspect the browser, but not any particular tab.
+      This is implemented by `cdp/targets/MainProcessTarget` and is instantiated on startup.
+    * Tab targets
+      Each opened tab will have a related `cdp/targets/TabTarget` instantiated on their opening,
+      or on server startup for already opened ones.
+    Each target aims at focusing on one particular context. This context is typically running in one
+    particular environment. This can be a particular process or thread.
+    In the future, we will most likely support targets for workers and add-ons.
+    All targets inherit from `cdp/targets/Target`.
+
+## Connecting to Websocket endpoints
+
+Each target's websocket URL will be registered as a HTTP endpoint via `server/HTTPD:registerPathHandler` (This registration is done from `RemoteAgentParentProcess:#listen`).
+Once a HTTP request happens, `server/HTTPD` will call the `handle` method on the object passed to `registerPathHandler`.
+For static endpoints registered by `JSONHandler`, this will call `JSONHandler:handle` and return a JSON string as HTTP body.
+For target's endpoint, it is slightly more complicated as it requires a special handshake to morph the HTTP connection into a WebSocket one.
+The WebSocket is then going to be long lived and be used to inspect the target over time.
+When a request is made to a target URL, `cdp/targets/Target:handle` is called and:
+
+* delegate the complex HTTP to WebSocket handshake operation to `server/WebSocketHandshake:upgrade`
+    In return we retrieve a WebSocket object.
+
+* hand over this WebSocket to `server/WebSocketTransport` and get a transport
+  object in return.  The transport implements a basic JSON stream over WebSocket.
+  With that, you can send and receive JSON objects over a WebSocket connection.
+
+* hand over the transport to a freshly instantiated `Connection`.  The Connection has two goals:
+  * Interpret incoming CDP packets by reading the JSON object attribute (`id`, `method`, `params` and `sessionId`).  This is done in `Connection:onPacket`.
+  * Format outgoing CDP packets by writing the right JSON object for command response (`id`, `result` and `sessionId`) and events (`method`, `params` and `sessionId`).
+  * Redirect CDP packet from/to the right session.
+    A connection may have more than one session attached to it.
+
+  * Instantiate the default session.
+    The session is specific to each target kind and all of them inherit from `cdp/session/Session`.
+    For example, tabs targets uses `cdp/session/TabSession` and the main browser target uses `cdp/session/MainProcessSession`.
+    Which session class is used is defined by the Target subclass’ constructor, which pass a session class reference to `cdp/targets/Target:constructor`.
+    A session is mostly responsible of accommodating the eventual cross process/cross thread aspects of the target.
+    The code we are currently describing (`cdp/targets/Target:handle`) is running in the parent process.
+    The session class receive CDP commands from the connection and first try to execute the Domain commands in the parent process.
+    Then, if the target actually runs in some other context, the session tries to forward this command to this other context, which can be a thread or a process.
+    Typically, the `cdp/sessions/TabSession` forward the CDP command to the content process where the tab is running.
+    It also redirects back the command response as well as Domain events from that process back to the parent process in order to
+    forward them to the connection.
+    Sessions will be using the `DomainCache` class as a helper to manage a list of Domain implementations in a given context.
+
+## Debugging additional Targets
+
+From a given connection you can know about the other potential targets.
+You typically do that via `Target.setDiscoverTargets()`, which will emit `Target.targetCreated` events providing a target ID.
+You may create a new session for the new target by handing the ID to `Target.attachToTarget()`, which will return a session ID.
+"Target" here is a reference to the CDP Domain implemented in `cdp/domains/parent/Target.jsm`. That is different from `cdp/targets/Target`
+class which is an implementation detail of the Remote Agent.
+
+Then, there is two ways to communicate with the other targets:
+
+* Use `Target.sendMessageToTarget()` and `Target.receivedMessageFromTarget`
+  You will manually send commands via the `Target.sendMessageToTarget()` command and receive command's response as well as events via `Target.receivedMessageFromTarget`.
+  In both cases, a session ID attribute is passed in the command or event arguments in order to select which additional target you are communicating with.
+
+* Use `Target.attachToTarget({ flatten: true })` and include `sessionId` in CDP packets
+  This requires a special client, which will use the `sessionId` returned by `Target.attachToTarget()` in order to spawn a distinct client instance.
+  This client will reuse the same WebSocket connection, but every single CDP packet will contain an additional `sessionId` attribute.
+  This helps distinguish packets which relate to the original target as well as the multiple additional targets you may attach to.
+
+In both cases, `Target.attachToTarget()` is special as it will spawn `cdp/session/TabSession` for the tab you are attaching to.
+This is the code path creating non-default session. The default session is related to the target you originally connected to,
+so that you don't need any ID for this one. When you want to debug more than one target over a single connection
+you need additional sessions, which will have a unique ID.
+`Target.attachToTarget` will compute this ID and instantiate a new session bound to the given target.
+This additional session will be managed by the `Connection` class, which will then redirect CDP packets to the
+right session when you are using flatten session.
+
+## Cross Process / Layers
+
+Because targets may runs in different contexts, the Remote Agent code runs in different processes.
+The main and startup code of the Remote Agent code runs in the parent process.
+The handling of the command line as well as all the HTTP and WebSocket work is all done in the parent process.
+The browser target is also all implemented in the parent process.
+But when it comes to a tab target, as the tab runs in the content process, we have to run code there as well.
+Let's start from the `cdp/sessions/TabSession` class, which has already been described.
+We receive here JSON packets from the WebSocket connection and we are in the parent process.
+In this class, we route the messages to the parent process domains first.
+If there is no implementation of the domain or the particular method,
+we forward the command to a `cdp/session/ContentProcessSession` which runs in the tab's content process.
+These two Session classes will interact with each other in order to forward back the returned value
+of the method we just called, as well as piping back any event being sent by a Domain implemented in any
+of the two processes.
+
+## Organizational chart of all the classes
+
+```text
+            ┌─────────────────────────────────────────────────┐
+            │                                                 │
+          1 ▼                                                 │
+    ┌───────────────┐     1 ┌───────────────┐     1..n┌───────────────┐
+    │  RemoteAgent  │──────▶│  HttpServer   │◀───────▶│  JsonHandler  │
+    └───────────────┘       └───────────────┘ 1       └───────────────┘
+            │
+            │
+            │              1 ┌────────────────┐ 1
+            └───────────────▶│  TargetList    │◀─┐
+                             └────────────────┘  │
+                                    │            │
+                                    ▼ 1..n       │
+                             ┌────────────┐      │
+           ┌─────────────────│ Target  [1]│      │
+           │                 └────────────┘      │
+           │                        ▲ 1          │
+           ▼ 1..n                   │            │
+    ┌────────────┐       1..n┌────────────┐      │
+    │ Connection │◀─────────▶│ Session [2]│──────┘
+    └────────────┘ 1         └────────────┘
+           │                      1 ▲
+           │                        │
+           ▼ 1                      ▼ 1
+┌────────────────────┐       ┌──────────────┐         1..n┌────────────┐
+│ WebSocketTransport │       │  DomainCache | │──────────▶│ Domain  [3]│
+└────────────────────┘       └──────────────┘             └────────────┘
+```
+
+[1] Target is inherited by TabTarget and MainProcessTarget.
+[2] Session is inherited by TabSession and MainProcessSession.
+[3] Domain is inherited by Log, Page, Browser, Target.... i.e. all domain implementations. From both cdp/domains/parent and cdp/domains/content folders.