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Diffstat (limited to 'devtools/docs/backend')
| -rw-r--r-- | devtools/docs/backend/actor-best-practices.md | 38 | ||||
| -rw-r--r-- | devtools/docs/backend/actor-e10s-handling.md | 108 | ||||
| -rw-r--r-- | devtools/docs/backend/actor-hierarchy.md | 183 | ||||
| -rw-r--r-- | devtools/docs/backend/actor-registration.md | 39 | ||||
| -rw-r--r-- | devtools/docs/backend/backward-compatibility.md | 90 | ||||
| -rw-r--r-- | devtools/docs/backend/client-api.md | 252 | ||||
| -rw-r--r-- | devtools/docs/backend/debugger-api.md | 5 | ||||
| -rw-r--r-- | devtools/docs/backend/protocol.js.md | 577 | ||||
| -rw-r--r-- | devtools/docs/backend/protocol.md | 1609 |
9 files changed, 2901 insertions, 0 deletions
diff --git a/devtools/docs/backend/actor-best-practices.md b/devtools/docs/backend/actor-best-practices.md new file mode 100644 index 0000000000..3e29f51f60 --- /dev/null +++ b/devtools/docs/backend/actor-best-practices.md @@ -0,0 +1,38 @@ +# Actor Best Practices + +Some aspects of front and actor design can be tricky to understand, even for experienced engineers. +The following are several best practices you should keep in mind when adding new actors and fronts. + +## Actor Should Clean Up Itself, Don't Wait For the Client + +In the past, some actors would wait for the client to send a "you are done now" message when the toolbox closes to shutdown the actor. +This seems reasonable at first, but keep in mind that the connection can disappear at any moment. +It may not be possible for the client to send this message. + +A better choice is for the actor to do all clean up itself when it's notified that the connection goes away. +Then there's no need for the client to send any clean up message, and we know the actor will be in a good state no matter what. + +## Actor Destruction + +Ensure that the actor's destroy is really destroying everything that it should. Here's an example from the animation actor: + +```js +destroy: function() { + Actor.prototype.destroy.call(this); + this.targetActor.off("will-navigate", this.onWillNavigate); + this.targetActor.off("navigate", this.onNavigate); + + this.stopAnimationPlayerUpdates(); + this.targetActor = this.observer = this.actors = null; +}, +``` + +## Child Actors + +With protocol.js actors, if your creates child actors for further functionality, in most cases you should call: + +```js +this.manage(child); +``` + +in the parent after constructing the child, so that the child is destroyed when the parent is. diff --git a/devtools/docs/backend/actor-e10s-handling.md b/devtools/docs/backend/actor-e10s-handling.md new file mode 100644 index 0000000000..af532b0ac5 --- /dev/null +++ b/devtools/docs/backend/actor-e10s-handling.md @@ -0,0 +1,108 @@ +# How to handle E10S in actors + +In multi-process environments, most devtools actors are created and initialized in the child content process, to be able to access the resources they are exposing to the toolbox. But sometimes, these actors need to access things in the parent process too. Here's why and how. + +{% hint style="danger" %} + +This documentation page is **deprecated**. `setupInParent` relies on the message manager which is being deprecated. Furthermore, communications between parent and content processes should be avoided for security reasons. If possible, the client should be responsible for calling actors both on the parent and content process. + +This page will be removed when all actors relying on this API are removed. + +{% endhint %} + +## Use case and examples + +Some actors need to exchange messages between the parent and the child process (typically when some components aren't available in the child process). + +To that end, there's a parent/child setup mechanism at `DevToolsServer` level that can be used. + +When the actor is loaded for the first time in the `DevToolsServer` running in the child process, it may decide to run a setup procedure to load a module in the parent process with which to communicate. + +Example code for the actor running in the child process: + +``` + const {DevToolsServer} = require("devtools/server/devtools-server"); + + // Setup the child<->parent communication only if the actor module + // is running in a child process. + if (DevToolsServer.isInChildProcess) { + setupChildProcess(); + } + + function setupChildProcess() { + // `setupInParent` is defined on DevToolsServerConnection, + // your actor receives a reference to one instance in its constructor. + conn.setupInParent({ + module: "devtools/server/actors/module-name", + setupParent: "setupParentProcess" + }); + // ... + } +``` + +The `setupChildProcess` helper defined and used in the previous example uses the `DevToolsServerConnection.setupInParent` to run a given setup function in the parent process DevTools Server. + +With this, the `DevToolsServer` running in the parent process will require the requested module and call its `setupParentProcess` function (which should be exported on the module). + +The `setupParentProcess` function will receive a parameter that contains a reference to the **MessageManager** and a prefix that should be used to send/receive messages between the child and parent processes. + +See below an example implementation of a `setupParent` function in the parent process: + +``` +exports.setupParentProcess = function setupParentProcess({ mm, prefix }) { + // Start listening for messages from the actor in the child process. + setMessageManager(mm); + + function handleChildRequest(msg) { + switch (msg.json.method) { + case "get": + return doGetInParentProcess(msg.json.args[0]); + break; + case "list": + return doListInParentProcess(); + break; + default: + console.error("Unknown method name", msg.json.method); + throw new Error("Unknown method name"); + } + } + + function setMessageManager(newMM) { + if (mm) { + // Remove listener from old message manager + mm.removeMessageListener("debug:some-message-name", handleChildRequest); + } + // Switch to the new message manager for future use + // Note: Make sure that any other functions also use the new reference. + mm = newMM; + if (mm) { + // Add listener to new message manager + mm.addMessageListener("debug:some-message-name", handleChildRequest); + } + } + + return { + onDisconnected: () => setMessageManager(null), + }; +}; +``` + +The server will call the `onDisconnected` method returned by the parent process setup flow to give the actor modules the chance to cleanup their handlers registered on the disconnected message manager. + +## Summary of the setup flow + +In the child process: + +* The `DevToolsServer` loads an actor module, +* the actor module checks `DevToolsServer.isInChildProcess` to know whether it runs in a child process or not, +* the actor module then uses the `DevToolsServerConnection.setupInParent` helper to start setting up a parent-process counterpart, +* the `DevToolsServerConnection.setupInParent` helper asks the parent process to run the required module's setup function, +* the actor module uses the `DevToolsServerConnection.parentMessageManager.sendSyncMessage` and `DevToolsServerConnection.parentMessageManager.addMessageListener` helpers to send or listen to message. + +In the parent process: + +* The DevToolsServer receives the `DevToolsServerConnection.setupInParent` request, +* tries to load the required module, +* tries to call the `module[setupParent]` function with the frame message manager and the prefix as parameters `{ mm, prefix }`, +* the `setupParent` function then uses the mm to subscribe the message manager events, +* the `setupParent` function returns an object with a `onDisconnected` method which the server can use to notify the module of various lifecycle events diff --git a/devtools/docs/backend/actor-hierarchy.md b/devtools/docs/backend/actor-hierarchy.md new file mode 100644 index 0000000000..123f86a8fe --- /dev/null +++ b/devtools/docs/backend/actor-hierarchy.md @@ -0,0 +1,183 @@ +# How actors are organized + +To start with, actors are living within devtools/server/actors folder. +They are organized in a hierarchy for easier lifecycle and memory management: +once a parent is removed from the pool, its children are removed as well. +(See actor-registration.md for more information about how to implement one) + +The overall hierarchy of actors looks like this: + +``` +RootActor: First one, automatically instantiated when we start connecting. + | Mostly meant to instantiate new actors. + | + |-- Global-scoped actors: + | Actors exposing features related to the main process, that are not + | specific to any particular target (document, tab, add-on, or worker). + | These actors are registered with `global: true` in + | devtools/server/main.js. + | Examples include: + | PreferenceActor (for Firefox prefs) + | + \-- Target actors: + Actors that represent the main "thing" being targeted by a given toolbox, + such as a tab, frame, worker, add-on, etc. and track its lifetime. + Generally, there is a target actor for each thing you can point a + toolbox at. + Examples include: + FrameTargetActor (for a frame, such as a tab) + WorkerTargetActor (for various kind of workers) + | + \-- Target-scoped actors: + Actors exposing one particular feature set. They are children of a + given target actor and the data they return is filtered to reflect + the target. + These actors are registered with `target: true` in + devtools/server/main.js. + Examples include: + WebConsoleActor + InspectorActor + These actors may extend this hierarchy by having their own children, + like LongStringActor, WalkerActor, etc. +``` + +## RootActor + +The root actor is special. It is automatically created when a client connects. +It has a special `actorID` which is unique and is "root". +All other actors have an `actorID` which is computed dynamically, +so that you need to ask an existing actor to create an Actor +and returns its `actorID`. That's the main role of RootActor. + +``` +RootActor (root.js) + | + |-- TabDescriptorActor (descriptors/tab.js) + | Targets frames (such as a tab) living in the parent or child process. + | Note that this is just a proxy for FrameTargetActor, which is loaded via + | the frame's message manager as a frame script in the process containing + | the frame content. This proxy via message manager is always used, even + | when the content happens to be in the same process. + | Returned by "listTabs" or "getTab" requests. + | | + | \-- FrameTargetActor (frame.js) + | The "real" target actor for a frame (such as a tab) which runs in + | whichever process holds the content. FrameTargetActorProxy + | communicates with this via the frame's message manager. + | Extends the abstract class BrowsingContextTargetActor. + | Returned by "connect" server method on FrameTargetActorProxy. + | + |-- WorkerTargetActor (worker.js) + | Targets a worker (applies to various kinds like web worker, service + | worker, etc.). + | Returned by "listWorkers" request to the root actor to get all workers. + | Returned by "listWorkers" request to a FrameTargetActorProxy to get + | workers for a specific frame. + | Returned by "listWorkers" request to a ContentProcessTargetActor to get + | workers for the chrome of the child process. + | + |-- ChromeWindowTargetActor (chrome-window.js) + | Targets a single window, such as a browser window in Firefox, but it can + | be used to reach any window in the parent process. + | Extends the abstract class BrowsingContextTargetActor. + | Returned by "getWindow" request to the root actor. + | + |-- ParentProcessTargetActor (parent-process.js) + | Targets all resources in the parent process of Firefox (chrome documents, + | JSMs, JS XPCOM, etc.). + | Extends the abstract class BrowsingContextTargetActor. + | Extended by WebExtensionTargetActor. + | Returned by "getProcess" request without any argument. + | + |-- ContentProcessTargetActor (content-process.js) + | Targets all resources in a content process of Firefox (chrome sandboxes, + | frame scripts, documents, etc.) + | Returned by "getProcess" request with a id argument, matching the + | targeted process. + | + \-- WebExtensionActor (addon/webextension.js) + Represents a WebExtension add-on in the parent process. This gives some + metadata about the add-on and watches for uninstall events. This uses a + proxy to access the actual WebExtension in the WebExtension process via + the message manager. + Returned by "listAddons" request. + | + \-- WebExtensionTargetActor (targets/webextension.js) + Targets a WebExtension add-on. This runs in the WebExtension process. + The client issues an additional "connect" request to + WebExtensionActor to get this actor, which is different from the + approach used for frame target actors. + Extends ParentProcessTargetActor. + Returned by "connect" request to WebExtensionActor. +``` + +## Target Actors + +Those are the actors exposed by the root actors which are meant to track the +lifetime of a given target: tab, process, add-on, or worker. It also allows to +fetch the target-scoped actors connected to this target, which are actors like +console, inspector, thread (for debugger), style inspector, etc. + +Some target actors inherit from BrowsingContextTargetActor (defined in +browsing-context.js) which is meant for "browsing contexts" which present +documents to the user. It automatically tracks the lifetime of the targeted +browsing context, but it also tracks its iframes and allows switching the +target to one of its iframes. + +For historical reasons, target actors also handle creating the ThreadActor, used +to manage breakpoints in the debugger. Actors inheriting from +BrowsingContextTargetActor expose `attach`/`detach` requests, that allows to +start/stop the ThreadActor. + +Target-scoped actors are accessed via the target actor's RDP form which contains +the `actorID` for each target-scoped actor. + +The target-scoped actors expect to find the following properties on the target +actor: + - threadActor: + ThreadActor instance for the given target, + only defined once `attach` request is called, or on construction. + - isRootActor: (historical name) + Always false, except on ParentProcessTargetActor. + Despite the attribute name, it is being used to accept all resources + (like chrome one) instead of limiting only to content resources. + - makeDebugger: + Helper function used to create Debugger object for the target. + (See actors/utils/make-debugger.js for more info) + +In addition to this, the actors inheriting from BrowsingContextTargetActor, +expose many other attributes and events: + - window: + Reference to the window global object currently targeted. + It can change over time if we switch target to an iframe, so it + shouldn't be stored in a variable, but always retrieved from the actor. + - windows: + List of all document globals including the main window object and all + iframes. + - docShell: + Primary docShell reference for the targeted document. + - docShells: + List of all docShells for the targeted document and all its iframes. + - chromeEventHandler: + The chrome event handler for the current target. Allows to listen to events + that can be missing/cancelled on this document itself. + +See BrowsingContextTargetActor documentation for more details. + +## Target-scoped actors + +Each of these actors focuses on providing one particular feature set. They are +children of a given target actor. + +The data they return is filtered to reflect the target. For example, the +InspectorActor that you fetch from a FrameTargetActor gives you information +about the markup and styles for only that frame. + +These actors may extend this hierarchy by having their own children, like +LongStringActor, WalkerActor, etc. + +To improve performance, target-scoped actors are created lazily. The target +actor lists the actor ID for each one, but the actor modules aren't actually +loaded and instantiated at that point. Once the first request for a given +target-scoped actor is received by the server, that specific actor is +instantiated just in time to service the request. diff --git a/devtools/docs/backend/actor-registration.md b/devtools/docs/backend/actor-registration.md new file mode 100644 index 0000000000..c43fef29a7 --- /dev/null +++ b/devtools/docs/backend/actor-registration.md @@ -0,0 +1,39 @@ +# How to register an actor + +## Target-scoped actors vs. global actors + +Target-scoped actors are the most common types of actors. That's the type of actors you will most probably be adding. + +Target-scoped actors target a document, this could be a tab in Firefox or a remote document in Firefox for Android. + +Global actors however are for the rest, for things not related to any particular document but instead for things global to the whole Firefox/Chrome/Safari instance the toolbox is connected to (e.g. the preference actor). + +## The ActorRegistry.registerModule function + +To register a target-scoped actor: + +``` +ActorRegistry.registerModule("devtools/server/actors/webconsole", { + prefix: "console", + constructor: "WebConsoleActor", + type: { target: true } +}); +``` + +To register a global actor: + +``` +ActorRegistry.registerModule("devtools/server/actors/preference", { + prefix: "preference", + constructor: "PreferenceActor", + type: { global: true } +}); +``` + +If you are adding a new built-in actor, you should be registering it using `ActorRegistry.registerModule` in `addBrowserActors` or `addTargetScopedActors` in `/devtools/server/actors/utils/actor-registry.js`. + +## A note about lazy registration + +The `ActorRegistry` loads and creates all of the actors lazily to keep the initial memory usage down (which is extremely important on lower end devices). + +It becomes especially important when debugging pages with e10s when there are more than one process, because that's when we need to spawn a `DevToolsServer` per process (it may not be immediately obvious that the server in the main process is mostly only here for piping messages to the actors in the child process). diff --git a/devtools/docs/backend/backward-compatibility.md b/devtools/docs/backend/backward-compatibility.md new file mode 100644 index 0000000000..8166d75dd8 --- /dev/null +++ b/devtools/docs/backend/backward-compatibility.md @@ -0,0 +1,90 @@ +# Backward Compatibility + +## Overview + +When making changes to the DevTools, there are certain backward compatibility requirements that we should keep in mind. + +In general, we should strive to maintain feature support for existing servers as we continue to make changes to the code base. At times, this can be difficult to achieve, however. + +## Specific Guidelines + +The important compatibility scenarios are: + +- Nightly desktop client **MUST** maintain existing compatibility back to release channel servers. + +This is mainly to simplify cross-platform use cases, i.e. desktop Nightly with release Fennec. + +- Servers **MAY** use traits to state a feature is not supported yet. + +This helps us support alternate environments, which does not implement every possible server feature. + +Certainly when a new feature needs a new actor method to function, it won't work with servers that don't support it. But we should still ensure the client doesn't explode when using unrelated, existing features, at least until the above time windows have elapsed. + +## Testing + +The harder part of this currently is that there is no automated testing to ensure the above guidelines have been met. While we hope to have this at some point, for now manual testing is needed here. + +The easiest way to test this is to check your work against a Firefox for Android device on release channel to ensure existing features in the area you are changing continue to function. That doesn't cover every case, but it's a great start. + +Alternatively, you can connect to a Firefox release server. This can be done in multiple steps: + +1. Start Firefox release from the command line, specifying the `--start-debugger-server` with an available port (e.g. `/Applications/Firefox.app/Contents/MacOS/firefox --start-debugger-server 6081`) +2. Navigate to a page where you can check that the part of DevTools which is impacted by the patch still works. +3. Build and run Firefox locally with the patch you want to check +4. In this build, open an `about:debugging` tab +5. On the `Network Location` section, fill in the host with localhost and the debugger server port you started the Firefox release instance with (e.g. `localhost:6081`) and hit Enter (or the `Add` button) +6. A new item will appear in the sidebar, click on its `Connect` button. +7. Accept the `Incoming connection` popup that appears +8. Click on the on sidebar item again. You will now see a list of the tabs and workers running in the Firefox release instance. Click on the `Inspect` button next to them to open a toolbox that is connected to the older server. + +## Feature Detection + +Starting with Firefox 36 (thanks to [bug 1069673](https://bugzilla.mozilla.org/show_bug.cgi?id=1069673)), you can use actor feature detection to determine which actors exist. + +### Target hasActor helper + +Detecting if the server has an actor: all you need is access to the `Toolbox` instance, which all panels do, when they get instantiated. Then you can do: + +```js +let hasPerformanceActor = toolbox.target.hasActor("performance"); +``` + +The `hasActor` method returns a boolean synchronously. + +### Traits + +Expose traits on an Actor in order to flag certain features as available or not. For instance if a new method "someMethod" is added to an Actor, expose a "supportsSomeMethod" flag in the traits object for the Actor, set to true. When debugging older servers, the flag will be missing and will default to false. + +Traits need to be forwarded to the client, and stored or used by the corresponding Front. There is no unique way of exposing traits, but there are still a few typical patterns found in the codebase. + +For Actors using a "form()" method, for which the Front is automatically created by protocol.js, the usual pattern is to add a "traits" property to the form, that contains all the traits for the actor. The Front can then read the traits in its corresponding "form()" method. Example: + +- [NodeActor form method](https://searchfox.org/mozilla-central/rev/e75e8e5b980ef18f4596a783fbc8a36621de7d1e/devtools/server/actors/inspector/node.js#209) +- [NodeFront form method](https://searchfox.org/mozilla-central/rev/e75e8e5b980ef18f4596a783fbc8a36621de7d1e/devtools/client/fronts/node.js#145) + +For other Actors, there are two options. First option is to define the trait on the Root actor. Those traits will be available both via TargetMixin::getTrait(), and on DevToolsClient.traits. The second option is to implement a "getTraits()" method on the Actor, which will return the traits for the Actor. Example: + +- [CompatibilityActor getTraits method](https://searchfox.org/mozilla-central/rev/e75e8e5b980ef18f4596a783fbc8a36621de7d1e/devtools/shared/specs/compatibility.js#40) +- [CompatibilitySpec getTraits definition](https://searchfox.org/mozilla-central/rev/e75e8e5b980ef18f4596a783fbc8a36621de7d1e/devtools/shared/specs/compatibility.js#40-43) +- [CompatibilityFront getTraits method](https://searchfox.org/mozilla-central/rev/e75e8e5b980ef18f4596a783fbc8a36621de7d1e/devtools/client/fronts/compatibility.js#41-47) + +Ironically, "getTraits" needs to be handled with backwards compatibility. But there is no way to check that "getTraits" is available on the server other than performing a try catch around the method. See the CompatibilityFront example. + +Whenever traits are added, make sure to add a relevant backward compatibility comment so that we know when the trait can be removed. + +## Maintaining backward compatibility code + +When introducing backward compatibility code, a comment should be added for extra information. +In order to simplify future code cleanups, the comment should follow the following syntax: +`// @backward-compat { version XX } Detailed comment`, where `XX` is the Firefox version this code was added in. + +Below is a made-up example of what it should look like: + +```js +// @backward-compat { version 85 } For older server which don't have the AwesomeActor, +// we have to do this another way. +if (!toolbox.target.hasActor("awesome")) { +``` + +Backward compatibility code can be safely removed when the revision it was added in reaches the release channel. +So if something landed in Firefox Nightly 85, it can be removed when Firefox 85 is released, i.e. when Firefox Nightly is 87. Search for the corresponding `@backward-compat` entries to retrieve all the code that can be removed. diff --git a/devtools/docs/backend/client-api.md b/devtools/docs/backend/client-api.md new file mode 100644 index 0000000000..818ab54bcd --- /dev/null +++ b/devtools/docs/backend/client-api.md @@ -0,0 +1,252 @@ +# Client API + +DevTools has a client module that allows applications to be written that debug or inspect web pages using the [Remote Debugging Protocol](protocol.md). + +## Starting communication + +In order to communicate, a client and a server instance must be created and a protocol connection must be established. The connection can be either over a TCP socket or an nsIPipe. The `start` function displayed below establishes an nsIPipe-backed connection: + +```javascript +const { DevToolsServer } = require("devtools/server/devtools-server"); +const { DevToolsClient } = require("devtools/client/devtools-client"); + +function start() { + // Start the server. + DevToolsServer.init(); + DevToolsServer.registerAllActors(); + + // Listen to an nsIPipe + let transport = DevToolsServer.connectPipe(); + + // Start the client. + client = new DevToolsClient(transport); + + client.connect((type, traits) => { + // Now the client is connected to the server. + debugTab(); + }); +} +``` + +If a TCP socket is required, the function should be split in two parts, a server-side and a client-side, like this: + +```javascript +const { DevToolsServer } = require("devtools/server/devtools-server"); +const { DevToolsClient } = require("devtools/client/devtools-client"); + +function startServer() { + // Start the server. + DevToolsServer.init(); + DevToolsServer.registerAllActors(); + + // For an nsIServerSocket we do this: + DevToolsServer.openListener(2929); // A connection on port 2929. +} + +async function startClient() { + let transport = await DevToolsClient.socketConnect({ host: "localhost", port: 2929 }); + + // Start the client. + client = new DevToolsClient(transport); + + client.connect((type, traits) => { + // Now the client is connected to the server. + debugTab(); + }); +} +``` + +## Shutting down + +When the application is finished, it has to notify the client to shut down the protocol connection. This makes sure that memory leaks are avoided and the server is terminated in an orderly fashion. Shutting down is as simple as it gets: + +```javascript +function shutdown() { + client.close(); +} +``` + +## Attaching to a browser tab + +Attaching to a browser tab requires enumerating the available tabs and attaching to one: + +```javascript +function attachToTab() { + // Get the list of tabs to find the one to attach to. + client.mainRoot.listTabs().then(tabs => { + // Find the active tab. + let targetFront = tabs.find(tab => tab.selected); + + // Attach to the tab. + targetFront.attach().then(() => { + // Now the targetFront is ready and can be used. + + // Attach listeners for client events. + targetFront.on("tabNavigated", onTab); + }); + }); +} +``` + +The devtools client will send event notifications for a number of events the application may be interested in. These events include state changes in the debugger, like pausing and resuming, stack frames or source scripts being ready for retrieval, etc. + +## Handling location changes + +When the user navigates away from a page, a `tabNavigated` event will be fired. The proper way to handle this event is to detach from the previous thread and tab and attach to the new ones: + +```javascript +async function onTab() { + // Detach from the previous tab. + await targetFront.detach(); + // Start debugging the new tab. + start(); +} +``` + +## Debugging JavaScript running in a browser tab + +Once the application is attached to a tab, it can attach to its thread in order to interact with the JavaScript debugger: + +```javascript +// Assuming the application is already attached to the tab, and response is the first +// argument of the attachTarget callback. + +client.attachThread(response.threadActor).then(function(threadFront) { + if (!threadFront) { + return; + } + + // Attach listeners for thread events. + threadFront.on("paused", onPause); + threadFront.on("resumed", fooListener); + + // Debugger is now ready and debuggee is running. +}); +``` + +## Debugger application example + +Here is the source code for a complete debugger application: + +```javascript +/* + * Debugger API demo. + */ +const { DevToolsServer } = require("devtools/server/devtools-server"); +const { DevToolsClient } = require("devtools/client/devtools-client"); + +let client; +let threadFront; + +function startDebugger() { + // Start the server. + DevToolsServer.init(); + DevToolsServer.registerAllActors(); + // Listen to an nsIPipe + let transport = DevToolsServer.connectPipe(); + // For an nsIServerSocket we do this: + // DevToolsServer.openListener(port); + // ...and this at the client: + // let transport = debuggerSocketConnect(host, port); + + // Start the client. + client = new DevToolsClient(transport); + client.connect((type, traits) => { + // Now the client is connected to the server. + debugTab(); + }); +} + +function shutdownDebugger() { + client.close(); +} + +/** + * Start debugging the current tab. + */ +async function debugTab() { + // Get the list of tabs to find the one to attach to. + const tabs = await client.mainRoot.listTabs(); + // Find the active tab. + let targetFront = tabs.find(tab => tab.selected); + // Attach to the tab. + await targetFront.attach(); + // Attach to the thread (context). + const threadFront = await targetFront.attachThread(); + // Attach listeners for thread events. + threadFront.on("paused", onPause); + threadFront.on("resumed", fooListener); + // Debugger is now ready and debuggee is running. +} + +/** + * Handler for location changes. + */ +function onTab() { + // Detach from the previous tab. + client.detach().then(() => { + // Start debugging the new tab. + debugTab(); + }); +} + +/** + * Helper function to inspect the provided frame. + */ +function inspectFrame(frame) { + // Get the "this" object. + if (frame["this"]) { + getObjectProperties(frame["this"]); + } + + // Add "arguments". + if (frame.arguments && frame.arguments.length > 0) { + // frame.arguments is a regular Array. + dump("frame.arguments: " + frame.arguments.toSource() + "\n"); + + // Add variables for every argument. + let objClient = client.activeThread.pauseGrip(frame.callee); + objClient.getSignature(response => { + for (let i = 0; i < response.parameters.length; i++) { + let name = response.parameters[i]; + let value = frame.arguments[i]; + + if (typeof value == "object" && value.type == "object") { + getObjectProperties(value); + } + } + }); + } +} + +/** + * Helper function that retrieves the specified object's properties. + */ +function getObjectProperties(object) { + let thisClient = client.activeThread.pauseGrip(object); + thisClient.getPrototypeAndProperties(response => { + // Get prototype as a protocol-specified grip. + if (response.prototype.type != "null") { + dump("__proto__: " + response.prototype.toSource() + "\n"); + } + + // Get the rest of the object's own properties as protocol-specified grips. + for (let prop of Object.keys(response.ownProperties)) { + dump(prop + ": " + response.ownProperties[prop].toSource() + "\n"); + } + }); +} + +/** + * Generic event listener. + */ +function fooListener(event) { + dump(event + "\n"); +} + +// Run the program. +startDebugger(); + +// Execute the following line to stop the program. +//shutdownDebugger(); +``` diff --git a/devtools/docs/backend/debugger-api.md b/devtools/docs/backend/debugger-api.md new file mode 100644 index 0000000000..344cf7eddc --- /dev/null +++ b/devtools/docs/backend/debugger-api.md @@ -0,0 +1,5 @@ +# Debugger API + +The Debugger API is a low-level API that provides methods for introspecting and affecting a target environment like a page. You can find JavaScript sources, set breakpoints on them, and more. + +This API is completely documented on the [Debugger API](https://developer.mozilla.org/en-US/docs/Tools/Debugger-API) MDN page. diff --git a/devtools/docs/backend/protocol.js.md b/devtools/docs/backend/protocol.js.md new file mode 100644 index 0000000000..568344209e --- /dev/null +++ b/devtools/docs/backend/protocol.js.md @@ -0,0 +1,577 @@ +Writing an Actor +================ + +A Simple Hello World +-------------------- + +Here's a simple Hello World actor. It is a global actor (not associated with a given browser tab). +It has two parts: a spec and an implementation. The spec would go somewhere like +`devtools/shared/specs/hello-world.js` and would look like: + + const {Arg, RetVal, generateActorSpec} = require("devtools/shared/protocol"); + + const helloWorldSpec = generateActorSpec({ + typeName: "helloWorld", // I'll explain types later, I promise. + + methods: { + sayHello: { + // The request packet template. There are no arguments, so + // it is empty. The framework will add the "type" and "to" + // request properties. + request: {}, + + // The response packet template. The return value of the function + // will be plugged in where the RetVal() appears in the template. + response: { + greeting: RetVal("string") // "string" is the return value type. + } + }, + }, + }); + + // Expose the spec so it can be imported by the implementation. + exports.helloWorldSpec = helloWorldSpec; + +The actor implementation would go somewhere like +`devtools/server/actors/hello-world.js` and would look like: + + const protocol = require("devtools/shared/protocol"); + const {helloWorldSpec} = require("devtools/shared/specs/hello-world"); + + const HelloActor = protocol.ActorClassWithSpec(helloWorldSpec, { + initialize: function (conn) { + protocol.Actor.prototype.initialize.call(this, conn); // This is the worst part of heritage. + }, + + sayHello: function () { + return "hello"; + }, + }); + + // You also need to export the actor class in your module for discovery. + exports.HelloActor = HelloActor; + +To activate your actor, register it in the `addBrowserActors` method in `server/actors/utils/actor-registry.js`. +The registration code would look something like this: + + this.registerModule("devtools/server/actors/hello-world", { + prefix: "hello", + constructor: "HelloActor", + type: { global: true } + }); + +Your spec allows the actor to support a `sayHello` request. +A request/reply will look like this: + + -> { to: <actorID>, type: "sayHello" } + <- { from: <actorID>, greeting: "hello" } + +Now we can create a client side object. We call these *front* objects and +they typically go in `devtools/client/fronts/`. + +Here's the front for the HelloActor: + + const HelloFront = protocol.FrontClassWithSpec(helloWorldSpec, { + initialize: function (client, form) { + protocol.Front.prototype.initialize.call(this, client, form); + // This call may not be required but it's a good idea. It will + // guarantee that your instance is managed in the pool. + this.manage(this); + } + }); + +Note that there is no `sayHello` method. The FrontClass will generate a method on the Front object that matches the method declaration in the Actor class. + +The generated methods will return a Promise. That promise will resolve to the RetVal of the actor method. + +So if we have a reference to a HelloFront object, we can issue a `sayHello` request: + + hello.sayHello().then(greeting => { + console.log(greeting); + }); + +How do you get an initial reference to the front? That's a bit tricky, but basically there are two ways: + +* Manually +* Magically + +Manually - If you're using a DevToolsClient instance, you can discover the actorID manually and create a Front for it: + + let hello = new HelloFront(this.client, { actor: <hello actorID> }); + +Magically - Once you have an initial reference to a protocol.js object, it can return other protocol.js objects and fronts will automatically be created. + +Arguments +--------- + +`sayHello` has no arguments, so let's add a method that does take arguments. +Here's an adjustment to the spec: + + methods: { + echo: { + request: { echo: Arg(0, "string") }, + response: { echoed: RetVal("string") } + } + } + +Here's an adjustment to the implementation: + + echo: function (str) { + return str + "... " + str + "..."; + } + +This tells the library to place the 0th argument, which should be a string, in the `echo` property of the request packet. + + +This will generate a request handler whose request and response packets look like this: + + { to: <actorID>, type: "echo", echo: <str> } + { from: <actorID>, echoed: <str> } + +The client usage should be predictable: + + hello.echo("hello").then(str => { assert(str === "hello... hello...") }) + +The library tries hard to make using fronts feel like natural javascript (or as natural as you believe promises are, I guess). When building the response it will put the return value of the function where RetVal() is specified in the response template, and on the client side it will use the value in that position when resolving the promise. + +Returning JSON +-------------- + +Maybe your response is an object. Here's an example of a spec: + + methods: { + addOneTwice: { + request: { a: Arg(0, "number"), b: Arg(1, "number") }, + response: { ret: RetVal("json") } + } + } + +Here's an example implementation: + + addOneTwice: function (a, b) { + return { a: a + 1, b: b + 1 }; + } + +This will generate a response packet that looks like: + + { from: <actorID>, ret: { a: <number>, b: <number> } } + +That's probably unnecessary nesting (if you're sure you won't be returning an object with 'from' as a key!), so you can just replace `response` with: + + response: RetVal("json") + +and now your packet will look like: + + { from: <actorID>, a: <number>, b: <number> } + +Types and Marshalling +--------------------- + +Things have been pretty simple up to this point - all the arguments we've passed in have been javascript primitives. But for some types (most importantly Actor types, which I'll get to eventually), we can't just copy them into a JSON packet and expect it to work, we need to marshal things ourselves. + +Again, the protocol lib tries hard to provide a natural API to actors and clients, and sometime that natural API might involve object APIs. I'm going to use a wickedly contrived example, bear with me. Let's say I have a small object that contains a number and has a few methods associated with it: + + let Incrementor = function (i) { + this.value = value; + } + Incrementor.prototype = { + increment: function () { this.value++ }, + decrement: function () { this.value-- } + }; + + +and I want to return it from a backend function: + + // spec: + methods: { + getIncrementor: { + request: { number: Arg(0, "number") }, + response: { value: RetVal("incrementor") } // We'll define "incrementor" below. + } + } + + // implementation: + getIncrementor: function (i) { + return new Incrementor(i) + } + +I want that response to look like `{ from: <actorID>, value: <number> }`, but the client side needs to know to return an Incrementor, not a primitive number. So let's tell the protocol lib about Incrementors: + + protocol.types.addType("incrementor", { + // When writing to a protocol packet, just send the value + write: (v) => v.value, + + // When reading from a protocol packet, wrap with an Incrementor + // object. + read: (v) => new Incrementor(v) + }); + +And now our client can use the API as expected: + + front.getIncrementor(5).then(incrementor => { + incrementor.increment(); + assert(incrementor.value === 6); + }); + +You can do the same thing with arguments: + + // spec: + methods: { + passIncrementor: { + request: { Arg(0, "incrementor") }, + } + } + + // implementation: + passIncrementor: function (inc) { + w.increment(); + assert(incrementor.value === 6); + } + + front.passIncrementor(new Incrementor(5)); + +The library provides primitiive `boolean`, `number`, `string`, and `json` types. + +Moving right along, let's say you want to pass/return an array of Incrementors. You can just prepend `array:` to the type name: + + // spec: + methods: { + incrementAll: { + request: { incrementors: Arg(0, "array:incrementor") }, + response: { incrementors: RetVal("array:incrementor") } + } + } + + // implementation: + incrementAll: function (incrementors) { + incrementors.forEach(incrementor => { + incrementor.increment(); + } + return incrementors; + } + +You can use an iterator in place of an array as an argument or return value, and the library will handle the conversion automatically. + +Or maybe you want to return a dictionary where one item is a incrementor. To do this you need to tell the type system which members of the dictionary need custom marshallers: + + protocol.types.addDictType("contrivedObject", { + incrementor: "incrementor", + incrementorArray: "array:incrementor" + }); + + // spec: + methods: { + reallyContrivedExample: { + response: RetVal("contrivedObject") + } + } + + // implementations: + reallyContrivedExample: function () { + return { + /* a and b are primitives and so don't need to be called out specifically in addDictType */ + a: "hello", b: "world", + incrementor: new Incrementor(1), + incrementorArray: [new Incrementor(2), new Incrementor(3)] + } + } + + front.reallyContrivedExample().then(obj => { + assert(obj.a == "hello"); + assert(obj.b == "world"); + assert(incrementor.i == 1); + assert(incrementorArray[0].i == 2); + assert(incrementorArray[1].i == 3); + }); + +Nullables +--------- + +If an argument, return value, or dict property can be null/undefined, you can prepend `nullable:` to the type name: + + "nullable:incrementor", // Can be null/undefined or an incrementor + "array:nullable:incrementor", // An array of incrementors that can have holes. + "nullable:array:incrementor" // Either null/undefined or an array of incrementors without holes. + + +Actors +------ + +Probably the most common objects that need custom martialing are actors themselves. These are more interesting than the Incrementor object, but by default they're somewhat easy to work with. Let's add a ChildActor implementation that will be returned by the HelloActor (which is rapidly becoming the OverwhelminglyComplexActor): + + // spec: + const childActorSpec = generateActorSpec({ + actorType: "childActor", + methods: { + getGreeting: { + response: { greeting: RetVal("string") }, + } + } + }); + + // implementation: + const ChildActor = protocol.ActorClassWithSpec(childActorSpec, { + initialize: function (conn, id) { + protocol.Actor.prototype.initialize.call(this, conn); + this.greeting = "hello from " + id; + }, + getGreeting: function () { + return this.greeting; + }, + }); + + exports.ChildActor = ChildActor; + + const ChildFront = protocol.FrontClassWithSpec(childActorSpec, { + initialize: function (client, form) { + protocol.Front.prototype.initialize.call(this, client, form); + }, + }); + +The library will register a marshaller for the actor type itself, using typeName as its tag. + +So we can now add the following code to HelloActor: + + // spec: + methods: { + getChild: { + request: { id: Arg(0, "string") }, + response: { child: RetVal("childActor") } + } + } + + // implementation: + getChild: function (id) { + return ChildActor(this.conn, id); + } + + front.getChild("child1").then(childFront => { + return childFront.getGreeting(); + }).then(greeting => { + assert(id === "hello from child1"); + }); + +The conversation will look like this: + + { to: <actorID>, type: "getChild", id: "child1" } + { from: <actorID>, child: { actor: <childActorID> }} + { to: <childActorID>, type: "getGreeting" } + { from: <childActorID>, greeting: "hello from child1" } + +But the ID is the only interesting part of this made-up example. You're never going to want a reference to a ChildActor without checking its ID. Making an extra request just to get that id is wasteful. You really want the first response to look like `{ from: <actorID>, child: { actor: <childActorID>, greeting: "hello from child1" } }` + +You can customize the marshalling of an actor by providing a `form` method in the `ChildActor` class: + + form: function () { + return { + actor: this.actorID, + greeting: this.greeting + } + }, + +And you can demarshal in the `ChildFront` class by implementing a matching `form` method: + + form: function (form) { + this.actorID = form.actor; + this.greeting = form.greeting; + } + +Now you can use the id immediately: + + front.getChild("child1").then(child => { assert(child.greeting === "child1) }); + +You may come across a situation where you want to customize the output of a `form` method depending on the operation being performed. For example, imagine that ChildActor is a bit more complex, with a, b, c, and d members: + + ChildActor: + form: function () { + return { + actor: this.actorID, + greeting: this.greeting, + a: this.a, + b: this.b, + c: this.c, + d: this.d + } + } + ChildFront: + form: function (form) { + this.actorID = form.actorID; + this.id = form.id; + this.a = form.a; + this.b = form.b; + this.c = form.c; + this.d = form.d; + } + +And imagine you want to change 'c' and return the object: + + // Oops! If a type is going to return references to itself or any other + // type that isn't fully registered yet, you need to predeclare the type. + types.addActorType("childActor"); + + ... + + // spec: + methods: { + changeC: { + request: { newC: Arg(0) }, + response: { self: RetVal("childActor") } + } + } + + // implementation: + changeC: function (newC) { + c = newC; + return this; + } + + ... + + childFront.changeC('hello').then(ret => { assert(ret === childFront); assert(childFront.c === "hello") }); + +Now our response will look like: + + { from: <childActorID>, self: { actor: <childActorID>, greeting: <id>, a: <a>, b: <b>, c: "hello", d: <d> } + + +Lifetimes +--------- + +No, I don't want to talk about lifetimes quite yet. + +Events +------ + +Your actor has great news! + +Actors are subclasses of jetpack `EventTarget`, so you can just emit events. +Here's how you'd set it up in a spec: + + events: { + "good-news": { + type: "goodNews", // event target naming and packet naming are at odds, and we want both to be natural! + news: Arg(0) + } + } + + methods: { + giveGoodNews: { + request: { news: Arg(0) } + } + } + +Here's how the implementation would look: + + const EventEmitter = require("devtools/shared/event-emitter"); + + // In your protocol.ActorClassWithSpec definition: + giveGoodNews: function (news) { + EventEmitter.emit(this, "good-news", news); + } + +Now you can listen to events on a front: + + front.on("good-news", news => { + console.log(`Got some good news: ${news}\n`); + }); + front.giveGoodNews().then(() => { console.log("request returned.") }); + +If you want to modify the argument that will be passed to event listeners callbacks, you +can use `before(eventName, fn)` in the front definition. This can only be used once for a +given `eventName`. The `fn` function will be called before emitting the event via +the EventEmitter API on the Front, and its return value will be passed to the event +listener callbacks. If `fn` is async, the event will only be emitted after `fn` call resolves. + + // In front file, most probably in the constructor: + this.before("good-news", function(news) { + return news.join(" - "); + }); + + // In any consumer + front.on("good-news", function(news) { + console.log(news); + }); + +So if the server sent the following array: `[1, 2, 3]`, the console.log in the consumer +would print `1 - 2 - 3`. + +On a somewhat related note, not every method needs to be request/response. Just like an actor can emit a one-way event, a method can be marked as a one-way request. Maybe we don't care about giveGoodNews returning anything: + + // spec: + methods: { + giveGoodNews: { + request: { news: Arg(0, "string") }, + oneway: true + } + } + + // implementation: + giveGoodNews: function (news) { + emit(this, "good-news", news); + } + +Lifetimes +--------- + +No, let's talk about custom front methods instead. + +Custom Front Methods +-------------------- + +You might have some bookkeeping to do before issuing a request. Let's say you're calling `echo`, but you want to count the number of times you issue that request. Just use the `custom` tag in your front implementation: + + echo: custom(function (str) { + this.numEchos++; + return this._echo(str); + }, { + impl: "_echo" + }) + +This puts the generated implementation in `_echo` instead of `echo`, letting you implement `echo` as needed. If you leave out the `impl`, it just won't generate the implementation at all. You're on your own. + +Lifetimes +--------- + +OK, I can't think of any more ways to put this off. The remote debugging protocol has the concept of a *parent* for each actor. This is to make distributed memory management a bit easier. Basically, any descendents of an actor will be destroyed if the actor is destroyed. + +Other than that, the basic protocol makes no guarantees about lifetime. Each interface defined in the protocol will need to discuss and document its approach to lifetime management (although there are a few common patterns). + +The protocol library will maintain the child/parent relationships for you, but it needs some help deciding what the child/parent relationships are. + +The default parent of an object is the first object that returns it after it is created. So to revisit our earlier HelloActor `getChild` implementation: + + // spec: + methods: { + getChild: { + request: { id: Arg(0) }, + response: { child: RetVal("childActor") } + } + } + + // implementation: + getChild: function (id) { + return new ChildActor(this.conn, id); + } + +The ChildActor's parent is the HelloActor, because it's the one that created it. + +You can customize this behavior in two ways. The first is by defining a `marshallPool` property in your actor. Imagine a new ChildActor method: + + // spec: + methods: { + getSibling: { + request: { id: Arg(0) }, + response: { child: RetVal("childActor") } + } + } + + // implementation: + getSibling: function (id) { + return new ChildActor(this.conn, id); + } + +This creates a new child actor owned by the current child actor. But in this example we want all actors created by the child to be owned by the HelloActor. So we can define a `defaultParent` property that makes use of the `parent` property provided by the Actor class: + + get marshallPool() { return this.parent } + +The front needs to provide a matching `defaultParent` property that returns an owning front, to make sure the client and server lifetimes stay synced. diff --git a/devtools/docs/backend/protocol.md b/devtools/docs/backend/protocol.md new file mode 100644 index 0000000000..199a0f5ffe --- /dev/null +++ b/devtools/docs/backend/protocol.md @@ -0,0 +1,1609 @@ +# Remote Debugging Protocol + +The Mozilla debugging protocol allows a debugger to connect to a browser, discover what sorts of things are present to debug or inspect, select JavaScript threads to watch, and observe and modify their execution. The protocol provides a unified view of JavaScript, DOM nodes, CSS rules, and the other technologies used in client-side web applications. The protocol ought to be sufficiently general to be extended for use with other sorts of clients (profilers, say) and servers (mail readers; random XULrunner applications). + +All communication between debugger (client) and browser (server) is in the form of JSON objects. This makes the protocol directly readable by humans, capable of graceful evolution, and easy to implement using stock libraries. In particular, it should be easy to create mock implementations for testing and experimentation. + +The protocol operates at the JavaScript level, not at the C++ or machine level, and assumes that the JavaScript implementation itself is healthy and responsive. The JavaScript program being executed may well have gone wrong, but the JavaScript implementation's internal state must not be corrupt. Bugs in the implementation may cause the debugger to fail; bugs in the interpreted program must not. + +## General Conventions + +### Actors + +An **actor** is something on the server that can exchange JSON packets with the client. Every packet from the client specifies the actor to which it is directed, and every packet from the server indicates which actor sent it. + +Each server has a root actor, with which the client first interacts. The root actor can explain what sort of thing the server represents (browser; mail reader; etc.), and enumerate things available to debug: tabs, chrome, and so on. Each of these, in turn, is represented by an actor to which requests can be addressed. Both artifacts of the program being debugged, like JavaScript objects and stack frames, and artifacts of the debugging machinery, like breakpoints and watchpoints, are actors with whom packets can be exchanged. + +For example, a debugger might connect to a browser, ask the root actor to list the browser's tabs, and present this list to the developer. If the developer chooses some tabs to debug, then the debugger can send `attach` requests to the actors representing those tabs, to begin debugging. + +Actor names are JSON strings, containing no spaces or colons. The name of the root actor is `"root"`. + +To allow the server to reuse actor names and the resources they require, actors have limited lifetimes. All actors in a server form a tree, whose root is the root actor. Closing communications with an actor automatically closes communications with its descendants. For example, the actors representing a thread's stack frames are children of the actor representing the thread itself, so that when a debugger detaches from a thread, which closes the thread's actor, the frames' actors are automatically closed. This arrangement allows the protocol to mention actors liberally, without making the client responsible for explicitly closing every actor that has ever been mentioned. + +When we say that some actor *A* is a child of some actor *B*, we mean that *A* is a direct child of *B*, not a grandchild, great-grandchild, or the like. Similarly, **parent** means "direct parent". We use the terms **ancestor** and **descendent** to refer to those looser relationships. + +The root actor has no parent, and lives as long as the underlying connection to the client does; when that connection is closed, all actors are closed. + +Note that the actor hierarchy does not, in general, correspond to any particular hierarchy appearing in the debuggee. For example, although web workers are arranged in a hierarchy, the actors representing web worker threads are all children of the root actor: one might want to detach from a parent worker while continuing to debug one of its children, so it doesn't make sense to close communications with a child worker simply because one has closed communications with its parent. + +*(We are stealing the "actor" terminology from Mozilla's IPDL, to mean, roughly, "things participating in the protocol". However, IPDL does much more with the idea than we do: it treats both client and server as collections of actors, and uses that detail to statically verify properties of the protocol. In contrast, the debugging protocol simply wants a consistent way to indicate the entities to which packets are directed.)* + +### Packets + +The protocol is carried by a reliable, bi-directional byte stream; data sent in both directions consists of JSON objects, called packets. A packet is a top-level JSON object, not contained inside any other value. + +Every packet sent from the client has the form: + +``` +{ "to":actor, "type":type, ... } +``` + +where `actor` is the name of the actor to whom the packet is directed and `type` is a string specifying what sort of packet it is. Additional properties may be present, depending on `type`. + +Every packet sent from the server has the form: + +``` +{ "from":actor, ... } +``` + +where `actor` is the name of the actor that sent it. The packet may have additional properties, depending on the situation. + +If a packet is directed to an actor that no longer exists, the server sends a packet to the client of the following form: + +``` +{ "from":actor, "error":"noSuchActor" } +``` + +where `actor` is the name of the non-existent actor. (It is strange to receive messages from actors that do not exist, but the client evidently believes that actor exists, and this reply allows the client to pair up the error report with the source of the problem.) + +Clients should silently ignore packet properties they do not recognize. We expect that, as the protocol evolves, we will specify new properties that can appear in existing packets, and experimental implementations will do the same. + +### Common Patterns of Actor Communication + +Each type of actor specifies which packets it can receive, which it might send, and when it can do each. Although in principle these interaction rules could be complex, in practice most actors follow one of two simple patterns: + +* **Request/Reply**: Each packet sent to the actor ("request") elicits a single packet in response ("reply"). +* **Request/Reply/Notify**: Like Request/Reply, but the actor may send packets that are not in response to any specific request ("notification"), perhaps announcing events that occur spontaneously in the debuggee. + +These patterns are described in more detail below. + +Some actors require more complicated rules. For example, the set of packets accepted by a [Thread-like actor](#interacting-with-thread-like-actors) depends on which one of four states it occupies. The actor may spontaneously transition from one state to another, and not all state transitions produce notification packets. Actors like this require careful specification. + +#### The Request/Reply Pattern + +In this specification, if we call a packet a **request**, then it is a packet sent by the client, which always elicits a single packet from the actor in return, the **reply**. These terms indicate a simple pattern of communication: the actor processes packets in the order they are received, and the client can trust that the *i*'th reply corresponds to the *i*'th request. + +An [error reply packet](#error-packets) from a request/reply actor constitutes a reply. + +Note that it is correct for a client to send several requests to a request/reply actor without waiting for a reply to each request before sending the next; requests can be pipelined. However, as the pending requests consume memory, the client should ensure that only a bounded number of requests are outstanding at any one time. + +#### The Request/Reply/Notify Pattern + +Some actors follow the request/reply pattern, but may also send the client ***notification*** packets, not in reply to any particular request. For example, if the client sends the root actor a `["listTabs"](#listing-browser-tabs)` request, then the root actor sends a reply. However, since the client has now expressed an interest in the list of open tabs, the root actor may subsequently send the client a `"tabListChanged"` notification packet, indicating that the client should re-fetch the list of tabs if it is interested in the latest state. + +There should be a small upper bound on the number of notification packets any actor may send between packets received from the client, to ensure that the actor does not flood the client. In the example above, the root actor sends at most one `"tabListChanged"` notification after each `"listTabs"` request. + +#### Error Packets + +Any actor can reply to a packet it is unable to process with an **error reply** of the form: + +``` +{ "from":actor, "error":name, "message":message } +``` + +where *name* is a JSON string naming what went wrong, and *message* is an English error message. Error *names* are specified by the protocol; the client can use the name to identify which error condition arose. The *message* may vary from implementation to implementation, and should only be displayed to the user as a last resort, as the server lacks enough information about the user interface context to provide appropriate messages. + +If an actor receives a packet whose type it does not recognize, it sends an error reply of the form: + +``` +{ "from":actor, "error":"unrecognizedPacketType", "message":message } +``` + +where *message* provides details to help debugger developers understand what went wrong: what kind of actor actor is; the packet received; and so on. + +If an actor receives a packet which is missing needed parameters (say, an `"autocomplete"` packet with no `"text"` parameter), it sends an error reply of the form: + +``` +{ "from":actor, "error":"missingParameter", "message":message } +``` + +where *message* provides details to help debugger developers fix the problem. + +If an actor receives a packet with a parameter whose value is inappropriate for the operation, it sends an error reply of the form: + +``` +{ "from":actor, "error":"badParameterType", "message":message } +``` + +where *message* provides details to help debugger developers fix the problem. (Some packets' descriptions specify more specific errors for particular circumstances.) + +### Grips + +A grip is a JSON value that refers to a specific JavaScript value in the debuggee. Grips appear anywhere an arbitrary value from the debuggee needs to be conveyed to the client: stack frames, object property lists, lexical environments, `paused` packets, and so on. + +For mutable values like objects and arrays, grips do not merely convey the value's current state to the client. They also act as references to the original value, by including an actor to which the client can send messages to modify the value in the debuggee. + +A grip has one of the following forms: + +``` +value +``` + +where value is a string, a number, or a boolean value. For these types of values, the grip is simply the JSON form of the value. + +``` +{ "type":"null" } +``` + +This represents the JavaScript `null` value. (The protocol does not represent JavaScript `null` simply by the JSON `null`, for the convenience of clients implemented in JavaScript: this representation allows such clients to use `typeof(grip) == "object"` to decide whether the grip is simple or not.) + +``` +{ "type":"undefined" } +``` + +This represents the JavaScript `undefined` value. (`undefined` has no direct representation in JSON.) + +``` +{ "type":"Infinity" } +``` + +This represents the JavaScript `Infinity` value. (`Infinity` has no direct representation in JSON.) + +``` +{ "type":"-Infinity" } +``` + +This represents the JavaScript `-Infinity` value. (`-Infinity` has no direct representation in JSON.) + +``` +{ "type":"NaN" } +``` + +This represents the JavaScript `NaN` value. (`NaN` has no direct representation in JSON.) + +``` +{ "type":"-0" } +``` + +This represents the JavaScript `-0` value. (`-0` stringifies to JSON as 0.) + +``` +{ "type":"object", "class":className, "actor":actor } +``` + +This represents a JavaScript object whose class is `className`. (Arrays and functions are treated as objects for the sake of forming grips.) Actor can be consulted for the object's contents, as explained below. + +If the class is "Function", the grip may have additional properties: + +``` +{ "type":"object", "class":"Function", "actor":actor, + "name":name, "displayName":displayName, + "userDisplayName":userDisplayName, + "url":url, "line":line, "column":column } +``` + +These additional properties are: + +***Name*** + +The function's name (as given in the source code, following the `function` keyword), as a string. If the function is anonymous, the `name` property is omitted. + +***displayName*** + +A name the system has inferred for the function (say, `"Foo.method"`). If the function has a given name (appearing in the grip as the `"name"` property), or if the system was unable to infer a suitable name for it, the `displayName` property is omitted. + +***userDisplayName*** + +If the function object has a `"displayName"` value property whose value is a string, this is that property's value. (Many JavaScript development tools consult such properties, to give developers a way to provide their own meaningful names for functions.) + +***url*** + +The URL of the function's source location (see [Source Locations](#source-locations)); + +***line*** + +The line number of the function's source location (see [Source Locations](#source-locations)); + +***column*** + +The column number of the function's source location (see [Source Locations](#source-locations)); + +``` +{ "type":"longString", "initial":initial, "length":length, "actor":actor } +``` + +This represents a very long string, where "very long" is defined at the server's discretion. `Initial` is some initial portion of the string, `length` is the string's full length, and actor can be consulted for the rest of the string, as explained below. + +For example, the following table shows some JavaScript expressions and the grips that would represent them in the protocol: + +| JavaScript Expression | Grip | +|:--------------------------------------------------------:|:---------------------------------------------------------------------------------------------:| +| 42 | 42 | +| true | true | +| "nasu" | "nasu" | +| (void 0) | `{ "type":"undefined" }` | +| ({x:1}) | `{ "type":"object", "class":"Object", "actor":"24" }` | +| "Arms and the man I sing, who, *[much, much more text]*" | `{ "type":"longString", "initial":"Arms and the man I sing", "length":606647, "actor":"25" }` | + +Garbage collection will never free objects visible to the client via the protocol. Thus, actors representing JavaScript objects are effectively garbage collection roots. + +#### Objects + +While a thread is paused, the client can send requests to the actors appearing in object grips to examine the objects they represent in more detail. + +##### Property Descriptors + +Protocol requests that describe objects' properties to the client often use **descriptors**, JSON values modeled after ECMAScript 5's property descriptors, to describe individual properties. + +A descriptor has the form: + +``` +{ "enumerable":<enumerable>, "configurable":<configurable>, ... } +``` + +where *enumerable* and *configurable* are boolean values indicating whether the property is enumerable and configurable, and additional properties are present depending on what sort of property it is. + +A descriptor for a data property has the form: + +``` +{ "enumerable":<enumerable>, "configurable":<configurable>, + "value":<value>, "writeable":<writeable> } +``` + +where *value* is a grip on the property's value, and *writeable* is a boolean value indicating whether the property is writeable. + +A descriptor for an accessor property has the form: + +``` +{ "enumerable":<enumerable>, "configurable":<configurable>, + "get":<getter>, "set":<setter> } +``` + +where *getter* and *setter* are grips on the property's getter and setter functions. These may be `{ "type":"undefined" }` if the property lacks the given accessor function. + +A **safe getter value descriptor** provides a value that an inherited accessor returned when applied to an instance. (See [Finding An Object's Prototype And Properties](#finding-an-objects-prototype-and-properties) for an explanation of why and when such descriptors are used.) Such a descriptor has the form: + +``` +{ "getterValue": <value>, "getterPrototypeLevel": <level>, + "enumerable":<enumerable>, "writable":<writable> } +``` + +where *value* is a grip on the value the getter returned, *level* is the number of steps up the object's prototype chain one must take to find the object on which the getter appears as an own property. If the getter appears directly on the object, *level* is zero. The *writable* property is true if the inherited accessor has a setter, and false otherwise. + +For example, if the JavaScript program being debugged evaluates the expression: + +``` +({x:10, y:"kaiju", get a() { return 42; }}) +``` + +then a grip on this value would have the form: + +``` +{ "type":"object", "class":"Object", "actor":<actor> } +``` + +and sending a ["prototypeAndProperties"](#finding-an-objects-prototype-and-properties) request to *actor* would produce the following reply: + +``` +{ "from":<actor>, "prototype":{ "type":"object", "class":"Object", "actor":<objprotoActor> }, + "ownProperties":{ "x":{ "enumerable":true, "configurable":true, "writeable":true, "value":10 }, + "y":{ "enumerable":true, "configurable":true, "writeable":true, "value":"kaiju" }, + "a":{ "enumerable":true, "configurable":true, + "get":{ "type":"object", "class":"Function", "actor":<getterActor> }, + "set":{ "type":"undefined" } + } + } +} +``` + + +Sending a ["prototypeAndProperties"](#finding-an-objects-prototype-and-properties) request to an object actor referring to a DOM mouse event might produce the following reply: + +``` +{ "from":<mouseEventActor>, "prototype":{ "type":"object", "class":"MouseEvent", "actor":<mouseEventProtoActor> }, + "ownProperties":{ } + "safeGetterValues":{ "screenX": { "getterValue": 1000, "getterPrototypeLevel": 1, + "enumerable": true, "writable": false }, + "screenY": { "getterValue": 1000, "getterPrototypeLevel": 1, + "enumerable": true, "writable": false }, + "clientX": { "getterValue": 800, "getterPrototypeLevel": 1, + "enumerable": true, "writable": false }, + "clientY": { "getterValue": 800, "getterPrototypeLevel": 1, + "enumerable": true, "writable": false }, + ... + } +} +``` + +##### Finding An Object's Prototype And Properties + +To examine an object's prototype and properties, a client can send the object's grip's actor a request of the form: + +``` +{ "to":<gripActor>, "type":"prototypeAndProperties" } +``` + +to which the grip actor replies: + +``` +{ "from":<gripActor>, "prototype":<prototype>, "ownProperties":<ownProperties> } +``` + +where *prototype* is a grip on the object's prototype (possibly `{ "type":"null" }`), and *ownProperties* has the form: + +``` +{ name:<descriptor>, ... } +``` + +with a *name*:<descriptor> pair for each of the object's own properties. + +The web makes extensive use of inherited accessor properties; for example, the `clientX` and `clientY`> properties of a mouse click event are actually accessor properties which the event object inherits from its prototype chain. It can be very valuable to display such properties' values directly on the object (taking care to distinguish them from true "own" properties), if the server can determine that the getters can be called without side effects. + +To this end, when possible, the server may provide safe getter value descriptors for an object, as described in [Property Descriptors](#property-descriptors) above, reporting the values that getter functions found on the object's prototype chain return when applied to that object. If the server chooses to provide any, the reply includes a `"safeGetterValues"` property of the form: + +``` +{ name:<descriptor>, ... } +``` + +with a *name*:<descriptor> pair for each safe getter the object inherits from its prototype chain, or that appears directly on the object. Each *descriptor* here is a safe getter value descriptor. + +*TODO: What about objects with many properties?* + +##### Finding an Object's Prototype + + +To find an object's prototype, a client can send the object's grip's actor a request of the form: + +``` +{ "to":<gripActor>, "type":"prototype" } +``` + +to which the grip actor replies: + +``` +{ "from":<gripActor>, "prototype":<prototype> } +``` + +where *prototype* is a grip on the object's prototype (possibly `{ "type":"null" }`). + + +##### Listing an Object's Own Properties' Names + +To list an object's own properties' names, a client can send the object's grip's actor a request of the form: + +``` +{ "to":<gripActor>, "type":"ownPropertyNames" } +``` + +to which the grip actor replies: + +``` +{ "from":<gripActor>, "ownPropertyNames":[ <name>, ... ] } +``` + +where each *name* is a string naming an own property of the object. + +##### Finding Descriptors For Single Properties + +To obtain a descriptor for a particular property of an object, a client can send the object's grip's actor a request of the form: + +``` +{ "to":<gripActor>, "type":"property", "name":<name> } +``` + +to which the grip actor replies: + +``` +{ "from":<gripActor>, "descriptor":<descriptor> } +``` + +where *descriptor* is a descriptor for the own property of the object named *name*, or `null` if the object has no such own property. + +A property descriptor has the form: + +``` +{ "configurable":<configurable>, "enumerable":<enumerable>, ... } +``` + +where *configurable* and *enumerable* are boolean values. *Configurable* is true if the property can be deleted or have its attributes changed. *Enumerable* is true if the property will be enumerated by a `for-in` enumeration. + +Descriptors for value properties have the form: + +``` +{ "configurable":<configurable>, "enumerable":<enumerable>, + "writable":<writable>, "value":<value> } +``` + +where *writable* is `true` if the property's value can be written to; *value* is a grip on the property's value; and *configurable* and *enumerable* are as described above. + +Descriptors for accessor properties have the form: + +``` +{ "configurable":<configurable>, "enumerable":<enumerable>, + "get":<get>, "set":<set> } +``` + +where *get* and *set* are grips on the property's getter and setter functions; either or both are omitted if the property lacks the given accessor function. *Configurable* and *enumerable* are as described above. + +*TODO: assign to value property* + +*TODO: special stuff for arrays* + +*TODO: special stuff for functions* + +*TODO: find function's source position* + +*TODO: get function's named arguments, in order* + +*TODO: descriptors for Harmony proxies* + +##### Functions + +If an object's class as given in the grip is `"Function"`, then the grip's actor responds to the messages given here. + +``` +{ "to":<functionGripActor>, "type":"parameterNames" } +``` + +This requests the names of the parameters of the function represented by *functionGripActor*. The reply has the form: + +``` +{ "from":<functionGripActor>, "parameterNames":[ <parameter>, ... ] } +``` + +where each *parameter* is the name of a formal parameter to the function as a string. If the function takes destructuring arguments, then *parameter* is a structure of JSON array and object forms matching the form of the destructuring arguments. + +``` +{ "to":<functionGripActor>, "type":"scope" } +``` + +Return the lexical environment over which the function has closed. The reply has the form: + +``` +{ "from":<functionGripActor>, "scope":<environment> } +``` + +where *environment* is a [lexical environment](#lexical-environments). Note that the server only returns environments of functions in a context being debugged; if the function's global scope is not the browsing context to which we are attached, the function grip actor sends an error reply of the form: + +``` +{ "from":<functionGripActor>, "error":"notDebuggee", "message":<message> } +``` + +where *message* is text explaining the problem. + +``` +{ "to":<functionGripActor>, "type":"decompile", "pretty":<pretty> } +``` + +Return JavaScript source code for a function equivalent to the one represented by *functionGripActor*. If the optional `pretty` parameter is present and *pretty* is `true`, then produce indented source code with line breaks. The reply has the form: + +``` +{ "from":<functionGripActor>, "decompiledCode":<code> } +``` + +where *code* is a string. + +If *functionGripActor*'s referent is not a function, or is a function proxy, the actor responds to these requests with an error reply of the form: + +``` +{ "from":<functionGripActor>, "error":"objectNotFunction", message:<message> } +``` + +where *message* is a string containing any additional information that would be helpful to debugger developers. + +#### Long Strings + +The client can find the full contents of a long string by sending a request to the long string grip actor of the form: + +``` +{ "to":<gripActor>, "type":"substring", "start":<start>, "end":<end> } +``` + +where *start* and *end* are integers. This requests the substring starting at the *start*'th character, and ending before the *end*'th character. The actor replies as follows: + +``` +{ "from":<gripActor>, "substring":<string> } +``` + +where *string* is the requested portion of the string the actor represents. Values for *start* less than zero are treated as zero; values greater than the length of the string are treated as the length of the string. Values for *end* are treated similarly. If *end* is less than *start*, the two values are swapped. (This is meant to be the same behavior as JavaScript's `String.prototype.substring`.) + +As with any other actor, the client may only send messages to a long string grip actor while it is alive: for [pause-lifetime grips](#grip-lifetimes), until the debuggee is resumed; or for [thread-lifetime grips](#grip-lifetimes), until the thread is detached from or exits. However, unlike object grip actors, the client may communicate with a long string grip actor at any time the actor is alive, regardless of whether the debuggee is paused. (Since strings are immutable values in JavaScript, the responses from a long string grip actor cannot depend on the actions of the debuggee.) + +#### Grip Lifetimes + +Most grips are **pause-lifetime** grips: they last only while the JavaScript thread is paused, and become invalid as soon as the debugger allows the thread to resume execution. (The actors in pause-lifetime grips are children of an actor that is closed when the thread resumes, or is detached from.) This arrangement allows the protocol to use grips freely in responses without requiring the client to remember and close them all. + +However, in some cases the client may wish to retain a reference to an object or long string while the debuggee runs. For example, a panel displaying objects selected by the user must update its view of the objects each time the debuggee pauses. To carry this out, the client can promote a pause-lifetime grip to a **thread-lifetime** grip, which lasts until the thread is detached from or exits. Actors in thread-lifetime grips are children of the thread actor. When the client no longer needs a thread-lifetime grip, it can explicitly release it. + +Both pause-lifetime and thread-lifetime grips are garbage collection roots. + +To promote a pause-lifetime grip to a thread-lifetime grip, the client sends a packet of the form: + +``` +{ "to":<gripActor>, "type":"threadGrip" } +``` + +where *gripActor* is the actor from the existing pause-lifetime grip. The grip actor will reply: + +``` +{ "from":<gripActor>, "threadGrip":<threadGrip> } +``` + +where *threadGrip* is a new grip on the same object, but whose actor is parented by the thread actor, not the pause actor. + +The client can release a thread-lifetime grip by sending the grip actor a request of the form: + +``` +{ "to":<gripActor>, "type":"release" } +``` + +The grip actor will reply, simply: + +``` +{ "from":<gripActor> } +``` + +This closes the grip actor. The `"release"` packet may only be sent to thread-lifetime grip actors; if a pause-lifetime grip actor receives a `"release"` packet, it sends an error reply of the form: + +``` +{ "from":<gripActor>, "error":"notReleasable", "message":<message> } +``` + +where each *gripActor* is the name of a child of *thread* that should be freed. The thread actor will reply, simply: + +``` +{ "from":<thread> } +``` + +Regardless of the lifetime of a grip, the client may only send messages to object grip actors while the thread to which they belong is paused; the client's interaction with mutable values cannot take place concurrently with the thread. + +### Completion Values + +Some packets describe the way a stack frame's execution completed using a **completion value**, which takes one of the following forms: + +``` +{ "return":<grip> } +``` + +This indicates that the frame completed normally, returning the value given by *grip*. + +``` +{ "throw":<grip> } +``` + +This indicates that the frame threw an exception; *grip* is the exception value thrown. + +``` +{ "terminated":true } +``` + +This indicates that the frame's execution was terminated, as by a "slow script" dialog box or running out of memory. + +### Source Locations + +Many packets refer to particular locations in source code: breakpoint requests specify where the breakpoint should be set; stack frames show the current point of execution; and so on. + +Descriptions of source code locations (written as *location* in packet descriptions) can take one of the following forms: + +``` +{ "url":<url>, "line":<line>, "column":<column> } +``` + +This refers to line *line*, column *column* of the source code loaded from *url*. Line and column numbers start with 1. If *column* or *line* are omitted, they default to 1. + +``` +{ "eval":<location>, "id":<id>, "line":<line>, "column":<column> } +``` + +This refers to line *line*, column *column* of the source code passed to the call to eval at *location*. To distinguish the different texts passed to eval, each is assigned a unique integer, *id*. + +``` +{ "function":<location>, "id":<id>, "line":<line>, "column":<column> } +``` + +This refers to line *line*, column *column* of the source code passed to the call to the `Function` constructor at *location*. To distinguish the different texts passed to the `Function` constructor, each is assigned a unique integer, *id*. + +As indicated, locations can be nested. A location like this one: + +``` +{ "eval":{ "eval":{ "url":"file:///home/example/sample.js", "line":20 } + "id":300, "line":30 } + "id":400, "line":40 } +``` + +refers to line 40 of the code passed to the call to eval occurring on line 30 of the code passed to the call to eval on line 20 of `file:///home/example/sample.js`. + +## The Root Actor + +When the connection to the server is opened, the root actor opens the conversation with the following packet: + +``` +{ "from":"root", "applicationType":<appType>, "traits":<traits>, ...} +``` + +The root actor's name is always `"root"`. *appType* is a string indicating what sort of program the server represents. There may be more properties present, depending on *appType*. + +*traits* is an object describing protocol variants this server supports that are not convenient for the client to detect otherwise. The property names present indicate what traits the server has; the properties' values depend on their names. If *traits* would have no properties, the `"traits"` property of the packet may be omitted altogether. This version of the protocol defines no traits, so if the `"traits"` property is present at all, its value must be an object with no properties, `{}`. + +For web browsers, the introductory packet should have the following form: + +``` +{ "from":"root", "applicationType":"browser", "traits":<traits> } +``` + +### Listing Browser Tabs + +To get a list of the tabs currently present in a browser, a client sends the root actor a request of the form: + +``` +{ "to":"root", "type":"listTabs" } +``` + +The root actor replies: + +``` +{ "from":"root", "tabs":[<tab>, ...], "selected":<selected> } +``` + +where each *tab* describes a single open tab, and *selected* is the index in the array of tabs of the currently selected tab. This form may have other properties describing other global actors; for one example, see [Chrome Debugging](#chrome-debugging). + +Each *tab* has the form: + +``` +{ "actor":<targetActor>, "title":<title>, "url":<URL> } +``` + +where *targetActor* is the name of an actor representing the tab, and *title* and *URL* are the title and URL of the web page currently visible in that tab. This form may have other properties describing other tab-specific actors. + +To attach to a *targetActor*, a client sends a message of the form: + +``` +{ "to":<targetActor>, "type":"attach" } +``` + +The target actor replies: + +``` +{ "from":<targetActor>, "threadActor":<tabThreadActor> } +``` + +where *tabThreadActor* is the name of a thread-like actor representing the tab's current content. If the user navigates the tab, *tabThreadActor* switches to the new content; we do not create a separate thread-like actor each page the tab visits. + +If the user closes the tab before the client attaches to it, *targetActor* replies: + +``` +{ "from":<targetActor>, "error":"exited" } +``` + +When the client is no longer interested in interacting with the tab, the client can request: + +``` +{ "to":<targetActor>, "type":"detach" } +``` + +The *targetActor* replies: + +``` +{ "from":<targetActor>, "type":"detached" } +``` + +If the client was not already attached to *targetActor*, *targetActor* sends an error reply of the form: + +``` +{ "from":<targetActor>, "error":"wrongState" } +``` + +While the client is attached, *targetActor* sends notifications to the client whenever the user navigates the tab to a new page. When navigation begins, *targetActor* sends a packet of the form: + +``` +{ "from":<targetActor>, "type":"tabNavigated", "state":"start", + "url":<newURL> } +``` + +This indicates that the tab has begun navigating to *newURL*; JavaScript execution in the tab's prior page is suspended. When navigation is complete, *targetActor* sends a packet of the form: + +``` +{ "from":<targetActor>, "type":"tabNavigated", "state":"stop", + "url":<newURL>, "title":<newTitle> } +``` + +where *newURL* and *newTitle* are the URL and title of the page the tab is now showing. The *tabThreadActor* given in the response to the original `"attach"` packet is now debugging the new page's code. + +If the user closes a tab to which the client is attached, its *targetActor* sends a notification packet of the form: + +``` +{ "from":<targetActor>, "type":"tabDetached" } +``` + +The client is now detached from the tab. + +### Chrome Debugging + +If the server supports debugging chrome code, the root actor's reply to a `"listTabs"` request includes a property named `"chromeDebugger"`, whose value is the name of a thread-like actor to which the client can attach to debug chrome code. + +## Interacting with Thread-Like Actors + +Actors representing independent threads of JavaScript execution, like browsing contexts and web workers, are collectively known as "threads". Interactions with actors representing threads follow a more complicated communication pattern. + +A thread is always in one of the following states: + +* **Detached**: the thread is running freely, and not presently interacting with the debugger. Detached threads run, encounter errors, and exit without exchanging any sort of messages with the debugger. A debugger can attach to a thread, putting it in the **Paused** state. Or, a detached thread may exit on its own, entering the **Exited** state. + +* **Running**: the thread is running under the debugger's observation, executing JavaScript code or possibly blocked waiting for input. It will report exceptions, breakpoint hits, watchpoint hits, and other interesting events to the client, and enter the **Paused** state. The debugger can also interrupt a running thread; this elicits a response and puts the thread in the **Paused** state. A running thread may also exit, entering the **Exited** state. + +* **Paused**: the thread has reported a pause to the client and is awaiting further instructions. In this state, a thread can accept requests and send replies. If the client asks the thread to continue or step, it returns to the **Running** state. If the client detaches from the thread, it returns to the **Detached** state. + +* **Exited**: the thread has ceased execution, and will disappear. The resources of the underlying thread may have been freed; this state merely indicates that the actor's name is not yet available for reuse. When the actor receives a "release" packet, the name may be reused. + + + +These interactions are meant to have certain properties: + +* At no point may either client or server send an unbounded number of packets without receiving a packet from its counterpart. This avoids deadlock without requiring either side to buffer an arbitrary number of packets per actor. +* In states where a transition can be initiated by either the debugger or the thread, it is always clear to the debugger which state the thread actually entered, and for what reason.<br>For example, if the debugger interrupts a running thread, it cannot be sure whether the thread stopped because of the interruption, paused of its own accord (to report a watchpoint hit, say), or exited. However, the next packet the debugger receives will either be "paused", or "exited", resolving the ambiguity.<br>Similarly, when the debugger attaches to a thread, it cannot be sure whether it has succeeded in attaching to the thread, or whether the thread exited before the "attach" packet arrived. However, in either case the debugger can expect a disambiguating response: if the attach succeeded, it receives an "attached" packet; and in the second case, it receives an "exit" packet.<br>To support this property, the thread ignores certain debugger packets in some states (the "interrupt" packet in the **Paused** and **Exited** states, for example). These cases all handle situations where the ignored packet was preempted by some thread action. + +Note that the rules here apply to the client's interactions with each thread actor separately. A client may send an "interrupt" to one thread actor while awaiting a reply to a request sent to a different thread actor. + +*TODO: What about user selecting nodes in displayed content? Should those be eventy things the client can receive in the "paused" state? What does that mean for the "request"/"reply" pattern?* + +### Attaching To a Thread + +To attach to a thread, the client sends a packet of the form: + +``` +{ "to":<thread>, "type":"attach" } +``` + +Here, *thread* is the actor representing the thread, perhaps a browsing context from a "listContexts" reply. This packet causes the thread to pause its execution, if it does not exit of its own accord first. The thread responds in one of two ways: + +``` +{ "from":<thread>, "type":"paused", "why":{ "type":"attached" }, ... } +``` + +The thread is now in the **Paused** state, because the client has attached to it. The actor name *thread* remains valid until the client detaches from the thread or acknowledges a thread exit. This is an ordinary `"paused"` packet, whose form and additional properties are as described in [Thread Pauses](#thread-pauses), below. + +``` +{ "from":<thread>, "type":"exited" } +``` + +This indicates that the thread exited on its own before receiving the "attach" packet. The thread is now in the **Exited** state. The client should follow by sending a "release" packet; see [Exiting Threads](#exiting-threads), below. + +If the client sends an `"attach"` packet to a thread that is not in the **Detached** or **Exited** state, the actor sends an error reply of the form: + +``` +{ "from":<thread>, "error":"wrongState", "message":<message> } +``` + +where *message* details which state the thread was in instead (to make debugging debuggers easier). In this case, the thread's state is unaffected. + +### Detaching From a Thread + +To detach from a thread, the client sends a packet of the form: + +``` +{ "to":<thread>, "type":"detach" } +``` + +The thread responds in one of three ways: + +``` +{ "from":<thread>, "type":"detached" } +``` + +This indicates that the client has detached from the thread. The thread is now in the **Detached** state: it can run freely, and no longer reports events to the client. Communications with *thread* are closed, and the actor name is available for reuse. If the thread had been in the **Paused** state, the pause actor is closed (because the pause actor is a child of *thread*). + +``` +{ "from":<thread>, "type":"paused", ... } +{ "from":<thread>, "type":"detached" } +``` + +This series of packets indicates that the thread paused of its own accord (for the reason given by the additional properties of the "paused" packet), and only then received the "detach" packet. As above, this indicates that the thread is in the **Detached** state, the just-created pause actor is closed, and the actor name is available for reuse. + +``` +{ "from":<thread>, "type":"exited" } +``` + +This indicates that the thread exited on its own before receiving the "detach" packet. The client should follow by sending a "release" packet; see [Exiting Threads](#exiting-threads), below. + +Detaching from a thread causes all breakpoints, watchpoints, and other debugging-related state to be forgotten. + +If the client sends a `"detach"` packet to a thread that is not in the **Running**, **Paused**, or **Exited** state, the actor sends an error reply of the form: + +``` +{ "from":<thread>, "error":"wrongState", "message":<message> } +``` + +where *message* details which state the thread was in instead (to make debugging debuggers easier). In this case, the thread's state is unaffected. + +### Running Threads + +Once the client has attached to a thread, it is in the **Running** state. In this state, four things can happen: + +* The thread can hit a breakpoint or watchpoint, or encounter some other condition of interest to the client. +* The thread can exit. +* The client can detach from the thread. +* The client can interrupt the running thread. + +Note that a client action can occur simultaneously with a thread action. The protocol is designed to avoid ambiguities when both client and thread act simultaneously. + +### Thread Pauses + +If the thread pauses to report an interesting event to the client, it sends a packet of the form: + +``` +{ "from":<thread>, "type":"paused", "actor":<pauseActor>, "why":<reason>, + "currentFrame":<frame>, "poppedFrames":[<poppedFrame>...] } +``` + +This indicates that the thread has entered the **Paused** state, and explains where and why. + +*PauseActor* is a "pause actor", representing this specific pause of the thread; it lives until the thread next leaves the **Paused** state. The pause actor parents actors referring to values and other entities uncovered during this pause; when the thread resumes, those actors are automatically closed. This relieves the client from the responsibility to explicitly close every actor mentioned during the pause. + +Since actors in value grips are parented by the pause actor, this means that those grips become invalid when the thread resumes, or is detached from; it is not possible to take a grip from one pause and use it in the next. To create a grip that remains valid between pauses, see [Grip Lifetimes](#grip-lifetimes). + +The *currentFrame* value describes the top frame on the JavaScript stack; see [Listing Stack Frames](#listing-stack-frames), below. + +The `"poppedFrames"` property is an array of frame actor names, listing the actors for all frames that were live as of the last pause, but have since been popped. If no frames have been popped, or if this is the first pause for this thread, then this property's value is the empty array. + +The *reason* value describes why the thread paused. It has one of the following forms: + +``` +{ "type":"attached" } +``` + +The thread paused because the client attached to it. + +``` +{ "type":"interrupted" } +``` + +The thread stopped because it received an "interrupt" packet from the client. + +``` +{ "type":"resumeLimit" } +``` + +The client resumed the thread with a `"resume"` packet that included a `resumeLimit` property, and the thread paused because the given *limit* was met. Execution remains in the frame the thread was resumed in, and that frame is not about to be popped. + +``` +{ "type":"resumeLimit", "frameFinished":<completion> } +``` + +The client resumed the thread with a `"resume"` packet that included a `resumeLimit` property, and the thread paused because the frame is about to be popped. *Completion* is a [completion value](#completion-values) describing how the frame's execution ended. The frame being popped is still the top frame on the stack, but subsequent `"resume"` operations will run in the calling frame. + +``` +{ "type":"debuggerStatement" } +``` + +The thread stopped because it executed a JavaScript "debugger" statement. + +``` +{ "type":"breakpoint", "actors":[<breakpointActor>...] } +``` + +The thread stopped at the breakpoints represented by the given actors. + +``` +{ "type":"watchpoint", "actors":[<watchpointActor>...] } +``` + +The thread stopped at the watchpoints represented by the given actors. + +*TODO: This should provide more details about the watchpoint in the packet, instead of incurring another round-trip before we can display anything helpful.* + +``` +{ "type":"clientEvaluated", "frameFinished":<completion> } +``` + +The expression given in the client's prior `clientEvaluate` command has completed execution; *completion* is a [completion value](#completion-values) describing how it completed. The frame created for the `clientEvaluate` resumption has been popped from the stack. See [Evaluating Source-Language Expressions](#evaluating-source-language-expressions) for details. + +### Resuming a Thread + +If a thread is in the **Paused** state, the client can resume it by sending a packet of the following form: + +``` +{ "to":<thread>, "type":"resume" } +``` + +This puts the thread in the **Running** state. The thread will pause again for breakpoint hits, watchpoint hits, throw watches, frame pop watches, and other standing pause requests. + +To step a thread's execution, the client can send a packet of the form: + +``` +{ "to":<thread>, "type":"resume", "resumeLimit":<limit> } +``` + +*Limit* must have one of the following forms: + +``` +{ "type":"next" } +``` + +The thread should pause: + +* just before the current frame is popped, whether by throwing an exception or returning a value; or +* when control in the current frame reaches a different statement than the one it is currently at. + +Note that execution in frames younger than the current frame never meets these conditions, so a `"next"` limit steps over calls, generator-iterator invocations, and so on. + +``` +{ "type":"step" } +``` + +The thread should pause: + +* just before the current frame is popped, whether by throwing an exception or returning a value; or +* just after a new frame is pushed; or +* when control in the current frame reaches a different statement than the one it is currently at. + +This is the same as `"next"`, except that it steps into calls. + +To resume the thread but have it stop when the current frame is about to be popped, the client can send a packet of the form: + +``` +{ "to":<thread>, "type":"resume", "resumeLimit":{ "type":"finish" } } +``` + +Here, the thread should pause just before the current frame is popped, whether by throwing an exception, returning a value, or being terminated. + +When a thread pauses because a limit was reached, the "paused" packet's *reason* will have a type of `"resumeLimit"`. + +A resume limit applies only to the current resumption; once the thread pauses, whether because the limit was reached or some other event occurred—a breakpoint hit, for example—the resume limit is no longer in effect. + +If no `"resumeLimit"` property appears in the `"resume"` packet, then the thread should run until some standing pause condition is met (a breakpoint is hit; a watchpoint triggers; or the like). + +To force the current frame to end execution immediately, the client can send a packet of the form: + +``` +{ "to":<thread>, "type":"resume", "forceCompletion":<completion> } +``` + +where *completion* is a [completion value](#completion-values) indicating whether the frame should return a value, throw an exception, or be terminated. Execution resumes in the current frame's caller, in the manner appropriate for *completion*. + +To request that execution pause when an exception is thrown, the client may send a request of the form: + +``` +{ "to":<thread>, "type":"resume", "pauseOnExceptions": true } +``` + +If `pauseOnExceptions` has the value `false` or is omitted, execution will continue in the face of thrown exceptions. When a thread pauses because an exception was thrown, the "paused" packet's *reason* will have the following form: + +``` +{ "type":"exception", "exception":<exception> } +``` + +where *exception* is a grip on the exception object. + +To request that execution pause on a DOM event, the client may send a request of the form: + +If a `"forceCompletion"` property is present in a `"resume"` packet, along with `"resumeLimit"`, or `"pauseOnExceptions"`, the thread will respond with an error: + +``` +{ "from":<thread>, "error":"badParameterType", "message":<message> } +``` + +A `"resume"` packet closes the pause actor the client provided in the "paused" packet that began the pause. + +If the client sends a `"resume"` packet to a thread that is not in the **Paused** state, the actor sends an error reply of the form: + +``` +{ "from":<thread>, "error":"wrongState", "message":<message> } +``` + +where *message* details which state the thread was in instead (to make debugging debuggers easier). In this case, the thread's state is unaffected. + +### Interrupting a Thread + +If a thread is in the **Running** state, the client can cause it to pause where it is by sending a packet of the following form: + +``` +{ "to":<thread>, "type":"interrupt" } +``` + +The thread responds in one of two ways: + +``` +{ "from":<thread>, "type":"paused", "why":<reason>, ... } +``` + +This indicates that the thread stopped, and is now in the **Paused** state. If *reason* is `{ "type":"interrupted" }`, then the thread paused due to the client's *interrupt* packet. Otherwise, the thread paused of its own accord before receiving the *interrupt* packet, and will ignore the *interrupt* packet when it receives it. In either case, this is an ordinary `"paused"` packet, whose form and additional properties are as described in [Thread Pauses](#thread-pauses), above. + +``` +{ "from":<thread>, "type":"exited" } +``` + +This indicates that the thread exited before receiving the client's *interrupt* packet, and is now in the **Exited** state. See [Exiting Threads](#exiting-threads), below. + +If the client sends an `"interrupt"` packet to a thread that is not in the **Running**, **Paused**, or **Exited** state, the actor sends an error reply of the form: + +``` +{ "from":<thread>, "error":"wrongState", "message":<message> } +``` + +where *message* details which state the thread was in instead (to make debugging debuggers easier). In this case, the thread's state is unaffected. + +### Exiting Threads + +When a thread in the **Running** state exits, it sends a packet of the following form: + +``` +{ "from":<thread>, "type":"exited" } +``` + +At this point, the thread can no longer be manipulated by the client, and most of the thread's resources may be freed; however, the thread actor name must remain alive, to handle stray `interrupt` and `detach` packets. To allow the last trace of the thread to be freed, the client should send a packet of the following form: + +``` +{ "to":<thread>, "type":"release" } +``` + +This acknowledges the exit and allows the thread actor name, *thread*, to be reused for other actors. + +## Inspecting Paused Threads + +When a thread is in the **Paused** state, the debugger can make requests to inspect its stack, lexical environment, and values. + +Only those packets explicitly defined to do so can cause the thread to resume execution. JavaScript features like getters, setters, and proxies, which could normally lead inspection operations like enumerating properties and examining their values to run arbitrary JavaScript code, are disabled while the thread is paused. If a given protocol request is not defined to let the thread run, but carrying out the requested operation would normally cause it to do so—say, fetching the value of a getter property—the actor sends an error reply of the form: + +``` +{ "from":<actor>, "error":"threadWouldRun", "message":<message>, "cause":<cause> } +``` + +where *message* is text that could be displayed to users explaining why the operation could not be carried out. *Cause* is one of the following strings: + +| *cause* value | meaning | +|:-------------:|:------------------------------------------------------------------------:| +| "proxy" | Carrying out the operation would cause a proxy handler to run. | +| "getter" | Carrying out the operation would cause an object property getter to run. | +| "setter" | Carrying out the operation would cause an object property setter to run. | + +(Taken together, the `"threadWouldRun"` error name and the *cause* value should allow the debugger to present an appropriately localized error message.) + +### Loading Script Sources + +To get a snapshot of all sources currently loaded by the thread actor, the client can send the following packet: + +``` +{ to: <threadActorID>, type: "sources" } +``` + +The response packet has the form: + +``` +{ from: <threadActorID>, sources: [<sourceForm1>, <sourceForm2>, ..., <sourceFormN>] } +``` + +Where each *sourceForm* has the following form: + +``` +{ actor: <sourceActorID>, + url: <sourceURL>, + isBlackBoxed: <isBlackBoxed> } +``` + +* *sourceActorID* is the source actor's id +* *sourceURL* is the URL of the source represented by the source actor +* *isBlackBoxed* is a boolean specifying whether the source actor's 'black-boxed' flag is set. See [Black Boxing Sources](#black-boxing-sources). + +Each source actor exists throughout the thread's whole lifetime. + +To get the contents of a source, send the corresponding source actor the following packet: + +``` +{ to: <sourceActorID>, type: "source" } +``` + +And the source actor replies with a packet of the following form: + +``` +{ from: <sourceActorID>, source: <contentsOfSource> } +``` + +where *contentsOfSource* is a grip representing the string of source code: either a JSON string, or a long string grip. (See [Grips](#grips) for a description of long string grips.) + +#### Black-Boxing Sources + +When debugging a web application that uses large off-the-shelf JavaScript libraries, it may help the developer focus on their own code to treat such libraries as "black boxes", whose internal details are omitted or simplified in the user interface. For example, the user interface could display a sub-chain of stack frames within a black-boxed library as a single element; breakpoints set in a black-boxed library could be disabled; and so on. + +Each source actor has a 'black-boxed' flag, and understands requests to set and clear the flag. When a source actor is black-boxed, the debugger does not pause when it hits breakpoints or `debugger` statements inside that source. If pausing on exceptions is enabled and an exception is thrown inside a black-boxed source, the debugger does not pause until the stack has unwound to a frame in a source that is not black-boxed. + +Thread actors still list black-boxed source actors in `"sources"` replies; and include stack frames running black-boxed code in `"frames"` requests. However, each *sourceForm* includes an `"isBlackBoxed"` property, giving the client all the information it needs to implement the black-boxing behavior in the user interface. + +To set a source actor's 'black-boxed' flag: + +``` +{ "to": <sourceActor>, "type": "blackbox" } +``` + +The *sourceActor* responds with a blank response on success: + +``` +{ "from": <sourceActor> } +``` + +Or an error response on failure: + +``` +{ "from": <sourceActor>, "error": <reason> } +``` + +To clear a source actor's 'black-boxed' flag: + +``` +{ "to": <sourceActor>, "type": "unblackbox" } +``` + +And once again, the *sourceActor* responds with a blank response on success: + +``` +{ "from": <sourceActor> } +``` + +Or an error response on failure: + +``` +{ "from": <sourceActor>, "error": <reason> } +``` + +### Listing Stack Frames + +To inspect the thread's JavaScript stack, the client can send the following request: + +``` +{ "to":<thread>, "type":"frames", "start":<start>, "count":<count> } +``` + +The `start` and `count` properties are optional. If present, *start* gives the number of the youngest stack frame the reply should describe, where the youngest frame on the stack is frame number zero; if absent, *start* is taken to be zero. If present, *count* specifies the maximum number of frames the reply should describe; if absent, it is taken to be infinity. (Clients should probably avoid sending `frames` requests with no *count*, to avoid being flooded by frames from unbounded recursion.) + +The thread replies as follows: + +``` +{ "from":<thread>, "frames":[<frame> ...] } +``` + +where each *frame* has the form: + +``` +{ "actor": <actor>, + "depth": <depth>, + "type": <type>, + "this": <this>, + ... } +``` + +where: + +* *actor* is the name of an actor representing this frame; +* *depth* is the number of this frame, starting with zero for the youngest frame on the stack; +* *type* is a string indicating what sort of frame this is; and +* *this* is a grip on the value of `this` for this call. + +The frame may have other properties, depending on *type*. + +All actors mentioned in the frame or grips appearing in the frame (*actor*, *callee*, *environment*, and so on) are parented by the thread actor. + +#### Global Code Frames + +A frame for global code has the form: + +``` +{ "actor":<actor>, + "depth":<depth>, + "type":"global", + "this":<this>, + "where":<location>, + "source":<source>, + "environment":<environment> } +``` + +where: + +* *location* is the source location of the current point of execution in the global code (see [Source Locations](#source-locations)); +* *environment* is a value representing the lexical environment of the current point of execution (see [Lexical Environments](#lexical-environments)); +* *source* is a source form as described in [Loading Script Sources](#loading-script-sources) + +and other properties are as above. + +#### Function Call Frames + +A frame for an ordinary JavaScript function call has the form: + +``` +{ "actor":<actor>, "depth":<depth>, "type":"call", "this":<this>, + "where":<location>, "environment":<environment>, + "callee":<callee>, "arguments":<arguments> } +``` + +where: + +* *callee* is a grip on the function value being called; +* *arguments* is an array of grips on the actual values passed to the function; + +and other properties are as above. + +If the callee is a host function, or a function scoped to some global other than the one to which we are attached, the `"where"` and `"environment"` properties are absent. + +The argument list may be incomplete or inaccurate, for various reasons. If the program has assigned to its formal parameters, the original values passed may have been lost, and compiler optimizations may drop some argument values. + +#### Eval Frames + +A frame for a call to `eval` has the form: + +``` +{ "actor":<actor>, "depth":<depth>, "type":"eval", "this":<this>, + "where":<location>, "environment":<environment> } +``` + +where the properties are as defined above. + +#### Client Evaluation Frames + +When the client evaluates an expression with an `clientEvaluate` packet, the evaluation appears on the stack as a special kind of frame, of the form: + +``` +{ "actor":<actor>, "depth":<depth>, "type":"clientEvaluate", "this":<this>, + "where":<location>, "environment":<environment> } +``` + +where the properties are as defined above. In this case, *where* will be a location inside the expression provided by the debugger. + +### Popping Stack Frames + +The client can remove frames from the stack by sending a request of the form: + +``` +{ "to":<frameActor>, "type":"pop", "completionValue":<completion> } +``` + +where *frameActor* is the actor representing the stack frame to pop, and *completion* is a [completion value](#completion-values) describing how the frame should appear to have finished execution. All younger stack frames are also popped. The thread remains paused. The frame actor will reply: + +``` +{ "from":<frameActor>, "watches":[<watchActor> ...] } +``` + +where each *watchActor* is the name of a frame pop watch actor that has been triggered in the process of popping the given frame. If no frame pop watches are triggered, the `watches` property may be omitted. + +*TODO: specify the error to return if the frame cannot be popped --- can host (C++) function frames be popped?* + +### Evaluating Source-Language Expressions + +To evaluate a source-language expression in a thread, the client sends a specialized `"resume"` packet of the form: + +``` +{ "to":<thread>, "type":"clientEvaluate", "expression":<expr>, "frame":<frame> } +``` + +This resumes the thread just as an ordinary `"resume"` packet does, but, rather than continuing execution where the pause took place, has the thread begin evaluation of the source-language expression given by *expr*, a string. The evaluation takes place in a new [Client Evaluation Frame](#client-evaluation-frames), pushed on top of *thread*'s current stack, using the environment of *frame*. *Frame* must be a live actor for one of *thread*'s frames, and the given frame must be one from which we can retrieve a lexical environment; that is, it must not be the frame for a call to a non-debuggee function. When evaluation of *expr* completes, the client will report a `clientEvaluate` pause containing the expression's value. + +If evaluating *expr* completes abruptly, this outcome is still reported via an `clientEvaluated` pause, so it is not necessary for the client to take explicit steps to catch exceptions thrown by the expression. + +If *frame* is not the name of an actor for a frame currently on *thread*'s stack, the thread actor sends a reply of the form: + +``` +{ "from":<thread>, "error":"unknownFrame", "message":<message> } +``` + +where *message* provides any details that would be helpful to the debugger developers. In this case, the thread's state is unaffected. + +If *frame* is not a frame whose environment we can access, the thread actor sends an error reply of the form: + +``` +{ "from":<thread>, "error":"notDebuggee", "message":<message> } +``` + +where *message* provides further appropriate details. + +If the client sends a `"clientEvaluate"` packet to a thread that is not in the **Paused** state, the actor sends an error reply of the form: + +``` +{ "from":<thread>, "error":"wrongState", "message":<message> } +``` + +where *message* details which state the thread was in instead (to make debugging debuggers easier). In this case, the thread's state is unaffected. + +*TODO: evaluate with given grips bound to given identifiers* + +## Lexical Environments + +A lexical environment (written as *environment* in packet descriptions) records the identifier bindings visible at a particular point in the program. An environment has one of the following forms: + +``` +{ "type":"object", "actor":<actor>, "object":<object>, "parent":<parentEnvironment> } +``` + +This represents a scope chain element whose identifier bindings reflect the properties of *object* (a grip). This could be the global object (`window` in a browser), or a DOM element (for event handler content attributes, which have the input element, form, and document on their scope chain along with the `window`). + +*Actor* is the name of an actor representing this lexical environment. The requests it can answer are described below. + +*ParentEnvironment* is a lexical environment describing the next enclosing environment; the `parent` property is omitted on the outermost environment. + +``` +{ "type":"function", "actor":<actor>, "function":<function>, + "bindings":<bindings>, "parent":<parentEnvironment> } +``` + +This represents the variable environment created by a call to *function* (a grip). *Bindings* describes the bindings in scope, including the function's arguments, the `arguments` object, and local `var` and function bindings; its form is described in detail below. The other properties are as described above. + +``` +{ "type":"with", "actor":<actor>, "object":<object>, "parent":<parentEnvironment> } +``` + +This represents an environment introduced by a `with` statement whose operand is *object* (a grip). The other properties are as described above. + +``` +{ "type":"block", "actor":<actor>, "bindings":<bindings>, "parent":<parentEnvironment> } +``` + +This represents an environment introduced by a `let` block, `for-in` statement, `catch` block, or the like. The properties are as described above. + +A *bindings* value has the form: + +``` +{ "arguments":[ { name:<descriptor> }, ... ], + "variables":{ name:<descriptor>, ... } } +``` + +Each *name* is the name of a bound identifier, as a string. Each *descriptor* is a [property descriptor](#property-descriptors) for the variable, presenting the variable's value as the descriptor's `"value"` property, and the variable's mutability as the descriptor's `"writable"` property. The descriptor's `"configurable"` property reflects whether the environment supports deleting and adding variables. Each descriptor's `"enumerable"` property is `true`. + +The `"arguments"` list appears only in bindings for `"function"` environments. It lists the arguments in the order they appear in the function's definition. (The same name may appear several times in the list, as permitted by JavaScript; the name's last appearance is the one in scope in the function.) + +Note that language implementations may omit some environment records from a function's scope if it can determine that the function would not use them. This means that it may be impossible for a debugger to find all the variables that ought to be in scope. + +To fully enumerate the bindings introduced by any lexical environment, the client can send a request of the following form to the environment's actor: + +``` +{ "to":<envActor>, "type":"bindings" } +``` + +The actor will reply as follows: + +``` +{ "from":<envActor>, "bindings":<bindings> } +``` + +Note that this request elicits a `"threadWouldRun"` error reply when *envActor* refers to an object environment whose object is a proxy. + +To change the value of a variable bound in a particular lexical environment, the client can send a request to the environment's actor: + +``` +{ "to":<envActor>, "type":"assign", "name":<name>, "value":<value> } +``` + +This changes the value of the identifier whose name is *name* (a string) to that represented by *value* (a grip). The actor will reply as follows, simply: + +``` +{ "from":<envActor> } +``` + +If the named identifier is immutable, the actor will send an error reply of the form: + +``` +{ "from":<envActor>, "error":"immutableBinding", "message":<message> } +``` + +If *envActor* refers to an object environment whose object is a proxy, or whose property named *name* has a setter function, this request elicits a `"threadWouldRun"` error reply. + +### Lexical Environment Examples + +For example, if we have the following JavaScript code: + +``` +function f(x) { + function g(y) { + var z = "value of z"; + alert(x + y); + } +} +``` + +we set a breakpoint on the line containing the call to `alert`, and then evaluate the expression: + +``` +f("argument to f")("argument to g") +``` + +then we would hit that breakpoint, eliciting a packet like the following: + +``` +{ "from":<thread>, "type":"paused", "actor":<pauseActor>, + "why":{ "type":"breakpoint", "actors":[<breakpointActor>] }, + "frame":{ "actor":<frameActor>, "depth":1, + "type":"call", "where":{ "url":"sample.js", "line":3 }, + "environment":{ "type":"function", "actor":<gFrameActor>, + "function":{ "type":"object", "class":"Function", "actor":<gActor> }, + "functionName":"g", + "bindings":{ arguments: [ { "y": { "value":"argument to g", "configurable":"false", + "writable":true, "enumerable":true } } ] }, + "parent":{ "type":"function", "actor":<fFrameActor>, + "function":{ "type":"object", "class":"Function", "actor":<fActor> }, + "functionName":"f", + "bindings": { arguments: [ { "x": { "value":"argument to f", "configurable":"false", + "writable":true, "enumerable":true } } ], + variables: { "z": { "value":"value of z", "configurable":"false", + "writable":true, "enumerable":true } } }, + "parent":{ "type":"object", "actor":<globalCodeActor>, + "object":{ "type":"object", "class":"Global", + "actor":<globalObjectActor> } + } + } + }, + "callee":<gActor>, "calleeName":"g", + "this":{ "type":"object", "class":"Function", "actor":<gActor> }, + "arguments":["argument to g"] + } +} +``` + +You can see here the three nested environment forms, starting with the `environment` property of the top stack frame, reported in the pause: + +* The first environment form shows the environment record created by the call to `g`, with the string `"argument to g"` passed as the value of `y`. +* Because `g` is nested within `f`, each function object generated for `g` captures the environment of a call to the enclosing function `f`. Thus, the next thing on `g`'s scope chain is an environment form for the call to `f`, where `"argument to f"` was passed as the vale of `x`. +* Because `f` is a top-level function, the (only) function object for `f` closes over the global object. This is the "type":"object" environment shown as the parent of `f`'s environment record. +* Because the global object is at the end of the scope chain, its environment form has no `parent` property. + +## Breakpoints + +While a thread is paused, a client can set breakpoints in the thread's code by sending requests of the form: + +``` +{ "to":<thread>, "type":"setBreakpoint", "location":<location> } +``` + +where *location* is a [source location](#source-locations). If the thread is able to establish a breakpoint at the given location, it replies: + +``` +{ "from":<thread>, "actor":<actor>, "actualLocation":<actualLocation> } +``` + +where *actor* is an actor representing the breakpoint (a child of the thread actor), and *actualLocation* is the location at which the breakpoint was really set. If *location* and *actualLocation* are the same, then the `actualLocation` property can be omitted. + +If the thread cannot find the script referred to in *location*, it sends an error reply of the form: + +``` +{ "from":<thread>, "error":"noScript" } +``` + +If *location* refers to a line and column at which the given script has no program code, and no reasonable alternative location can be chosen (say, by skipping forward), then the thread sends an error reply of the form: + +``` +{ "from":<thread>, "error":"noCodeAtLineColumn" } +``` + +To delete a breakpoint, the client can send the breakpoint's actor a message of the form: + +``` +{ "to":<breakpointActor>, "type":"delete" } +``` + +to which the breakpoint actor will reply, simply: + +``` +{ "from":<breakpointActor> } +``` + +This closes communications with *breakpointActor*. + +## Event Listeners + +To request a list of all the event listeners and event handlers (see [DOM Event Handlers](https://developer.mozilla.org/docs/Web/Guide/DOM/Events/Event_handlers#Definitions) for definitions of the two terms) attached to the page, the client sends a request of the form: + +``` +{ "to":<thread>, "type":"eventListeners" } +``` + +The thread replies with a response of the form: + +``` +{ "from":<thread>, "listeners":[ <listener>, ... ] } +``` + +Such requests can be sent when the thread is either paused or running. A *listener* value has the form: + +``` +{ "node":{ "selector":<node-selector>, "object":<node> }, + "type":<type>, + "capturing":<capturing>, + "allowsUntrusted":<allowsUntrusted>, + "inSystemEventGroup":<inSystemEventGroup>, + "isEventHandler":<isEventHandler>, + "function":<function> } +``` + +The values for these properties are: + +***node-selector*** + +A unique CSS selector of the DOM element on which the event handler is attached, or `"window"` if the handler is attached on the window. + +***node*** + +A grip on the DOM element on which the event handler is attached. + +***type*** + +The type of the DOM event as specified in the DOM specification (see [nsIEventListenerInfo](https://developer.mozilla.org/docs/XPCOM_Interface_Reference/nsIEventListenerInfo#Attributes)). + +***capturing*** + +A boolean flag indicating whether the event listener is in capture mode (see [nsIEventListenerInfo](https://developer.mozilla.org/docs/XPCOM_Interface_Reference/nsIEventListenerInfo#Attributes)). + +***allowsUntrusted*** + +A boolean flag that indicates whether the listener allows untrusted events (see [nsIEventListenerInfo](https://developer.mozilla.org/docs/XPCOM_Interface_Reference/nsIEventListenerInfo#Attributes)). + +***inSystemEventGroup*** + +A boolean flag that indicates whether or not the event listener is in the system event group (see [nsIEventListenerInfo](https://developer.mozilla.org/docs/XPCOM_Interface_Reference/nsIEventListenerInfo#Attributes)). + +***isEventHandler*** + +A boolean flag indicating whether this is an event handler or an event listener (see [DOM Event Handlers](https://developer.mozilla.org/docs/Web/Guide/DOM/Events/Event_handlers#Definitions) for definitions of the two terms). For HTML attribute handlers or assignments to WebIDL properties this flag would be true. + +***function*** + +A grip on the function object. + +## Stream Transport + +The debugging protocol is specified in terms of packets exchanged between a client and server, where each packet is either a JSON text or a block of bytes (a "bulk data" packet). The protocol does not specify any particular mechanism for carrying packets from one party to the other. Implementations may choose whatever transport they like, as long as packets arrive reliably, undamaged, and in order. + +This section describes the Mozilla Remote Debugging Protocol Stream Transport, a transport layer suitable for carrying Mozilla debugging protocol packets over a reliable, ordered byte stream, like a TCP/IP stream or a pipe. Debugger user interfaces can use it to exchange packets with debuggees in other processes (say, for debugging Firefox chrome code), or on other machines (say, for debugging Firefox OS apps running on a phone or tablet). + +(The Stream Transport is not the only transport used by Mozilla. For example, when using Firefox's built-in script debugger, the client and server are in the same process, so for efficiency they use a transport that simply exchanges the JavaScript objects corresponding to the JSON texts specified by the protocol, and avoid serializing packets altogether.) + +### Packets + +Once the underlying byte stream is established, transport participants may immediately begin sending packets, using the forms described here. The transport requires no initial handshake or setup, and no shutdown exchange: the first bytes on the stream in each direction are those of the first packet, if any; the last bytes on the stream in each direction are the final bytes of the last packet sent, if any. + +The transport defines two types of packets: JSON and bulk data. + +#### JSON Packets + +A JSON packet has the form: + +``` +length:JSON +``` + +where *length* is a series of decimal ASCII digits, *JSON* is a well-formed JSON text (as defined in [RFC 4627](http://www.ietf.org/rfc/rfc4627.txt)) encoded in UTF-8, and *length*, interpreted as a number, is the length of *JSON* in bytes. + +#### Bulk Data Packets + +A bulk data packet has the form: + +``` +bulk actor type length:data +``` + +where: + +* The keyword `bulk` is encoded in ASCII, and the spaces are always exactly one ASCII space +* *actor* is a sequence of Unicode characters, encoded in UTF-8, containing no spaces or colons +* *type* is a sequence of Unicode characters, encoded in UTF-8, containing no spaces or colons +* *length* is a sequence of decimal ASCII digits +* *data* is a sequence of bytes whose length is *length* interpreted as a number + +The *actor* field is the name of the actor sending or receiving the packet. (Actors are server-side entities, so if the packet was sent by the client, *actor* names the recipient; and if the packet was sent by the server, *actor* names the sender.) The protocol imposes the same syntactic restrictions on actor names that we require here. + +Which actor names are valid at any given point in an exchange is established by the remote debugging protocol. + +The *type* field defines the type of the packet, which may be used with the actor name to route the packet to its destination properly. The protocol provides more detail about the type, which remains in effect here. + +The content of a bulk data packet is exactly the sequence of bytes appearing as *data*. Data is not UTF-8 text. + +### Stream Requirements + +The Stream Transport requires the underlying stream to have the following properties: + +* It must be **transparent**: each transmitted byte is carried to the recipient without modification. Bytes whose values are ASCII control characters or fall outside the range of ASCII altogether must be carried unchanged; line terminators are left alone. +* It must be **reliable**: every transmitted byte makes it to the recipient, or else the connection is dropped altogether. Errors introduced by hardware, say, must be detected and corrected, or at least reported (and the connection dropped). The Stream Transport includes no checksums of its own; those are the stream's responsibility. (So, for example, a plain serial line is not suitable for use as an underlying stream.) +* It must be **ordered**: bytes are received in the same order they are transmitted, and bytes are not duplicated. (UDP packets, for example, may be duplicated or arrive out of order.) + +TCP/IP streams and USB streams meet these requirements. + +### Implementation Notes + +#### Constant-Overhead Bulk Data + +Mozilla added bulk data packets to the protocol to let devices with limited memory upload performance profiling and other large data sets more efficiently. Profiling data sets need to be as large as possible, as larger data sets can cover a longer period of time or more frequent samples. However, converting a large data set to a JavaScript object, converting that object to a JSON text, and sending the text over the connection entails making several temporary complete copies of the data; on small devices, this limits how much data the profiler can collect. Avoiding these temporary copies would allow small devices to collect and transmit larger profile data sets. Since it seemed likely that other sorts of tools would need to exchange large binary blocks efficiently as well, we wanted a solution usable by all protocol participants, rather than one tailored to the profiler's specific case. + +In our implementation of this Stream Transport, when a participant wishes to transmit a bulk data packet, it provides the actor name, the type, the data's length in bytes, and a callback function. When the underlying stream is ready to send more data, the transport writes the packet's `bulk actor type length:` header, and then passes the underlying `nsIOutputStream` to the callback, which then writes the packet's data portion directly to the stream. Similarly, when a participant receives a bulk data packet, the transport parses the header, and then passes the actor name, type, and the transport's underlying `nsIInputStream` to a callback function, which consumes the data directly. Thus, while the callback functions may well use fixed-size buffers to send and receive data, the transport imposes no overhead proportional to the full size of the data. |