[Extensions Bindings] Add some documentation

extensions/renderer/bindings.md

+# Extension Bindings
+
+[TOC]
+
+## What Is It?
+
+The Bindings System is responsible for creating the JS entry points for APIs.
+It creates the `chrome` object (if it does not exist) and adds the API objects
+(e.g. `tabs`) that should be accessible to the context.
+
+## Initialization
+
+Bindings are initialized by creating an ObjectTemplate from an API specification
+and stamping out copies of this template.  This means that once an API is
+instantiated once, further instantiations within that same process are
+significantly faster.  The API itself is specified from a .json or .idl file in
+extensions/common/api or chrome/common/extensions/api.
+
+This is slightly complicated because APIs may have features (such as specific
+methods or events) that are restricted in certain contexts, even if the rest of
+the API is available.  As a result, after object instantiation, there’s a chance
+we may have to alter the object in order to remove these unavailable features.
+
+## API Features
+
+A "feature" of an API is a property on the API object to expose some
+functionality.  There are three main types of features exposed on APIs.
+
+* __Functions__:
+Functions are the main type of feature exposed on APIs.  They allow callers to
+interact with the browser and trigger behavior.
+
+* __Events__:
+Most events are dispatched when something happens to inform an interested party
+of the instance.  Callers subscribe to the events they are interested in, and
+are notified only for subscribed events.  While most events do not influence
+behavior change in the browser, declarative events may.
+
+* __Properties__:
+Certain APIs have exposed properties that are accessed directly on the API
+object.  These are frequently constants (including enum definitions), but are
+also sometimes properties relating to the state of the context.
+
+## Restriction
+
+Not all APIs are available to all contexts; we restrict which capabilities are
+exposed based on multiple factors.
+
+### Scope
+
+Features may be restricted at multiple scopes.  The most common is at the
+API-scope - where none of the API will be made available if the requirements
+aren’t met.  In this case, the chrome.<apiName> property will simply be
+undefined.  However, we also have the ability to restrict features on a more
+granular scope, such as at the method or event level.  In this case, even though
+most of an API may be available, a certain function might not be; or,
+conversely, only a small subset of features may be available while the rest of
+the API is restricted.
+
+### Restricting Properties
+Feature restrictions are based on a specific v8::Context.  Different
+contexts within the same frame may have different API availabilities (this is
+significantly different than the web platform, where features are exposed at the
+frame-level).  The bindings system takes into account context type, associated
+extensions, URL, and more when evaluating features; for more information, see
+the [feature documentation](/chrome/common/extensions/api/_features.md).
+
+## Typical Function Flow
+
+The typical flow for all API methods is the same.  A JS entry point (the method
+on the API object) leads to a common native implementation.  This implementation
+has the following steps:
+
+* __Argument Parsing__:
+
+Passed arguments are parsed against an expected signature defined in the API
+specification.  If the passed arguments match the signature, the arguments are
+normalized and converted to a serialized format (base::Value).
+* __Request Dispatch__:
+A request is dispatched with the parsed arguments and other information about
+the request (such as requesting context and user gesture status).  If a callback
+is included in the arguments, it is stored (along with other information about
+the request) until the response is received.
+* __Request Response__:
+A response is provided asynchronously, indicating success or failure, along with
+any return values (to pass to a provided callback) or an error message.  The
+pending request is removed.
+
+## Custom Function Hooks
+
+Certain APIs need to deviate from this typical flow in order to customize
+behavior.  We provide the following general custom hooks for APIs to modify the
+typical behavior.
+
+* __updateArgumentsPreValidate__:
+Allows an API implementation to modify passed arguments before the argument
+signature is validated.  This can be useful in the case of undocumented
+(internal) parameters or properties, such as a generated ID.
+* __updateArgumentsPostValidate__:
+Allows an API implementation to modify passed arguments after the argument
+signature is validated, but before the request is handled.  Note: this is
+usually bad practice, as any modification means that the arguments no longer
+match the expected signature.  This can cause headaches when we attempt to
+deserialize these values.
+* __handleRequest__:
+Allows an API implementation to internally handle a request.  This is useful
+when the request itself should not go through the normal flow, such as when the
+logic requires a greater level of involvement on the renderer, or is entirely
+handled without needing to message the browser.
+* __customCallback__:
+Allows an API implementation to add a callback that should be called with the
+result of an API function call before the caller’s callback is invoked.  It is
+the responsibility of the custom callback to invoke the original callback, which
+is passed as an argument.  This is useful when the return results should be
+mutated before returning to the caller (which can be necessary when the eventual
+result could be a renderer-specific concept, such as a DOMWindow).
+
+An API implementation may use one or more of these hooks.
+
+### Registering Hooks
+
+Custom Hooks can be registered through either native or JS bindings. In native
+bindings, APIs can subclass APIBindingHooksDelegate and register themselves with
+the bindings system. This typically happens during the bootstrapping of the
+renderer process. Native binding hooks are the preferred approach for new
+bindings.
+
+We also expose hooks in JS through the APIBindingBridge object, which provides
+a registerCustomHook method to allow APIs to create hooks in JS. This style of
+custom hooks is __not preferred__ and will be __deprecated__. These are bad
+because a) JS is much more susceptible to untrusted code and b) since these run
+on each object instantiation, the performance cost is significantly higher.
+
+## Events
+
+Events are dispatched when the associated action occurs.
+
+### Types
+
+There are three types of events.
+
+* __Regular__:
+These events are dispatched to the subscriber when something happens, and merely
+serve as a notification to allow the subscriber to react.
+* __Declarative__:
+Declarative events allow a subscriber to specify some action to be taken when an
+event occurs.  For instance, the declarativeContent API allows a subscriber to
+indicate that an action should be shown whenever a certain URL pattern or CSS
+rule is matched.  For these events, the subscriber is not notified when the
+event happens; rather, the browser immediately takes the specified action.  By
+virtue of not notifying the subscriber, we help preserve the user’s privacy; if
+a subscriber says "do X when the user visits example.com", it does not know
+whether the user visited example.com.  (Note: subsequent actions, such as a user
+interacting with the action on a given page, can expose this.)
+* __Imperative__:
+A few events are designed to be dispatched and to return a response from the
+subscriber, indicating an action the browser should take.  These are
+predominantly used in the webRequest API, where a subscriber can register events
+for navigations, receive notifications of those navigations, and return a result
+of whether the navigation should continue, cancel, or redirect.  These events
+are generally discouraged for performance reasons, and declarative events are
+preferred.
+
+### Filters
+
+Certain events also allow the registration of filters, which allow subscribers
+to only be notified of a subset of events.  For example, the webNavigation and
+webRequest APIs allow filtering by URL pattern, so that uninteresting
+navigations are ignored.
+
+## Legacy JavaScript Implementations
+
+The prior bindings system was implemented primarily in JavaScript, rather than
+utilizing native code.  There were many reasons for this, but they include ease
+of coding and more limited interactions with Blink (WebKit at the time) and V8.
+Unfortunately, this led to numerous security vulnerabilities (because untrusted
+code can run in the same context) and performance issues (because bindings were
+set up per context, and could not be cached in any way).
+
+While the native bindings system replaces the core functionality with a native
+implementation, individual APIs may still be implemented in JavaScript custom
+bindings, or hooks.  These should eventually be replaced by native-only
+implementations.
+
+## Differences Between Web/Blink Bindings
+
+There are a number of differences between the Extensions Bindings System and
+Blink Bindings.
+
+### Common Implementation to Optimize Binary Size
+
+Most Extension APIs are implemented in the browser process after a common flow
+in the renderer.  This allows us to optimize the renderer implementation for
+space and have the majority of APIs lead to a single entry point, which can
+match an API against an expected schema.  This is contrary to Blink Bindings,
+which set up a distinct separate entry point for each API, and then individually
+parses the expected results.
+
+The Blink implementation provides greater speed, but comes at a larger generated
+code cost, since each API has its own generated parsing and handling code.
+Since most Blink/open web APIs are implemented in the renderer, this cost is not
+as severe - each API would already require specialized code in the renderer.
+
+Extension APIs, on the other hand, are predominantly implemented in the browser;
+this means we can optimize space by having a single parsing/handling point.
+This is also beneficial because many extension APIs are exposed on a more
+limited basis, where only a handful of contexts need access to them, and thus
+the binary size savings is more valuable, and the speed cost less harmful.
+
+### Signature Matching
+
+Signature matching differs significantly between WebIDL and Extension APIs.
+
+#### Optional Inner Parameters
+
+Unlike OWP APIs, Extension APIs allow for optional inner parameters.  For
+instance, if an API has the signature `(integer, optional string, optional
+function)`, it may be invoked with `(integer, function)` - which would not be
+valid in the OWP.  This also allows for inner parameters to be optional with
+subsequent required parameters, such as `(integer, optional string, function)` -
+again, something which would be disallowed on the OWP.
+
+#### Unknown Properties
+
+Unknown properties on objects are, by default, unallowed.  That is, if a
+function accepts an object that has properties of `foo` and `bar`, passing
+`{foo: <foo>, bar: <bar>, baz: <baz>}` is invalid.