Add runtime.getContexts() proposal

proposals/runtime_get_contexts.md

@@ -0,0 +1,233 @@
+# New API: runtime.getContexts()
+
+## Background / Summary
+
+Chromium currently has the
+[`extension.getViews()`](https://developer.chrome.com/docs/extensions/reference/extension/#method-getViews)
+API method that allows an extension to get information about the "views" that
+are active for the extension. A "view" in this context is any HTML frame in the
+extension's process that commits to the extension origin; this may not be all
+the extension owns, such as in the case of incognito-mode frames. The returned
+values are a set of `HTMLWindow` objects, which the extension has permission to
+reach directly into (as they are same-origin).
+
+Some common reasons for calling this method are: determining if a toolbar
+popup, tab, or options page is open; directly interacting with those pages by
+reaching into their `HTMLWindow`; etc.
+
+Chromium's implementation of Manifest V2 (MV2) allows for
+[background pages](https://developer.chrome.com/docs/extensions/mv2/background_pages/)
+to call this API. This is possible because these pages are themselves an
+extension frame (albeit one that isn't visibly rendered) and run on the main
+thread of the renderer; this allows them easy access to the JavaScript
+[`Window`](http://go/mdn/API/Window#instance_properties) objects provided by
+`extension.getViews()`.
+
+## Problem
+
+With the migration to Manifest V3 (MV3), background pages
+[no longer exist](https://developer.chrome.com/docs/extensions/mv3/migrating_to_service_workers/);
+instead, an extension's background context is
+[service worker](https://developer.chrome.com/docs/workbox/service-worker-overview/)-based.
+For technical reasons<sup>[1](#footnotes)</sup>, service workers cannot access
+the [`Window`](http://go/mdn/API/Window#instance_properties) objects that
+`extension.getViews()` provides and it is not feasible to implement that with
+our browser design. Due to this, we cannot provide access to the JavaScript
+context for these views, but we can allow an extension to query for them
+(determining if they exist) and target them for messaging purposes.
+
+## Solution
+
+Considering the above situation, we'd like to propose a new extension API
+method, `runtime.getContexts()`, to asynchronously provide metadata about
+associated contexts that is still useful for an extension. This will allow
+extension background scripts to identify the active contexts in the extension.
+For example, this can be used to target messages to send using
+[`runtime.sendMessage()`](https://developer.chrome.com/docs/extensions/reference/runtime/#method-sendMessage),
+etc.). Introducing this API will allow an easier migration from MV2.
+
+This also obviates the need for introducing multiple one-off APIs, such as
+separate APIs to query for the extension popup, offscreen documents, etc.
+
+## API Proposal
+
+This method will return an array of matching contexts, represented by a new
+`ExtensionContext` type. This will be defined as:
+
+```
+runtime.Context = {
+  // Context type -- tab, popup, etc.
+  contextType: ContextType,
+  // A unique identifier for this context.
+  contextId: string,
+  // ID of the tab for this context, or -1 if this context is not hosted in a
+  // tab.
+  tabId: int,
+  // ID of the window for this context, or -1 if this context is not hosted in a
+  // window.
+  windowId: int,
+  // ID of the DOM document for this context, or undefined if this context is
+  // not associated with a document.
+  documentId?: string,
+  // ID of the frame for this context, or -1 if this context is not hosted in a
+  // frame.
+  frameId: int,
+  // The current URL of the document, or undefined if this context is not
+  // hosted in a document.
+  documentUrl?: string,
+  // The current origin of the document, or undefined if this context is not
+  // hosted in a document.
+  documentOrigin?: string,
+  // Whether the context is for an incognito profile.
+  incognito: boolean,
+}
+```
+
+`ContextType` will indicate the type of context retrieved. It is an enum
+defined as:
+
+```
+extension.ContextType = {
+  // Tabs the extension is running in.
+  TAB: 'TAB',
+  // Toolbar popups created by the extension.
+  // TODO: Should this be `TOOLBAR_POPUP` to avoid ambiguity with web popup
+  // windows? Or perhaps `ACTION_POPUP` to avoid tying it to a particular UI
+  // surface?
+  POPUP: 'POPUP',
+  // The background context for the extension (in Chromium, the extension
+  // service worker).
+  BACKGROUND: 'BACKGROUND',
+  // Offscreen documents for the extension.
+  OFFSCREEN_DOCUMENT: 'OFFSCREEN_DOCUMENT',
+};
+```
+
+This enum will be expanded in the future as more context types are added.
+
+The method signature will be defined as:
+
+```
+runtime.getContexts(
+  filter?: ContextFilter
+): Promise<ExtensionContext[]>;
+```
+
+The `filter` argument will be used to filter down to a particular type of
+context. It will share the same properties as `ExtensionContext`, but will
+have all fields be optional. Any omitted field matches all available contexts.
+
+### Additional Considerations
+
+#### Context ID
+
+Each extension context will have a unique context ID, represented by a string.
+This is necessary to uniquely identify a context, since other fields may be
+non-unique (such as URL) or absent (such as documentId).
+
+Like `documentId`s, the extension `contextId` will update on (non-same-page)
+navigation.
+
+#### Incognito mode
+
+[Split-mode](https://developer.chrome.com/docs/extensions/mv3/manifest/incognito/#split)
+extensions will _not_ have access to the contexts from their corresponding
+profile. That is, the incognito extension process will not be able to access
+contexts from the non-incognito extension process, and vice versa.
+
+#### Sandboxed Pages
+
+Sandboxed pages will not be included in the returned contexts. They are
+a separate origin (`"null"`) and do not have access to extension APIs.
+
+#### TOCTOU
+
+Time-of-Check vs Time-of-Use (TOCTOU) issues are an unavoidable aspect of this
+API, given its asynchronous nature. An extension may call `getContexts()` and,
+by the time it receives the result, the values (such as available contexts or
+URLs of those contexts) may be different.
+
+This is something extension developers already need to worry about with any
+asynchronous API. API calls should handle references to non-existent contexts
+gracefully, and extension developers can leverage concepts like `documentId`
+(which is updated when a frame navigates, allowing extensions to identify if a
+given context is the same as when they queried it).
+
+#### Naming
+
+*   *ContextType*: While there already exist more "Context" references in APIs
+    (such as `ContextType` on the `contextMenus` API), the namespace (`runtime`)
+    helps differentiate these.
+*   *`documentUrl` et al*: We use `documentUrl` on contexts (as opposed to
+    `url`) to indicate the URL is that of the document associated with the
+    context. This is relevant for cases where there may not be a document URL
+    (such as service workers) and allows for potential future expansion where
+    other URLs (such as script URLs) may exist.
+
+#### Default / Absent Values
+
+There is an inconsistency in how we represent a value for a context that doesn't
+have the associated trait, such as a `tabId` or `documentId` for an extension's
+background service worker context (which has neither an associated tab nor
+document). Some of these values -- `tabId`, `windowId`, and `frameId` -- use
+constant values to indicate "no state", such as `-1` for `tabId`. Others --
+`documentId`, `documentUrl`, and `documentOrigin` -- use undefined to indicate
+this.
+
+This is an artifact of existing APIs and precedence. Since many existing APIs
+use the constant integer values, we want to be consistent with those. However,
+for newly-introduced fields, we use the more intuitive `undefined` state.
+
+## Future Work
+
+### Messaging APIs Support `ContextId`s as Target
+
+In practice, many extension messages are meant for a single target, but are
+broadcast to all extension contexts. With the ability to uniquely identify a
+single extension context, we will modify messaging APIs (such as
+`runtime.sendMessage()` and `runtime.connect()`) to allow specifying specific
+targets that should receive a message.
+
+### Multi Page Architecture Fields (`DocumentLifeCycle`, `FrameType`, and `parentDocumentId`, and `parentFrameId`)
+
+[Multi Page Architecture](https://docs.google.com//1NginQ8k0w3znuwTiJ5qjYmBKgZDekvEPC22q0I4swxQ#heading=h.w1qo2n6sr8wn)
+caused
+[multiple changes](https://developer.chrome.com/blog/extension-instantnav/) to
+Chromium's
+[tabs API](https://developer.chrome.com/docs/extensions/reference/tabs/),
+[scripting API](https://developer.chrome.com/docs/extensions/reference/scripting/),
+and
+[web navigation API](https://developer.chrome.com/docs/extensions/reference/webNavigation/).
+
+Among these changes are the additions of `DocumentLifecycle`, `FrameType`, and
+`parentDocumentId`. If there is sufficient demand, we can consider adding these
+fields to the `ExtensionContext` type.
+
+### runtime.getCurrentContext()
+
+We would like to provide an additional API, `runtime.getCurrentContext()`, to
+return the calling context.
+
+### Content Script Contexts
+
+[Content scripts](https://developer.chrome.com/docs/extensions/mv3/content_scripts/)
+run in a separate
+[Renderer](https://developer.chrome.com/blog/inside-browser-part3/) (and
+process) from the extension process. In this first version of the API, we will
+not include these contexts due to the complexity it entails. However, we would
+like to add these contexts in the future.
+
+With the content script additions, we may add new fields to `ExtensionContext`,
+such as `scriptUrl` (to indicate the content script's source).
+
+## Footnotes
+
+<sup>1</sup>: Non-main threads in a
+[Renderer](https://developer.chrome.com/blog/inside-browser-part3/) (where
+service workers run in Chromium) cannot access DOM concepts directly (they are
+only accessible from the main renderer thread). Service workers thus cannot
+synchronously access the JavaScript
+[`Window`](http://go/mdn/API/Window#instance_properties) objects provided by
+`extension.getViews()`. Supporting this access would take engineering years to
+change, and is likely undesirable due to the complexity and considerations it
+would introduce (threading and locking, slowing down main thread execution).