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Cookies and Prerender

Shared local storage such as DOM storage, IndexedDB, and HTTP cookies present challenges for prerendering. Ideally mutations made by the prerendered page should not be visible to other tabs until after the user has activated the page. Mutations made by other pages should be reflected in the prerendered page, which may have already read from local storage before the other pages made the changes.

There is some thought about adding a transactional cookie store which would be memory only for the prerendered page. Unlike some of the HTML5 storage mechanisms, cookies primarily exist in HTTP and setter's may not have access to the Visibility API to change behavior. Basically, the transactional store would exist in memory only. If the user never uses a prerendered page, any mutations will just disappear. If the user tries to navigate to a page, mutations in the transactional cookie store are compared against mutations in the profile's main cookie store. If there are any conflicts, the prerendered page is cancelled and the mutations disappear. Otherwise, the mutations are merged into the profile's main cookie store and the page is swapped in.

Prerendered requests will use a ChromeURLRequestContext which has a new CookieStore interface, but is otherwise the same as the current profile’s ChromeURLRequestContext. If the PrerenderContents are discarded without being used, the changes made to the CookieStore interface go away. Otherwise, the deltas will be committed to the main CookieStore for the profile. If there is a merge problem, the prerendered page is discarded and a fresh request is issued.

There are a couple of possible approaches to implementing this:

a) Do a complete copy of the canonical cookie store for the prerendered page. Both the canonical cookie store and the prerendered cookie store make mutations locally, and both maintain a log of mutations. Abandoning is simple: just delete the new cookie store. Conflict detection happens by examining the logs of the canonical store and the prerender store, and merges are simply applying the logs of the prerendered store to the canonical store if no detection detected. The main advantage is that this does not change the current "get all cookies for this request" path at all. The downsides are memory cost plus memcpy (back-of-the-envelope estimate is 600KB for store) and some overhead for Set-Cookie in non prerender case due to log.

b) Add ability for CookieMonster's to be chained together. The prerendered version starts out with 0 entries, but points to the canonical backing store. Set-Cookie in the prerendered page only adds entries to the prerendered version. "Get all cookies" in the prerendered page will need to merge together cookies from the prerendered cookiemonster and the backing cookiemonster. Abandoning is easy - just remove the prerendered cookie monster. Merge will look at last modified times for entries and any newer ones in the backing store will cause conflict detection. The main concern is how to detect a clearing of Set-Cookie in the backing store at the time of merge. The main advantage of this case is low overhead at startup and fairly minimial logic changes. The concerns are the complexity of "get all cookies" for the prerendered case, as well as detecting deleted cookies during the merge.

c) Add a new CookieStore implementation which stores diffs and has a CookieMonster backing store. This is similar to b), but could handle the deleted cookie case better. The downside is the potential for lots of duplicated logic, and very different lookup paths. Also, a fair bit of code does downcasting of CookieStore to CookieMonster, so the CookieStore interface will need to change for this to work.

A few other concerns:
 - Are cookies ever cached in the render process? For example, does document.cookies do an IPC over to the CookieMonster to retrieve the values each time, or is there a cache that we will have to deal with?
 - How big are the cookiestore's typically? Could we get away with the copy-and-log approach?

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