* Memoize promise listeners to prevent exponential growth
Previously, React would attach a new listener every time a promise is
thrown, regardless of whether the same listener was already attached
during a previous render. Because React attempts to render every time
a promise resolves, the number of listeners grows quickly.
This was especially bad in synchronous mode because the renders that
happen when the promise pings are not batched together. So if a single
promise has multiple listeners for the same root, there will be multiple
renders, which in turn results in more listeners being added to the
remaining unresolved promises. This results in exponential growth in
the number of listeners with respect to the number of IO-bound
components in a single render.
Fixes#14220
* Memoize on the root and Suspense fiber instead of on the promise
* Add TODO to fix persistent mode tests
* BUG: ReactPartialRenderer / New Context polutes mutable global state
The new context API stores the provided values on the shared context instance. When used in a synchronous context, this is not an issue. However when used in an concurrent context this can cause a "push provider" from one react render to have an effect on an unrelated concurrent react render.
I've encountered this bug in production when using renderToNodeStream, which asks ReactPartialRenderer for bytes up to a high water mark before yielding. If two Node Streams are created and read from in parallel, the state of one can polute the other.
I wrote a failing test to illustrate the conditions under which this happens.
I'm also concerned that the experimental concurrent/async React rendering on the client could suffer from the same issue.
* Use unique thread ID for each partial render to access Context
This first adds an allocator that keeps track of a unique ThreadID index
for each currently executing partial renderer. IDs are not just growing
but are reused as streams are destroyed.
This ensures that IDs are kept nice and compact.
This lets us use an "array" for each Context object to store the current
values. The look up for these are fast because they're just looking up
an offset in a tightly packed "array".
I don't use an actual Array object to store the values. Instead, I rely
on that VMs (notably V8) treat storage of numeric index property access
as a separate "elements" allocation.
This lets us avoid an extra indirection.
However, we must ensure that these arrays are not holey to preserve this
feature.
To do that I store the _threadCount on each context (effectively it takes
the place of the .length property on an array).
This lets us first validate that the context has enough slots before we
access the slot. If not, we fill in the slots with the default value.
Andrew Clark
Sebastian Markbåge
Brian Vaughn
Dan Abramov