Bug: A sync ping in render phase unwinds the stack

I found this bug when working on a different task.

`pingSuspendedRoot` sometimes calls `prepareFreshStack` to interupt the
work-in-progress tree and force a restart from the root. The idea is 
that if the current render is already in a state where it be blocked
from committing, and there's new data that could unblock it, we might
as well restart from the beginning.

The problem is that this is only safe to do if `pingSuspendedRoot` is
called from a non-React task, like an event handler or a microtask.
While this is usually the case, it's entirely possible for a thenable
to resolve (i.e. to call `pingSuspendedRoot`) synchronously while the
render phase is already executing. If that happens, and work loop
attempts to unwind the stack, it causes the render phase to crash.

This commit adds a regression test that reproduces one of
these scenarios.

packages/react-reconciler/src/__tests__/ReactSuspenseWithNoopRenderer-test.js

@@ -4218,4 +4218,80 @@ describe('ReactSuspenseWithNoopRenderer', () => {
     });
     expect(Scheduler).toHaveYielded(['Unmount Child']);
   });
+
+  // @gate enableLegacyCache
+  it(
+    'regression test: pinging synchronously within the render phase ' +
+      'does not unwind the stack',
+    async () => {
+      // This is a regression test that reproduces a very specific scenario that
+      // used to cause a crash.
+      const thenable = {
+        then(resolve) {
+          resolve('hi');
+        },
+        status: 'pending',
+      };
+
+      function ImmediatelyPings() {
+        if (thenable.status === 'pending') {
+          thenable.status = 'fulfilled';
+          throw thenable;
+        }
+        return <Text text="Hi" />;
+      }
+
+      function App({showMore}) {
+        return (
+          <div>
+            <Suspense fallback={<Text text="Loading..." />}>
+              {showMore ? (
+                <>
+                  <AsyncText text="Async" />
+                </>
+              ) : null}
+            </Suspense>
+            {showMore ? (
+              <Suspense>
+                <ImmediatelyPings />
+              </Suspense>
+            ) : null}
+          </div>
+        );
+      }
+
+      // Initial render. This mounts a Suspense boundary, so that in the next
+      // update we can trigger a "suspend with delay" scenario.
+      const root = ReactNoop.createRoot();
+      await act(async () => {
+        root.render(<App showMore={false} />);
+      });
+      expect(Scheduler).toHaveYielded([]);
+      expect(root).toMatchRenderedOutput(<div />);
+
+      // Update. This will cause two separate trees to suspend. The first tree
+      // will be inside an already mounted Suspense boundary, so it will trigger
+      // a "suspend with delay". The second tree will be a new Suspense
+      // boundary, but the thenable that is thrown will immediately call its
+      // ping listener.
+      //
+      // Before the bug was fixed, this would lead to a `prepareFreshStack` call
+      // that unwinds the work-in-progress stack. When that code was written, it
+      // was expected that pings always happen from an asynchronous task (or
+      // microtask). But this test shows an example where that's not the case.
+      //
+      // The fix was to check if we're in the render phase before calling
+      // `prepareFreshStack`.
+      await act(async () => {
+        root.render(<App showMore={true} />);
+      });
+      expect(Scheduler).toHaveYielded(['Suspend! [Async]', 'Loading...', 'Hi']);
+      expect(root).toMatchRenderedOutput(
+        <div>
+          <span prop="Loading..." />
+          <span prop="Hi" />
+        </div>,
+      );
+    },
+  );
 });