fix: make sure that the patched fetch cache set promise is properly awaited (vercel#75971)

The [promise created for updating the incremental cache on successful
fetches](https://github.com/vercel/next.js/blob/106e17ca8f093899d300885938eab863ffa478a7/packages/next/src/server/lib/patch-fetch.ts#L687-L728)
is not used, returned nor awaited in any way. I imagine that this can be
fine in environments such as Node.js servers where the dangling promise
will eventually run, in other environments however the promise might be
cancelled causing the cache update not to happen, so it would be
appropriate, when possible that the promise is awaited using `waitUntil`
(for more context, I've encountered this issue in the [OpenNext
project](https://github.com/opennextjs)).

So in this PR I'm adding the promise to the `pendingRevalidates` map so
that it will eventually be properly awaited.

cc. @vicb, @conico974

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Co-authored-by: JJ Kasper <jj@jjsweb.site>

Author: dario-piotrowicz

packages/next/src/server/lib/patch-fetch.ts

@@ -676,7 +676,7 @@ export function createPatchedFetcher(
                   // We are dynamically rendering including dev mode. We want to return
                   // the response to the caller as soon as possible because it might stream
                   // over a very long time.
-                  cloned1
+                  const cacheSetPromise = cloned1
                     .arrayBuffer()
                     .then(async (arrayBuffer) => {
                       const bodyBuffer = Buffer.from(arrayBuffer)
@@ -710,6 +710,26 @@ export function createPatchedFetcher(
                     )
                     .finally(handleUnlock)
 
+                  const pendingRevalidateKey = `cache-set-${cacheKey}`
+                  workStore.pendingRevalidates ??= {}
+                  if (pendingRevalidateKey in workStore.pendingRevalidates) {
+                    // there is already a pending revalidate entry that
+                    // we need to await to avoid race conditions
+                    await workStore.pendingRevalidates[pendingRevalidateKey]
+                  }
+                  workStore.pendingRevalidates[pendingRevalidateKey] =
+                    cacheSetPromise.finally(() => {
+                      // If the pending revalidate is not present in the store, then
+                      // we have nothing to delete.
+                      if (
+                        !workStore.pendingRevalidates?.[pendingRevalidateKey]
+                      ) {
+                        return
+                      }
+
+                      delete workStore.pendingRevalidates[pendingRevalidateKey]
+                    })
+
                   return cloned2
                 }
               }