Only share incremental cache for edge in next start (#79386)

When in minimal mode we should not try leveraging a shared
`IncrementalCache` instance across invocations as we pass in request
headers which are request specific. The only time we should share this
instance is in `next start` mode where the sandbox needs to be able to
access the filesystem-cache.

x-ref: [slack
thread](https://vercel.slack.com/archives/C07CGAA2RS6/p1747687927600699?thread_ts=1747142004.153739&cid=C07CGAA2RS6)

Author: ijjk

packages/next/src/server/base-server.ts

@@ -1467,6 +1467,8 @@ export default abstract class Server<
 
         incrementalCache.resetRequestCache()
         addRequestMeta(req, 'incrementalCache', incrementalCache)
+        // This is needed for pages router to leverage unstable_cache
+        // TODO: re-work this handling to not use global and use a AsyncStore
         ;(globalThis as any).__incrementalCache = incrementalCache
       }
 
@@ -2457,13 +2459,15 @@ export default abstract class Server<
 
     // use existing incrementalCache instance if available
     const incrementalCache: import('./lib/incremental-cache').IncrementalCache =
-      (globalThis as any).__incrementalCache ||
-      (await this.getIncrementalCache({
-        requestHeaders: Object.assign({}, req.headers),
-        requestProtocol: protocol.substring(0, protocol.length - 1) as
-          | 'http'
-          | 'https',
-      }))
+      process.env.NEXT_RUNTIME === 'edge' &&
+      (globalThis as any).__incrementalCache
+        ? (globalThis as any).__incrementalCache
+        : await this.getIncrementalCache({
+            requestHeaders: Object.assign({}, req.headers),
+            requestProtocol: protocol.substring(0, protocol.length - 1) as
+              | 'http'
+              | 'https',
+          })
 
     // TODO: investigate, this is not safe across multiple concurrent requests
     incrementalCache.resetRequestCache()

packages/next/src/server/web/adapter.ts

@@ -186,7 +186,10 @@ export async function adapter(
   }
 
   if (
-    !(globalThis as any).__incrementalCache &&
+    // If we are inside of the next start sandbox
+    // leverage the shared instance if not we need
+    // to create a fresh cache instance each time
+    !(globalThis as any).__incrementalCacheShared &&
     (params as any).IncrementalCache
   ) {
     ;(globalThis as any).__incrementalCache = new (

packages/next/src/server/web/sandbox/sandbox.ts

@@ -77,6 +77,7 @@ export async function getRuntimeContext(
   })
 
   if (params.incrementalCache) {
+    runtime.context.globalThis.__incrementalCacheShared = true
     runtime.context.globalThis.__incrementalCache = params.incrementalCache
   }