refactor: make sure promises as context would be preserved (#6277)

This addresses potential issues similar to one reported about
Moleculer, where the promise is used for context in the library.

Author: BridgeAR

packages/datadog-instrumentations/src/mongodb-core.js

@@ -199,17 +199,15 @@ function instrumentPromise (operation, command, instance, args, server, ns, ops,
   return startCh.runStores(ctx, () => {
     const promise = command.apply(instance, args)
 
-    return promise.then(function (res) {
+    promise.then(function (res) {
       ctx.result = res
-      return finishCh.runStores(ctx, () => {
-        return res
-      })
+      finishCh.publish(ctx)
     }, function (err) {
       ctx.error = err
       errorCh.publish(ctx)
       finishCh.publish(ctx)
-
-      throw err
     })
+
+    return promise
   })
 }

packages/datadog-instrumentations/src/next.js

@@ -162,14 +162,16 @@ function instrument (req, res, handler, error) {
 
       // promise should only reject when propagateError is true:
       // https://github.com/vercel/next.js/blob/cee656238a/packages/next/server/api-utils/node.ts#L547
-      return promise.then(
+      promise.then(
         result => finish(ctx, result),
         err => finish(ctx, null, err)
       )
+      return promise
     } catch (e) {
       // this will probably never happen as the handler caller is an async function:
       // https://github.com/vercel/next.js/blob/cee656238a/packages/next/server/api-utils/node.ts#L420
-      return finish(ctx, null, e)
+      finish(ctx, null, e)
+      throw e
     }
   })
 }
@@ -198,12 +200,6 @@ function finish (ctx, result, err) {
   }
 
   finishChannel.publish(ctx)
-
-  if (err) {
-    throw err
-  }
-
-  return result
 }
 
 // also wrapped in dist/server/future/route-handlers/app-route-route-handler.js