[misc] exportProject: collect and send user ids of updates in trailer

services/track-changes/app/js/HttpController.js

@@ -209,7 +209,7 @@ module.exports = HttpController = {
     logger.log({ project_id }, 'exporting project history')
     UpdatesManager.exportProject(project_id, function (
       err,
-      updates,
+      { updates, userIds },
       confirmWrite
     ) {
       const abortStreaming = req.aborted || res.finished || res.destroyed
@@ -236,6 +236,7 @@ module.exports = HttpController = {
         // The first write will emit the 200 status, headers and start of the
         //  response payload (open array)
         res.setHeader('Content-Type', 'application/json')
+        res.setHeader('Trailer', 'X-User-Ids')
         res.writeHead(200)
         res.write('[')
       }
@@ -255,7 +256,8 @@ module.exports = HttpController = {
 
       if (isLastWrite) {
         // The last write will have no updates and will finish the response
-        //  payload (close array).
+        //  payload (close array) and emit the userIds as trailer.
+        res.addTrailers({ 'X-User-Ids': JSON.stringify(userIds) })
         res.end(']')
       }
     })

services/track-changes/app/js/UpdatesManager.js

@@ -641,6 +641,8 @@ module.exports = UpdatesManager = {
       PackManager.makeProjectIterator(projectId, before, (err, iterator) => {
         if (err) return consumer(err)
 
+        const accumulatedUserIds = new Set()
+
         async.whilst(
           () => !iterator.done(),
 
@@ -654,13 +656,26 @@ module.exports = UpdatesManager = {
                 //  call.
                 return cb()
               }
+              updatesFromASinglePack.forEach((update) => {
+                accumulatedUserIds.add(
+                  // Super defensive access on update details.
+                  String(update && update.meta && update.meta.user_id)
+                )
+              })
               // Emit updates and wait for the consumer.
-              consumer(null, updatesFromASinglePack, cb)
+              consumer(null, { updates: updatesFromASinglePack }, cb)
             }),
 
           (err) => {
             if (err) return consumer(err)
-            consumer(null, [])
+
+            // Adding undefined can happen for broken updates.
+            accumulatedUserIds.delete('undefined')
+
+            consumer(null, {
+              updates: [],
+              userIds: Array.from(accumulatedUserIds).sort()
+            })
           }
         )
       })