avoid blocking when fetching redis keys

use scan instead of keys method

services/track-changes/app/coffee/RedisManager.coffee

@@ -33,6 +33,23 @@ module.exports = RedisManager =
 	getDocIdsWithHistoryOps: (project_id, callback = (error, doc_ids) ->) ->
 		rclient.smembers docsWithHistoryOpsKey(project_id), callback
 
+	# iterate over keys asynchronously using redis scan (non-blocking)
+	_getKeys: (pattern, callback) ->
+		cursor = 0  # redis iterator
+		keySet = {} # use hash to avoid duplicate results
+		# scan over all keys looking for pattern
+		doIteration = (cb) ->
+			rclient.scan cursor, "MATCH", pattern, "COUNT", 1000, (error, reply) ->
+				return callback(error) if error?
+				[cursor, keys] = reply
+				for key in keys
+					keySet[key] = true
+				if cursor == '0'  # note redis returns string result not numeric
+					return callback(null, Object.keys(keySet))
+				else
+					doIteration()
+		doIteration()
+
 	# extract ids from keys like DocsWithHistoryOps:57fd0b1f53a8396d22b2c24b
 	_extractIds: (keyList) ->
 		ids = (key.split(":")[1] for key in keyList)
@@ -40,7 +57,7 @@ module.exports = RedisManager =
 
 	# this will only work on single node redis, not redis cluster
 	getProjectIdsWithHistoryOps: (callback = (error, project_ids) ->) ->
-		rclient.keys docsWithHistoryOpsKey("*"), (error, project_keys) ->
+		RedisManager._getKeys docsWithHistoryOpsKey("*"), (error, project_keys) ->
 			return callback(error) if error?
 			project_ids = RedisManager._extractIds project_keys
 			callback(error, project_ids)
@@ -49,7 +66,7 @@ module.exports = RedisManager =
 	getAllDocIdsWithHistoryOps: (callback = (error, doc_ids) ->) ->
 		# return all the docids, to find dangling history entries after
 		# everything is flushed.
-		rclient.keys rawUpdatesKey("*"), (error, doc_keys) ->
+		RedisManager._getKeys rawUpdatesKey("*"), (error, doc_keys) ->
 			return callback(error) if error?
 			doc_ids = RedisManager._extractIds doc_keys
 			callback(error, doc_ids)