don't search for more than 100 merge candidates

closes #CORE-2249

Change-Id: I4f857b84dcf0fccb43661c0d908b7786c871b88e
Reviewed-on: https://gerrit.instructure.com/175251
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
QA-Review: James Williams <jamesw@instructure.com>
Product-Review: James Williams <jamesw@instructure.com>

app/messages/merge_email_communication_channel.email.erb

@@ -1,5 +1,5 @@
 <%
-  current_host = (HostUrl.context_host(asset.merge_candidates.first.pseudonym.account) rescue nil) || HostUrl.default_host
+  current_host = (HostUrl.context_host(asset.merge_candidates(true).first.pseudonym.account) rescue nil) || HostUrl.default_host
   new_host     = (HostUrl.context_host((asset.pseudonym || asset.user.pseudonym).account) rescue nil) || HostUrl.default_host
 %>
 

app/messages/merge_email_communication_channel.email.html.erb

@@ -1,5 +1,5 @@
 <%
-  current_host = (HostUrl.context_host(asset.merge_candidates.first.pseudonym.account) rescue nil) || HostUrl.default_host
+  current_host = (HostUrl.context_host(asset.merge_candidates(true).first.pseudonym.account) rescue nil) || HostUrl.default_host
   new_host     = (HostUrl.context_host((asset.pseudonym || asset.user.pseudonym).account) rescue nil) || HostUrl.default_host
 %>
 

app/models/communication_channel.rb

@@ -459,8 +459,13 @@ class CommunicationChannel < ActiveRecord::Base
     scope = CommunicationChannel.active.by_path(self.path).of_type(self.path_type)
     merge_candidates = {}
     Shard.with_each_shard(shards) do
-      scope = scope.shard(Shard.current)
-      scope.where("user_id<>?", self.user_id).preload(:user).map(&:user).select do |u|
+      scope = scope.shard(Shard.current).where("user_id<>?", self.user_id)
+
+      limit = Setting.get("merge_candidate_search_limit", "100").to_i
+      ccs = scope.preload(:user).limit(limit + 1).to_a
+      return [] if ccs.count > limit # just bail if things are getting out of hand
+
+      ccs.map(&:user).select do |u|
         result = merge_candidates.fetch(u.global_id) do
           merge_candidates[u.global_id] = (u.all_active_pseudonyms.length != 0)
         end

lib/sis/user_importer.rb

@@ -269,11 +269,16 @@ module SIS
             # find all CCs for this user, and active conflicting CCs for all users
             # unless we're deleting this user, then only find CCs for this user
             if status_is_active
-              ccs = CommunicationChannel.where("workflow_state='active' OR user_id=?", user)
+              cc_scope = CommunicationChannel.where("workflow_state='active' OR user_id=?", user)
             else
-              ccs = user.communication_channels
+              cc_scope = user.communication_channels
+            end
+            cc_scope = cc_scope.email.by_path(user_row.email)
+            limit = Setting.get("merge_candidate_search_limit", "100").to_i
+            ccs = cc_scope.limit(limit + 1).to_a
+            if ccs.count > limit
+              ccs = cc_scope.where(:user_id => user).to_a # don't bother with merge candidates anymore
             end
-            ccs = ccs.email.by_path(user_row.email).to_a
 
             # sis_cc could be set from the previous user, if we're not on a transaction boundary,
             # and the previous user had an sis communication channel, and this user doesn't have one