Use inReplyTo to find context.

lib/pleroma/web/ostatus/ostatus.ex

@@ -44,13 +44,19 @@ def handle_note(entry, doc \\ nil) do
 
     [author] = :xmerl_xpath.string('//author[1]', doc)
     {:ok, actor} = find_make_or_update_user(author)
+    inReplyTo = string_from_xpath("/entry/thr:in-reply-to[1]/@ref", entry)
 
     context = (string_from_xpath("/entry/ostatus:conversation[1]", entry) || "") |> String.trim
-    context = if String.length(context) > 0 do
-      context
-    else
-      ActivityPub.generate_context_id
-    end
+
+    context = with %{data: %{"context" => context}} <- Object.get_cached_by_ap_id(inReplyTo) do
+                context
+              else _e ->
+                if String.length(context) > 0 do
+                  context
+                else
+                  ActivityPub.generate_context_id
+                end
+              end
 
     to = [
       "https://www.w3.org/ns/activitystreams#Public"
@@ -74,8 +80,6 @@ def handle_note(entry, doc \\ nil) do
       "actor" => actor.ap_id
     }
 
-    inReplyTo = string_from_xpath("/entry/thr:in-reply-to[1]/@ref", entry)
-
     object = if inReplyTo do
       Map.put(object, "inReplyTo", inReplyTo)
     else

test/fixtures/incoming_reply_mastodon.xml

+<?xml version="1.0"?>
+<entry xmlns="http://www.w3.org/2005/Atom" xmlns:thr="http://purl.org/syndication/thread/1.0" xmlns:activity="http://activitystrea.ms/spec/1.0/" xmlns:poco="http://portablecontacts.net/spec/1.0" xmlns:media="http://purl.org/syndication/atommedia" xmlns:ostatus="http://ostatus.org/schema/1.0" xmlns:mastodon="http://mastodon.social/schema/1.0">
+  <id>tag:mastodon.social,2017-05-02:objectId=4901603:objectType=Status</id>
+  <published>2017-05-02T18:33:06Z</published>
+  <updated>2017-05-02T18:33:06Z</updated>
+  <title>New status by lambadalambda</title>
+  <author>
+    <id>https://mastodon.social/users/lambadalambda</id>
+    <activity:object-type>http://activitystrea.ms/schema/1.0/person</activity:object-type>
+    <uri>https://mastodon.social/users/lambadalambda</uri>
+    <name>lambadalambda</name>
+    <email>lambadalambda@mastodon.social</email>
+    <link rel="alternate" type="text/html" href="https://mastodon.social/@lambadalambda"/>
+    <link rel="avatar" type="image/gif" media:width="120" media:height="120" href="https://files.mastodon.social/accounts/avatars/000/000/264/original/1429214160519.gif"/>
+    <link rel="header" type="" media:width="700" media:height="335" href="/headers/original/missing.png"/>
+    <poco:preferredUsername>lambadalambda</poco:preferredUsername>
+    <poco:displayName>Critical Value</poco:displayName>
+    <mastodon:scope>public</mastodon:scope>
+  </author>
+  <activity:object-type>http://activitystrea.ms/schema/1.0/comment</activity:object-type>
+  <activity:verb>http://activitystrea.ms/schema/1.0/post</activity:verb>
+  <content type="html" xml:lang="el">&lt;p&gt;&lt;span class="h-card"&gt;&lt;a href="https://pleroma.soykaf.com/users/lain" class="u-url mention"&gt;@&lt;span&gt;lain&lt;/span&gt;&lt;/a&gt;&lt;/span&gt; hey&lt;/p&gt;</content>
+  <link rel="mentioned" ostatus:object-type="http://activitystrea.ms/schema/1.0/person" href="https://pleroma.soykaf.com/users/lain"/>
+  <link rel="mentioned" ostatus:object-type="http://activitystrea.ms/schema/1.0/collection" href="http://activityschema.org/collection/public"/>
+  <mastodon:scope>public</mastodon:scope>
+  <link rel="alternate" type="text/html" href="https://mastodon.social/users/lambadalambda/updates/2224923"/>
+  <link rel="self" type="application/atom+xml" href="https://mastodon.social/users/lambadalambda/updates/2224923.atom"/>
+  <thr:in-reply-to ref="https://pleroma.soykaf.com/objects/c237d966-ac75-4fe3-a87a-d89d71a3a7a4" href=""/>
+</entry>

test/web/ostatus/ostatus_test.exs

@@ -2,6 +2,7 @@ defmodule Pleroma.Web.OStatusTest do
   use Pleroma.DataCase
   alias Pleroma.Web.OStatus
   alias Pleroma.Web.XML
+  alias Pleroma.{Object, Repo}
 
   test "don't insert create notes twice" do
     incoming = File.read!("test/fixtures/incoming_note_activity.xml")
@@ -32,6 +33,22 @@ test "handle incoming notes - GS, subscription" do
     assert activity.data["object"]["content"] == "Will it blend?"
   end
 
+  test "handle incoming notes - Mastodon, salmon, reply" do
+    # It uses the context of the replied to object
+    Repo.insert!(%Object{
+          data: %{
+            "id" => "https://pleroma.soykaf.com/objects/c237d966-ac75-4fe3-a87a-d89d71a3a7a4",
+            "context" => "2hu"
+          }})
+    incoming = File.read!("test/fixtures/incoming_reply_mastodon.xml")
+    {:ok, [activity]} = OStatus.handle_incoming(incoming)
+
+    assert activity.data["type"] == "Create"
+    assert activity.data["object"]["type"] == "Note"
+    assert activity.data["object"]["actor"] == "https://mastodon.social/users/lambadalambda"
+    assert activity.data["context"] == "2hu"
+  end
+
   test "handle incoming notes - GS, subscription, reply" do
     incoming = File.read!("test/fixtures/ostatus_incoming_reply.xml")
     {:ok, [activity]} = OStatus.handle_incoming(incoming)