activitypub discovery link in header of individual posts

daemon_get_post.py

@@ -33,6 +33,7 @@ from httpcodes import http_401
 from httpcodes import http_403
 from httpcodes import http_404
 from httpheaders import set_headers
+from httpheaders import set_html_post_headers
 from httpheaders import login_headers
 from httpheaders import redirect_headers
 from httprequests import request_http
@@ -161,8 +162,9 @@ def _show_post_from_file(self, post_filename: str, liked_by: str,
                                  auto_cw_cache)
         msg = msg.encode('utf-8')
         msglen = len(msg)
-        set_headers(self, 'text/html', msglen,
-                          cookie, calling_domain, False)
+        set_html_post_headers(self, msglen,
+                              cookie, calling_domain, False,
+                              post_json_object)
         write2(self, msg)
         fitness_performance(getreq_start_time, fitness,
                             '_GET', 'show_post_from_file',

httpheaders.py

@@ -14,6 +14,7 @@ from utils import string_contains
 from utils import get_instance_url
 from utils import data_dir
 from utils import save_json
+from utils import remove_id_ending
 
 
 def login_headers(self, file_format: str, length: int,
@@ -164,6 +165,25 @@ def set_headers(self, file_format: str, length: int, cookie: str,
     self.end_headers()
 
 
+def set_html_post_headers(self, length: int, cookie: str,
+                          calling_domain: str, permissive: bool,
+                          post_json_object: {}) -> None:
+    """A HTML format post representing an individual ActivityPub post
+    """
+    _set_headers_base(self, 'text/html', length, cookie, calling_domain,
+                      permissive)
+    if post_json_object.get('id'):
+        # Discover activitypub version of the html post
+        # https://swicg.github.io/activitypub-html-discovery/#http-link-header
+        post_id = remove_id_ending(post_json_object['id'])
+        ap_link = \
+            '<' + post_id + '>; rel="alternate"; ' + \
+            'type="application/activity+json"'
+        self.send_header('Link', ap_link)
+
+    self.end_headers()
+
+
 def set_headers_head(self, file_format: str, length: int, etag: str,
                      calling_domain: str, permissive: bool,
                      last_modified_time_str: str) -> None: