comp AP gateway: signature checking, caching and threads management:

- HTTP signature is checked for incoming messages
- AP actor can now be followed using pubsub subscription. When following is accepted, the
  node is cached
- replies to posts are put in cached pubsub comment nodes, with a `comments_max_depth`
  option to limit the number of comment nodes for a root message (documentation will come
  to explain this).

ticket 364

sat/plugins/plugin_comp_ap_gateway/constants.py

@@ -22,6 +22,7 @@ CONF_SECTION = f"component {IMPORT_NAME}"
 CONTENT_TYPE_AP = "application/activity+json; charset=utf-8"
 TYPE_ACTOR = "actor"
 TYPE_INBOX = "inbox"
+TYPE_SHARED_INBOX = "shared_inbox"
 TYPE_OUTBOX = "outbox"
 TYPE_ITEM = "item"
 MEDIA_TYPE_AP = "application/activity+json"
@@ -30,7 +31,39 @@ AP_MB_MAP = {
     "content": "content_xhtml",
 
 }
-AP_REQUEST_TYPES = {"actor", "outbox"}
+AP_REQUEST_TYPES = {
+    "GET": {"actor", "outbox"},
+    "POST": {"inbox"},
+}
+# headers to check for signature
+SIGN_HEADERS = {
+    # headers needed for all HTTP methods
+    None: [
+        # tuples are equivalent headers/pseudo headers, one of them must be present
+        ("date", "(created)"),
+        ("digest", "(request-target)"),
+    ],
+    b"GET": ["host"],
+    b"POST": ["digest"]
+}
 PAGE_SIZE = 10
+HS2019 = "hs2019"
+# delay after which a signed request is not accepted anymore
+SIGN_EXP = 12*60*60  # 12 hours (same value as for Mastodon)
 
 LRU_MAX_SIZE = 200
+ACTIVITY_TYPES = (
+    "Accept", "Add", "Announce", "Arrive", "Block", "Create", "Delete", "Dislike", "Flag",
+    "Follow", "Ignore", "Invite", "Join", "Leave", "Like", "Listen", "Move", "Offer",
+    "Question", "Reject", "Read", "Remove", "TentativeReject", "TentativeAccept",
+    "Travel", "Undo", "Update", "View"
+)
+ACTIVITY_TYPES_LOWER = [a.lower() for a in ACTIVITY_TYPES]
+ACTIVITY_OBJECT_MANDATORY = (
+    "Create", "Update", "Delete", "Follow", "Add", "Remove", "Like", "Block", "Undo"
+)
+ACTIVITY_TARGET_MANDATORY = ("Add", "Remove")
+# activities which can be used with Shared Inbox (i.e. with not account specified)
+ACTIVIY_NO_ACCOUNT_ALLOWED = ("create",)
+# maximum number of parents to retrieve when comments_max_depth option is set
+COMMENTS_MAX_PARENTS = 100