bug#40354: [PATCH] Fix Libravatar federation handling

["Philip K." <philipk@posteo.net>]

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Lars Ingebrigtsen <larsi@gnus.org> writes:

> Lars Ingebrigtsen <larsi@gnus.org> writes:
>
>> Philip K <philip@warpmail.net> writes:
>>
>>> Previous implementation didn't correctly handle the result of
>>> dns-query, and didn't implement the necessary record selection
>>> algorithm as specified on the wiki (https://wiki.libravatar.org/api/,
>>> "Federated servers").
>>>
>>> * lisp/image/gravatar.el (gravatar--service-libravatar): Fix
>>> libravatar image host resolver algorithm.
>>
>> Looks good to me, although this didn't apply either, since the code has
>> been rewritten to be asynchronous.  Could you re-spin this patch, too,
>> and I'll get it applied...
>
> Philip, have you had time to look at this?

The patch below should implement the fixes from bug#40354. I found a
libravatar user here[0], and could test it by evaluating

        (gravatar--service-libravatar "who@zerokspot.com" #'message)

or

        (gravatar--service-libravatar "who@gnu.org" #'message)

as an example for a domain that doesn't use libravatar. Both work as
expected, and don't lag.

[0]: https://zerokspot.com/weblog/2020/05/26/avatars-without-gravatar/

-- 
	Philip K.


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From 9b345b965cce8f79aa31b6a8989ff466c84b6798 Mon Sep 17 00:00:00 2001
From: Philip K <philipk@posteo.net>
Date: Tue, 18 Aug 2020 20:50:07 +0200
Subject: [PATCH] Fix Libravatar federation handling

Previous implementation didn't correctly handle the result of
dns-query, and didn't implement the necessary record selection
algorithm as specified on the wiki (https://wiki.libravatar.org/api/,
"Federated servers").

* lisp/image/gravatar.el (gravatar--service-libravatar): Implement
  correct algorithm
---
 lisp/image/gravatar.el | 49 ++++++++++++++++++++++++++++++++++++------
 1 file changed, 42 insertions(+), 7 deletions(-)

diff --git a/lisp/image/gravatar.el b/lisp/image/gravatar.el
index e917033562..cca737656c 100644
--- a/lisp/image/gravatar.el
+++ b/lisp/image/gravatar.el
@@ -158,18 +158,53 @@ gravatar--service-libravatar
         (setq func
               (lambda (result)
                 (cond
-                 (result
-                  (funcall callback (format "%s://%s/avatar"
-                                            (cdar records) result)))
-                 ((> (length records) 1)
-                  (pop records)
+                 ((and result           ;there is a result
+                       (let* ((data (mapcar (lambda (record)
+                                              (dns-get 'data (cdr record)))
+                                            (dns-get 'answers result)))
+                              (priorities (mapcar (lambda (r) (dns-get 'priority r))
+                                                  data))
+                              (max-priority (if priorities
+                                                (apply #'max priorities)
+                                              0))
+                              (sum 0) top)
+                         ;; Attempt to find all records with the same maximal
+                         ;; priority, and calculate the sum of their weights.
+                         (dolist (ent data)
+                           (when (= max-priority (dns-get 'priority ent))
+                             (setq sum (+ sum (dns-get 'weight ent)))
+                             (push ent top)))
+                         ;; In case there is more than one maximal priority
+                         ;; record, choose one at random, while taking the
+                         ;; individual record weights into consideration.
+                         (catch 'done
+                           (dolist (ent top)
+                             (when (and (or (= 0 sum)
+                                            (<= 0 (random sum) (dns-get 'weight ent)))
+                                        ;; Ensure that port and domain data are
+                                        ;; valid. In case non of the results
+                                        ;; were valid, `catch' will evaluate to
+                                        ;; nil, and the next cond clause will be
+                                        ;; tested.
+                                        (<= 1 (dns-get 'port ent) 65535)
+                                        (string-match-p "\\`[-.0-9A-Za-z]+\\'"
+                                                        (dns-get 'target ent)))
+                               (funcall callback
+                                        (url-normalize-url (format "%s://%s:%s/avatar"
+                                                                   (cdar records)
+                                                                   (dns-get 'target ent)
+                                                                   (dns-get 'port ent))))
+                               (throw 'done t))
+                             (setq sum (- sum (dns-get 'weight ent))))))))
+                 ((setq records (cdr records)) ;in case there are at least two methods
                   (dns-query-asynchronous
                    (concat (caar records) "._tcp." domain)
                    func 'SRV))
-                 (t
+                 (t                     ;fallback
                   (funcall callback "https://seccdn.libravatar.org/avatar")))))
         (dns-query-asynchronous
-         (concat (caar records) "._tcp." domain) func 'SRV)))))
+         (concat (caar records) "._tcp." domain)
+         func 'SRV t)))))
 
 (defun gravatar-hash (mail-address)
   "Return the Gravatar hash for MAIL-ADDRESS."
-- 
2.26.2