1 files changed, 46 insertions, 19 deletions
diff --git a/lisp/gnus/gnus-sum.el b/lisp/gnus/gnus-sum.el
index b0f9ed4c6f0..5bd58b690af 100644
--- a/lisp/gnus/gnus-sum.el
+++ b/lisp/gnus/gnus-sum.el
@@ -5658,10 +5658,21 @@ or a straight list of headers."
 	  (setf (mail-header-subject header) subject))))))
 
 (defun gnus-fetch-headers (articles &optional limit force-new dependencies)
-  "Fetch headers of ARTICLES."
+  "Fetch headers of ARTICLES.
+This calls the `gnus-retrieve-headers' function of the current
+group's backend server.  The server can do one of two things:
+
+1. Write the headers for ARTICLES into the
+   `nntp-server-buffer' (the current buffer) in a parseable format, or
+2. Return the headers directly as a list of vectors.
+
+In the first case, `gnus-retrieve-headers' returns a symbol
+value, either `nov' or `headers'.  This value determines which
+parsing function is used to read the headers.  It is also stored
+into the variable `gnus-headers-retrieved-by', which is consulted
+later when possibly building full threads."
   (gnus-message 7 "Fetching headers for %s..." gnus-newsgroup-name)
-  (prog1
-      (pcase (setq gnus-headers-retrieved-by
+  (let ((res (setq gnus-headers-retrieved-by
 		   (gnus-retrieve-headers
 		    articles gnus-newsgroup-name
 		    (or limit
@@ -5671,22 +5682,34 @@ or a straight list of headers."
 				  (not (eq gnus-fetch-old-headers 'some))
 				  (not (numberp gnus-fetch-old-headers)))
 				 (> (length articles) 1))
-			     gnus-fetch-old-headers))))
-    ('nov
-     (gnus-get-newsgroup-headers-xover
-      articles force-new dependencies gnus-newsgroup-name t))
-    ('headers
-     (gnus-get-newsgroup-headers dependencies force-new))
-    ((pred listp)
-     (let ((dependencies
-	    (or dependencies
-		(with-current-buffer gnus-summary-buffer
-		  gnus-newsgroup-dependencies))))
-     (delq nil (mapcar   #'(lambda (header)
-			     (gnus-dependencies-add-header
-			      header dependencies force-new))
-			 gnus-headers-retrieved-by)))))
-  (gnus-message 7 "Fetching headers for %s...done" gnus-newsgroup-name)))
+			     gnus-fetch-old-headers))))))
+    (prog1
+	(pcase res
+	  ('nov
+	   (gnus-get-newsgroup-headers-xover
+	    articles force-new dependencies gnus-newsgroup-name t))
+	  ;; For now, assume that any backend returning its own
+	  ;; headers takes some effort to do so, so return `headers'.
+	  ((pred listp)
+	   (setq gnus-headers-retrieved-by 'headers)
+	   (let ((dependencies
+		  (or dependencies
+		      (buffer-local-value
+		       'gnus-newsgroup-dependencies gnus-summary-buffer))))
+	     (when (functionp gnus-alter-header-function)
+	       (mapc gnus-alter-header-function res))
+	     (mapc (lambda (header)
+		     ;; The agent or the cache may have already
+		     ;; registered this header in the dependency
+		     ;; table.
+		     (unless (gethash (mail-header-id header) dependencies)
+		       (gnus-dependencies-add-header
+			header dependencies force-new)))
+		   res)
+	     res))
+	  (_ (gnus-get-newsgroup-headers dependencies force-new)))
+      (gnus-message 7 "Fetching headers for %s...done"
+		    gnus-newsgroup-name))))
 
 (defun gnus-select-newsgroup (group &optional read-all select-articles)
   "Select newsgroup GROUP.
@@ -6443,6 +6466,10 @@ The resulting hash table is returned, or nil if no Xrefs were found."
 	(unless (gnus-ephemeral-group-p group)
 	  (gnus-group-update-group group t))))))
 
+;; FIXME: Refactor this with `gnus-get-newsgroup-headers-xover' and
+;; extract the necessary bits for the direct-header-return case.  Also
+;; look at this and see how similar it is to
+;; `nnheader-parse-naked-head'.
 (defun gnus-get-newsgroup-headers (&optional dependencies force-new)
   (let ((dependencies
 	 (or dependencies