>From d871f95325ef230f0fda7aeecf1c2b6461d23183 Mon Sep 17 00:00:00 2001
From: Eric Abrahamsen <eric@ericabrahamsen.net>
Date: Thu, 31 Oct 2019 14:14:44 -0700
Subject: [PATCH] WIP on allowing Gnus backends to return headers directly

* lisp/gnus/gnus-sum.el (gnus-fetch-headers): Allow the
  gnus-retrieve-headers backend function to directly return a list of
  headers, instead of inserting data to parse in the
  nntp-server-buffer.
* lisp/gnus/nnir.el (nnir-retrieve-headers): Handle the same case when
  nnir calls the "real" backend function.
---
 lisp/gnus/gnus-sum.el | 37 ++++++++++++++++++++-----------------
 lisp/gnus/nnir.el     | 35 +++++++++++++++++++++--------------
 2 files changed, 41 insertions(+), 31 deletions(-)

diff --git a/lisp/gnus/gnus-sum.el b/lisp/gnus/gnus-sum.el
index f21bc7584e..e86781bd1a 100644
--- a/lisp/gnus/gnus-sum.el
+++ b/lisp/gnus/gnus-sum.el
@@ -5631,23 +5631,26 @@ gnus-summary-remove-list-identifiers
 (defun gnus-fetch-headers (articles &optional limit force-new dependencies)
   "Fetch headers of ARTICLES."
   (gnus-message 7 "Fetching headers for %s..." gnus-newsgroup-name)
-  (prog1
-      (if (eq 'nov
-	      (setq gnus-headers-retrieved-by
-		    (gnus-retrieve-headers
-		     articles gnus-newsgroup-name
-		     (or limit
-			 ;; We might want to fetch old headers, but
-			 ;; not if there is only 1 article.
-			 (and (or (and
-				   (not (eq gnus-fetch-old-headers 'some))
-				   (not (numberp gnus-fetch-old-headers)))
-				  (> (length articles) 1))
-			      gnus-fetch-old-headers)))))
-	  (gnus-get-newsgroup-headers-xover
-	   articles force-new dependencies gnus-newsgroup-name t)
-	(gnus-get-newsgroup-headers dependencies force-new))
-    (gnus-message 7 "Fetching headers for %s...done" gnus-newsgroup-name)))
+  (let ((res (setq gnus-headers-retrieved-by
+		   (gnus-retrieve-headers
+		    articles gnus-newsgroup-name
+		    (or limit
+			;; We might want to fetch old headers, but
+			;; not if there is only 1 article.
+			(and (or (and
+				  (not (eq gnus-fetch-old-headers 'some))
+				  (not (numberp gnus-fetch-old-headers)))
+				 (> (length articles) 1))
+			     gnus-fetch-old-headers))))))
+    (prog1
+	(pcase res
+	  ('nov
+	   (gnus-get-newsgroup-headers-xover
+	    articles force-new dependencies gnus-newsgroup-name t))
+	  ((pred listp) res)
+	  ;; 'headers is the other likely value.
+	  (_ (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.
diff --git a/lisp/gnus/nnir.el b/lisp/gnus/nnir.el
index 7cb2d1615a..48af6067f0 100644
--- a/lisp/gnus/nnir.el
+++ b/lisp/gnus/nnir.el
@@ -719,22 +719,29 @@ nnir-retrieve-headers
 	     (setq parsefunc 'nnheader-parse-nov))
 	    ('headers
 	     (setq parsefunc 'nnheader-parse-head))
+	    ((pred listp)
+	     (setq headers gnus-headers-retrieved-by))
 	    (_ (error "Unknown header type %s while requesting articles \
                     of group %s" gnus-headers-retrieved-by artgroup)))
-	  (goto-char (point-min))
-	  (while (not (eobp))
-	    (let* ((novitem (funcall parsefunc))
-		   (artno (and novitem
-			       (mail-header-number novitem)))
-		   (art (car (rassq artno articleids))))
-	      (when art
-		(setf (mail-header-number novitem) art)
-		(push novitem headers))
-	      (forward-line 1)))))
-      (setq headers
-	    (sort headers
-		  (lambda (x y)
-		    (< (mail-header-number x) (mail-header-number y)))))
+	  (unless headers
+	    (goto-char (point-min))
+	    (while (not (eobp))
+	      (push (funcall parsefunc) headers)
+	      (forward-line 1)))
+	  (setq headers
+		(sort
+		 (delq nil
+		       (mapcar
+			(lambda (novitem)
+			  (let* ((artno (and novitem
+					     (mail-header-number novitem)))
+				 (art (car-safe (rassq artno articleids))))
+			    (when art
+			      (setf (mail-header-number novitem) art)
+			      novitem)))
+			headers))
+		 (lambda (x y)
+		   (< (mail-header-number x) (mail-header-number y)))))))
       (erase-buffer)
       (mapc 'nnheader-insert-nov headers)
       'nov)))
-- 
2.23.0