Add a new variable to control Gnus Agent caching

* doc/misc/gnus.texi (Agent Variables): Document it.

* lisp/gnus/gnus-art.el (gnus-request-article-this-buffer): Ditto.

* lisp/gnus/gnus-async.el (gnus-async-article-callback): Use it.

* lisp/gnus/gnus.el (gnus-agent-eagerly-store-articles): New variable.

Includes work from Madhu <enometh@meer.net>.

doc/misc/gnus.texi

@@ -19489,6 +19489,11 @@ Variable to control whether use the locally stored @acronym{NOV} and
 articles when plugged, e.g., essentially using the Agent as a cache.
 The default is non-@code{nil}, which means to use the Agent as a cache.
 
+@item gnus-agent-eagerly-store-articles
+@vindex gnus-agent-eagerly-store-articles
+If non-@code{nil} (which is the default), store all articles read in
+agentized groups in the Agent cache.
+
 @item gnus-agent-go-online
 @vindex gnus-agent-go-online
 If @code{gnus-agent-go-online} is @code{nil}, the Agent will never

etc/NEWS

@@ -434,6 +434,11 @@ tags to be considered as well.
 
 ** Gnus
 
++++
+*** New user option 'gnus-agent-eagerly-store-articles'.
+If non-nil (which is the default), the Gnus Agent will store all read
+articles in the Agent cache.
+
 +++
 *** New user option 'gnus-global-groups'.
 Gnus handles private groups differently from public (i.e., NNTP-like)

lisp/gnus/gnus-art.el

@@ -7094,6 +7094,7 @@ If given a prefix, show the hidden text instead."
 		      (gnus-backlog-enter-article
 		       group article (current-buffer)))
 		    (when (and gnus-agent
+			       gnus-agent-eagerly-store-articles
 			       (gnus-agent-group-covered-p group))
 		      (gnus-agent-store-article article group)))
 		  (setq result 'article))

lisp/gnus/gnus-async.el

@@ -227,6 +227,7 @@ that was fetched."
 	  (narrow-to-region mark (point-max))
 	  ;; Put the articles into the agent, if they aren't already.
 	  (when (and gnus-agent
+		     gnus-agent-eagerly-store-articles
 		     (gnus-agent-group-covered-p group))
 	    (save-restriction
 	      (narrow-to-region mark (point-max))

lisp/gnus/gnus.el

@@ -2285,6 +2285,14 @@ a string, be sure to use a valid format, see RFC 2616."
     (gnus-message 1 "Edit your init file to make this change permanent.")
     (sit-for 2)))
 
+(defcustom gnus-agent-eagerly-store-articles t
+  "If non-nil, cache articles eagerly.
+
+When using the Gnus Agent and reading an agentized newsgroup,
+automatically cache the article in the agent cache."
+  :type 'boolean
+  :version "28.1")
+
 
 ;;; Internal variables