GNU bug report logs - #15377
24.1; erc-mode-line-update-buffer can mishandle updates when there is no topic

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Vivek Dasmohapatra <vivek <at> etla.org>
Subject: bug#15377: closed (Re: bug#15377: 24.1; erc-mode-line-update-buffer
 can mishandle updates when there is no topic)
Date: Sat, 14 Sep 2013 23:34:03 +0000

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Your bug report

#15377: 24.1; erc-mode-line-update-buffer can mishandle updates when there is no topic

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 15377 <at> debbugs.gnu.org.

-- 
15377: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=15377
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Glenn Morris <rgm <at> gnu.org>
To: 15377-done <at> debbugs.gnu.org
Subject: Re: bug#15377: 24.1;
 erc-mode-line-update-buffer can mishandle updates when there is no
 topic
Date: Sat, 14 Sep 2013 19:33:47 -0400

Version: 24.4

Thanks; applied.

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From: Vivek Dasmohapatra <vivek <at> etla.org>
To: bug-gnu-emacs <at> gnu.org
Subject: 24.1;
 erc-mode-line-update-buffer can mishandle updates when there is no
 topic
Date: Sat, 14 Sep 2013 01:32:40 +0100

Very rarely, erc can start throwing lots of errors inside its process
sentinel - a couple of users on #emacs also reported it recently:

[conversation edited for noise]
<xxxx> sigh, periodically, my erc … "error in process
       filter: Invalid format character: `%o'"

<yyyy> xxxx: I've seen this error a few times before--typically when my
       bouncer has just experienced connection difficulties and is in a
       disconnect/reconnect thrash.

I tracked it down to:

erc-header-line-format: "%n on %t (%m,%l) %o"

And in erc-update-mode-line-buffer:

    (let ((spec (format-spec-make
                 ⋮
                 ?o (erc-controls-strip erc-channel-topic)

And then:

    (format-spec erc-header-line-format spec)

But erc-controls-strip can return nil, which does make format-spec
choke on "%o", since it does not differentiate between a spec with
no ?o element and one containing ?o nil.

The following should fix it:

--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -6210,7 +6210,7 @@ if `erc-away' is non-nil."
 		 ?m (erc-format-channel-modes)
 		 ?n (or (erc-current-nick) "")
 		 ?N (erc-format-network)
-		 ?o (erc-controls-strip erc-channel-topic)
+		 ?o (or (erc-controls-strip erc-channel-topic) "")
 		 ?p (erc-port-to-string erc-session-port)
 		 ?s (erc-format-target-and/or-server)
 		 ?S (erc-format-target-and/or-network)

I have no real test recipe, since the error is intermittent and
I don't know what makes the erc-channel-topic degrade to nil,
but you can try this:

M-: (let (erc-channel-topic) (erc-update-mode-line-buffer (current-buffer)))

with an erc-buffer current.

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