Emacs 21 hangs in vm if scroll-margin=5 and Emacs.LineSpacing=1

[Dobler@kis.uni-freiburg.de]

The following message is a courtesy copy of an article
that has been posted to gnu.emacs.vm.bug as well.

I am not perfectly sure where to post this -- it might be a problem of
Emacs or of VM -- but I want problem and workaround to be documented
somewhere on the web.

I am using VM to read my email and had it hang up Emacs every now and
then; the X window was still updated, but Emacs wouldn't react to any key
press and just run at 99% CPU usage for at least 10 minutes (after which I
killed the process). According to strace, it was doing

  --- SIGALRM (Alarm clock) ---
  gettimeofday({1037458979, 140315}, NULL) = 0
  gettimeofday({1037458979, 149317}, NULL) = 0
  gettimeofday({1037458979, 149872}, NULL) = 0
  setitimer(ITIMER_REAL, {it_interval={0, 0}, it_value={0, 90443}}, NULL) = 0
  sigreturn()                             = ? (mask now [])

and so on.

I found that the problem was caused by setting

  (setq scroll-margin 5)

and at closer inspection the hang occured (reproducibly) when I was in the
following situation:

  +-------------------------------------------------------------+
  |   86    Some@sender   Nov 15    8/389  "Some subject"       |
  |   87    Some@sender   Nov 15   13/472  "Some subject"       |
  |   88    Some@sender   Nov 15   17/676  "Some subject"       |
  |   89    Some@sender   Nov 15   74/162  "Some subject"       |
  |   90    Some@sender   Nov 15   16/488  "Some subject"       |
  |-> 91    Some@sender   Nov 16   41/139  "Some subject"       |
  |   92    Some@sender   Nov 16  908/369  "Some subject"       |
  |   93  U Some@sender   Nov 16   23/996  "Some subject"       |
  |   94  U Some@sender   Nov 16    8/229  "Some subject"       |
  |   95  U Some@sender   Nov 16    9/275  "Some subject"       |
  |   96  U Some@sender   Nov 16    8/363  "Some subject"       |
  |   97  D Some@sender   Nov 16   21/777  "Some subject"       |
  |__--_VM_7.07:_INBOX_Summary_92_(of_98)__read___85%___16:49___|
  |From: Some@body                                              |
  |Sender: Some@body                                            |
  |To: me@here                                                  |
  |Subject: Some subject                                        |
  |Date: Some date                                              |
  |                                                             |
  |Some                                                         |
  |content                                                      |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |...                                                          |
  |__**_VM_7.07:_INBOX_________91_(of_98)__read___All___16:49___|
  +-------------------------------------------------------------+

and pressed `n' to get to the next message. This is the point where the
last message line (that for message No.98) would appear in the
INBOX-Summary buffer -- but instead Emacs just hung up.

This problem does not occur with Xemacs and at closer inspection it only
occurs when I set the X11 resource
  Emacs.LineSpacing:  1
(I never got Xemacs to use an extra line spacing). Setting
Emacs.LineSpacing to 0 or not setting 'scroll-margin makes the problem
disappear. Also, I found no problem in a console window, only uner X.

Is that a bug of the rendering engine?


W o l f g a n g


System:
  SuSE Linux 8.0, kernel 2.4.18-4GB, Pentium 4 @ 1700MHz
  XFree86 v. 4.2.0
  Fvwm2 v.2.4.5
  GNU Emacs 21.1.1
  VM 7.07 (had the same problem with 7.05)
  Xresources:
    Emacs*background:       grey95
    Emacs*font:     -*-courier-medium-r-*-*-12-*-*-*-*-*-iso8859-1
    Emacs.ToolBar:  0
    Emacs.TopToolBarHeight: 0
    Emacs.geometry: 81x69+10+0
    Emacs.mode-line.AttributeBox:   off
    emacs.waitForWM:        false

--

 -------------------------------------------------------------------------
|  Wolfgang Dobler              Kiepenheuer Institute for Solar Physics  |
 -------------------------------------------------------------------------