PuTTY bug unclean-close-hang

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We've had numerous reports that if a PuTTY session on Windows Vista or Windows 7 closes with a network error -- for instance, "Connection timed out" at startup, or "Software caused connection abort" in an established session -- then after hitting OK, and after the window title has changed to "PuTTY (inactive)", the PuTTY window becomes unresponsive, and the PuTTY process consumes a lot of CPU, and must be killed via Task Manager.

This apparently happens with PuTTY's default setting of "Close window on exit": "Only on clean exit". Changing this to "Always" is reported to avoid the problem.

Unfortunately, while this is readily reproducible for sufferers, none of the development team who have tried it on a variety of Vista and Win7 systems have managed to reproduce the problem.

Intriguingly, some of the reports hint that dwm.exe -- Windows' Desktop Window Manager -- also consumes significant CPU until PuTTY is killed. One report appears to say that stopping the 'uxsms' service ("Desktop Window Manager Session Manager") made the problem go away, but that probably isn't a good idea as a workaround. (On this developer's machine at least, that service is running and I still can't reproduce the problem.)

There are also a couple of reports of very similar behaviour on XP, but nowhere near as many as Vista / Windows 7. It could be that this is a manifestation of unclean-close-crash that's just particularly reproducible on the new versions of Windows.

(Note that we did fix another Vista-specific CPU-bound hang in r8030, after 0.60 was released, but that was specific to processing of disconnection messages sent by SSH servers -- for instance after entering an incorrect password too many times -- and can't be implicated in these instances of network errors.)

Reports on Vista and Windows 7:

• 4802EEDB.7050908@gmail.com
  Vista SP1; hints of RAM munching also
• 42453A7809FE8F4F99CD7697CEF964CD0E0816964C@hpserver-exch.gadellnet.local
  Vista SP1; PuTTY 0.60
• f685e3ca0807272327r19c24587t722e9278063dd845@mail.gmail.com
  Vista 32-bit; PuTTY 0.60; dwm.exe also taking lots of CPU
• Pine.LNX.4.64.0805122143270.20049@mulva.they.org
  Vista Home Premium; PuTTY 0.60, 2008-07-02
• D1598C2AF60F433E85E85BC86938AB71@blue
  Vista Home Premium; PuTTY 0.60
• 21aed3f90812142037y224ebb9fw71737392bc6517cd@mail.gmail.com
  Vista Home Premium SP1; PuTTY 0.59, 0.60, 2008-12-15
• 4980A115.50709@gmail.com
  Vista; PuTTY 0.60, 2009-01-28; explorer.exe also taking lots of CPU
• 283365400AEE054DA4089467FDA6C78B09643568@PTEEXB02.mtsallstream.com
  Vista 64-bit
• 44c111ef0911171632r7d515cd9r72bf83ba1e9a3af4@mail.gmail.com
  Vista 32/64-bit and Windows 7 64-bit (but not XP 32-bit)
  PuTTY 0.59, 0.60, 2009-11-17
• CA6A6D14F091ED4B9BAED858B52EE9950132158F@mail0.niksoftware.loc
  Windows 7 64-bit
• AANLkTik4o6rvjnu0RHC5U_G0g8GuyvcUq0PDmXJNH5tS@mail.gmail.com
  Windows 7 64-bit; PuTTY 2010-05-24, 2010-07-28
• SNT138-w35C7A1ABCA375AA953BC9B82840@phx.gbl
  Windows 7
• AANLkTinrDMEJSBiTy3pXg6mr_Fd+R81O8eVgwaNsDx4J@mail.gmail.com
  Windows 7; PuTTY 0.60, 2010-09-20
• Windows Client TechCenter
  Windows 7 Pro 32-bit; PuTTY 0.60; using Aero
  Reported to make the taskbar unresponsive too
  Stopping the 'uxsms' service (Desktop Window Manager Session Manager) is reported to stop the problem

Other reports that don't fit the pattern in some way:

• 003901c77dd0$44baf080$0100a8c0@cgusa
  XP SP2; PuTTY 0.59, 2007-04-13
• 98D6DBD179F61A46AF5C064829A832A0023BD076E5@erebus.totalmanaged.com
  XP 32-bit; PuTTY 0.59, 0.60, 2008-07-01

Update, 2011-03-02: Committed r9115, which looks like a probable cause of this; it's made it go away for at least two people, so we've counted it as fixed despite never being able to reproduce it; if anyone still sees the problem with a version after r9115, please let us know.

Audit trail for this bug.