[Tcsh] tcsh 6.22.00 does not merge history (was: Re: tcsh Deadlock with SIGHUP)

Brett Frankenberger rbf at rbfnet.com
Sun Feb 23 20:55:56 UTC 2020

So I replaced my custom tcsh with the current source and confirmed that
the fix you implemented prevents the deadlock, but in the process of
testing, found another bug -- tcsh no longer (as of 6.21) properly
merges history even when savehist is configured to do so.  (This is
unrelated to the fix for the SIGHUP deadlock.)

For example, if you do the following (with savehist = (100 merge lock))
(although the lock is, I think, not important here):

  (1) Launch two sessions.
  (2) In the first session, enter the command "testcommand1".
  (2) In the second session, enter the command "testcommand2".
  (3) Exit the first session.
  (4) Exit the second session.
  (5) Launch a new session.

Then, in the new session, the history will only include "testcommand2"; 
it will not include "testcommmand1", because when the second sesson
exited (step 4 above), the history file written by the first session  
(which exited in step 3 above) is not merged.

This appears to have broken between 6.21.00 and 6.22.00; I believe this
   4. Avoid infinite loops during history loads when merging, print a better
      error for errors during history load.
is the cause; specifically, this code in sh.hist.c:
   - loadhist(fname, 1);
   + getexit(osetexit);
   + if (setexit())
   +     loadhist(fname, 1);
   + resexit(osetexit);

It looks to me like "if (setexit())" should be "if (!setexit())". 
(setexit() is ultimately a wrapper around setjmp() or sigsetjmp(), both
of which return 0 on their first invocation.  So the code as written is
skipping loadhist() the first time through, which appears to be the
opposite of what was intended.)

Replacing "setexit()" with "!setexit()" fixed the merge issue for me. 

--- tcsh-6.22.02.orig/sh.hist.c
+++ tcsh-6.22.02/sh.hist.c
@@ -1292,7 +1292,7 @@ rechist(Char *fname, int ref)
-           if (setexit())
+           if (!setexit())
                loadhist(fname, 1);

     -- Brett

On Mon, Jan 20, 2020 at 11:40:49AM -0500, Christos Zoulas wrote:
> Thanks, it should be fixed now.
> christos
> > On Jan 20, 2020, at 9:08 AM, Brett Frankenberger <rbf at rbfnet.com> wrote:
> > 
> > (Resending what I posted last August ... let me know if there's
> > anything I can do to get this (or a differnet fix for the same issue)
> > into the tree.)
> > 
> > tcsh can deadlock with itself if savehist is confgured with "merge" and
> > "lock", and two SIGHUPs are received in rapid succession.  The
> > mechanism of the deadlock is the first SIGHUP triggers a rechist() and
> > while that rechist() is executing (and after it has created the lock
> > file), the second SIGHUP triggers a another rechist() which then waits
> > forever for the lock the the first rechist() created to be released
> > (which will never happen).

    [ . . . ]

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