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), another SIGHUP triggers a another rechist() which then waits
forever for the lock the the first rechist() created to be released
(which will never happen).

---
 tcsh-6.21.00/sh.hist.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- tcsh-6.21.00/sh.hist.c
+++ tcsh-6.21.00/sh.hist.c	2019-08-22 12:05:25.800474245 +0000
@@ -1219,7 +1219,7 @@ void
 rechist(Char *fname, int ref)
 {
     Char    *snum, *rs;
-    int     fp, ftmp, oldidfds;
+    int     fp, ftmp, oldidfds, phup_disabled_tmp;
     struct varent *shist;
     char path[MAXPATHLEN];
     struct stat st;
@@ -1227,6 +1227,10 @@ rechist(Char *fname, int ref)
 
     if (fname == NULL && !ref) 
 	return;
+
+    phup_disabled_tmp = phup_disabled;
+    phup_disabled = 1;
+
     /*
      * If $savehist is just set, we use the value of $history
      * else we use the value in $savehist
@@ -1301,6 +1305,7 @@ rechist(Char *fname, int ref)
     if (fp == -1) {
 	didfds = oldidfds;
 	cleanup_until(fname);
+	phup_disabled = phup_disabled_tmp;
 	return;
     }
     /* Try to preserve ownership and permissions of the original history file */
@@ -1325,6 +1330,7 @@ rechist(Char *fname, int ref)
     (void)ReplaceFile( short2str(fname),path,NULL,0,NULL,NULL);
 #endif
     cleanup_until(fname);
+    phup_disabled = phup_disabled_tmp;
 }