From https://ftp.gnu.org/gnu/bash/bash-4.4-patches/bash44-020

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

			     BASH PATCH REPORT
			     =================

Bash-Release:	4.4
Patch-ID:	bash44-020

Bug-Reported-by:	Graham Northup <northug@clarkson.edu>
Bug-Reference-ID:	<537530c3-61f0-349b-9de6-fa4e2487f428@clarkson.edu>
Bug-Reference-URL:	http://lists.gnu.org/archive/html/bug-bash/2017-02/msg00025.html

Bug-Description:

In circumstances involving long-running scripts that create and reap many
processes, it is possible for the hash table bash uses to store exit
statuses from asynchronous processes to develop loops. This patch fixes
the loop causes and adds code to detect any future loops.

Patch (apply with `patch -p0'):

*** bash-4.4-patched/jobs.c	2016-11-11 13:42:55.000000000 -0500
--- b/jobs.c	2017-02-22 15:16:28.000000000 -0500
***************
*** 813,818 ****
    struct pidstat *ps;
  
!   bucket = pshash_getbucket (pid);
!   psi = bgp_getindex ();
    ps = &bgpids.storage[psi];
  
--- b/796,815 ----
    struct pidstat *ps;
  
!   /* bucket == existing chain of pids hashing to same value
!      psi = where were going to put this pid/status */
! 
!   bucket = pshash_getbucket (pid);	/* index into pidstat_table */
!   psi = bgp_getindex ();		/* bgpids.head, index into storage */
! 
!   /* XXX - what if psi == *bucket? */
!   if (psi == *bucket)
!     {
! #ifdef DEBUG
!       internal_warning ("hashed pid %d (pid %d) collides with bgpids.head, skipping", psi, pid);
! #endif
!       bgpids.storage[psi].pid = NO_PID;		/* make sure */
!       psi = bgp_getindex ();			/* skip to next one */
!     }
! 
    ps = &bgpids.storage[psi];
  
***************
*** 842,845 ****
--- b/839,843 ----
  {
    struct pidstat *ps;
+   ps_index_t *bucket;
  
    ps = &bgpids.storage[psi];
***************
*** 847,856 ****
      return;
  
!   if (ps->bucket_next != NO_PID)
      bgpids.storage[ps->bucket_next].bucket_prev = ps->bucket_prev;
!   if (ps->bucket_prev != NO_PID)
      bgpids.storage[ps->bucket_prev].bucket_next = ps->bucket_next;
    else
!     *(pshash_getbucket (ps->pid)) = ps->bucket_next;
  }
  
--- b/845,861 ----
      return;
  
!   if (ps->bucket_next != NO_PIDSTAT)
      bgpids.storage[ps->bucket_next].bucket_prev = ps->bucket_prev;
!   if (ps->bucket_prev != NO_PIDSTAT)
      bgpids.storage[ps->bucket_prev].bucket_next = ps->bucket_next;
    else
!     {
!       bucket = pshash_getbucket (ps->pid);
!       *bucket = ps->bucket_next;	/* deleting chain head in hash table */
!     }
! 
!   /* clear out this cell, just in case */
!   ps->pid = NO_PID;
!   ps->bucket_next = ps->bucket_prev = NO_PIDSTAT;
  }
  
***************
*** 859,863 ****
       pid_t pid;
  {
!   ps_index_t psi;
  
    if (bgpids.storage == 0 || bgpids.nalloc == 0 || bgpids.npid == 0)
--- b/864,868 ----
       pid_t pid;
  {
!   ps_index_t psi, orig_psi;
  
    if (bgpids.storage == 0 || bgpids.nalloc == 0 || bgpids.npid == 0)
***************
*** 865,871 ****
  
    /* Search chain using hash to find bucket in pidstat_table */
!   for (psi = *(pshash_getbucket (pid)); psi != NO_PIDSTAT; psi = bgpids.storage[psi].bucket_next)
!     if (bgpids.storage[psi].pid == pid)
!       break;
  
    if (psi == NO_PIDSTAT)
--- b/870,883 ----
  
    /* Search chain using hash to find bucket in pidstat_table */
!   for (orig_psi = psi = *(pshash_getbucket (pid)); psi != NO_PIDSTAT; psi = bgpids.storage[psi].bucket_next)
!     {
!       if (bgpids.storage[psi].pid == pid)
! 	break;
!       if (orig_psi == bgpids.storage[psi].bucket_next)	/* catch reported bug */
! 	{
! 	  internal_warning ("bgp_delete: LOOP: psi (%d) == storage[psi].bucket_next", psi);
! 	  return 0;
! 	}
!     }
  
    if (psi == NO_PIDSTAT)
***************
*** 905,909 ****
       pid_t pid;
  {
!   ps_index_t psi;
  
    if (bgpids.storage == 0 || bgpids.nalloc == 0 || bgpids.npid == 0)
--- b/917,921 ----
       pid_t pid;
  {
!   ps_index_t psi, orig_psi;
  
    if (bgpids.storage == 0 || bgpids.nalloc == 0 || bgpids.npid == 0)
***************
*** 911,917 ****
  
    /* Search chain using hash to find bucket in pidstat_table */
!   for (psi = *(pshash_getbucket (pid)); psi != NO_PIDSTAT; psi = bgpids.storage[psi].bucket_next)
!     if (bgpids.storage[psi].pid == pid)
!       return (bgpids.storage[psi].status);
  
    return -1;
--- b/923,936 ----
  
    /* Search chain using hash to find bucket in pidstat_table */
!   for (orig_psi = psi = *(pshash_getbucket (pid)); psi != NO_PIDSTAT; psi = bgpids.storage[psi].bucket_next)
!     {
!       if (bgpids.storage[psi].pid == pid)
! 	return (bgpids.storage[psi].status);
!       if (orig_psi == bgpids.storage[psi].bucket_next)	/* catch reported bug */
! 	{
! 	  internal_warning ("bgp_search: LOOP: psi (%d) == storage[psi].bucket_next", psi);
! 	  return -1;
! 	}
!     }
  
    return -1;
*** bash-4.4/patchlevel.h	2016-06-22 14:51:03.000000000 -0400
--- b/patchlevel.h	2016-10-01 11:01:28.000000000 -0400
***************
*** 26,30 ****
     looks for to find the patch level (for the sccs version string). */
  
! #define PATCHLEVEL 19
  
  #endif /* _PATCHLEVEL_H_ */
--- b/26,30 ----
     looks for to find the patch level (for the sccs version string). */
  
! #define PATCHLEVEL 20
  
  #endif /* _PATCHLEVEL_H_ */