Commits on Jun 13, 2020

1. Implement a better fix for unsetting special env vars …

   The regression this commit fixes was first introduced in ksh93t
   2008-07-25. It was previously worked around in 6f0e008 by forking
   subshells if any special environment variable is unset.
   
   The reason why this problem doesn't occur in ksh93s+ is because in
   that version of ksh sh_assignok never moves nodes, it only clones
   them. The second argument doesn't set NV_MOVE, which makes
   `sh_assignok(np,0)` is similar to `sh_assignok(np,1)`. In ksh93t and
   higher, setting the second argument to zero causes the node to be moved
   with NV_MOVE, which causes the discipline function associated with
   the variable node to be removed when `np->nvfun` is set to zero (i.e.
   NULL). This is why a command like `(unset LC_NUMERIC; LC_NUMERIC=invalid)`
   doesn't print a diagnostic, as it looses its discipline function.
   
   This patch fixes the problem by cloning the node with sh_assignok
   if it is a special variable with a discipline function. This allows
   special variables to work as expected in virtual subshells. The
   original workaround has been kept for the $PATH variable only, as
   hash tables are still broken in virtual subshells. It has been updated
   accordingly to only fork subshells if it detects the variable node
   for PATH. I have added two more regression tests for changing the
   PATH in subshells to make sure hash tables continue working as
   expected with this fix.
   
   src/cmd/ksh93/bltins/typeset.c:
    - Only fork virtual subshells if the PATH will be changed. If a
      variable is a special variable with a discipline function, it
      should be just be cloned, not moved.
   
   src/cmd/ksh93/sh/nvdisc.c:
    - Add a comment to clarify that NV_MOVE will delete the discipline
      function associated with the node.
   
   src/cmd/ksh93/tests/subshells.sh:
    - Add two more regression tests for unsetting the PATH in subshells,
      one for if PATH is being pointed to by a nameref. Condense the
      hash table tests by moving the main test into a single function.

   

   JohnoKing committed Jun 13, 2020
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