Re: [RFC or so] Add HASH_LOOKUP option

[Peter Stephenson <Peter.Stephenson@csr.com>]

On Mon, 23 Aug 2010 16:09:45 +0200
Mikael Magnusson <mikachu@gmail.com> wrote:
> When this is unset, external commands are always resolved with a full
> path search, but still inserted into the hash for spell correction if
> those options are on.

Looks a reasonable thing to be able to do.  The word on the streets (for a
tiny, tiny subset of streets) has for some time been that modern operating
systems do their own optimisation of path lookup such that hashing in the
shell doesn't gain you very much.  The main reason we haven't done anything
about it is that we need the hash for other reasons.  You've sidestepped
that simply by always searching and still hashing it (depending on the
existing options).  That's a very minor performance hit beyond the path
search, so I can't see any objection.

Telling people to turn it off is probably the big problem.  I'd be
sympathetic to turning it off by default if there's enough evidence the
performance hit is minor, but that's all rather system-specific, so don't
hold your breath.  I'd be wary of turning it off by default on Cygwin with
paths to remote shares, for example.

It would be useful if you could write some tests to go in
Test/E01options.ztst.  Basically you just need to test the right version
of a command is found with it on and off.  It would be good to do a test
along the lines of

  mkdir dir1 dir2
  mkcmd() {
    print "#!/bin/sh\necho Command $1" >dir$1/cmd
    chmod +x dir$1/cmd
  }
  mkcmd 2
  for opt in on off on off; do
    if [[ $opt = on ]]; then
      setopt hashlookup
    else
      unsetopt hashlookup
    fi
    rm -rf dir1/cmd
    (path=($PWD/dir1 $PWD/dir2)
     cmd
     mkcmd 1
     cmd)
  done

and verify that when the option is off the second cmd prints
"Command 1" each time round and when it's on it doesn't.  (I haven't tested
this code.)

-- 
Peter Stephenson <pws@csr.com>            Software Engineer
Tel: +44 (0)1223 692070                   Cambridge Silicon Radio Limited
Churchill House, Cambridge Business Park, Cowley Road, Cambridge, CB4 0WZ, UK


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