[tor-bugs] #14350 [Tor]: configure.ac: fix disabling systemd notification support

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#14350: configure.ac: fix disabling systemd notification support
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     Reporter:  blueness  |      Owner:
         Type:  defect    |     Status:  needs_review
     Priority:  normal    |  Milestone:  Tor: 0.2.6.x-final
    Component:  Tor       |    Version:
   Resolution:            |   Keywords:
Actual Points:            |  Parent ID:
       Points:            |
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Comment (by blueness):

 Okay a couple of points about how autoconf works.

 AC_ARG_ENABLE(systemd ...) automatically sets $enable_systemd to "yes" if
 --enable-systemd is passed, to "no" if --disable-systemd is given and to
 "foo" if -enable-systemd="foo" is given.  Its equal to "" if niether
 --enable-systemd  nor --disable-systemd are given.  So the code is
 inconsistent but works int that it uses $systemd in one palce and
 $enable_systemd in another.  I can make that consistant.

 You should never use $enableval becuase it is not reset between instances
 of AC_ARG_ENABLE().  However, in this case it works because you are uisng
 it in the 3rd parameter which is only run because you gave
 --{enable,disable}-systemd.  So its net effect is to set shadow variable
 $systemd to true or false and make sure soemone didn't pass something like
 --enable-systemd="silly".

 Convince yourself of how AC_ARG_ENABLE() works by playing with

 AC_PREREQ([2.69])
 AC_INIT([x],[1],[])

 AC_ARG_ENABLE(a, [],

   [case "${enableval}" inyes) echo "a true" ;;no)  echo "a false" ;;*)
 echo "a bad" ;;esac], [a=true])

 echo "enableval="${enableval}
 echo "enable_a="${enable_a}

 AC_ARG_ENABLE(b, [],

   [case "${enableval}" inyes) echo "b true" ;;no)  echo "b false" ;;*)
 echo "b bad" ;;esac], [b=true])

 echo "enableval="${enableval}"
 echo "enable_b="${enable_b}"
 AC_OUTPUT

 Try running it with `./configure --enable-a` and you'll see the second
 echo of ${enableval} is yes because it was never reset.

 Looking at the whole configure.a, there are a few places you make use of
 these shadowing variables like $systemd and $enable_system probably
 because whoever wrote that isn't aware of what AC_ARG_ENABLE() does behind
 the scenes.  If you want, I can clean that up, but I try not to be too
 intrusive in how upstream likes to do things.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/14350#comment:2>
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