Re: [PATCH 05/26] configure, meson: convert pam detection to meson

[Daniel P . Berrangé]

On Tue, Jun 08, 2021 at 12:45:51PM -0700, Richard Henderson wrote:
> On 6/8/21 4:22 AM, Paolo Bonzini wrote:
> > +pam = not_found
> > +if not get_option('auth_pam').auto() or have_system
> > +  pam = cc.find_library('pam', has_headers: ['security/pam_appl.h'],
> 
> The condition doesn't look right.
> Why are we looking for pam if --disable-pam-auth?
> 
> Surely
> 
>   if not get_option('auth_pam').disabled() and have_system

This isn't entirely obvious at first glance, but the line after
the one you quote with the 'required' param makes it "do the
right thing (tm)".

The 'auth_pam' option is a tri-state taking 'enabled', 'disabled'
and 'auto', with 'auto' being the default state. When a tri-state
value is passed as the value of the 'required' parameter, then

   required==enabled   is interpreted as 'required=true'
   required==auto      is interpreted as 'required=false'
   required==disabled  means the entire call is a no-op

So this logic:

 if not get_option('auth_pam').auto() or have_system
    pam = cc.find_library('pam', has_headers: ['security/pam_appl.h'],
                          required: get_option('auth_pam'),
                          ...)

Means

  => If 'auto' is set, then only look for the library if we're
     building system emulators. In this case 'required:' will
     evaluate to 'false', and so we'll gracefully degrade
     if the library is missing.


  => If 'enabled' is set, then we'll look for the library
     and if it is missing then it is a fatal error as
     'required' will evaluate to 'true'.

  => If 'disabled' is set, then the 'find_library' call
     will not look for anything, immediately return a
     'not found' result and let the caller carry on.


Regards,
Daniel
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