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

[Richard Henderson <richard.henderson@linaro.org>]

On 6/8/21 1:20 PM, Daniel P. Berrangé wrote:
> 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 not have_system, there's no point in looking for pam *at all* regardless of 
get_option().

>    => 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.

This is not true.  If 'required: false', find_library *will* look for the 
library, but it will allow it to be missing.

 From the meson docs:

>     meson.get_compiler('c').find_library('m', required: false),
...
> The `required: false` means the build will continue when using another C library
> that does not separate the maths library. See [Add math library (-lm)
> portably](howtox.md#add-math-library-lm-portably).
So if auto, and not system, we will look for (and maybe find) pam.  But then... 
I guess through a complex sequence of events arrives at

   softmmu_ss.add(authz, blockdev, chardev, crypto, io, qmp)

So pam doesn't get included into the user-only binaries anyway, but, geez.


r~