Mike,

I see what you mean here, but I believe it is absolutely fine to implement DebugCommonLib.h interface within DebugLib without depending on DebugCommonLib if one absolutely desires it (thought I do not think it is a good idea).
In this case, perhaps we can avoid adding DebugCommonLib.h header at all and leave it all in DebugLib.h?
So that DebugCommonLib library is entirely private thing to DebugLib, which can either use it or not.

Best wishes,
Vitaly

> 12 мая 2020 г., в 21:18, Kinney, Michael D <michael.d.kinney@intel.com> написал(а):
> 
> Laszlo,
> 
> A library implementation that uses another library class
> lists those library classes in the library INF.  This is
> a way a module can inherit the use of a second lib without
> listing the second lib in the module INF.  This is the
> type of inheritance that is supported by the EDK II build
> system and the EDK II meta data files.
> 
> What is not supported today is indirect inheritance from
> the libclass .h file itself.  There is no mechanism for
> the libclass .h file to declare it is using another library
> class.  This would require extensions to the meta data
> files to declare this type dependency.
> 
> How does the build system know to add DebugCommonLib to
> the link command for a module that only lists DebugLib
> in its library classes section?  What if there is a
> DebugLib implementation of the DebugLib class that
> does not depend on DebugCommonLib.  The module link
> will then fail when the module writer followed all the
> rules.  This is why including a libclass from another
> libclass is bad idea.
> 
> Mike
> 
>> -----Original Message-----
>> From: devel@edk2.groups.io <devel@edk2.groups.io> On
>> Behalf Of Laszlo Ersek
>> Sent: Tuesday, May 12, 2020 2:51 AM
>> To: Kinney, Michael D <michael.d.kinney@intel.com>;
>> Vitaly Cheptsov <cheptsov@ispras.ru>;
>> devel@edk2.groups.io
>> Cc: Andrew Fish <afish@apple.com>; Marvin Häuser
>> <mhaeuser@outlook.de>; Gao, Liming
>> <liming.gao@intel.com>; Gao, Zhichao
>> <zhichao.gao@intel.com>
>> Subject: Re: [edk2-devel] [PATCH V4 00/27] Disabling
>> safe string constraint assertions
>> 
>> On 05/12/20 00:40, Kinney, Michael D wrote:
>>> Vitaly,
>>> 
>>> Thank you for the contribution.
>>> 
>>> There are a couple points about this approach that
>> need to be discussed.
>>> 
>>> You have included the <Library/DebugCommonLib.h> from
>>> MdePkg/Include/Library/DebugLib.h.
>> 
>> Right, I've noticed it. I agree it's unusual. I didn't
>> think it was wrong.
>> 
>>> It is very rare for a
>>> lib class to include another lib class.  This means
>> that a module
>>> that has a dependency on the DebugLib class inherits
>> a hidden
>>> dependency on the DebugCommonLib class.
>> 
>> I agree.
>> 
>> I think it should be fine. Any header H1 should include
>> such further
>> headers H2, H3, ... Hn that are required for making the
>> interfaces
>> declared in H1 usable in client modules.
>> 
>>> For module INF files,
>>> we require the INF file to list all the lib classes
>> that the
>>> module sources directly use.
>> 
>> Yes, keyword being "directly".
>> 
>>> Since a module that uses the
>>> DebugLib uses the ASSERT() and DEBUG() macros, all
>> the APIs
>>> that the ASSERT() and DEBUG() macros use are also
>> directly
>>> used by the module.
>> 
>> I believe this is where I disagree. The replacement
>> texts of the
>> ASSERT() and DEBUG() function-like macros are internals
>> of the
>> DebugLib.h lib class header, in my opinion. Those
>> internals place
>> requirements on specific DebugLib instances, not on
>> DebugLib class
>> consumers.
>> 
>> In other words, when writing a new DebugLib instance,
>> the implementor
>> has to ensure that the ASSERT() and DEBUG() macros, as
>> defined in the
>> DebugLib class header, will continue working in
>> DebugLib consumer
>> modules. The implementor may then choose to make the
>> new DebugLib
>> instance dependent on the (singleton) DebugCommonLib
>> instance, for
>> example. (This is being done in patches #3, #4, #16,
>> maybe more.) The
>> DebugLib consumer module will inherit that dependency,
>> and everything
>> will work.
>> 
>> Just because ASSERT() and DEBUG() are function-like
>> macros and not
>> actual functions, I don't think the INF file
>> requirements in
>> DebugLib-consumer modules should change.
>> 
>>> With this patch series, these macros
>>> now use the DebugCommonLib class APIs, which means
>> any module
>>> that uses the DebugLib also directly uses the
>> DebugCommonLib.
>> 
>> In my opinion: indirectly.
>> 
>> Thanks,
>> Laszlo
>> 
>> 
>> 
>