[LEDE-DEV] [PATCH] metadata: compile dependencies only when the package is selected

[Yousong Zhou]

On 28 February 2018 at 18:58, Felix Fietkau <nbd at nbd.name> wrote:
> On 2018-02-28 11:48, Yousong Zhou wrote:
>> On 28 February 2018 at 16:13, Felix Fietkau <nbd at nbd.name> wrote:
>>> On 2018-02-28 06:07, Yousong Zhou wrote:
>>>> This is intended to reduce build time for situations like the following
>>>> where python and python-six and their dependencies could still be built
>>>> as long as any subpackage within the srcpackage was selected regardless
>>>> of the selection state of openvswitch-python
>>>>
>>>>     define Package/openvswitch-python
>>>>       ...
>>>>       DEPENDS:=+python +python-six
>>>>     endef
>>>>
>>>> Previously we work around this by specifying the dependency as
>>>> +PACKAGE_openvswitch-python:python, which is unintuitive
>>>>
>>>> Signed-off-by: Yousong Zhou <yszhou4tech at gmail.com>
>>> The current behavior is intentional. The idea is that many packages
>>> currently do not use PKG_CONFIG_DEPENDS properly, or specify enable or
>>> disable of extra library support via configure arguments.
>>>
>>> This means that if the dependency depends on the package selection, we
>>> will get a lot of random package build failures depending on the build
>>> order.
>>>
>>
>> Hi Felix, can you describe a concrete example where failure can
>> happen?  PKG_CONFIG_DEPENDS and the change here seems orthogonal to
>> each other.
> Let's take the openvswitch package as an example. If you modify it to
> remove the PKG_CONFIG_DEPENDS and PKG_BUILD_DEPENDS, theoretically the
> following scenario could happen:
>
> You make a clean build with openvswitch-python and python itself not
> selected. Since python doesn't get built, openvswitch detects that
> python is not available and can't build its language binding.
> Now you decide that you want python support after all, so you select
> openvswitch-python and run make again.
> It won't rebuild anything in openvswitch, since no stampfiles are
> affected, it will just try to package openvswitch-python.
> This will now fail, because the openvswitch-python package bindings
> could not be built the first time around.

Current openvswitch does not have this issue as it explicitly selects
"+python" for openvswitch-python.  That put aside, even if the said
situation happened where openvswitch-python does not depends on or
select python, it's a flaw in the packaging process that will affect
later usage when e.g. installing it with openvswitch-python as users
will expect the dependency should already be in place.

>
> There are several other packages that have support for plugins that
> depend on various libraries. If the maintainers of those packages are
> not careful about either handling reconfiguration, or specifying
> everything as build dependencies, you can get spurious rebuild bugs like
> this.

If you meat situations like "+CONFIG_openvpn_use_mbedtls:libmbedtls
+CONFIG_openvpn_use_openssl:libopenssl" where packagers were expected
to add these two "CONFIG_openvpn_use_xx" symbols to
PKG_CONFIG_DEPENDS.

The suggested change here won't worse the situation: as long as
openvpn got selected, these libmbedtls and libopenssl would still have
their chance to be built.  Whether re-configure should happen depends
solely on PKG_CONFIG_DEPENDS.  It should still work as before even in
the situation where users switched from use_mbedtls to use_openssl in
which case libopenssl will be rebuilt and  it won't intervene with
configure and use of the new lib.  That's why I think the change is
orthogonal to the reconfigure issue.  Please correct me if I am wrong
or missing something.

Regards,
                yousong