handling target specific flags in smb_build

[Andrew Tridgell]

Metze and Jelmer,

I am hoping to get your help in making a change to the smb_build
system in order to be able to support the in-tree heimdal build, and
also better support the in-tree building of tdb, ldb, talloc, ejs and
other libraries that have both in-tree and out-of-tree lives.

The things I am hoping to change are:

  - turn subsystems into separate make targets
  - target specific cflags
  - let make do its own expansion of dependencies and object lists

The reason we need this is that when building the heimdal part of the
code we need different CFLAGS than when building the rest of the
code. Specifically, we need a bunch of -I options so that we don't
need to change the #include paths in the heimdal code. We can't just
add these to the global CFLAGS, as we have at least one header file
with the same name (config.h), plus it would be rather untidy to have
to have a single huge CFLAGS.

I initially tried to do this by using the following syntax:

 target: TARGET_CFLAGS=-Isome/path -Isome/other/path

which works fine in gnu make, but not sun make. On sun make you can do
the same thing with:

 target:= TARGET_CFLAGS=-Isome/path -Isome/other/path

but neither works on aix make. So instead I am proposing that if a
subsystem have a TARGET_CFLAGS override it will end up with this:

target: $(TARGET_DEPS)
	TARGET_CFLAGS="some extra flags" $(MAKE) $(TARGET_DEPS)

that is recursive make of course, which we have tried to avoid, but I
can't see any other portable way to do this. It isn't the worst kind
of recursive make though, as at least it doesn't change makefile or
change directory, which is largely what makes the usual recursive make
systems so nasty.

In order to make all this work we will need subsystem targets. So at
the moment the SUBSYSTEM::SERVER_SERVICE exists as just a bunch of
macro defines in the makefile. I'd like to see it like this:

subsystem_SERVER_SERVICE: $(SERVER_SERVICE_DEPS)

that makes it possible to do 'make subsystem_SERVER_SERVICE', and also
makes it possible to do the target specific cflags, using the method
above.

It should also make it possible to reduce the size of the generated
Makefile a lot. For example, the generated fragment for building
bin/smbd is currently over 380 lines long, due to the expansion of all
the dependencies and object lists. It would be much more compact if it
looked like this:

BINARY_smbd_DEPEND_LIST = subsystem_PROCESS_MODEL \
			  subsystem_SERVER_SERVICE \
			  subsystem_CONFIG \
			  subsystem_LIBCMDLINE \
			  subsystem_LIBBASIC

which matches what is in smbd/config.mk. The normal make rules for
dependencies then take care of the expansion for us.

I started to have a look at how to make these changes in
build/smb_build/ but got a big bogged down with how the dependency and
object lists are built up. Do either of you two have time to have a
look at this?

Jelmer and I also discussed the possibility of adopting 'cons' as our
build system. That is certainly an option, although it doesn't seem to
support 'virtual' targets - it only seems to support targets that
exist as a directory of a file. The whole of the Samba4 build is full
of virtual targets so I'm not sure cons is a good fit. What do you
think Jelmer, any ideas on how to cope with that?

We could also adopt scons, but I am very reluctant to be reliant on
python for building Samba.

Cheers, Tridge

PS: please don't suggest automake