kbuild: deb-pkg: set DEB_* variables if debian/rules is directly executed

Since commit 491b146 ("kbuild: builddeb: Eliminate debian/arch
use"), direct execution of debian/rules results in the following error:

  dpkg-architecture: error: unknown option 'DEB_HOST_MULTIARCH'

The current code:

  dpkg-architecture -a$DEB_HOST_ARCH -qDEB_HOST_MULTIARCH

... does not look sensible because:

 - For this code to work correctly, DEB_HOST_ARCH must be pre-defined,
   which is true when the packages are built via dpkg-buildpackage.
   In this case, DEB_HOST_MULTIARCH is also likely defined, hence there
   is no need to query DEB_HOST_MULTIARCH in the first place.

 - If DEB_HOST_MULTIARCH is undefined, DEB_HOST_ARCH is likely undefined
   too. So, you cannot query DEB_HOST_MULTIARCH in this way. This is
   mostly the case where debian/rules is directly executed.

When debian/rules is directly executed, querying DEB_HOST_MUCHARCH is
not enough because we need to know DEB_{BUILD,HOST}_GNU_TYPE as well.

All DEB_* variables are defined when the package build is initiated by
dpkg-buildpackage, but otherwise, let's call dpkg-architecture to set
all DEB_* environment variables.

This requires dpkg 1.20.6 or newer because --print-format option
was added in dpkg commit 7c54fa2b232e ("dpkg-architecture: Add a
--print-format option").

Signed-off-by: Masahiro Yamada <[email protected]>
Reviewed-by: Nicolas Schier <[email protected]>

Author: masahir0y

scripts/package/builddeb

@@ -171,9 +171,8 @@ install_libc_headers () {
 
 	# move asm headers to /usr/include/<libc-machine>/asm to match the structure
 	# used by Debian-based distros (to support multi-arch)
-	host_arch=$(dpkg-architecture -a$DEB_HOST_ARCH -qDEB_HOST_MULTIARCH)
-	mkdir $pdir/usr/include/$host_arch
-	mv $pdir/usr/include/asm $pdir/usr/include/$host_arch/
+	mkdir "$pdir/usr/include/${DEB_HOST_MULTIARCH}"
+	mv "$pdir/usr/include/asm" "$pdir/usr/include/${DEB_HOST_MULTIARCH}"
 }
 
 rm -f debian/files

scripts/package/debian/rules

@@ -30,5 +30,16 @@ build-arch:
 
 .PHONY: clean
 clean:
-	rm -rf debian/files debian/linux-*
+	rm -rf debian/files debian/linux-* debian/deb-env.vars*
 	$(MAKE) -f $(srctree)/Makefile ARCH=$(ARCH) clean
+
+# If DEB_HOST_ARCH is empty, it is likely that debian/rules was executed
+# directly. Run 'dpkg-architecture --print-set --print-format=make' to
+# generate a makefile construct that exports all DEB_* variables.
+ifndef DEB_HOST_ARCH
+include debian/deb-env.vars
+
+debian/deb-env.vars:
+	dpkg-architecture -a$$(cat debian/arch) --print-set --print-format=make > $@.tmp
+	mv $@.tmp $@
+endif