Backport PARALLEL_MAKE_IMAGE support

Add support for running the final make_image step in parallel

to make best use of larger machines with SSD storage. Use
PARALLEL_MAKE_IMAGE=<n> to specify how many to build in parallel.

CONF.sh

@@ -428,6 +428,11 @@ if [ -d "/etc/ssl/ca-debian" ]; then
 fi
 export WGET="wget $WGET_OPTS"
 
+# Run the make_image step in parallel? Specify the number of calls to
+# use in parallel here if desired. Don't go too high - this *will*
+# thrash your IO!
+#export PARALLEL_MAKE_IMAGE=4
+
 # Set this to force the Release file(s) to say "stable". Used in first
 # Etch builds to allow us to build before the archive updated
 #EARLY_BUILD_HACK=1

Makefile

@@ -491,6 +491,12 @@ image-trees: ok genlist
 images: ok $(OUT) $(BDIR)/md5-check
 	$(Q)$(make_image) "$(BDIR)" "$(ARCHES)" "$(OUT)" "$(DEBVERSION)" "$(MIRROR)" "$(MKISOFS)" "$(MKISOFS_OPTS)" "$(JIGDO_OPTS)" "$(jigdo_cleanup)"
 
+parallel_images: $(subst volid,image,$(notdir $(wildcard $(BDIR)/*.volid)))
+	echo $^
+
+%.image: ok $(OUT) $(BDIR)/checksum-check
+	$(Q)CD=$(subst .image,,$@) $(make_image)  "$(BDIR)" "$(ARCHES)" "$(OUT)" "$(DEBVERSION)" "$(MIRROR)" "$(MKISOFS)" "$(MKISOFS_OPTS)" "$(JIGDO_OPTS)" "$(jigdo_cleanup)"
+
 check-number-given:
 	@test -n "$(CD)" || (echo "Give me a CD=<num> parameter !" && false)
 

build.sh

@@ -80,7 +80,12 @@ else
 fi
 echo "Building $NUMJIGDOS jigdos and $NUMISOS isos for $ARCHES $DISKTYPE"
 
-make $IMAGETARGET
+if [ "$IMAGETARGET" = "official_images" ] && [ ! -z "$PARALLEL_MAKE_IMAGE" ]; then
+    make ok init packagelists image-trees
+    make -j $PARALLEL_MAKE_IMAGE parallel_images
+else
+    make $IMAGETARGET
+fi
 
 if [ "$IMAGESUMS"x = 1x ]; then
 	make imagesums

debian/changelog

@@ -11,6 +11,10 @@ debian-cd (3.1.26-buster) UNRELEASED; urgency=medium
     + tools/generate_di_list: Only include kernel udebs for the
       highest ABI we know about. Don't waste space on new installation
       media with older udebs from the archive.
+    + Add support for running the final make_image step in parallel to
+      make best use of larger machines with SSD storage. Use
+      PARALLEL_MAKE_IMAGE=<n> to specify how many to build in
+      parallel.
 
  -- Samuel Thibault <sthibault@debian.org>  Fri, 23 Aug 2019 21:51:50 +0200