include/download.mk: generate reproducable SCM tarballs

Apply a number of changes to the tarball generation in order to produce
identical files on different systems:

1) Use an explicit `gzip -cn` to avoid storing file mtime in the gzip header
2) Instruct `tar` to unconditionally use uid and gid 0 for archive members
3) Instruct `tar` to sort archive members by file name
4) For SCMs that do not preserve file modification times like Git or Mercurial,
   use the date of the last commit to the repository and pass it as `--mtime`
   value to `tar`

After these changes, locally produced tarballs generated from SCM checkouts
should be identical on any system, simplifying the mirroring of cache archives.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

include/download.mk

@@ -33,9 +33,9 @@ $(strip \
 endef
 
 # code for creating tarballs from cvs/svn/git/bzr/hg/darcs checkouts - useful for mirror support
-dl_pack/bz2=$(TAR) cjf $(1) $(2)
+dl_pack/bz2=$(TAR) --owner=0 --group=0 --sort=name $$$${TAR_TIMESTAMP:+--mtime="$$$$TAR_TIMESTAMP"} -cjf $(1) $(2)
-dl_pack/gz=$(TAR) czf $(1) $(2)
+dl_pack/gz=$(TAR) --owner=0 --group=0 --sort=name $$$${TAR_TIMESTAMP:+--mtime="$$$$TAR_TIMESTAMP"} -c $(2) | gzip -nc > $(1)
-dl_pack/xz=$(TAR) c $(2) | xz -zc > $(1)
+dl_pack/xz=$(TAR) --owner=0 --group=0 --sort=name $$$${TAR_TIMESTAMP:+--mtime="$$$$TAR_TIMESTAMP"} -c $(2) | xz -zc > $(1)
 dl_pack/unknown=echo "ERROR: Unknown pack format for file $(1)"; false
 define dl_pack
 	$(if $(dl_pack/$(call ext,$(1))),$(dl_pack/$(call ext,$(1))),$(dl_pack/unknown))
@@ -79,6 +79,7 @@ define DownloadMethod/svn
 		svn export --non-interactive --trust-server-cert -r$(VERSION) $(URL) $(SUBDIR) || \
 		svn export --non-interactive -r$(VERSION) $(URL) $(SUBDIR) ) && \
 		echo "Packing checkout..." && \
+		export TAR_TIMESTAMP="" && \
 		$(call dl_pack,$(TMP_DIR)/dl/$(FILE),$(SUBDIR)) && \
 		mv $(TMP_DIR)/dl/$(FILE) $(DL_DIR)/ && \
 		rm -rf $(SUBDIR); \
@@ -96,6 +97,7 @@ define DownloadMethod/git
 		(cd $(SUBDIR) && git checkout $(VERSION) && \
 		git submodule update --init --recursive) && \
 		echo "Packing checkout..." && \
+		export TAR_TIMESTAMP=`cd $(SUBDIR) && git log -1 --format='@%ct'` && \
 		rm -rf $(SUBDIR)/.git && \
 		$(call dl_pack,$(TMP_DIR)/dl/$(FILE),$(SUBDIR)) && \
 		mv $(TMP_DIR)/dl/$(FILE) $(DL_DIR)/ && \
@@ -110,8 +112,9 @@ define DownloadMethod/bzr
 		cd $(TMP_DIR)/dl && \
 		rm -rf $(SUBDIR) && \
 		[ \! -d $(SUBDIR) ] && \
-		bzr export -r$(VERSION) $(SUBDIR) $(URL) && \
+		bzr export --per-file-timestamps -r$(VERSION) $(SUBDIR) $(URL) && \
 		echo "Packing checkout..." && \
+		export TAR_TIMESTAMP="" && \
 		$(call dl_pack,$(TMP_DIR)/dl/$(FILE),$(SUBDIR)) && \
 		mv $(TMP_DIR)/dl/$(FILE) $(DL_DIR)/ && \
 		rm -rf $(SUBDIR); \
@@ -126,6 +129,7 @@ define DownloadMethod/hg
 		rm -rf $(SUBDIR) && \
 		[ \! -d $(SUBDIR) ] && \
 		hg clone -r $(VERSION) $(URL) $(SUBDIR) && \
+		export TAR_TIMESTAMP=`cd $(SUBDIR) && hg log --template '@{date}' -l 1` && \
 		find $(SUBDIR) -name .hg | xargs rm -rf && \
 		echo "Packing checkout..." && \
 		$(call dl_pack,$(TMP_DIR)/dl/$(FILE),$(SUBDIR)) && \
@@ -142,6 +146,7 @@ define DownloadMethod/darcs
 		rm -rf $(SUBDIR) && \
 		[ \! -d $(SUBDIR) ] && \
 		darcs get -t $(VERSION) $(URL) $(SUBDIR) && \
+		export TAR_TIMESTAMP=`cd $(SUBDIR) && LC_ALL=C darcs log --last 1 | sed -ne 's!^Date: \+!!p'` && \
 		find $(SUBDIR) -name _darcs | xargs rm -rf && \
 		echo "Packing checkout..." && \
 		$(call dl_pack,$(TMP_DIR)/dl/$(FILE),$(SUBDIR)) && \