[PATCH 1/3] Documentation: allow overriding timestamps of generated asciidoc

[Jonathan Nieder <jrnieder@gmail.com>]

From: Anders Kaseorg <andersk@mit.edu>
Date: Wed, 30 Nov 2016 22:21:15 -0500

Allow overriding the timestamp in generated documentation by setting
SOURCE_DATE_EPOCH to the number of seconds since 1970-01-01 00:00:00
UTC to use.

This makes the generated documentation reproducible from the source
code as long as that variable is set, without losing the last-modified
dates in the default build.

With this change, the package passes Debian's build reproducibility
test (https://wiki.debian.org/ReproducibleBuilds/TimestampsProposal).

The goal is to make it easier to verify that binary packages of open
source projects were built from the source they were claimed to have
been built from.  https://reproducible-builds.org/ has more details.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
---
Perhaps this should e.g. be taking the latest timestamp of all its
inputs.  That would be straightforward to do, but what's here is what
we've been running with for the past year, so I'd rather stick to it,
at least as a starting point.

Another tweak I'd be interested in is allowing asciidoc to take the
timestamp as a parameter instead of inferring it from mtimes.
Asciidoc accepts an "--attribute footer-style=none" parameter, but I'm
not aware of an "--attribute footer-date=<foo>" parameter to keep the
footer but override its date.

 Documentation/Makefile               | 7 +++++--
 Documentation/technical/api-index.sh | 5 +++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/Documentation/Makefile b/Documentation/Makefile
index 2ab65561af..dfec29f36f 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -410,6 +410,7 @@ $(patsubst %.txt,%.texi,$(MAN_TXT)): %.texi : %.xml
 howto-index.txt: howto-index.sh $(wildcard howto/*.txt)
 	$(QUIET_GEN)$(RM) $@+ $@ && \
 	'$(SHELL_PATH_SQ)' ./howto-index.sh $(sort $(wildcard howto/*.txt)) >$@+ && \
+	$(if $(SOURCE_DATE_EPOCH),touch -d '@$(SOURCE_DATE_EPOCH)' $@+ &&) \
 	mv $@+ $@
 
 $(patsubst %,%.html,$(ARTICLES)) : %.html : %.txt
@@ -420,8 +421,10 @@ WEBDOC_DEST = /pub/software/scm/git/docs
 howto/%.html: ASCIIDOC_EXTRA += -a git-relative-html-prefix=../
 $(patsubst %.txt,%.html,$(wildcard howto/*.txt)): %.html : %.txt
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	sed -e '1,/^$$/d' $< | \
-	$(TXT_TO_HTML) - >$@+ && \
+	sed -e '1,/^$$/d' $< > $<+ && \
+	$(if $(SOURCE_DATE_EPOCH),touch -d '@$(SOURCE_DATE_EPOCH)' $<+ &&) \
+	$(TXT_TO_HTML) -o $@+ $<+ && \
+	rm $<+ && \
 	mv $@+ $@
 
 install-webdoc : html
diff --git a/Documentation/technical/api-index.sh b/Documentation/technical/api-index.sh
index 9c3f4131b8..07b3909627 100755
--- a/Documentation/technical/api-index.sh
+++ b/Documentation/technical/api-index.sh
@@ -20,6 +20,11 @@
 	sed -n -e '/^\/\/ table of contents end/,$p' "$skel"
 ) >api-index.txt+
 
+if test "${SOURCE_DATE_EPOCH:+set}"
+then
+	touch -d "@$SOURCE_DATE_EPOCH" api-index.txt+
+fi
+
 if test -f api-index.txt && cmp api-index.txt api-index.txt+ >/dev/null
 then
 	rm -f api-index.txt+
-- 
2.15.0.448.gf294e3d99a