[syslinux] [PATCH 3/3] Make configure an order-only prerequisite of aconfig.h.in

[Ron Lee]

From: Ron <ron at debian.org>

On filesystems with subsecond resolution, like ext4, we can't trust the
timestamp of aconfig.h.in since autoheader leaves it truncated to second
resolution (apparently touch -r and cp -p can do this at the very least)
while configure has full subsecond resolution, so it can look newer even
when it was cleanly created first, leading to the build system looping
trying to recreate all of that again ...

So in the initial stage of a clean build we can get something like this:

$ make spotless
$ make autoconf
rm -rf MCONFIG configure config.log aconfig.h *.cache
autoconf
rm -f aconfig.h.in aconfig.h
autoheader

$ stat configure aconfig.h.in
  File: ‘configure’
Device: 807h/2055d	Inode: 9443466     Links: 1
Access: 2014-07-31 03:27:27.599293442 +0930
Modify: 2014-07-31 03:27:27.711290270 +0930
Change: 2014-07-31 03:27:27.711290270 +0930

  File: ‘aconfig.h.in’
Device: 807h/2055d	Inode: 9443467     Links: 1
Access: 2014-07-31 03:27:27.000000000 +0930
Modify: 2014-07-31 03:27:27.000000000 +0930
Change: 2014-07-31 03:27:27.903284841 +0930

And with a parallel build, that can then leave 'make all' racing to
remove and recreate aconfig.h (and possibly more things), while it
begins to build the first targets.  Which then fail horribly like
we see here:

https://buildd.debian.org/status/fetch.php?pkg=tftp-hpa&arch=i386&ver=5.2%2B20140608-1&stamp=1406736363

Possibly we also need to move the actual build job into the rule for
the 'all' target, so that the build system update prerequisites are
guaranteed to be completed before it runs (as opposed to running in
parallel with them), but this change might be enough for now.

Signed-off-by: Ron Lee <ron at debian.org>
---
 Makefile | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 9ff12d8..75a2042 100644
--- a/Makefile
+++ b/Makefile
@@ -58,8 +58,12 @@ aconfig.h: MCONFIG
 
 # Adding "configure" to the dependencies serializes this with running
 # autoconf, because there are apparently race conditions between
-# autoconf and autoheader.
-aconfig.h.in: configure.in configure aclocal.m4
+# autoconf and autoheader.  And worse than that, even when autoconf
+# cleanly returns first, autoheader will truncate the timestamp of
+# aconfig.h.in to second resolution, so on a filesystem with subsecond
+# resolution it can appear older than configure (which isn't truncated).
+# So make it an order-only prerequisite to avoid looping regenerating it.
+aconfig.h.in: configure.in aclocal.m4 | configure
 	rm -f aconfig.h.in aconfig.h
 	autoheader
 
-- 
2.0.0