split brain cache: Add a dummy ref update when necessary

git-update-ref does not make a new reflog entry if the ref is not
actually being changed.

This can mean that `dgit build' might fail to find a good cache entry,
but also fail to generate one that push will accept.

In this case, generate a dummy reflog update.  We make a spurious
child of the $dgitview, as that's straightforward.  Giving it the same
tree and a parent means that human use of reflog commands will produce
empty results rather than lots of confusing output.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

2 files changed, 20 insertions, 0 deletions

diff --git a/debian/changelog b/debian/changelog
index 99e724d..08b3e32 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -6,6 +6,10 @@ dgit (2.4~) unstable; urgency=low
     Closes:#841100.
   * split brain cache: Fix a wrong implicit reference to $_.
     Closes:#841383.
+  * split brain cache: Make sure to write reflog entries for cache updates
+    even if the eventual tree (and therefore commit) is the same.
+    Otherwise, after updating dgit, the cache might have the right answer
+    but not be refreshed even by a build.
 
  --
 
diff --git a/dgit b/dgit
index 9fcd1fc..7ca97c5 100755
--- a/dgit
+++ b/dgit
@@ -3917,6 +3917,22 @@ END
     ensuredir ".git/logs/refs/dgit-intern";
     my $makelogfh = new IO::File ".git/logs/refs/$splitbraincache", '>>'
       or die $!;
+
+    my $oldcache = git_get_ref "refs/$splitbraincache";
+    if ($oldcache eq $dgitview) {
+	my $tree = cmdoutput qw(git rev-parse), "$dgitview:";
+	# git update-ref doesn't always update, in this case.  *sigh*
+	my $dummy = make_commit_text <<END;
+tree $tree
+parent $dgitview
+author Dgit <dgit\@example.com> 1000000000 +0000
+committer Dgit <dgit\@example.com> 1000000000 +0000
+
+Dummy commit - do not use
+END
+	runcmd @git, qw(update-ref -m), "dgit $our_version - dummy",
+	    "refs/$splitbraincache", $dummy;
+    }
     runcmd @git, qw(update-ref -m), $cachekey, "refs/$splitbraincache",
 	$dgitview;