From c09523372b70830cf31de0a21348c89c2b680fdc Mon Sep 17 00:00:00 2001
From: Ian Jackson <ijackson@chiark.greenend.org.uk>
Date: Fri, 25 Aug 2017 10:45:29 +0100
Subject: [PATCH] git-debrebase: classify: tolerate backwards breakwater merges

---
 git-debrebase | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/git-debrebase b/git-debrebase
index a3733a98..8f2e9223 100755
--- a/git-debrebase
+++ b/git-debrebase
@@ -390,8 +390,6 @@ sub classify ($) {
     my ($stype, $series) = git_cat_file "$t:debian/patches/series";
     my $haspatches = $stype ne 'missing' && $series =~ m/^\s*[^#\n\t ]/m;
 
-@p = reverse @p; #xxx
-
     # How to decide about l/r ordering of breakwater merges ?  git
     # --topo-order prefers to expand 2nd parent first.  There's
     # already an easy rune to look for debian/ history anyway (git log
@@ -399,17 +397,25 @@ sub classify ($) {
     # way also there's also an easy rune to look for the upstream
     # patches (--topo-order).
 
-    my $prevbrw = 0;
-
-    if (@p == 2 &&
-	!$haspatches &&
-	!$p[$prevbrw]{IsOrigin} && # breakwater never starts with an origin
-	!($p[$prevbrw]{Differs} & ~D_DEB) &&
-	!($p[!$prevbrw]{Differs} & ~D_UPS)) {
-	return $classify->(qw(BreakwaterUpstreamMerge),
-			   OrigParents => [ $p[!$prevbrw] ]);
+    # The above tells us which way *we* will generate them.  But we
+    # might encounter ad-hoc breakwater merges generated manually,
+    # which might be the other way around.  In principle, in some odd
+    # situations, a breakwater merge might have two identical parents.
+    # In that case we guess which way round it is (ie, which parent
+    # has the upstream history).  The order of the 2-iteration loop
+    # controls which guess we make.
+
+    foreach my $prevbrw (qw(0 1)) {
+	if (@p == 2 &&
+	    !$haspatches &&
+	    !$p[$prevbrw]{IsOrigin} && # breakwater never starts with an origin
+	    !($p[$prevbrw]{Differs} & ~D_DEB) &&
+	    !($p[!$prevbrw]{Differs} & ~D_UPS)) {
+	    return $classify->(qw(BreakwaterUpstreamMerge),
+			       OrigParents => [ $p[!$prevbrw] ]);
+	}
+	# xxx multi-.orig upstreams
     }
-    # xxx multi-.orig upstreams
 
     return $unknown->("complex merge");
 }
-- 
2.30.2