X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=dgit.git;a=blobdiff_plain;f=git-debrebase;h=e8dfbf99dad4e747490b2a304f66124ed8fce96e;hp=b3b5ea691e37cee8775134adefa4818ac4957996;hb=ec2d9d708e0d45cfc4623b20057d042127690599;hpb=80a062713ecedac03c5ba113c596ee85cd71e5ec

diff --git a/git-debrebase b/git-debrebase
index b3b5ea69..e8dfbf99 100755
--- a/git-debrebase
+++ b/git-debrebase
@@ -18,6 +18,10 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+#    git-debrebase new-upstreams-v0 \
+#             NEW-VERSION ORIG-COMMITISH
+#            [EXTRA-ORIG-NAME EXTRA-ORIG-COMMITISH ...]
+
 # usages:
 #    git-debrebase status
 #    git-debrebase start       # like ffqrebase start + debrebase launder
@@ -104,12 +108,17 @@
 
 use strict;
 
+use Debian::Dgit qw(:DEFAULT :playground);
+setup_sigwarn();
+
 use Memoize;
 use Carp;
 use POSIX;
 use Data::Dumper;
+use Getopt::Long qw(:config posix_default gnu_compat bundling);
+use Dpkg::Version;
 
-use Debian::Dgit qw(:DEFAULT $wa);
+our ($opt_force);
 
 sub badusage ($) {
     my ($m) = @_;
@@ -128,23 +137,118 @@ sub cfg ($) {
 
 memoize('cfg');
 
+sub dd ($) {
+    my ($v) = @_;
+    my $dd = new Data::Dumper [ $v ];
+    Terse $dd 1; Indent $dd 0; Useqq $dd 1;
+    return Dump $dd;
+}
+
 sub get_commit ($) {
     my ($objid) = @_;
-    my ($type,$data) = git_cat_file $objid;
-    die unless $type eq 'commit';
-    $data =~ m/(?<=\n)\n/;
+    my $data = git_cat_file $objid, 'commit';
+    $data =~ m/(?<=\n)\n/ or die "$objid ($data) ?";
     return ($`,$');
 }
 
-sub D_DEB ()     { return 0x1; } # debian/ (not including debian/patches/)
-sub D_UPS ()     { return 0x2; } # upstream files
-sub D_PAT_ADD () { return 0x4; } # debian/patches/ extra patches at end
-sub D_PAT_OTH () { return 0x8; } # debian/patches other changes
+sub D_UPS ()      { 0x02; } # upstream files
+sub D_PAT_ADD ()  { 0x04; } # debian/patches/ extra patches at end
+sub D_PAT_OTH ()  { 0x08; } # debian/patches other changes
+sub D_DEB_CLOG () { 0x10; } # debian/ (not patches/ or changelog)
+sub D_DEB_OTH ()  { 0x20; } # debian/changelog
+sub DS_DEB ()     { D_DEB_CLOG | D_DEB_OTH; } # debian/ (not patches/)
+
+our $playprefix = 'debrebase';
+our $rd;
+our $workarea;
 
-our $rd = ".git/git-debrebase";
-our $ud = "$rd/work";
 our @git = qw(git);
 
+sub in_workarea ($) {
+    my ($sub) = @_;
+    changedir $workarea;
+    my $r = eval { $sub->(); };
+    { local $@; changedir $maindir; }
+    die $@ if $@;
+}
+
+sub fresh_workarea () {
+    $workarea = fresh_playground "$playprefix/work";
+    in_workarea sub { playtree_setup };
+}
+
+sub get_differs ($$) {
+    my ($x,$y) = @_;
+    # This resembles quiltify_trees_differ, in dgit, a bit.
+    # But we don't care about modes, or dpkg-source-unrepresentable
+    # changes, and we don't need the plethora of different modes.
+    # Conversely we need to distinguish different kinds of changes to
+    # debian/ and debian/patches/.
+
+    my $differs = 0;
+
+    my $rundiff = sub {
+	my ($opts, $limits, $fn) = @_;
+	my @cmd = (@git, qw(diff-tree -z --no-renames));
+	push @cmd, @$opts;
+	push @cmd, "$_:" foreach $x, $y;
+	push @cmd, @$limits;
+	my $diffs = cmdoutput @cmd;
+	foreach (split /\0/, $diffs) { $fn->(); }
+    };
+
+    $rundiff->([qw(--name-only)], [], sub {
+        $differs |= $_ eq 'debian' ? DS_DEB : D_UPS;
+    });
+
+    if ($differs & DS_DEB) {
+	$differs &= ~DS_DEB;
+	$rundiff->([qw(--name-only -r)], [qw(debian)], sub {
+            $differs |=
+		m{^debian/patches/}      ? D_PAT_OTH  :
+		$_ eq 'debian/changelog' ? D_DEB_CLOG :
+		                           D_DEB_OTH;
+	});
+	die "mysterious debian changes $x..$y"
+	    unless $differs & (D_PAT_OTH|DS_DEB);
+    }
+
+    if ($differs & D_PAT_OTH) {
+	my $mode;
+	$differs &= ~D_PAT_OTH;
+	my $pat_oth = sub {
+	    $differs |= D_PAT_OTH;
+	    no warnings qw(exiting);  last;
+	};
+	$rundiff->([qw(--name-status -r)], [qw(debian/patches/)], sub {
+            no warnings qw(exiting);
+            if (!defined $mode) {
+		$mode = $_;  next;
+	    }
+	    die unless s{^debian/patches/}{};
+	    my $ok;
+	    if ($mode eq 'A' && !m/\.series$/s) {
+		$ok = 1;
+	    } elsif ($mode eq 'M' && $_ eq 'series') {
+		my $x_s = git_cat_file "$x:debian/patches/series", 'blob';
+		my $y_s = git_cat_file "$y:debian/patches/series", 'blob';
+		chomp $x_s;  $x_s .= "\n";
+		$ok = $x_s eq substr($y_s, 0, length $x_s);
+	    } else {
+		# nope
+	    }
+	    $mode = undef;
+	    $differs |= $ok ? D_PAT_ADD : D_PAT_OTH;
+        });
+	die "mysterious debian/patches changes $x..$y"
+	    unless $differs & (D_PAT_ADD|D_PAT_OTH);
+    }
+
+    printdebug sprintf "get_differs %s, %s = %#x\n", $x, $y, $differs;
+
+    return $differs;
+}
+
 sub commit_pr_info ($) {
     my ($r) = @_;
     return Data::Dumper->dump([$r], [qw(commit)]);
@@ -158,6 +262,17 @@ sub calculate_committer_authline () {
     return $&;
 }
 
+sub rm_subdir_cached ($) {
+    my ($subdir) = @_;
+    runcmd @git, qw(rm --quiet -rf --cached --ignore-unmatch), $subdir;
+}
+
+sub read_tree_subdir ($$) {
+    my ($subdir, $new_tree_object) = @_;
+    rm_subdir_cached $subdir;
+    runcmd @git, qw(read-tree), "--prefix=$subdir/", $new_tree_object;
+}
+
 # classify returns an info hash like this
 #   CommitId => $objid
 #   Hdr => # commit headers, including 1 final newline
@@ -176,6 +291,7 @@ sub calculate_committer_authline () {
 #
 # Types:
 #   Packaging
+#   Changelog
 #   Upstream
 #   AddPatches
 #   Mixed
@@ -192,41 +308,67 @@ sub calculate_committer_authline () {
 #
 #   BreakwaterUpstreamMerge
 #     has additional entry in classification result
-#       OrigParents = [ subset of Parents ]
+#       OrigParents = [ subset of Parents ]  # singleton list
 
-sub classify ($) {
-    my ($objid) = @_;
+sub parsecommit ($;$) {
+    my ($objid, $p_ref) = @_;
+    # => hash with                   CommitId Hdr Msg Tree Parents
+    #    Parents entries have only   Ix CommitId
+    #    $p_ref, if provided, must be [] and is used as a base for Parents
+
+    $p_ref //= [];
+    die if @$p_ref;
 
     my ($h,$m) = get_commit $objid;
 
     my ($t) = $h =~ m/^tree (\w+)$/m or die $objid;
-    my (@ph) = $h =~ m/^parent (\w+)$/m;
-    my @p;
+    my (@ph) = $h =~ m/^parent (\w+)$/mg;
 
     my $r = {
 	CommitId => $objid,
 	Hdr => $h,
 	Msg => $m,
         Tree => $t,
-	Parents => \@p,
+	Parents => $p_ref,
     };
 
     foreach my $ph (@ph) {
-	push @p, {
-            Ix => $#p,
+	push @$p_ref, {
+            Ix => scalar @$p_ref,
             CommitId => $ph,
-            Differs => (get_differs $t, $ph),
         };
     }
 
+    return $r;
+}    
+
+sub classify ($) {
+    my ($objid) = @_;
+
+    my @p;
+    my $r = parsecommit($objid, \@p);
+    my $t = $r->{Tree};
+
+    foreach my $p (@p) {
+	$p->{Differs} = (get_differs $p->{CommitId}, $t),
+    }
+
+    printdebug "classify $objid \$t=$t \@p",
+	(map { sprintf " %s/%#x", $_->{CommitId}, $_->{Differs} } @p),
+	"\n";
+
     my $classify = sub {
 	my ($type, @rest) = @_;
 	$r = { %$r, Type => $type, @rest };
+	if ($debuglevel) {
+	    printdebug " = $type ".(dd $r)."\n";
+	}
 	return $r;
     };
     my $unknown = sub {
 	my ($why) = @_;
 	$r = { %$r, Type => qw(Unknown) };
+	printdebug " ** Unknown\n";
 	return $r;
     };
 
@@ -236,10 +378,14 @@ sub classify ($) {
 	    return $classify->(qw(AddPatches));
 	} elsif ($d & (D_PAT_ADD|D_PAT_OTH)) {
 	    return $unknown->("edits debian/patches");
-	} elsif ($d == D_DEB) {
-	    my ($ty,$dummy) = git_cat_file "$ph[0]:debian";
+	} elsif ($d & DS_DEB and !($d & ~DS_DEB)) {
+	    my ($ty,$dummy) = git_cat_file "$p[0]{CommitId}:debian";
 	    if ($ty eq 'tree') {
-		return $classify->(qw(Packaging));
+		if ($d == D_DEB_CLOG) {
+		    return $classify->(qw(Changelog));
+		} else {
+		    return $classify->(qw(Packaging));
+		}
 	    } elsif ($ty eq 'missing') {
 		return $classify->(qw(BreakwaterStart));
 	    } else {
@@ -247,7 +393,7 @@ sub classify ($) {
 	    }
 	} elsif ($d == D_UPS) {
 	    return $classify->(qw(Upstream));
-	} elsif ($d == (D_DEB|D_UPS)) {
+	} elsif ($d & DS_DEB and $d & D_UPS and !($d & ~(DS_DEB|D_UPS))) {
 	    return $classify->(qw(Mixed));
 	} elsif ($d == 0) {
 	    return $unknown->("no changes");
@@ -279,12 +425,12 @@ sub classify ($) {
 			   Contributor => $bytime[1]);
     }
     foreach my $p (@p) {
-	my ($p_h, $p_m) = get_commit $p;
+	my ($p_h, $p_m) = get_commit $p->{CommitId};
 	$p->{IsOrigin} = $p_h !~ m/^parent \w+$/m;
 	($p->{IsDgitImport},) = $p_m =~ m/^\[dgit import ([0-9a-z]+) .*\]$/m;
     }
     my @orig_ps = grep { ($_->{IsDgitImport}//'X') eq 'orig' } @p;
-    my $m2 = $m;
+    my $m2 = $r->{Msg};
     if (!(grep { !$_->{IsOrigin} } @p) and
 	(@orig_ps >= @p - 1) and
 	$m2 =~ s{^\[(dgit import unpatched .*)\]$}{[was: $1]}m) {
@@ -302,26 +448,41 @@ sub classify ($) {
     # debian/) so debian breakwater branch should be 1st parent; that
     # way also there's also an easy rune to look for the upstream
     # patches (--topo-order).
-    if (@p == 2 &&
-	!$haspatches &&
-	!$p[0]{IsOrigin} && # breakwater merge never starts with an origin
-	!($p[0]{Differs} & ~D_DEB) &&
-	!($p[1]{Differs} & ~D_UPS)) {
-	return $classify->(qw(BreakwaterUpstreamMerge),
-			   OrigParents => [ $p[1] ]);
+
+    # 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} & ~DS_DEB) && # no non-debian changess
+	    !($p[$prevbrw]{Differs} & ~D_UPS)) { # no non-upstream changes
+	    return $classify->(qw(BreakwaterUpstreamMerge),
+			       OrigParents => [ $p[!$prevbrw] ]);
+	}
     }
-    # xxx multi-.orig upstreams
+
+    # multi-orig upstreams are represented with a breakwater merge
+    # from a single upstream commit which combines the orig tarballs
 
     return $unknown->("complex merge");
 }
 
 sub walk ($;$$);
-sub walk {
+sub walk ($;$$) {
     my ($input,
 	$nogenerate,$report) = @_;
-    # => ($tip, $breakwater_tip)
+    # => ($tip, $breakwater_tip, $last_upstream_merge_in_breakwater)
     # (or nothing, if $nogenerate)
 
+    printdebug "*** WALK $input ".($nogenerate//0)." ".($report//'-')."\n";
+
     # go through commits backwards
     # we generate two lists of commits to apply:
     # breakwater branch and upstream patches
@@ -360,7 +521,9 @@ sub walk {
 	if ($nogenerate) {
 	    return (undef,undef);
 	}
-	die "commit $cur: Cannot cope with this commit";
+	die "commit $cur: Cannot cope with this commit (d.".
+	    (join ' ', map { sprintf "%#x", $_->{Differs} }
+	     @{ $cl->{Parents} }). ")";
     };
 
     my $build;
@@ -373,6 +536,8 @@ sub walk {
 	no warnings qw(exiting); last;
     };
 
+    my $last_upstream_update;
+
     for (;;) {
 	$cl = classify $cur;
 	my $ty = $cl->{Type};
@@ -385,11 +550,12 @@ sub walk {
 	    $cur = $p0;
 	    $rewrite_from_here->();
 	    next;
-	} elsif ($ty eq 'Packaging') {
+	} elsif ($ty eq 'Packaging' or $ty eq 'Changelog') {
 	    push @brw_cl, $cl;
 	    $cur = $p0;
 	    next;
 	} elsif ($ty eq 'BreakwaterStart') {
+            $last_upstream_update = $cur;
 	    $build_start->('FirstPackaging', $cur);
 	} elsif ($ty eq 'Upstream') {
 	    push @upp_cl, $cl;
@@ -398,7 +564,7 @@ sub walk {
 	} elsif ($ty eq 'Mixed') {
 	    my $queue = sub {
 		my ($q, $wh) = @_;
-		my $cls = { $cl, $xmsg->("split mixed commit: $wh part") };
+		my $cls = { %$cl, $xmsg->("split mixed commit: $wh part") };
 		push @$q, $cls;
 	    };
 	    $queue->(\@brw_cl, "debian");
@@ -407,12 +573,14 @@ sub walk {
 	    $cur = $p0;
 	    next;
 	} elsif ($ty eq 'Pseudomerge') {
-	    print $report " Contributor=$ty->{Contributor}" if $report;
+	    my $contrib = $cl->{Contributor}{CommitId};
+	    print $report " Contributor=$contrib" if $report;
 	    push @pseudomerges, $cl;
 	    $rewrite_from_here->();
-	    $cur = $ty->{Contributor};
+	    $cur = $contrib;
 	    next;
 	} elsif ($ty eq 'BreakwaterUpstreamMerge') {
+            $last_upstream_update = $cur;
 	    $build_start->("PreviousBreakwater", $cur);
 	} elsif ($ty eq 'DgitImportUnpatched') {
 	    my $pm = $pseudomerges[-1];
@@ -429,6 +597,7 @@ sub walk {
 		printf $report " PM=%s \@Overwr:%d", $pm, (scalar @$ovwrs)
 		    if $report;
 		if (@$ovwrs != 1) {
+                    printdebug "*** WALK BOMB DgitImportUnpatched\n";
 		    return $bomb->();
 		}
 		my $ovwr = $ovwrs->[0]{CommitId};
@@ -466,6 +635,7 @@ sub walk {
 		$prline->(" Import");
 		$rewrite_from_here->();
 		$upp_limit //= $#upp_cl; # further, deeper, patches discarded
+                die 'BUG $upp_limit is not used anywhere?';
 		$cur = $ovwr;
 		next;
 	    } else {
@@ -480,26 +650,27 @@ sub walk {
 	    }
 	    die "$ty ?";
         } else {
+            printdebug "*** WALK BOMB unrecognised\n";
 	    return $bomb->();
 	}
     }
     $prprdelim->();
+
+    printdebug "*** WALK prep done cur=$cur".
+        " brw $#brw_cl upp $#upp_cl proc $#processed pm $#pseudomerges\n";
+
     return if $nogenerate;
 
     # Now we build it back up again
 
-    workarea_fresh();
+    fresh_workarea();
 
     my $rewriting = 0;
 
-    my $rm_tree_cached = sub {
-	my ($subdir) = @_;
-	runcmd @git, qw(rm --quiet -rf --cached), $subdir;
-    };
     my $read_tree_debian = sub {
 	my ($treeish) = @_;
-	$rm_tree_cached->(qw(debian));
-	runcmd @git, qw(read-tree --prefix=debian/), "$treeish:debian";
+	read_tree_subdir 'debian', "$treeish:debian";
+        rm_subdir_cached 'debian/patches';
     };
     my $read_tree_upstream = sub {
 	my ($treeish) = @_;
@@ -509,9 +680,14 @@ sub walk {
  
     my $committer_authline = calculate_committer_authline();
 
+    printdebug "WALK REBUILD $build ".(scalar @processed)."\n";
+
+    confess "internal error" unless $build eq (pop @processed)->{CommitId};
+
     in_workarea sub {
 	mkdir $rd or $!==EEXIST or die $!;
 	my $current_method;
+	runcmd @git, qw(read-tree), $build;
 	foreach my $cl (qw(Debian), (reverse @brw_cl),
 			{ SpecialMethod => 'RecordBreakwaterTip' },
 			qw(Upstream), (reverse @upp_cl)) {
@@ -522,6 +698,8 @@ sub walk {
 	    my $method = $cl->{SpecialMethod} // $current_method;
 	    my @parents = ($build);
 	    my $cltree = $cl->{CommitId};
+	    printdebug "WALK BUILD ".($cltree//'undef').
+		" $method (rewriting=$rewriting)\n";
 	    if ($method eq 'Debian') {
 		$read_tree_debian->($cltree);
 	    } elsif ($method eq 'Upstream') {
@@ -534,20 +712,26 @@ sub walk {
 		next;
 	    } elsif ($method eq 'DgitImportDebianUpdate') {
 		$read_tree_debian->($cltree);
-		$rm_tree_cached->(qw(debian/patches));
+		rm_subdir_cached qw(debian/patches);
 	    } elsif ($method eq 'DgitImportUpstreamUpdate') {
 		$read_tree_upstream->($cltree);
 		push @parents, map { $_->{CommitId} } @{ $cl->{OrigParents} };
 	    } else {
 		confess "$method ?";
 	    }
-	    $rewriting ||= $cl ne pop @processed;
+	    if (!$rewriting) {
+		my $procd = (pop @processed) // 'UNDEF';
+		if ($cl ne $procd) {
+		    $rewriting = 1;
+		    printdebug "WALK REWRITING NOW cl=$cl procd=$procd\n";
+		}
+	    }
 	    my $newtree = cmdoutput @git, qw(write-tree);
 	    my $ch = $cl->{Hdr};
 	    $ch =~ s{^tree .*}{tree $newtree}m or confess "$ch ?";
 	    $ch =~ s{^parent .*\n}{}m;
 	    $ch =~ s{(?=^author)}{
-		map { "parent $_\n" } @parents
+		join '', map { "parent $_\n" } @parents
 	    }me or confess "$ch ?";
 	    if ($rewriting) {
 		$ch =~ s{^committer .*$}{$committer_authline}m
@@ -563,29 +747,45 @@ sub walk {
 	    my $newcommit = cmdoutput @cmd;
 	    confess "$ch ?" unless $rewriting or $newcommit eq $cl->{CommitId};
 	    $build = $newcommit;
+            if (grep { $method eq $_ } qw(DgitImportUpstreamUpdate)) {
+                $last_upstream_update = $cur;
+            }
 	}
     };
 
-    runcmd @git, qw(diff-tree --quiet), $input, $build;
+    my $final_check = get_differs $build, $input;
+    die sprintf "internal error %#x %s %s", $final_check, $build, $input
+	if $final_check & ~D_PAT_ADD;
 
-    return ($build, $breakwater);
+    my @r = ($build, $breakwater, $last_upstream_update);
+    printdebug "*** WALK RETURN @r\n";
+    return @r
 }
 
 sub get_head () { return git_rev_parse qw(HEAD); }
 
-sub update_head ($$) {
+sub update_head ($$$) {
+    my ($old, $new, $mrest) = @_;
+    runcmd @git, qw(update-ref -m), "debrebase: $mrest", 'HEAD', $new, $old;
+}
+
+sub update_head_checkout ($$$) {
     my ($old, $new, $mrest) = @_;
-    runcmd @git, qw(update-ref -m), "git-debrebase $mrest", $new, $old;
+    my $symref = git_get_symref();
+    runcmd @git, qw(checkout), $new, qw(.);
+    update_head $old, $new, $mrest;
 }
 
 sub cmd_launder () {
-    badusage "no arguments to launder allowed";
+    badusage "no arguments to launder allowed" if @ARGV;
     my $old = get_head();
-    my ($tip,$breakwater) = walk $old;
+    my ($tip,$breakwater,$last_upstream_merge) = walk $old;
     update_head $old, $tip, 'launder';
     # no tree changes except debian/patches
-    runcmd @git, qw(rm --quiet -rf debian/patches);
+    runcmd @git, qw(rm --quiet --ignore-unmatch -rf debian/patches);
     printf "# breakwater tip\n%s\n", $breakwater;
+    printf "# working tip\n%s\n", $tip;
+    printf "# last upstream merge\n%s\n", $last_upstream_merge;
 }
 
 sub cmd_analyse () {
@@ -598,13 +798,265 @@ sub cmd_analyse () {
 	$old = get_head();
     }
     my ($dummy,$breakwater) = walk $old, 1,*STDOUT;
-    print "$breakwater BREAKWATER\n";
     STDOUT->error and die $!;
 }
 
-my $toplevel = runcmd @git, qw(rev-parse --show-toplevel);
+sub cmd_new_upstream_v0 () {
+    # xxx would like to support more git-rebase options
+    badusage
+ "need NEW-VERSION UPS-COMMITISH [EXTRA-UPS-NAME EXTRA-UPS-COMMITISH...]"
+	unless @ARGV % 2 == 0 and @ARGV >= 2;
+    # tree should be clean and this is not checked
+    # automatically and unconditionally launders before rebasing
+    # if rebase --abort is used, laundering has still been done
+
+    my %pieces;
+
+    # parse args - low commitment
+    my $new_version = (new Dpkg::Version scalar(shift @ARGV), check => 1);
+    my $new_upstream_version = $new_version->version();
+
+    my $new_upstream = git_rev_parse shift @ARGV;
+
+    my $piece = sub {
+        my ($n, @x) = @_; # may be ''
+        my $pc = $pieces{$n} //= {
+    	    Name => $n,
+	    Desc => ($n ? "upstream piece $n" : "upstream (main piece"),
+	};
+	while (my $k = shift @x) { $pc->{$k} = shift @x; }
+    };
+
+    my @newpieces;
+    my $newpiece = sub {
+	my ($n, @x) = @_; # may be ''
+	my $pc = $piece->($n, @x, NewIx => (scalar @newpieces));
+	push @newpieces, $pc;
+    };
+
+    $newpiece->('',
+        OldIx => 0,
+        New => $new_upstream,
+    );
+    while (@ARGV) {
+	my $n = shift @ARGV;
+	my $c = git_rev_parse shift @ARGV;
+	die unless $n =~ m/^$extra_orig_namepart_re$/;
+	$newpiece->($n, New => $c);
+    }
+
+    # now we need to investigate the branch this generates the
+    # laundered version but we don't switch to it yet
+    my $old_head = get_head();
+    my ($old_laundered_tip,$old_bw,$old_upstream_update) = walk $old_head;
+
+    my $old_bw_cl = classify $old_bw;
+    my $old_upstream_update_cl = classify $old_upstream_update;
+    confess unless $old_upstream_update_cl->{OrigParents};
+    my $old_upstream = parsecommit
+        $old_upstream_update_cl->{OrigParents}[0]{CommitId};
+
+    my $problems = 0;
+    my $problem = sub {
+	my ($msg) = @_;
+	$problems++;
+	print STDERR "preflight check failed: $msg\n";
+    };
+
+    $piece->('', Old => $old_upstream->{CommitId});
+
+    if ($old_upstream->{Msg} =~ m{^\[git-debrebase }m) {
+	if ($old_upstream->{Msg} =~
+ m{^\[git-debrebase new-upstream combine \.((?: $extra_orig_namepart_re)+)\]}
+	   ) {
+	    my @oldpieces = ('', split / /, $1);
+	    my $parentix = -1 + scalar @{ $old_upstream->{Parents} };
+	    foreach my $i (0..$#oldpieces) {
+		my $n = $oldpieces[$i];
+		$piece->($n, Old => $old_upstream->{CommitId}.'^'.$parentix);
+	    }
+	} else {
+	    $problem->("previous upstream $old_upstream->{CommitId} is from".
+                 " git-debrebase but not a \`new-upstream combine' commit");
+	}
+    }
+
+    foreach my $pc (values %pieces) {
+	if (!$pc->{Old}) {
+	    $problem->("introducing upstream piece $pc->{Name}");
+	} elsif (!$pc->{New}) {
+	    $problem->("dropping upstream piece $pc->{Name}");
+	} elsif (!is_fast_fwd $pc->{Old}, $pc->{New}) {
+	    $problem->("not fast forward: $pc->{Name} $pc->{Old}..$pc->{New}");
+	}
+    }
+
+    if ($problems) {
+	if ($opt_force) {
+	    printf STDERR
+		"preflight check failures (%d) overriden by --force\n",
+		$problems;
+	} else {
+	    fail sprintf
+		"preflight check failures (%d) (you could --force)",
+		$problems;
+	}
+    }
+
+    my $new_bw;
+
+    fresh_workarea();
+    in_workarea sub {
+	my @upstream_merge_parents;
+
+	if (!$problems) {
+	    push @upstream_merge_parents, $old_upstream->{CommitId};
+	}
+
+	foreach my $pc (@newpieces) { # always has '' first
+	    if ($pc->{Name}) {
+		read_tree_subdir $pc->{Name}, $pc->{New};
+	    } else {
+		runcmd @git, qw(read-tree), $pc->{New};
+	    }
+	    push @upstream_merge_parents, $pc->{New};
+	}
+
+	# index now contains the new upstream
+
+	if (@newpieces > 1) {
+	    # need to make the upstream subtree merge commit
+	    my $us_tree = cmdoutput @git, qw(write-tree);
+	    my @cmd = (@git, qw(commit-tree), $us_tree);
+	    push @cmd, qw(-p), $_ foreach @upstream_merge_parents;
+	    push @cmd, qw(-m), "Combine upstreams for $new_upstream_version";
+	    push @cmd, qw(-m),
+		"[git-debrebase new-upstream combine . ".
+		(join " ", map { $_->{Name} } @newpieces[1..$#newpieces]).
+		 "]";
+	    $new_upstream = cmdoutput @cmd;
+	}
+
+	# $new_upstream is either the single upstream commit, or the
+	# combined commit we just made.  Either way it will be the
+	# "upstream" parent of the breakwater special merge.
+
+	read_tree_subdir 'debian', "$old_bw:debian";
+
+	# index now contains the breakwater merge contents
+
+	my $bw_tree = cmdoutput @git, qw(write_tree);
+	my @cmd = (@git, qw(commit-tree), $bw_tree);
+	push @cmd, qw(-p), $old_bw, qw(-p), $new_upstream;
+	push @cmd, qw(-m), "Update to upstream $new_upstream_version";
+	push @cmd, qw(-m),
+	    "[git-debrebase new-upstream breakwater $new_upstream_version]";
+	$new_bw = cmdoutput @git;
+
+	# Now we have to add a changelog stanza so the Debian version
+	# is right.
+
+	die if unlink "debian";
+	die unless $!==ENOTEMPTY;
+	unlink "debian/changelog" or die $!;
+	open CN, ">", "debian/changelog" or die $!;
+	my $oldclog = git_cat_file ":debian/changelog";
+	$oldclog =~ m/^($package_re) \(\S+\) / or
+	    fail "cannot parse old changelog to get package name";
+	my $p = $1;
+	print CN <<END, $oldclog or die $!;
+$p ($new_version) UNRELEASED; urgency=medium
+
+  * Update to new upstream version $new_upstream_version.
+
+ -- 
+
+END
+	close CN or die $!;
+	runcmd @git, qw(update-index --add --replace), 'debian/changelog';
+
+	# Now we have the final new breakwater branch in the index
+
+	$bw_tree = cmdoutput @git, qw(write_tree);
+	@cmd = (@git, qw(commit-tree), $bw_tree);
+	push @cmd, qw(-p), $new_bw;
+	push @cmd, qw(-m),
+	    "Update changelog for new upstream $new_upstream_version";
+	push @cmd, qw(-m),
+	    "[git-debrebase new-upstream changelog $new_upstream_version]";
+	$new_bw = cmdoutput @git;
+    };
+
+    # we have constructed the new breakwater. we now need to commit to
+    # the laundering output, because git-rebase can't easily be made
+    # to make a replay list which is based on some other branch
+
+    update_head $old_head, $old_laundered_tip, 'launder for new upstream';
+
+    my @cmd = (@git, qw(rebase --onto), $new_bw, $old_bw);
+    runcmd @cmd;
+    # now it's for the user to sort out
+}
+
+sub cmd_downstream_rebase_launder_v0 () {
+    badusage "needs 1 argument, the baseline" unless @ARGV==1;
+    my ($base) = @ARGV;
+    $base = git_rev_parse $base;
+    my $old_head = get_head();
+    my $current = $old_head;
+    my $topmost_keep;
+    for (;;) {
+	if ($current eq $base) {
+	    $topmost_keep //= $current;
+	    print " $current BASE stop\n";
+	    last;
+	}
+	my $cl = classify $current;
+	print " $current $cl->{Type}";
+	my $keep = 0;
+	my $p0 = $cl->{Parents}[0]{CommitId};
+	my $next;
+	if ($cl->{Type} eq 'Pseudomerge') {
+	    print " ^".($cl->{Contributor}{Ix}+1);
+	    $next = $cl->{Contributor}{CommitId};
+	} elsif ($cl->{Type} eq 'AddPatches' or
+		 $cl->{Type} eq 'Changelog') {
+	    print " strip";
+	    $next = $p0;
+	} else {
+	    print " keep";
+	    $next = $p0;
+	    $keep = 1;
+	}
+	print "\n";
+	if ($keep) {
+	    $topmost_keep //= $current;
+	} else {
+	    die "to-be stripped changes not on top of the branch\n"
+		if $topmost_keep;
+	}
+	$current = $next;
+    }
+    if ($topmost_keep eq $old_head) {
+	print "unchanged\n";
+    } else {
+	print "updating to $topmost_keep\n";
+	update_head_checkout
+	    $old_head, $topmost_keep,
+	    'downstream-rebase-launder-v0';
+    }
+}
+
+GetOptions("D+" => \$debuglevel,
+	   'force!') or die badusage "bad options\n";
+initdebug('git-debrebase ');
+enabledebug if $debuglevel;
+
+my $toplevel = cmdoutput @git, qw(rev-parse --show-toplevel);
 chdir $toplevel or die "chdir $toplevel: $!";
 
+$rd = fresh_playground "$playprefix/misc";
+
 my $cmd = shift @ARGV;
 my $cmdfn = $cmd;
 $cmdfn =~ y/-/_/;