X-Git-Url: https://www.chiark.greenend.org.uk/ucgi/~ianmdlvl/git?p=dgit.git;a=blobdiff_plain;f=git-debrebase;h=b70f8d91ff1f0824dd24cdd0fc3c1c643ddabf20;hp=a3733a989d62ee890af7a8c948fe54443c5846a5;hb=6f6ffad94a649300cea96914c4ee38f1c55f6f32;hpb=78dfec329693691f0ff468e444d7ae19aec220d1

diff --git a/git-debrebase b/git-debrebase
index a3733a98..b70f8d91 100755
--- a/git-debrebase
+++ b/git-debrebase
@@ -3,7 +3,7 @@
 # Script helping make fast-forwarding histories while still rebasing
 # upstream deltas when working on Debian packaging
 #
-# Copyright (C)2017 Ian Jackson
+# Copyright (C)2017,2018 Ian Jackson
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -18,99 +18,77 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+
 # usages:
-#    git-debrebase status
-#    git-debrebase start       # like ffqrebase start + debrebase launder
-#    git-debrebase new-upstream [stuff]  # see below
-#    git-debrebase <git-rebase options>  # does debrebase start if necessary
 #
-#    git-debrebase analyse
-#    git-debrebase launder     # prints breakwater tip
-#    git-debrebase create-new-upstream-breakwater [-f] <upstreaminfo>...
+#    git-debrebase [<options>] new-upstream-v0 \
+#             <new-version> <orig-commitish> \
+#            [<extra-orig-name> <extra-orig-commitish> ...] \
+#            [<git-rebase options>...]
+#
+#    git-debrebase [<options> --] [<git-rebase options...>]
+#    git-debrebase [<options>] analyse
+#    git-debrebase [<options>] launder         # prints breakwater tip etc.
+#    git-debrebase [<options>] downstream-rebase-launder-v0  # experimental
+#
+#    git-debrebase [<options>] gbp2debrebase-v0 \
+#             <upstream>
+
+# problems / outstanding questions:
+#
+#  *  dgit push with a `3.0 (quilt)' package means doing quilt
+#     fixup.  Usually this involves recommitting the whole patch
+#     series, one at a time, with dpkg-source --commit.  This is
+#     terribly terribly slow.  (Maybe this should be fixed in dgit.)
+#
+#  * dgit push usually needs to (re)make a pseudomerge.  The "first"
+#    git-debrebase stripped out the previous pseudomerge and could
+#    have remembeed the HEAD.  But it's not quite clear what history
+#    ought to be preserved and what should be discarded.  For now
+#    the user will have to tell dgit --overwrite.
+#
+#    To fix this, do we need a new push hook for dgit ?
+#
+#  * Workflow is currently clumsy.  Lots of spurious runes to type.
+#    There's not even a guide.
 #
-# <upstreaminfo> is
-#    [,][<subdir>:][+]<commitid>[,...]
+#  * There are no tests.
 #
-# if initial comma is supplied, entries are not positional.  Unspecified
-# <subdir> means root (and there may be only one).
-# xxx want auto branch names
-# xxx too complicated
-# how about for now
-#    [+]<commit> [<subdir/> [+]<commit>...]
-# ?  plus options
-#     --new-upstream-different-subtrees
+#  * new-upstream-v0 has a terrible UI.  You end up with giant
+#    runic command lines.
 #
-#  automatic case
-#       git-debrebase new-upstream
-#             - previous breakwater merge must be gdr-generated
-#             - orig set is the same as before
-#             - implicitly uses upstream branches according to orig set
-#             - not all upstream branches need be updated
-#             - insists on fast-forward of each branch, unless
-#                  --force (or --force=<subdir>[/])
-#  branch set adjustments
-#       git-debrebase new-upstream --add <subdir>/
-#       git-debrebase new-upstream --rm <subdir>/
-#       git-debrebase new-upstream / [<subdir>/ ...]
-#             - orig set is adjusted
-#             - otherwise like auto (--add is not checked for ffness, obv)
-#             - multiple --add and --rm may be specified
-#             - --add makes new upstream the last contributor
-#  explicit
-#       git-debrebase / [<rootcommitid>] [<subdir>/ [<commitid>] ...]
-#             - orig set is precisely as specified now
-#             - previous breakwater merge is irrelevant
-#             - no fast forward checks
-#  for now only explicit with commitids
-
-#         implicitly uses `upstream'
-#                                     # (or multiple other branches)
-#       git-debrebase new-upstream \
-#             [<subdir>/]=<commitid>
-
-#    UPSTREAM[,[[SUBDIR:]SUBUPSTREAM]
-#    default for SUBDIR: is from previous upstream merge[xxx terminology]
-#    
+#    One consequence of the lack of richness it can need --force in
+#    fairly sensible situations and there is no way to tell it what
+#    you are really trying to do, other than just --force.  There
+#    should be an interface with some default branch names.
 #
-#xxx
-# when starting must record original start (for ff)
-# and new rebase basis
+#  * There should be a standard convention for the version number,
+#    and unfinalised or not changelog, after new-upstream.
 #
-#    git-ffqrebase start [BASE]
-#                # records previous HEAD so it can be overwritten
-#                # records base for future git-ffqrebase
-#    git-ffqrebase set-base BASE
-#    git-ffqrebase <git-rebase options>
-#    git-ffqrebase finish
-#    git-ffqrebase status [BRANCH]
+#  * Handing of multi-orig dgit new-upstream .dsc imports is known to
+#    be broken.  They may be not recognised, improperly converted, or
+#    their conversion may be unrecognised.
 #
-#  refs/ffqrebase-prev/BRANCH    BRANCH may be refs/...; if not it means
-#  refs/ffqrebase-base/BRANCH      refs/heads/BRANCH
-#                               zero, one, or both of these may exist
+#  * Docs need writing and updating.  Even README.git-debrebase
+#    describes a design but may not reflect the implementation.
 #
-# git-debrebase without start, if already started, is willing
-# to strip pseudomerges provided that they overwrite exactly
-# the previous HEAD
-#  xxxx is this right ?  what matters is have we pushed
-#    I think in fact the right answer is:
-#       git-debrebase always strips out pseudomerges from its branch
-#       a pseudomerge is put in at the time we want to push
-#       at that time, we make a pseudomerge of the remote tracking
-#           branch (if raw git) or the dgit view (if dgit)
-#       for raw git git-ffqrebase, do want preciseley to record
-#           value of remote tracking branch or our branch, on start, so we
-#           overwrite only things we intend to
-#  the previous pseudomerge    check for tags and remote branches ?
+#  * We need to develop a plausible model that works for derivatives,
+#    who probably want to maintain their stack on top of Debian's.
+#    downstream-rebase-launder-v0 may be a starting point?
 
 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 :playground);
+our ($opt_force);
 
 sub badusage ($) {
     my ($m) = @_;
@@ -129,18 +107,26 @@ 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 $data = git_cat_file $objid, 'commit';
+    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 $rd;
@@ -152,7 +138,8 @@ sub in_workarea ($) {
     my ($sub) = @_;
     changedir $workarea;
     my $r = eval { $sub->(); };
-    changedir $maindir;
+    { local $@; changedir $maindir; }
+    die $@ if $@;
 }
 
 sub fresh_workarea () {
@@ -181,16 +168,19 @@ sub get_differs ($$) {
     };
 
     $rundiff->([qw(--name-only)], [], sub {
-        $differs |= $_ eq 'debian' ? D_DEB : D_UPS;
+        $differs |= $_ eq 'debian' ? DS_DEB : D_UPS;
     });
 
-    if ($differs & D_DEB) {
-	$differs &= ~D_DEB;
+    if ($differs & DS_DEB) {
+	$differs &= ~DS_DEB;
 	$rundiff->([qw(--name-only -r)], [qw(debian)], sub {
-            $differs |= $_ eq m{^debian/patches/} ? D_PAT_OTH : D_DEB;
+            $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|D_DEB);
+	    unless $differs & (D_PAT_OTH|DS_DEB);
     }
 
     if ($differs & D_PAT_OTH) {
@@ -207,11 +197,11 @@ sub get_differs ($$) {
 	    }
 	    die unless s{^debian/patches/}{};
 	    my $ok;
-	    if ($mode eq 'A' && !m/(?:^|\.)series$/s) {
+	    if ($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';
+		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 {
@@ -242,6 +232,46 @@ 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 ($$) {
+    my ($subdir, $new_tree_object) = @_;
+    rm_subdir_cached $subdir;
+    runcmd @git, qw(read-tree), "--prefix=$subdir/", $new_tree_object;
+}
+
+sub make_commit ($$) {
+    my ($parents, $message_paras) = @_;
+    my $tree = cmdoutput @git, qw(write-tree);
+    my @cmd = (@git, qw(commit-tree), $tree);
+    push @cmd, qw(-p), $_ foreach @$parents;
+    push @cmd, qw(-m), $_ foreach @$message_paras;
+    return cmdoutput @cmd;
+}
+
+our $fproblems;
+sub fproblem ($) {
+    my ($msg) = @_;
+    $fproblems++;
+    print STDERR "git-debrebase: safety catch tripped: $msg\n";
+}
+sub fproblems_maybe_bail () {
+    if ($fproblems) {
+	if ($opt_force) {
+	    printf STDERR
+		"safety catch trips (%d) overriden by --force\n",
+		$fproblems;
+	} else {
+	    fail sprintf
+		"safety catch trips (%d) (you could --force)",
+		$fproblems;
+	}
+    }
+}
+
 # classify returns an info hash like this
 #   CommitId => $objid
 #   Hdr => # commit headers, including 1 final newline
@@ -260,6 +290,7 @@ sub calculate_committer_authline () {
 #
 # Types:
 #   Packaging
+#   Changelog
 #   Upstream
 #   AddPatches
 #   Mixed
@@ -276,33 +307,51 @@ 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) = $h =~ m/^parent (\w+)$/mg;
-    my @p;
 
     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}, $_->{Differs} } @p),
 	"\n";
@@ -311,9 +360,7 @@ sub classify ($) {
 	my ($type, @rest) = @_;
 	$r = { %$r, Type => $type, @rest };
 	if ($debuglevel) {
-	    my $dd = new Data::Dumper [ $r ];
-	    Terse $dd 1; Indent $dd 0; Useqq $dd 1;
-	    printdebug " = $type ".(Dump $dd)."\n";
+	    printdebug " = $type ".(dd $r)."\n";
 	}
 	return $r;
     };
@@ -330,10 +377,14 @@ sub classify ($) {
 	    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 $classify->(qw(BreakwaterStart));
 	    } else {
@@ -341,7 +392,7 @@ sub classify ($) {
 	    }
 	} 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 $unknown->("no changes");
@@ -378,7 +429,7 @@ sub classify ($) {
 	($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: $1]}m) {
@@ -390,8 +441,6 @@ sub classify ($) {
     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 +448,27 @@ 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} & ~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 $unknown->("complex merge");
 }
@@ -418,9 +477,11 @@ sub walk ($;$$);
 sub walk ($;$$) {
     my ($input,
 	$nogenerate,$report) = @_;
-    # => ($tip, $breakwater_tip)
+    # => ($tip, $breakwater_tip, $last_upstream_merge_in_breakwater)
     # (or nothing, if $nogenerate)
 
+    printdebug "*** WALK $input ".($nogenerate//0)." ".($report//'-')."\n";
+
     # go through commits backwards
     # we generate two lists of commits to apply:
     # breakwater branch and upstream patches
@@ -459,7 +520,9 @@ sub walk ($;$$) {
 	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 $build;
@@ -472,6 +535,8 @@ sub walk ($;$$) {
 	no warnings qw(exiting); last;
     };
 
+    my $last_upstream_update;
+
     for (;;) {
 	$cl = classify $cur;
 	my $ty = $cl->{Type};
@@ -484,11 +549,12 @@ sub walk ($;$$) {
 	    $cur = $p0;
 	    $rewrite_from_here->();
 	    next;
-	} elsif ($ty eq 'Packaging') {
+	} elsif ($ty eq 'Packaging' or $ty eq 'Changelog') {
 	    push @brw_cl, $cl;
 	    $cur = $p0;
 	    next;
 	} elsif ($ty eq 'BreakwaterStart') {
+            $last_upstream_update = $cur;
 	    $build_start->('FirstPackaging', $cur);
 	} elsif ($ty eq 'Upstream') {
 	    push @upp_cl, $cl;
@@ -497,7 +563,7 @@ sub walk ($;$$) {
 	} 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;
 	    };
 	    $queue->(\@brw_cl, "debian");
@@ -513,6 +579,7 @@ sub walk ($;$$) {
 	    $cur = $contrib;
 	    next;
 	} elsif ($ty eq 'BreakwaterUpstreamMerge') {
+            $last_upstream_update = $cur;
 	    $build_start->("PreviousBreakwater", $cur);
 	} elsif ($ty eq 'DgitImportUnpatched') {
 	    my $pm = $pseudomerges[-1];
@@ -529,6 +596,7 @@ sub walk ($;$$) {
 		printf $report " PM=%s \@Overwr:%d", $pm, (scalar @$ovwrs)
 		    if $report;
 		if (@$ovwrs != 1) {
+                    printdebug "*** WALK BOMB DgitImportUnpatched\n";
 		    return $bomb->();
 		}
 		my $ovwr = $ovwrs->[0]{CommitId};
@@ -566,6 +634,7 @@ sub walk ($;$$) {
 		$prline->(" Import");
 		$rewrite_from_here->();
 		$upp_limit //= $#upp_cl; # further, deeper, patches discarded
+                die 'BUG $upp_limit is not used anywhere?';
 		$cur = $ovwr;
 		next;
 	    } else {
@@ -580,10 +649,15 @@ sub walk ($;$$) {
 	    }
 	    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
@@ -592,14 +666,10 @@ sub walk ($;$$) {
 
     my $rewriting = 0;
 
-    my $rm_tree_cached = sub {
-	my ($subdir) = @_;
-	runcmd @git, qw(rm --quiet -rf --cached --ignore-unmatch), $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) = @_;
@@ -609,9 +679,14 @@ 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 $cl (qw(Debian), (reverse @brw_cl),
 			{ SpecialMethod => 'RecordBreakwaterTip' },
 			qw(Upstream), (reverse @upp_cl)) {
@@ -622,6 +697,8 @@ sub walk ($;$$) {
 	    my $method = $cl->{SpecialMethod} // $current_method;
 	    my @parents = ($build);
 	    my $cltree = $cl->{CommitId};
+	    printdebug "WALK BUILD ".($cltree//'undef').
+		" $method (rewriting=$rewriting)\n";
 	    if ($method eq 'Debian') {
 		$read_tree_debian->($cltree);
 	    } elsif ($method eq 'Upstream') {
@@ -634,20 +711,26 @@ sub walk ($;$$) {
 		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) {
 		$ch =~ s{^committer .*$}{$committer_authline}m
@@ -663,29 +746,60 @@ sub walk ($;$$) {
 	    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 ($$$) {
     my ($old, $new, $mrest) = @_;
-    runcmd @git, qw(update-ref -m), "git-debrebase $mrest", $new, $old;
+    runcmd @git, qw(update-ref -m), "debrebase: $mrest", 'HEAD', $new, $old;
+}
+
+sub update_head_checkout ($$$) {
+    my ($old, $new, $mrest) = @_;
+    my $symref = git_get_symref();
+    runcmd @git, qw(checkout), $new, qw(.);
+    update_head $old, $new, $mrest;
+}
+
+sub update_head_postlaunder ($$$) {
+    my ($old, $tip, $reflogmsg) = @_;
+    return if $tip eq $old;
+    print "git-debrebase: laundered (head was $old)\n";
+    update_head $old, $tip, $reflogmsg;
+    # no tree changes except debian/patches
+    runcmd @git, qw(rm --quiet --ignore-unmatch -rf debian/patches);
 }
 
 sub cmd_launder () {
     badusage "no arguments to launder allowed" if @ARGV;
     my $old = get_head();
-    my ($tip,$breakwater) = walk $old;
-    update_head $old, $tip, 'launder';
-    # no tree changes except debian/patches
-    runcmd @git, qw(rm --quiet -rf debian/patches);
+    my ($tip,$breakwater,$last_upstream_merge) = walk $old;
+    update_head_postlaunder $old, $tip, 'launder';
     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 defaultcmd_rebase () {
+    my $old = get_head();
+    my ($tip,$breakwater) = walk $old;
+    update_head_postlaunder $old, $tip, 'launder for rebase';
+    @ARGV = qw(-i) unless @ARGV; # make configurable
+    runcmd @git, qw(rebase), @ARGV, $breakwater;
 }
 
 sub cmd_analyse () {
@@ -701,7 +815,232 @@ sub cmd_analyse () {
     STDOUT->error and die $!;
 }
 
-GetOptions("D+" => \$debuglevel) or die badusage "bad options\n";
+sub cmd_new_upstream_v0 () {
+    # 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;
+
+    badusage "need NEW-VERSION UPS-COMMITTISH" unless @ARGV >= 2;
+
+    # 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; }
+        $pc;
+    };
+
+    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 && $ARGV[0] !~ m{^-}) {
+	my $n = shift @ARGV;
+
+        badusage "for each EXTRA-UPS-NAME need EXTRA-UPS-COMMITISH"
+            unless @ARGV && $ARGV[0] !~ m{^-};
+
+	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};
+
+    $piece->('', Old => $old_upstream->{CommitId});
+
+    if ($old_upstream->{Msg} =~ m{^\[git-debrebase }m) {
+	if ($old_upstream->{Msg} =~
+ m{^\[git-debrebase (?:\w*-)?upstream combine \.((?: $extra_orig_namepart_re)+)\]}
+	   ) {
+	    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 {
+	    fproblem "previous upstream $old_upstream->{CommitId} is from".
+                 " git-debrebase but not an \`upstream combine' commit";
+	}
+    }
+
+    foreach my $pc (values %pieces) {
+	if (!$pc->{Old}) {
+	    fproblem "introducing upstream piece \`$pc->{Name}'";
+	} elsif (!$pc->{New}) {
+	    fproblem "dropping upstream piece \`$pc->{Name}'";
+	} elsif (!is_fast_fwd $pc->{Old}, $pc->{New}) {
+	    fproblem "not fast forward: $pc->{Name} $pc->{Old}..$pc->{New}";
+	}
+    }
+
+    printdebug "%pieces = ", (dd \%pieces), "\n";
+    printdebug "\@newpieces = ", (dd \@newpieces), "\n";
+
+    fproblems_maybe_bail();
+
+    my $new_bw;
+
+    fresh_workarea();
+    in_workarea sub {
+	my @upstream_merge_parents;
+
+	if (!$fproblems) {
+	    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
+            $new_upstream = make_commit \@upstream_merge_parents,
+                [ "Combine upstreams for $new_upstream_version",
+                  ("[git-debrebase new-upstream combine . ".
+                   (join " ", map { $_->{Name} } @newpieces[1..$#newpieces]).
+                   "]"),
+                ];
+	}
+
+	# $new_upstream is either the single upstream commit, or the
+	# combined commit we just made.  Either way it will be the
+	# "upstream" parent of the breakwater special merge.
+
+	read_tree_subdir 'debian', "$old_bw:debian";
+
+	# index now contains the breakwater merge contents
+        $new_bw = make_commit [ $old_bw, $new_upstream ],
+            [ "Update to upstream $new_upstream_version",
+              "[git-debrebase new-upstream breakwater $new_upstream_version]",
+            ];
+
+	# Now we have to add a changelog stanza so the Debian version
+	# is right.
+	die if unlink "debian";
+	die $! unless $!==ENOENT or $!==ENOTEMPTY;
+	unlink "debian/changelog" or $!==ENOENT or die $!;
+        mkdir "debian" 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
+        $new_bw = make_commit [ $new_bw ],
+            [ "Update changelog for new upstream $new_upstream_version",
+              "[git-debrebase new-upstream changelog $new_upstream_version]",
+            ];
+    };
+
+    # 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_postlaunder $old_head, $old_laundered_tip,
+        'launder for new upstream';
+
+    my @cmd = (@git, qw(rebase --onto), $new_bw, $old_bw, @ARGV);
+    runcmd @cmd;
+    # now it's for the user to sort out
+}
+
+sub cmd_downstream_rebase_launder_v0 () {
+    badusage "needs 1 argument, the baseline" unless @ARGV==1;
+    my ($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;
 
@@ -710,10 +1049,14 @@ chdir $toplevel or die "chdir $toplevel: $!";
 
 $rd = fresh_playground "$playprefix/misc";
 
-my $cmd = shift @ARGV;
-my $cmdfn = $cmd;
-$cmdfn =~ y/-/_/;
-$cmdfn = ${*::}{"cmd_$cmdfn"};
+if (!@ARGV || $ARGV[0] =~ m{^-}) {
+    defaultcmd_rebase();
+} else {
+    my $cmd = shift @ARGV;
+    my $cmdfn = $cmd;
+    $cmdfn =~ y/-/_/;
+    $cmdfn = ${*::}{"cmd_$cmdfn"};
 
-$cmdfn or badusage "unknown git-debrebase sub-operation $cmd";
-$cmdfn->();
+    $cmdfn or badusage "unknown git-debrebase sub-operation $cmd";
+    $cmdfn->();
+}