From 5320239ed0ef59ac44b83abeb80999dc2f4810fd Mon Sep 17 00:00:00 2001
From: Ian Jackson <ijackson@chiark.greenend.org.uk>
Date: Sun, 18 Feb 2018 13:36:27 +0000
Subject: git-debrebase: do most ref updates together, atomically

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
---
 git-debrebase | 69 ++++++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 42 insertions(+), 27 deletions(-)

diff --git a/git-debrebase b/git-debrebase
index a5329b6..9f7eabe 100755
--- a/git-debrebase
+++ b/git-debrebase
@@ -157,6 +157,28 @@ sub fresh_workarea () {
     in_workarea sub { playtree_setup };
 }
 
+our @deferred_updates;
+our @deferred_update_messages;
+
+sub run_deferred_updates ($) {
+    my ($mrest) = @_;
+
+    my @upd_cmd = (@git, qw(update-ref --stdin -m), "debrebase: $mrest");
+    debugcmd '>|', @upd_cmd;
+    open U, "|-", @upd_cmd or die $!;
+    foreach (@deferred_updates) {
+	printdebug ">= ", $_, "\n";
+	print U $_, "\n" or die $!;
+    }
+    printdebug ">\$\n";
+    close U or failedcmd @upd_cmd;
+
+    print $_, "\n" foreach @deferred_update_messages;
+
+    @deferred_updates = ();
+    @deferred_update_messages = ();
+}
+
 sub get_differs ($$) {
     my ($x,$y) = @_;
     # This resembles quiltify_trees_differ, in dgit, a bit.
@@ -840,7 +862,8 @@ sub get_head () {
 
 sub update_head ($$$) {
     my ($old, $new, $mrest) = @_;
-    runcmd @git, qw(update-ref -m), "debrebase: $mrest", 'HEAD', $new, $old;
+    push @deferred_updates, "update HEAD $new $old";
+    run_deferred_updates $mrest;
 }
 
 sub update_head_checkout ($$$) {
@@ -870,8 +893,8 @@ sub cmd_launder_v0 () {
 
 sub defaultcmd_rebase () {
     my $old = get_head();
-    my ($status, $message) = record_ffq_prev();
-    if ($status eq 'written' || $status eq 'exists') {
+    my ($status, $message) = record_ffq_prev_deferred();
+    if ($status eq 'deferred' || $status eq 'exists') {
     } else {
 	fproblem $status, "could not record ffq-prev: $message";
 	fproblems_maybe_bail();
@@ -909,17 +932,18 @@ sub ffq_prev_branchinfo () {
     return ('branch', undef, $current, $ffq_prev);
 }
 
-sub record_ffq_prev () {
+sub record_ffq_prev_deferred () {
     # => ('status', "message")
     # 'status' may be
-    #    written          message is undef
+    #    deferred          message is undef
     #    exists
     #    detached
     #    weird-symref
     #    notbranch
     # if not ff from some branch we should be ff from, is an fproblem
-    # if "written", will have printed something about that to stdout,
-    #   and also some messages about ff checks
+    # if "deferred", will have added something about that to
+    #   @deferred_update_messages, and also maybe printed (already)
+    #   some messages about ff checks
     my ($status, $message, $current, $ffq_prev) = ffq_prev_branchinfo();
     return ($status, $message) unless $status eq 'branch';
 
@@ -985,10 +1009,10 @@ sub record_ffq_prev () {
     }
 
     fproblems_maybe_bail();
-    runcmd @git, qw(update-ref -m), "record current head for preservation",
-	$ffq_prev, $currentval, $git_null_obj;
-    print "Recorded current head for preservation\n" or die $!;
-    return ('written', undef);
+
+    push @deferred_updates, "update $ffq_prev $currentval $git_null_obj";
+    push @deferred_update_messages, "Recorded current head for preservation";
+    return ('deferred', undef);
 }
 
 sub cmd_new_upstream_v0 () {
@@ -1171,10 +1195,11 @@ END
 
 sub cmd_record_ffq_prev () {
     badusage "no arguments allowed" if @ARGV;
-    my ($status, $msg) = record_ffq_prev();
+    my ($status, $msg) = record_ffq_prev_deferred();
     if ($status eq 'exists' && $opt_noop_ok) {
 	print "Previous head already recorded\n" or die $!;
-    } elsif ($status eq 'written') {
+    } elsif ($status eq 'deferred') {
+	run_deferred_updates 'record-ffq-prev';
     } else {
 	fail "Could not preserve: $msg";
     }
@@ -1199,6 +1224,8 @@ sub cmd_stitch () {
     if (!$prev) {
 	fail "No ffq-prev to stitch." unless $opt_noop_ok;
     }
+    push @deferred_updates, "delete $ffq_prev $prev";
+
     my $old_head = get_head();
     if (is_fast_fwd $old_head, $prev) {
 	my $differs = get_differs $old_head, $prev;
@@ -1206,9 +1233,7 @@ sub cmd_stitch () {
 	    # ffq-prev is ahead of us, and the only tree changes it has
 	    # are possibly addition of things in debian/patches/.
 	    # Just wind forwards rather than making a pointless pseudomerge.
-	    update_head_checkout $old_head, $prev,
-		"debrebase: stitch (fast forward)";
-	    runcmd @git, qw(update-ref -d), $ffq_prev; # should be atomic
+	    update_head_checkout $old_head, $prev, "stitch (fast forward)";
 	    return;
 	}
     }
@@ -1217,17 +1242,7 @@ sub cmd_stitch () {
 	'Declare fast forward / record previous work',
         "[git-debrebase pseudomerge: stitch$prose]",
     ];
-    my @upd_cmd = (@git, qw(update-ref --stdin -m), "debrebase: stitch");
-    debugcmd '>|', @upd_cmd;
-    open U, "|-", @upd_cmd or die $!;
-    my $u = <<END;
-update HEAD $new_head $old_head
-delete $ffq_prev $prev
-END
-    printdebug ">= ", $_, "\n" foreach split /\n/, $u;
-    print U $u;
-    printdebug ">\$\n";
-    close U or failedcmd @upd_cmd;
+    update_head $old_head, $new_head, "stitch";
 }
 
 sub cmd_convert_from_gbp () {
-- 
cgit v1.2.3