git: Do the revert operation in a secondary working tree

This avoids leaving the git directory in an inconsistent state if the
host system is rebooted while we are processing a revert.

diff --git a/IkiWiki/Plugin/git.pm b/IkiWiki/Plugin/git.pm
index 346808f0491dda4f91227cdc8a27039b4044d3e6..58d23ce919484e7dd60b6afc7c3daf09315ed659 100644 (file)
--- a/IkiWiki/Plugin/git.pm
+++ b/IkiWiki/Plugin/git.pm
@@ -5,6 +5,7 @@ use warnings;
 use strict;
 use IkiWiki;
 use Encode;
+use File::Path qw{remove_tree};
 use URI::Escape q{uri_escape_utf8};
 use open qw{:utf8 :std};
 
@@ -164,6 +165,40 @@ sub in_git_dir ($$) {
        return @ret;
 }
 
+# Loosely based on git-new-workdir from git contrib.
+sub create_temp_working_dir ($$) {
+       my $rootdir = shift;
+       my $branch = shift;
+       my $working = "$rootdir/.git/ikiwiki-temp-working";
+       remove_tree($working);
+
+       foreach my $dir ("", ".git") {
+               if (!mkdir("$working/$dir")) {
+                       error("Unable to create $working/$dir: $!");
+               }
+       }
+
+       # Hooks are deliberately not included: we will commit to the temporary
+       # branch that is used in the temporary working tree, and we don't want
+       # to run the post-commit hook there.
+       #
+       # logs/refs is not included because we don't use the reflog.
+       # remotes, rr-cache, svn are similarly excluded.
+       foreach my $link ("config", "refs", "objects", "info", "packed-refs") {
+               if (!symlink("../../$link", "$working/.git/$link")) {
+                       error("Unable to create symlink $working/.git/$link: $!");
+               }
+       }
+
+       open (my $out, '>', "$working/.git/HEAD") or
+               error("failed to write $working.git/HEAD: $!");
+       print $out "ref: refs/heads/$branch\n" or
+               error("failed to write $working.git/HEAD: $!");
+       close $out or
+               error("failed to write $working.git/HEAD: $!");
+       return $working;
+}
+
 sub safe_git (&@) {
        # Start a child process safely without resorting to /bin/sh.
        # Returns command output (in list content) or success state
@@ -993,7 +1028,6 @@ sub rcs_preprevert ($) {
 
                my $failure;
                my @ret;
-               # If it looks OK, do it for real, on a branch.
                eval {
                        IkiWiki::disable_commit_hook();
                        push @undo, sub {
@@ -1009,23 +1043,22 @@ sub rcs_preprevert ($) {
                        }
                        run_or_die('git', 'branch', $branch, $config{gitmaster_branch});
 
+                       my $working = create_temp_working_dir($rootdir, $branch);
+
                        push @undo, sub {
-                               if (!run_or_cry('git', 'checkout', '--quiet', $config{gitmaster_branch})) {
-                                       run_or_cry('git', 'checkout','-f', '--quiet', $config{gitmaster_branch});
-                               }
+                               remove_tree($working);
                        };
-                       run_or_die('git', 'checkout', '--quiet', $branch);
-
-                       run_or_die('git', 'revert', '--no-commit', $sha1);
-                       run_or_non('git', 'commit', '-m', "revert $sha1", '-a');
 
-                       # Re-switch to master.
-                       run_or_die('git', 'checkout', '--quiet', $config{gitmaster_branch});
+                       in_git_dir($working, sub {
+                               run_or_die('git', 'checkout', '--quiet', '--force', $branch);
+                               run_or_die('git', 'revert', '--no-commit', $sha1);
+                               run_or_die('git', 'commit', '-m', "revert $sha1", '-a');
+                       });
 
                        my @raw_lines;
                        @raw_lines = run_or_die('git', 'diff', '--pretty=raw',
                                '--raw', '--abbrev=40', '--always', '--no-renames',
-                               "ikiwiki_revert_${sha1}..");
+                               "..${branch}");
 
                        my $ci = {
                                details => [parse_changed_files(\@raw_lines)],