[PATCH 3/3] xcpdb: avoid race when shards are added

[Eric Wong <e@80x24.org>]

It's possible for the rename() sequence to cause read-only
daemons using ->xdb_shards_flat to load an incomplete set of
contiguous shards and get invalid docids for search results.

With this change, we favor the case where search is momentarily
unavailable rather than giving wrong results during the small
window where Xapcmd->commit_changes runs.
---
 lib/PublicInbox/Xapcmd.pm | 31 +++++++++++++++++--------------
 1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/lib/PublicInbox/Xapcmd.pm b/lib/PublicInbox/Xapcmd.pm
index daef896c..44e0f8e5 100644
--- a/lib/PublicInbox/Xapcmd.pm
+++ b/lib/PublicInbox/Xapcmd.pm
@@ -20,16 +20,23 @@ sub commit_changes ($$$$) {
 	my $reshard = $opt->{reshard};
 
 	$SIG{INT} or die 'BUG: $SIG{INT} not handled';
-	my @old_shard;
-	my $over_chg;
-	my $mode;
-
-	while (my ($old, $newdir) = each %$tmp) {
+	my (@old_shard, $over_chg);
+
+	# Sort shards highest-to-lowest, since ->xdb_shards_flat
+	# determines the number of shards to load based on the max;
+	# and we'd rather xdb_shards_flat to momentarily fail rather
+	# than load out-of-date shards
+	my @order = sort {
+		my ($x) = ($a =~ m!/([0-9]+)/*\z!);
+		my ($y) = ($b =~ m!/([0-9]+)/*\z!);
+		($y // -1) <=> ($x // -1) # we may have non-shards
+	} keys %$tmp;
+
+	my ($dname) = ($order[0] =~ m!(.*/)[^/]+/*\z!);
+	my $mode = (stat($dname))[2];
+	for my $old (@order) {
 		next if $old eq ''; # no invalid paths
-		$mode //= do {
-			my ($dname) = ($old =~ m!(.*/)[^/]+/*\z!);
-			(stat($dname))[2];
-		};
+		my $newdir = $tmp->{$old};
 		my $have_old = -e $old;
 		if (!$have_old && !defined($opt->{reshard})) {
 			die "failed to stat($old): $!";
@@ -57,11 +64,7 @@ sub commit_changes ($$$$) {
 					die "rename $old => $new/old: $!\n";
 		}
 		rename($new, $old) or die "rename $new => $old: $!\n";
-		if ($have_old) {
-			my $prev = "$old/old";
-			remove_tree($prev) or
-				die "failed to remove $prev: $!\n";
-		}
+		push @old_shard, "$old/old" if $have_old;
 	}
 
 	# trigger ->check_inodes in read-only daemons