[PATCH 3/3] lei completion: workaround old Perl bug

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

While `$argv[-1]' is `undef' on an empty @argv, using `$argv[-1]'
as a subroutine argument would fail incorrectly with:

    Modification of non-creatable array value attempted, subscript -1 at ...

...even though we'd never attempt to modify @_ itself in the
subroutines being called.  Work around the bug (tested on
5.16.3) by passing `undef' explicitly when `$argv[-1]' is
already `undef'.

Reported-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://public-inbox.org/meta/20210927124056.kj5okiefvs4ztk27@meerkat.local/
---
 lib/PublicInbox/LeiExportKw.pm        | 2 +-
 lib/PublicInbox/LeiImport.pm          | 4 ++--
 lib/PublicInbox/LeiLsMailSource.pm    | 4 ++--
 lib/PublicInbox/LeiRefreshMailSync.pm | 2 +-
 4 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/lib/PublicInbox/LeiExportKw.pm b/lib/PublicInbox/LeiExportKw.pm
index cea9beeb5694..0b65c2762633 100644
--- a/lib/PublicInbox/LeiExportKw.pm
+++ b/lib/PublicInbox/LeiExportKw.pm
@@ -131,7 +131,7 @@ sub _complete_export_kw {
 	my $match_cb = $lei->complete_url_prepare(\@argv);
 	# filter-out read-only sources:
 	my @k = grep(!m!(?://;AUTH=ANONYMOUS\@|\A(?:nntps?|s?news)://)!,
-			$lms->folders($argv[-1], 1));
+			$lms->folders($argv[-1] // undef, 1));
 	my @m = map { $match_cb->($_) } @k;
 	@m ? @m : @k;
 }
diff --git a/lib/PublicInbox/LeiImport.pm b/lib/PublicInbox/LeiImport.pm
index 397292d4c787..69d63ab6b397 100644
--- a/lib/PublicInbox/LeiImport.pm
+++ b/lib/PublicInbox/LeiImport.pm
@@ -122,11 +122,11 @@ sub lei_import { # the main "lei import" method
 sub _complete_import {
 	my ($lei, @argv) = @_;
 	my ($re, $cur, $match_cb) = $lei->complete_url_prepare(\@argv);
-	my @k = $lei->url_folder_cache->keys($argv[-1], 1);
+	my @k = $lei->url_folder_cache->keys($argv[-1] // undef, 1);
 	my @m = map { $match_cb->($_) } @k;
 	my %f = map { $_ => 1 } (@m ? @m : @k);
 	if (my $lms = $lei->lms) {
-		@k = $lms->folders($argv[-1], 1);
+		@k = $lms->folders($argv[-1] // undef, 1);
 		@m = map { $match_cb->($_) } @k;
 		if (@m) { @f{@m} = @m } else { @f{@k} = @k }
 	}
diff --git a/lib/PublicInbox/LeiLsMailSource.pm b/lib/PublicInbox/LeiLsMailSource.pm
index 1db15d5742b5..7c3c0cda18d6 100644
--- a/lib/PublicInbox/LeiLsMailSource.pm
+++ b/lib/PublicInbox/LeiLsMailSource.pm
@@ -107,11 +107,11 @@ sub lei_ls_mail_source {
 sub _complete_ls_mail_source {
 	my ($lei, @argv) = @_;
 	my $match_cb = $lei->complete_url_prepare(\@argv);
-	my @k = $lei->url_folder_cache->keys($argv[-1], 1);
+	my @k = $lei->url_folder_cache->keys($argv[-1] // undef, 1);
 	my @m = map { $match_cb->($_) } @k;
 	my %f = map { $_ => 1 } (@m ? @m : @k);
 	if (my $lms = $lei->lms) {
-		@k = $lms->folders($argv[-1], 1);
+		@k = $lms->folders($argv[-1] // undef, 1);
 		@m = map { $match_cb->($_) } grep(m!\A[a-z]+://!, @k);
 		if (@m) { @f{@m} = @m } else { @f{@k} = @k }
 	}
diff --git a/lib/PublicInbox/LeiRefreshMailSync.pm b/lib/PublicInbox/LeiRefreshMailSync.pm
index eb842843b51d..0cb9f3dccd48 100644
--- a/lib/PublicInbox/LeiRefreshMailSync.pm
+++ b/lib/PublicInbox/LeiRefreshMailSync.pm
@@ -101,7 +101,7 @@ sub _complete_refresh_mail_sync {
 	my ($lei, @argv) = @_;
 	my $lms = $lei->lms or return ();
 	my $match_cb = $lei->complete_url_prepare(\@argv);
-	my @k = $lms->folders($argv[-1], 1);
+	my @k = $lms->folders($argv[-1] // undef, 1);
 	my @m = map { $match_cb->($_) } @k;
 	@m ? @m : @k
 }