git: reload alternates file on missing blob

Since we'll be adding new repositories to the `alternates' file
in git, we must restart the `git cat-file --batch' process as
git currently does not detect changes to the alternates file
in long-running cat-file processes.

Don't bother with the `--batch-check' process since we won't be
using it with v2.

1 files changed, 25 insertions, 4 deletions

diff --git a/lib/PublicInbox/Git.pm b/lib/PublicInbox/Git.pm
index 6437643d..95df52ed 100644
--- a/lib/PublicInbox/Git.pm
+++ b/lib/PublicInbox/Git.pm
@@ -15,7 +15,19 @@ use PublicInbox::Spawn qw(spawn popen_rd);
 
 sub new {
         my ($class, $git_dir) = @_;
-        bless { git_dir => $git_dir }, $class
+        my @st;
+        $st[7] = $st[10] = 0;
+        bless { git_dir => $git_dir, st => \@st }, $class
+}
+
+sub alternates_changed {
+        my ($self) = @_;
+        my $alt = "$self->{git_dir}/objects/info/alternates";
+        my @st = stat($alt) or return 0;
+        my $old_st = $self->{st};
+        # 10 - ctime, 7 - size
+        return 0 if ($st[10] == $old_st->[10] && $st[7] == $old_st->[7]);
+        $self->{st} = \@st;
 }
 
 sub _bidi_pipe {
@@ -38,14 +50,23 @@ sub _bidi_pipe {
 
 sub cat_file {
         my ($self, $obj, $ref) = @_;
+        my ($retried, $in, $head);
 
+again:
         batch_prepare($self);
         $self->{out}->print($obj, "\n") or fail($self, "write error: $!");
 
-        my $in = $self->{in};
+        $in = $self->{in};
         local $/ = "\n";
-        my $head = $in->getline;
-        $head =~ / missing$/ and return undef;
+        $head = $in->getline;
+        if ($head =~ / missing$/) {
+                if (!$retried && alternates_changed($self)) {
+                        $retried = 1;
+                        cleanup($self);
+                        goto again;
+                }
+                return;
+        }
         $head =~ /^[0-9a-f]{40} \S+ (\d+)$/ or
                 fail($self, "Unexpected result from git cat-file: $head");