[PATCH] gcf2: workaround libgit2 alternates bug for extindex

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

While libgit2 handles alternates with relative paths properly
for v2 epochs; nesting them another layer with extindex uses
the wrong relative path expansion (and is inconsistent with
git(1) behavior).

Fortunately, it's possible to work around this libgit2 bug
entirely within Gcf2 and avoid further special cases throughout
the rest of our code to support extindex.

Link: https://bugs.debian.org/975607
---
 lib/PublicInbox/Gcf2.pm | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/lib/PublicInbox/Gcf2.pm b/lib/PublicInbox/Gcf2.pm
index 7983c841..041dffe7 100644
--- a/lib/PublicInbox/Gcf2.pm
+++ b/lib/PublicInbox/Gcf2.pm
@@ -56,17 +56,37 @@ undef $c_src;
 undef %CFG;
 undef $lockfh;
 
+sub add_alt ($$) {
+	my ($gcf2, $objdir) = @_;
+
+	# libgit2 (tested 0.27.7+dfsg.1-0.2 and 0.28.3+dfsg.1-1~bpo10+1
+	# in Debian) doesn't handle relative epochs properly when nested
+	# multiple levels.  Add all the absolute paths to workaround it,
+	# since $EXTINDEX_DIR/ALL.git/objects/info/alternates uses absolute
+	# paths to reference $V2INBOX_DIR/all.git/objects and
+	# $V2INBOX_DIR/all.git/objects/info/alternates uses relative paths
+	# to refer to $V2INBOX_DIR/git/$EPOCH.git/objects
+	#
+	# See https://bugs.debian.org/975607
+	if (open(my $fh, '<', "$objdir/info/alternates")) {
+		chomp(my @abs_alt = grep(m!^/!, <$fh>));
+		$gcf2->add_alternate($_) for @abs_alt;
+	}
+	$gcf2->add_alternate($objdir);
+}
+
 # Usage: $^X -MPublicInbox::Gcf2 -e 'PublicInbox::Gcf2::loop()'
 # (see lib/PublicInbox/Gcf2Client.pm)
 sub loop {
 	my $gcf2 = new();
+	my %seen;
 	STDERR->autoflush(1);
 	STDOUT->autoflush(1);
 
 	while (<STDIN>) {
 		chomp;
 		my ($oid, $git_dir) = split(/ /, $_, 2);
-		$gcf2->add_alternate("$git_dir/objects");
+		$seen{$git_dir}++ or add_alt($gcf2, "$git_dir/objects");
 		if (!$gcf2->cat_oid(1, $oid)) {
 			# retry once if missing.  We only get unabbreviated OIDs
 			# from SQLite or Xapian DBs, here, so malicious clients
@@ -74,7 +94,8 @@ sub loop {
 			warn "I: $$ $oid missing, retrying in $git_dir\n";
 
 			$gcf2 = new();
-			$gcf2->add_alternate("$git_dir/objects");
+			%seen = ($git_dir => 1);
+			add_alt($gcf2, "$git_dir/objects");
 
 			if ($gcf2->cat_oid(1, $oid)) {
 				warn "I: $$ $oid found after retry\n";