gcf2: libgit2-based git cat-file alternative

Having tens of thousands of inboxes and associated git processes
won't work well, so we'll use libgit2 to access the object DB
directly.  We only care about OID lookups and won't need to rely
on per-repo revision names or paths.

The Git::Raw XS package won't be used since its manpages don't
promise a stable API.  Since we already use Inline::C and have
experience with I::C when it comes to compatibility, this only
introduces libgit2 itself as a source of new incompatibilities.

This also provides an excuse for me to writev(2) to reduce
syscalls, but liburing is on the horizon for next year.

1 files changed, 56 insertions, 0 deletions

diff --git a/lib/PublicInbox/Gcf2.pm b/lib/PublicInbox/Gcf2.pm
new file mode 100644
index 00000000..6ac3aa18
--- /dev/null
+++ b/lib/PublicInbox/Gcf2.pm
@@ -0,0 +1,56 @@
+# Copyright (C) 2020 all contributors <meta@public-inbox.org>
+# License: AGPL-3.0+ <https://www.gnu.org/licenses/agpl-3.0.txt>
+#
+# git-cat-file based on libgit2
+package PublicInbox::Gcf2;
+use strict;
+use PublicInbox::Spawn qw(which popen_rd);
+use Fcntl qw(LOCK_EX);
+my (%CFG, $c_src, $lockfh);
+BEGIN {
+        # PublicInbox::Spawn will set PERL_INLINE_DIRECTORY
+        # to ~/.cache/public-inbox/inline-c if it exists
+        my $inline_dir = $ENV{PERL_INLINE_DIRECTORY} //
+                die 'PERL_INLINE_DIRECTORY not defined';
+        my $f = "$inline_dir/.public-inbox.lock";
+        open $lockfh, '>', $f or die "failed to open $f: $!\n";
+        my $pc = which($ENV{PKG_CONFIG} // 'pkg-config');
+        my ($dir) = (__FILE__ =~ m!\A(.+?)/[^/]+\z!);
+        my $rdr = {};
+        open $rdr->{2}, '>', '/dev/null' or die "open /dev/null: $!";
+        for my $x (qw(libgit2)) {
+                my $l = popen_rd([$pc, '--libs', $x], undef, $rdr);
+                $l = do { local $/; <$l> };
+                next if $?;
+                my $c = popen_rd([$pc, '--cflags', $x], undef, $rdr);
+                $c = do { local $/; <$c> };
+                next if $?;
+
+                # note: we name C source files .h to prevent
+                # ExtUtils::MakeMaker from automatically trying to
+                # build them.
+                my $f = "$dir/gcf2_$x.h";
+                if (open(my $fh, '<', $f)) {
+                        chomp($l, $c);
+                        local $/;
+                        $c_src = <$fh>;
+                        $CFG{LIBS} = $l;
+                        $CFG{CCFLAGSEX} = $c;
+                        last;
+                } else {
+                        die "E: $f: $!\n";
+                }
+        }
+        die "E: libgit2 not installed\n" unless $c_src;
+
+        # CentOS 7.x ships Inline 0.53, 0.64+ has built-in locking
+        flock($lockfh, LOCK_EX) or die "LOCK_EX failed on $f: $!\n";
+}
+
+# we use Capitalized and ALLCAPS for compatibility with old Inline::C
+use Inline C => Config => %CFG, BOOT => 'git_libgit2_init();';
+use Inline C => $c_src;
+undef $c_src;
+undef %CFG;
+undef $lockfh;
+1;