git: use --batch-command in git 2.36+ to save processes

`git cat-file --batch-command' combines the functionality of
`--batch' and `--batch-check' into a single process.  This
reduces the amount of running processes and is primarily
useful for coderepos (e.g. solver).

This also fixes prior use of `print { $git->{out} }' which is
a a potential (but unlikely) bug since commit d4ba8828ab23f278
(git: fix asynchronous batching for deep pipelines, 2023-01-04)

Lack of libgit2 on one of my test machines also uncovered fixes
necessary for t/imapd.t, t/nntpd.t and t/nntpd-v2.t.

3 files changed, 108 insertions, 55 deletions

diff --git a/lib/PublicInbox/Git.pm b/lib/PublicInbox/Git.pm
index 12f997dc..70adfd45 100644
--- a/lib/PublicInbox/Git.pm
+++ b/lib/PublicInbox/Git.pm
@@ -31,7 +31,10 @@ our $async_warn; # true in read-only daemons
 # 512: POSIX PIPE_BUF minimum (see pipe(7))
 # 3: @$inflight is flattened [ $OID, $cb, $arg ]
 # 65: SHA-256 hex size + "\n" in preparation for git using non-SHA1
-use constant MAX_INFLIGHT => 512 * 3 / 65;
+use constant {
+        MAX_INFLIGHT => 512 * 3 / (65 + length('contents ')),
+        BATCH_CMD_VER => (2 << 24 | 36 << 16), # git 2.36+
+};
 
 my %GIT_ESC = (
         a => "\a",
@@ -46,6 +49,24 @@ my %GIT_ESC = (
 );
 my %ESC_GIT = map { $GIT_ESC{$_} => $_ } keys %GIT_ESC;
 
+my $EXE_ST = ''; # pack('dd', st_ctime, st_size);
+my ($GIT_EXE, $GIT_VER);
+
+sub check_git_exe () {
+        $GIT_EXE = which('git') // die "git not found in $ENV{PATH}";
+        my @st = stat($GIT_EXE) or die "stat: $!";
+        my $st = pack('dd', $st[10], $st[7]);
+        if ($st ne $EXE_ST) {
+                my $rd = popen_rd([ $GIT_EXE, '--version' ]);
+                my $v = readline($rd);
+                $v =~ /\b([0-9]+(?:\.[0-9]+){2})/ or die
+                        "$GIT_EXE --version output: $v # unparseable";
+                my @v = split(/\./, $1, 3);
+                $GIT_VER = ($v[0] << 24) | ($v[1] << 16) | $v[2];
+                $EXE_ST = $st;
+        }
+}
+
 # unquote pathnames used by git, see quote.c::unquote_c_style.c in git.git
 sub git_unquote ($) {
         return $_[0] unless ($_[0] =~ /\A"(.*)"\z/);
@@ -122,25 +143,6 @@ sub _bidi_pipe {
                 return;
         }
 
-        state $EXE_ST = ''; # pack('dd', st_ctime, st_size);
-        my $exe = which('git') // die "git not found in $ENV{PATH}";
-        my @st = stat($exe) or die "stat: $!";
-        my $st = pack('dd', $st[10], $st[7]);
-        state $VER;
-        if ($st ne $EXE_ST) {
-                my $rd = popen_rd([ $exe, '--version' ]);
-                my $v = readline($rd);
-                $v =~ /\b([0-9]+(?:\.[0-9]+){2})/ or die
-                        "$exe --version output: $v # unparseable";
-                my @v = split(/\./, $1, 3);
-                $VER = ($v[0] << 24) | ($v[1] << 16) | $v[2];
-                $EXE_ST = $st;
-        }
-
-        # git 2.31.0+ supports -c core.abbrev=no, don't bother with
-        # core.abbrev=64 since not many releases had SHA-256 prior to 2.31
-        my $abbr = $VER < (2 << 24 | 31 << 16) ? 40 : 'no';
-
         pipe(my ($out_r, $out_w)) or $self->fail("pipe failed: $!");
         my $rdr = { 0 => $out_r, pgid => 0 };
         my $gd = $self->{git_dir};
@@ -148,10 +150,14 @@ sub _bidi_pipe {
                 $rdr->{-C} = $gd;
                 $gd = $1;
         }
-        my @cmd = ($exe, "--git-dir=$gd", '-c', "core.abbrev=$abbr",
-                        'cat-file', $batch);
+
+        # git 2.31.0+ supports -c core.abbrev=no, don't bother with
+        # core.abbrev=64 since not many releases had SHA-256 prior to 2.31
+        my $abbr = $GIT_VER < (2 << 24 | 31 << 16) ? 40 : 'no';
+        my @cmd = ($GIT_EXE, "--git-dir=$gd", '-c', "core.abbrev=$abbr",
+                        'cat-file', "--$batch");
         if ($err) {
-                my $id = "git.$self->{git_dir}$batch.err";
+                my $id = "git.$self->{git_dir}.$batch.err";
                 my $fh = tmpfile($id) or $self->fail("tmpfile($id): $!");
                 $self->{$err} = $fh;
                 $rdr->{2} = $fh;
@@ -163,7 +169,7 @@ sub _bidi_pipe {
         $out_w->autoflush(1);
         if ($^O eq 'linux') { # 1031: F_SETPIPE_SZ
                 fcntl($out_w, 1031, 4096);
-                fcntl($in_r, 1031, 4096) if $batch eq '--batch-check';
+                fcntl($in_r, 1031, 4096) if $batch eq 'batch-check';
         }
         $out_w->blocking(0);
         $self->{$out} = $out_w;
@@ -233,6 +239,16 @@ sub cat_async_retry ($$) {
         cat_async_step($self, $inflight); # take one step
 }
 
+# returns true if prefetch is successful
+sub async_prefetch {
+        my ($self, $oid, $cb, $arg) = @_;
+        my $inflight = $self->{inflight} or return;
+        return if @$inflight;
+        substr($oid, 0, 0) = 'contents ' if $self->{-bc};
+        write_all($self, $self->{out}, "$oid\n", \&cat_async_step, $inflight);
+        push(@$inflight, $oid, $cb, $arg);
+}
+
 sub cat_async_step ($$) {
         my ($self, $inflight) = @_;
         die 'BUG: inflight empty or odd' if scalar(@$inflight) < 3;
@@ -240,12 +256,22 @@ sub cat_async_step ($$) {
         my $rbuf = delete($self->{rbuf}) // \(my $new = '');
         my ($bref, $oid, $type, $size);
         my $head = my_readline($self->{in}, $rbuf);
+        my $cmd = ref($req) ? $$req : $req;
         # ->fail may be called via Gcf2Client.pm
+        my $bc = $self->{-bc};
         if ($head =~ /^([0-9a-f]{40,}) (\S+) ([0-9]+)$/) {
                 ($oid, $type, $size) = ($1, $2, $3 + 0);
-                $bref = my_read($self->{in}, $rbuf, $size + 1) or
-                        $self->fail(defined($bref) ? 'read EOF' : "read: $!");
-                chop($$bref) eq "\n" or $self->fail('LF missing after blob');
+                unless ($bc && $cmd =~ /\Ainfo /) { # --batch-command
+                        $bref = my_read($self->{in}, $rbuf, $size + 1) or
+                                $self->fail(defined($bref) ?
+                                                'read EOF' : "read: $!");
+                        chop($$bref) eq "\n" or
+                                        $self->fail('LF missing after blob');
+                }
+        } elsif ($bc && $cmd =~ /\Ainfo / &&
+                        $head =~ / (missing|ambiguous)\n/) {
+                $type = $1;
+                $oid = substr($cmd, 5);
         } elsif ($head =~ s/ missing\n//s) {
                 $oid = $head;
                 # ref($req) indicates it's already been retried
@@ -254,15 +280,23 @@ sub cat_async_step ($$) {
                         return cat_async_retry($self, $inflight);
                 }
                 $type = 'missing';
-                $oid = ref($req) ? $$req : $req if $oid eq '';
+                if ($oid eq '') {
+                        $oid = $cmd;
+                        $oid =~ s/\A(?:contents|info) // if $bc;
+                }
         } else {
                 my $err = $! ? " ($!)" : '';
                 $self->fail("bad result from async cat-file: $head$err");
         }
         $self->{rbuf} = $rbuf if $$rbuf ne '';
         splice(@$inflight, 0, 3); # don't retry $cb on ->fail
-        eval { $cb->($bref, $oid, $type, $size, $arg) };
-        async_err($self, $req, $oid, $@, 'cat') if $@;
+        if ($bc && $cmd =~ /\Ainfo /) {
+                eval { $cb->($oid, $type, $size, $arg, $self) };
+                async_err($self, $req, $oid, $@, 'check') if $@;
+        } else {
+                eval { $cb->($bref, $oid, $type, $size, $arg) };
+                async_err($self, $req, $oid, $@, 'cat') if $@;
+        }
 }
 
 sub cat_async_wait ($) {
@@ -274,7 +308,15 @@ sub cat_async_wait ($) {
 }
 
 sub batch_prepare ($) {
-        _bidi_pipe($_[0], qw(--batch in out pid));
+        my ($self) = @_;
+        check_git_exe();
+        if ($GIT_VER >= BATCH_CMD_VER) {
+                _bidi_pipe($self, qw(batch-command in out pid err_c));
+                $self->{-bc} = 1;
+        } else {
+                _bidi_pipe($self, qw(batch in out pid));
+        }
+        $self->{inflight} = [];
 }
 
 sub _cat_file_cb {
@@ -314,18 +356,24 @@ sub check_async_step ($$) {
 
 sub check_async_wait ($) {
         my ($self) = @_;
+        return cat_async_wait($self) if $self->{-bc};
         my $inflight_c = $self->{inflight_c} or return;
-        while (scalar(@$inflight_c)) {
-                check_async_step($self, $inflight_c);
-        }
+        check_async_step($self, $inflight_c) while (scalar(@$inflight_c));
 }
 
 sub check_async_begin ($) {
         my ($self) = @_;
-        cleanup($self) if alternates_changed($self);
-        _bidi_pipe($self, qw(--batch-check in_c out_c pid_c err_c));
         die 'BUG: already in async check' if $self->{inflight_c};
-        $self->{inflight_c} = [];
+        cleanup($self) if alternates_changed($self);
+        check_git_exe();
+        if ($GIT_VER >= BATCH_CMD_VER) {
+                _bidi_pipe($self, qw(batch-command in out pid err_c));
+                $self->{-bc} = 1;
+                $self->{inflight} = [];
+        } else {
+                _bidi_pipe($self, qw(batch-check in_c out_c pid_c err_c));
+                $self->{inflight_c} = [];
+        }
 }
 
 sub write_all {
@@ -345,10 +393,18 @@ sub write_all {
 
 sub check_async ($$$$) {
         my ($self, $oid, $cb, $arg) = @_;
-        my $inflight_c = $self->{inflight_c} // check_async_begin($self);
-        write_all($self, $self->{out_c}, $oid."\n",
-                \&check_async_step, $inflight_c);
-        push(@$inflight_c, $oid, $cb, $arg);
+        my $inflight = $self->{-bc} ?
+                        ($self->{inflight} // cat_async_begin($self)) :
+                        ($self->{inflight_c} // check_async_begin($self));
+        if ($self->{-bc}) {
+                substr($oid, 0, 0) = 'info ';
+                write_all($self, $self->{out}, "$oid\n",
+                                \&cat_async_step, $inflight);
+        } else {
+                write_all($self, $self->{out_c}, "$oid\n",
+                                \&check_async_step, $inflight);
+        }
+        push(@$inflight, $oid, $cb, $arg);
 }
 
 sub _check_cb { # check_async callback
@@ -389,6 +445,8 @@ sub async_abort ($) {
                         while (@$q) {
                                 my ($req, $cb, $arg) = splice(@$q, 0, 3);
                                 $req = $$req if ref($req);
+                                $self->{-bc} and
+                                        $req =~ s/\A(?:contents|info) //;
                                 $req =~ s/ .*//; # drop git_dir for Gcf2Client
                                 eval { $cb->(undef, $req, undef, undef, $arg) };
                                 warn "E: (in abort) $req: $@" if $@;
@@ -461,12 +519,10 @@ sub cleanup {
         return 1 if $lazy && (scalar(@{$self->{inflight_c} // []}) ||
                                 scalar(@{$self->{inflight} // []}));
         local $in_cleanup = 1;
-        delete $self->{async_cat};
-        delete $self->{async_chk};
+        delete @$self{qw(async_cat async_chk)};
         async_wait_all($self);
-        delete $self->{inflight};
-        delete $self->{inflight_c};
-        _destroy($self, qw(pid rbuf in out));
+        delete @$self{qw(inflight inflight_c -bc)};
+        _destroy($self, qw(pid rbuf in out err_c));
         _destroy($self, qw(pid_c rbuf_c in_c out_c err_c));
         undef;
 }
@@ -524,14 +580,14 @@ sub pub_urls {
 sub cat_async_begin {
         my ($self) = @_;
         cleanup($self) if $self->alternates_changed;
-        $self->batch_prepare;
         die 'BUG: already in async' if $self->{inflight};
-        $self->{inflight} = [];
+        batch_prepare($self);
 }
 
 sub cat_async ($$$;$) {
         my ($self, $oid, $cb, $arg) = @_;
         my $inflight = $self->{inflight} // cat_async_begin($self);
+        substr($oid, 0, 0) = 'contents ' if $self->{-bc};
         write_all($self, $self->{out}, $oid."\n", \&cat_async_step, $inflight);
         push(@$inflight, $oid, $cb, $arg);
 }
diff --git a/lib/PublicInbox/GitAsyncCat.pm b/lib/PublicInbox/GitAsyncCat.pm
index 6dda7340..49a3c602 100644
--- a/lib/PublicInbox/GitAsyncCat.pm
+++ b/lib/PublicInbox/GitAsyncCat.pm
@@ -78,6 +78,7 @@ sub async_check ($$$$) {
         my ($ibx, $oidish, $cb, $arg) = @_; # $ibx may be $ctx
         my $git = $ibx->{git} // $ibx->git;
         $git->check_async($oidish, $cb, $arg);
+        return watch_cat($git) if $git->{-bc}; # --batch-command
         $git->{async_chk} //= do {
                 my $self = bless { git => $git }, 'PublicInbox::GitAsyncCheck';
                 $git->{in_c}->blocking(0);
@@ -97,12 +98,8 @@ sub ibx_async_prefetch {
                         $oid .= " $git->{git_dir}\n";
                         return $GCF2C->gcf2_async(\$oid, $cb, $arg); # true
                 }
-        } elsif ($git->{async_cat} && (my $inflight = $git->{inflight})) {
-                if (!@$inflight) {
-                        print { $git->{out} } $oid, "\n" or
-                                                $git->fail("write error: $!");
-                        return push(@$inflight, $oid, $cb, $arg);
-                }
+        } elsif ($git->{async_cat}) {
+                return $git->async_prefetch($oid, $cb, $arg);
         }
         undef;
 }
diff --git a/t/nntpd.t b/t/nntpd.t
index 30233ce0..bebf4203 100644
--- a/t/nntpd.t
+++ b/t/nntpd.t
@@ -365,7 +365,7 @@ Date: Fri, 02 Oct 1993 00:00:00 +0000
                 ($^O =~ /\A(?:linux)\z/) or
                         skip "lsof /(deleted)/ check untested on $^O", 1;
                 my @lsof = xqx([$lsof, '-p', $td->{pid}], undef, $noerr);
-                my $d = [ grep(/\(deleted\)/, @lsof) ];
+                my $d = [ grep(/\(deleted\)/, grep(!/batch-command\.err/, @lsof)) ];
                 is_deeply($d, [], 'no deleted files') or diag explain($d);
         };
         SKIP: { test_watch($tmpdir, $host_port, $group) };