imap: introduce and use Git->async_prefetch

We can keep the git process more active by sending another
request to it while fetch_run_ops() is running.  This
parallelization speeds up mutt's initial FETCH for headers by
around ~35%(!).

1 files changed, 16 insertions, 0 deletions

diff --git a/lib/PublicInbox/Git.pm b/lib/PublicInbox/Git.pm
index ffc464eb..7b2ada24 100644
--- a/lib/PublicInbox/Git.pm
+++ b/lib/PublicInbox/Git.pm
@@ -410,6 +410,22 @@ sub cat_async ($$$;$) {
         push(@$inflight, $oid, $cb, $arg);
 }
 
+# this is safe to call inside $cb, but not guaranteed to enqueue
+# returns true if successful, undef if not.
+sub async_prefetch {
+        my ($self, $oid, $cb, $arg) = @_;
+        if (defined($self->{async_cat}) && (my $inflight = $self->{inflight})) {
+                # we could use MAX_INFLIGHT here w/o the halving,
+                # but lets not allow one client to monopolize a git process
+                if (scalar(@$inflight) < int(MAX_INFLIGHT/2)) {
+                        print { $self->{out} } $oid, "\n" or
+                                                fail($self, "write error: $!");
+                        return push(@$inflight, $oid, $cb, $arg);
+                }
+        }
+        undef;
+}
+
 sub extract_cmt_time {
         my ($bref, undef, undef, undef, $modified) = @_;