imap: limit ibx_async_prefetch to idle git processes

This improves fairness while having no measurable performance
impact for a single uncached IMAP client (mutt) opening a folder
for the first time.

I noticed this problem with the public-inbox.org IMAP server where
a few IMAP clients were unfairly monopolizing the -netd process.

1 files changed, 4 insertions, 5 deletions

diff --git a/lib/PublicInbox/GitAsyncCat.pm b/lib/PublicInbox/GitAsyncCat.pm
index cea3f539..6b7425f6 100644
--- a/lib/PublicInbox/GitAsyncCat.pm
+++ b/lib/PublicInbox/GitAsyncCat.pm
@@ -69,19 +69,18 @@ sub ibx_async_cat ($$$$) {
 }
 
 # this is safe to call inside $cb, but not guaranteed to enqueue
-# returns true if successful, undef if not.
+# returns true if successful, undef if not.  For fairness, we only
+# prefetch if there's no in-flight requests.
 sub ibx_async_prefetch {
         my ($ibx, $oid, $cb, $arg) = @_;
         my $git = $ibx->git;
         if (!defined($ibx->{topdir}) && $GCF2C) {
-                if (!$GCF2C->{wbuf}) {
+                if (!@{$GCF2C->{inflight} // []}) {
                         $oid .= " $git->{git_dir}\n";
                         return $GCF2C->gcf2_async(\$oid, $cb, $arg); # true
                 }
         } elsif ($git->{async_cat} && (my $inflight = $git->{inflight})) {
-                # we could use MAX_INFLIGHT here w/o the halving,
-                # but lets not allow one client to monopolize a git process
-                if (@$inflight < int(PublicInbox::Git::MAX_INFLIGHT/2)) {
+                if (!@$inflight) {
                         print { $git->{out} } $oid, "\n" or
                                                 $git->fail("write error: $!");
                         return push(@$inflight, $oid, $cb, $arg);