[PATCH 18/21] ds: guard against stack-not-refcounted quirk of Perl 5

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

The Perl 5 stack is weakly-referenced for performance reasons.
This means it's possible for items in the stack to be freed
while executing further down the stack.

In lei (and perhaps public-facing read-only daemons in the
future), we'll fork and call PublicInbox::DS->Reset in the child
process.  This causes %DescriptorMap to be clobbered, allowing
the $DescriptorMap{$fd} arg to be freed inside the child
process.

When Carp::confess or Carp::longmess is called to generate a
backtrace, it may access the @DB::args array.  This array access
is not protected by reference counting and is known to cause
segfaults and other weird errors.

While the caller of an unnecessary Carp::confess may be
eliminated in a future commit, we can't guarantee our
dependencies will be free of @DB::args access attempts
in the future.

So guard against this Perl 5 quirmk by defensively bumping the
refcount of any object we call ->event_step on.

cf. https://rt.perl.org/Public/Bug/Display.html?id=131046
    https://github.com/Perl/perl5/issues/15928
---
 lib/PublicInbox/DS.pm | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/PublicInbox/DS.pm b/lib/PublicInbox/DS.pm
index 40994fd4..2d312f0a 100644
--- a/lib/PublicInbox/DS.pm
+++ b/lib/PublicInbox/DS.pm
@@ -271,6 +271,7 @@ sub EventLoop {
     $Epoll //= _InitPoller();
     local $in_loop = 1;
     my @events;
+    my $obj; # guard stack-not-refcounted w/ Carp + @DB::args
     do {
         my $timeout = RunTimers();
 
@@ -281,7 +282,8 @@ sub EventLoop {
             # that ones in the front triggered unregister-interest actions.  if we
             # can't find the %sock entry, it's because we're no longer interested
             # in that event.
-            $DescriptorMap{$fd}->event_step;
+            $obj = $DescriptorMap{$fd};
+            $obj->event_step;
         }
     } while (PostEventLoop());
     _run_later();