searchidxskeleton: add a note about locking

It's tempting to rely on the atomicity of smaller-than-PIPE_BUF
writes, but it doesn't work if mixed with larger ones.

1 files changed, 2 insertions, 0 deletions

diff --git a/lib/PublicInbox/SearchIdxSkeleton.pm b/lib/PublicInbox/SearchIdxSkeleton.pm
index 063c83e8..506e566f 100644
--- a/lib/PublicInbox/SearchIdxSkeleton.pm
+++ b/lib/PublicInbox/SearchIdxSkeleton.pm
@@ -80,6 +80,8 @@ sub index_skeleton {
         $str = length($str) . "\n" . $str;
 
         # multiple processes write to the same pipe, so use flock
+        # We can't avoid this lock for <=PIPE_BUF writes, either,
+        # because those atomic writes can break up >PIPE_BUF ones
         $self->_lock_acquire;
         print $w $str or $err = $!;
         $self->_lock_release;