From 7720ea9679e9c0952edee5aaa3bd4d77e4cbbd33 Mon Sep 17 00:00:00 2001
From: Ken Brown <kbrown@cornell.edu>
Date: Mon, 17 May 2021 17:18:45 -0400
Subject: [PATCH] Cygwin: AF_UNIX: keep a reference count of open descriptors

Add a data member _ndesc to the af_unix_shmem_t class, along with
methods to increment and decrement it.

Increment it during socket/fork/exec/dup, and decrement it during
close.  When the last descriptor is closed, call shutdown (SHUT_RDWR)
and unlink the socket's message queue.

The shutdown call will make the socket's peer see EPIPE if it tries to
write.
---
 winsup/cygwin/fhandler.h              |  6 ++++++
 winsup/cygwin/fhandler_socket_unix.cc | 12 ++++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/winsup/cygwin/fhandler.h b/winsup/cygwin/fhandler.h
index e608551b3..45d622deb 100644
--- a/winsup/cygwin/fhandler.h
+++ b/winsup/cygwin/fhandler.h
@@ -956,6 +956,7 @@ class af_unix_shmem_t
   LONG _so_passcred;		/* SO_PASSCRED */
   LONG _reuseaddr;		/* dummy */
   int  _type;			/* socket type */
+  LONG _ndesc;			/* number of open descriptors */
   sun_name_t _sun_path;
   sun_name_t _peer_sun_path;
   struct ucred _sock_cred;	/* filled at listen time */
@@ -997,6 +998,9 @@ class af_unix_shmem_t
   void set_socket_type (int val) { _type = val; }
   int get_socket_type () const { return _type; }
 
+  LONG inc_ndesc () { return InterlockedIncrement (&_ndesc); }
+  LONG dec_ndesc () { return InterlockedDecrement (&_ndesc); }
+
   void sun_path (struct sockaddr_un *un, __socklen_t unlen)
     { _sun_path.set (un, unlen); }
   void peer_sun_path (struct sockaddr_un *un, __socklen_t unlen)
@@ -1076,6 +1080,8 @@ class fhandler_socket_unix : public fhandler_socket
   int reuseaddr () const { return shmem->reuseaddr (); }
   void set_socket_type (int val) { shmem->set_socket_type (val); }
   int get_socket_type () const { return shmem->get_socket_type (); }
+  LONG inc_ndesc () { return shmem->inc_ndesc (); }
+  LONG dec_ndesc () { return shmem->dec_ndesc (); }
 
   int create_shmem ();
   int reopen_shmem ();
diff --git a/winsup/cygwin/fhandler_socket_unix.cc b/winsup/cygwin/fhandler_socket_unix.cc
index 109cad4ba..6071061af 100644
--- a/winsup/cygwin/fhandler_socket_unix.cc
+++ b/winsup/cygwin/fhandler_socket_unix.cc
@@ -1122,6 +1122,7 @@ fhandler_socket_unix::fixup_helper ()
   connect_wait_thr = NULL;
   cwt_termination_evt = NULL;
   cwt_param = NULL;
+  inc_ndesc ();
 }
 
 /* ========================== public methods ========================= */
@@ -1209,6 +1210,7 @@ fhandler_socket_unix::dup (fhandler_base *child, int flags)
   fhs->connect_wait_thr = NULL;
   fhs->cwt_termination_evt = NULL;
   fhs->cwt_param = NULL;
+  inc_ndesc ();
   return 0;
 }
 
@@ -1285,6 +1287,7 @@ fhandler_socket_unix::socket (int af, int type, int protocol, int flags)
   set_handle (NULL);
   set_unique_id ();
   set_ino (get_unique_id ());
+  inc_ndesc ();
   return 0;
 }
 
@@ -1753,10 +1756,11 @@ fhandler_socket_unix::close ()
     ret = mq_close (get_mqd_in ());
   if (get_mqd_out () != (mqd_t) -1)
     ret |= mq_close (get_mqd_out ());
-  /* FIXME: Maybe we should keep a reference count on the mqueues and
-     unlink it after the last one is close.  OTOH, this will become
-     unnecessary if the mqueue implementation is changed to use
-     Windows shared memory. */
+  if (dec_ndesc () <= 0)
+    {
+      shutdown (SHUT_RDWR);
+      mq_unlink (get_mqueue_name ());
+    }
   return ret;
 }