[newlib-cygwin] Cygwin: FIFO: keep track of the number of readers

[Ken Brown <kbrown@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=365818a4a5b0140bbf37b1d91fbf82aa9e376a6b

commit 365818a4a5b0140bbf37b1d91fbf82aa9e376a6b
Author: Ken Brown <kbrown@cornell.edu>
Date:   Fri Mar 27 09:43:30 2020 -0400

    Cygwin: FIFO: keep track of the number of readers
    
    Add data and methods to the shared memory that keep track of the
    number of open readers.
    
    Increment this number in open, dup, fork, and exec.  Decrement it in
    close.  Reset read_ready if there are no readers left.

Diff:
---
 winsup/cygwin/fhandler.h       |  8 ++++++++
 winsup/cygwin/fhandler_fifo.cc | 22 ++++++++++++++--------
 2 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/winsup/cygwin/fhandler.h b/winsup/cygwin/fhandler.h
index 8d6b94021..b2ee7e6b6 100644
--- a/winsup/cygwin/fhandler.h
+++ b/winsup/cygwin/fhandler.h
@@ -1303,6 +1303,11 @@ struct fifo_client_handler
 /* Info needed by all readers of a FIFO, stored in named shared memory. */
 class fifo_shmem_t
 {
+  LONG _nreaders;
+
+public:
+  int inc_nreaders () { return (int) InterlockedIncrement (&_nreaders); }
+  int dec_nreaders () { return (int) InterlockedDecrement (&_nreaders); }
 };
 
 class fhandler_fifo: public fhandler_base
@@ -1342,6 +1347,9 @@ class fhandler_fifo: public fhandler_base
   int create_shmem ();
   int reopen_shmem ();
 
+  int inc_nreaders () { return shmem->inc_nreaders (); }
+  int dec_nreaders () { return shmem->dec_nreaders (); }
+
 public:
   fhandler_fifo ();
   bool hit_eof ();
diff --git a/winsup/cygwin/fhandler_fifo.cc b/winsup/cygwin/fhandler_fifo.cc
index 9a0db3f33..d87070ac7 100644
--- a/winsup/cygwin/fhandler_fifo.cc
+++ b/winsup/cygwin/fhandler_fifo.cc
@@ -570,8 +570,9 @@ fhandler_fifo::open (int flags, mode_t)
       SetEvent (read_ready);
       if (create_shmem () < 0)
 	goto err_close_writer_opening;
+      inc_nreaders ();
       if (!(cancel_evt = create_event ()))
-	goto err_close_shmem;
+	goto err_dec_nreaders;
       if (!(thr_sync_evt = create_event ()))
 	goto err_close_cancel_evt;
       new cygthread (fifo_reader_thread, this, "fifo_reader", thr_sync_evt);
@@ -680,7 +681,10 @@ err_close_reader:
   return 0;
 err_close_cancel_evt:
   NtClose (cancel_evt);
-err_close_shmem:
+err_dec_nreaders:
+  if (dec_nreaders () == 0)
+    ResetEvent (read_ready);
+/* err_close_shmem: */
   NtUnmapViewOfSection (NtCurrentProcess (), shmem);
   NtClose (shmem_handle);
 err_close_writer_opening:
@@ -1003,15 +1007,13 @@ fhandler_fifo::close ()
 {
   if (reader)
     {
+      if (dec_nreaders () == 0)
+	ResetEvent (read_ready);
       cancel_reader_thread ();
       if (cancel_evt)
 	NtClose (cancel_evt);
       if (thr_sync_evt)
 	NtClose (thr_sync_evt);
-      /* FIXME: There could be several readers open because of
-	 dup/fork/exec; we should only reset read_ready when the last
-	 one closes. */
-      ResetEvent (read_ready);
       if (shmem)
 	NtUnmapViewOfSection (NtCurrentProcess (), shmem);
       if (shmem_handle)
@@ -1116,8 +1118,8 @@ fhandler_fifo::dup (fhandler_base *child, int flags)
 	goto err_close_handlers;
       if (!(fhf->thr_sync_evt = create_event ()))
 	goto err_close_cancel_evt;
-      new cygthread (fifo_reader_thread, fhf, "fifo_reader",
-		     fhf->thr_sync_evt);
+      inc_nreaders ();
+      new cygthread (fifo_reader_thread, fhf, "fifo_reader", fhf->thr_sync_evt);
     }
   return 0;
 err_close_cancel_evt:
@@ -1161,6 +1163,7 @@ fhandler_fifo::fixup_after_fork (HANDLE parent)
 	api_fatal ("Can't create reader thread cancel event during fork, %E");
       if (!(thr_sync_evt = create_event ()))
 	api_fatal ("Can't create reader thread sync event during fork, %E");
+      inc_nreaders ();
       new cygthread (fifo_reader_thread, this, "fifo_reader", thr_sync_evt);
     }
 }
@@ -1180,6 +1183,9 @@ fhandler_fifo::fixup_after_exec ()
 	api_fatal ("Can't create reader thread cancel event during exec, %E");
       if (!(thr_sync_evt = create_event ()))
 	api_fatal ("Can't create reader thread sync event during exec, %E");
+      /* At this moment we're a new reader.  The count will be
+	 decremented when the parent closes. */
+      inc_nreaders ();
       new cygthread (fifo_reader_thread, this, "fifo_reader", thr_sync_evt);
     }
 }