Cygwin: FIFO: introduce a new type, fifo_reader_id_t

This uniquely identifies an fhandler_fifo open for reading in any
process.

Add a new data member 'me' of this type, which is set in open, dup,
fork, and exec.

winsup/cygwin/fhandler.h

@@ -1300,6 +1300,26 @@ struct fifo_client_handler
   int pipe_state ();
 };
 
+class fhandler_fifo;
+
+struct fifo_reader_id_t
+{
+  DWORD winpid;
+  fhandler_fifo *fh;
+
+  operator bool () const { return winpid != 0 || fh != NULL; }
+
+  friend operator == (const fifo_reader_id_t &l, const fifo_reader_id_t &r)
+  {
+    return l.winpid == r.winpid && l.fh == r.fh;
+  }
+
+  friend operator != (const fifo_reader_id_t &l, const fifo_reader_id_t &r)
+  {
+    return l.winpid != r.winpid || l.fh != r.fh;
+  }
+};
+
 /* Info needed by all readers of a FIFO, stored in named shared memory. */
 class fifo_shmem_t
 {
@@ -1329,6 +1349,7 @@ class fhandler_fifo: public fhandler_base
   af_unix_spinlock_t _fifo_client_lock;
   bool reader, writer, duplexer;
   size_t max_atomic_write;
+  fifo_reader_id_t me;
 
   HANDLE shmem_handle;
   fifo_shmem_t *shmem;
@@ -1362,6 +1383,8 @@ public:
   void fifo_client_lock () { _fifo_client_lock.lock (); }
   void fifo_client_unlock () { _fifo_client_lock.unlock (); }
 
+  fifo_reader_id_t get_me () const { return me; }
+
   int open (int, mode_t);
   off_t lseek (off_t offset, int whence);
   int close ();