9042 multiples of tty streams modules cause weirdness
Reviewed by: Randy Fishel <randyf@sibernet.com>
Reviewed by: Carlos Neira <cneirabustos@gmail.com>
Approved by: Hans Rosenfeld <rosenfeld@grumpf.hope-2000.org>

*** 23,32 ****
--- 23,33 ----
  
  
  /*
   * Copyright (c) 1988, 2010, Oracle and/or its affiliates. All rights reserved.
   * Copyright 2017 Joyent, Inc.
+  * Copyright 2018 OmniOS Community Edition (OmniOSce) Association.
   */
  
  #include <sys/types.h>
  #include <sys/sysmacros.h>
  #include <sys/param.h>
*** 3781,3790 ****
--- 3782,3815 ----
                          return (EINVAL);
  
                  TRACE_2(TR_FAC_STREAMS_FR, TR_I_PUSH,
                      "I_PUSH:fp %p stp %p", fp, stp);
  
+                 /*
+                  * If the module is flagged as single-instance, then check
+                  * to see if the module is already pushed. If it is, return
+                  * as if the push was successful.
+                  */
+                 if (fp->f_qflag & _QSINGLE_INSTANCE) {
+                         queue_t *q;
+ 
+                         claimstr(stp->sd_wrq);
+                         for (q = stp->sd_wrq->q_next; q; q = q->q_next) {
+                                 if (q->q_flag & QREADR) {
+                                         q = NULL;
+                                         break;
+                                 }
+                                 if (strcmp(mname, Q2NAME(q)) == 0)
+                                         break;
+                         }
+                         releasestr(stp->sd_wrq);
+                         if (q != NULL) {
+                                 fmodsw_rele(fp);
+                                 return (0);
+                         }
+                 }
+ 
                  if (error = strstartplumb(stp, flag, cmd)) {
                          fmodsw_rele(fp);
                          return (error);
                  }