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>

@@ -23,10 +23,11 @@
 
 
 /*
  * 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>

@@ -3781,10 +3782,34 @@
                         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);
                 }