From d50a44ddcd428e992b46d44c4b1404b88fb733a0 Mon Sep 17 00:00:00 2001
From: Chris Hanson <org/chris-hanson/cph>
Date: Fri, 28 Apr 1995 06:59:29 +0000
Subject: [PATCH] In primitive OS2-SELECT-REGISTRY-TEST, there was a window
 where input could be waiting to be read, but the primitive would be blocked
 waiting for some input.  This was because the primitive is composed of two
 phases, and input detected in the first phase was not used to prevent
 blocking in the second.

---
 v7/src/microcode/pros2io.c | 39 ++++++++++++++++++--------------------
 1 file changed, 18 insertions(+), 21 deletions(-)

diff --git a/v7/src/microcode/pros2io.c b/v7/src/microcode/pros2io.c
index 9f9e345d9..a43e84d66 100644
--- a/v7/src/microcode/pros2io.c
+++ b/v7/src/microcode/pros2io.c
@@ -1,6 +1,6 @@
 /* -*-C-*-
 
-$Id: pros2io.c,v 1.3 1995/01/16 20:58:18 cph Exp $
+$Id: pros2io.c,v 1.4 1995/04/28 06:59:29 cph Exp $
 
 Copyright (c) 1994-95 Massachusetts Institute of Technology
 
@@ -104,6 +104,8 @@ DEFINE_PRIMITIVE ("OS2-SELECT-REGISTRY-TEST", Prim_OS2_select_registry_test, 3,
     qid_t qid;
     int n;
 
+    /* This first phase checks the qid subqueues and OS2_scheme_tqueue
+       for any previously-queued input.  */
     for (qid = 0; (qid <= QID_MAX); qid += 1)
       {
 	(results [qid]) = 0;
@@ -119,34 +121,29 @@ DEFINE_PRIMITIVE ("OS2-SELECT-REGISTRY-TEST", Prim_OS2_select_registry_test, 3,
 	      break;
 	    }
       }
-    while (1)
-      {
-	for (qid = 0; (qid <= QID_MAX); qid += 1)
-	  (OS2_scheme_tqueue_avail_map [qid]) = 0;
-	n = (OS2_tqueue_select (OS2_scheme_tqueue, blockp));
-	if (n == (-1))
-	  break;
-	else if (n < 0)
-	  {
-	    interruptp = 1;
+    /* This second phase waits for input if necessary.  It does not
+       check the subqueues for previously-stored data, so it's
+       important that we already did this.  Otherwise we could end up
+       waiting for input when there was valid input ready.  */
+    if (blockp)
+      while (! (inputp || interruptp))
+	{
+	  for (qid = 0; (qid <= QID_MAX); qid += 1)
+	    (OS2_scheme_tqueue_avail_map [qid]) = 0;
+	  n = (OS2_tqueue_select (OS2_scheme_tqueue, blockp));
+	  if (n == (-1))
 	    break;
-	  }
-	else
-	  {
-	    int breakp = 0;
+	  else if (n < 0)
+	    interruptp = 1;
+	  else
 	    for (qid = 0; (qid <= QID_MAX); qid += 1)
 	      if (((registry [qid]) != 0)
 		  && (OS2_scheme_tqueue_avail_map [qid]))
 		{
 		  inputp = 1;
 		  (results [qid]) = 1;
-		  if (blockp)
-		    breakp = 1;
 		}
-	    if (breakp)
-	      break;
-	  }
-      }
+	}
     if (inputp)
       PRIMITIVE_RETURN (LONG_TO_UNSIGNED_FIXNUM (0));
     else if (!interruptp)
-- 
2.25.1