From: Chris Hanson <org/chris-hanson/cph>
Date: Fri, 28 Apr 1995 06:59:29 +0000 (+0000)
Subject: In primitive OS2-SELECT-REGISTRY-TEST, there was a window where input
X-Git-Tag: 20090517-FFI~6383
X-Git-Url: https://birchwood-abbey.net/git?a=commitdiff_plain;h=d50a44ddcd428e992b46d44c4b1404b88fb733a0;p=mit-scheme.git

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.
---

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)