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 9f9e345d9c86462ac1fe87a06e0077e4f69db7c4..a43e84d6620e107feabc9708c1d2cde0f4bbb7d5 100644 (file)
--- 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)