From a69a08ab9d276ce65d757f866b021c95d553bb41 Mon Sep 17 00:00:00 2001
From: "Guillermo J. Rozas" <edu/mit/csail/zurich/gjr>
Date: Wed, 9 Dec 1987 06:31:42 +0000
Subject: [PATCH] Fix bug in purify

Under some circumstances (prob = 1/512) a bufferfull was lost between
the pure copy and the constant copy, because the header overflowed the
free buffer, and the free buffer dumping code assumed that the buffer
would be dumped when scan was dumped, but scan was reinitialized
immediately.
---
 v7/src/microcode/bchpur.c | 52 ++++++++++++++++++++++++---------------
 1 file changed, 32 insertions(+), 20 deletions(-)

diff --git a/v7/src/microcode/bchpur.c b/v7/src/microcode/bchpur.c
index 540739467..abc6de0b8 100644
--- a/v7/src/microcode/bchpur.c
+++ b/v7/src/microcode/bchpur.c
@@ -1,6 +1,6 @@
 /* -*-C-*-
 
-$Header: /Users/cph/tmp/foo/mit-scheme/mit-scheme/v7/src/microcode/Attic/bchpur.c,v 9.35 1987/11/17 08:06:48 jinx Exp $
+$Header: /Users/cph/tmp/foo/mit-scheme/mit-scheme/v7/src/microcode/Attic/bchpur.c,v 9.36 1987/12/09 06:31:42 jinx Rel $
 
 Copyright (c) 1987 Massachusetts Institute of Technology
 
@@ -223,6 +223,29 @@ end_purifyloop:
   return Scan;
 }
 
+/* This is not paranoia!
+   The two words in the header may overflow the free buffer. 
+ */
+
+Pointer *
+purify_header_overflow(free_buffer)
+     Pointer *free_buffer;
+{
+  Pointer *scan_buffer;
+  long delta;
+
+  delta = (free_buffer - free_buffer_top);
+  free_buffer = dump_and_reset_free_buffer(delta, NULL);
+  scan_buffer = dump_and_reload_scan_buffer(0, NULL);
+  if ((scan_buffer + delta) != free_buffer)
+  {
+    fprintf(stderr,
+	    "\nPurify: Scan and Free do not meet at the end.\n");
+    Microcode_Termination(TERM_EXIT);
+  }
+  return (free_buffer);
+}
+
 Pointer
 purify(object, flag)
      Pointer object, flag;
@@ -233,15 +256,16 @@ purify(object, flag)
   Weak_Chain = NIL;
   free_buffer = initialize_free_buffer();
   block_start = Free_Constant;
+
   Free_Constant += 2;
   *free_buffer++ = NIL;		/* Pure block header. */
   *free_buffer++ = object;
-  /* This is paranoia, but... */
   if (free_buffer >= free_buffer_top)
   {
     free_buffer =
       dump_and_reset_free_buffer((free_buffer - free_buffer_top), NULL);
   }
+
   if (flag == TRUTH)
   {
     Result = purifyloop(initialize_scan_buffer(),
@@ -258,15 +282,15 @@ purify(object, flag)
   {
     pure_length = 3;
   }
+
   Free_Constant += 2;
   *free_buffer++ = Make_Non_Pointer(TC_MANIFEST_SPECIAL_NM_VECTOR, 1);
   *free_buffer++ = Make_Non_Pointer(CONSTANT_PART, pure_length);
-  /* This is paranoia, but... */
   if (free_buffer >= free_buffer_top)
   {
-    free_buffer =
-      dump_and_reset_free_buffer((free_buffer - free_buffer_top), NULL);
+    free_buffer = purify_header_overflow(free_buffer);
   }
+
   if (flag == TRUTH)
   {
     Result = purifyloop(initialize_scan_buffer(),
@@ -282,28 +306,16 @@ purify(object, flag)
     fprintf(stderr, "\nPurify: Constant Copy ended too early.\n");
     Microcode_Termination(TERM_BROKEN_HEART);
   }
+
   Free_Constant += 2;
   length = (Free_Constant - block_start);
   *free_buffer++ = Make_Non_Pointer(TC_MANIFEST_SPECIAL_NM_VECTOR, 1);
   *free_buffer++ = Make_Non_Pointer(END_OF_BLOCK, (length - 1));
-
-  /* This is not paranoia!
-     The last two words may overflow the free buffer. 
-   */
   if (free_buffer >= free_buffer_top)
   {
-    long delta;
-
-    delta = (free_buffer - free_buffer_top);
-    free_buffer =
-      dump_and_reset_free_buffer(delta, NULL);
-    Result = dump_and_reload_scan_buffer(0, NULL);
-    if ((Result + delta) != free_buffer)
-    {
-      fprintf(stderr, "\nPurify: Scan and Free do not meet at the end.\n");
-      Microcode_Termination(TERM_EXIT);
-    }
+    free_buffer = purify_header_overflow(free_buffer);
   }
+
   end_transport(NULL);
 
   if (!Test_Pure_Space_Top(Free_Constant))
-- 
2.25.1