Subject: bug#9273: 23.3; malloc initialization should (sometimes) happen at runtime

From: Ken Brown <[email-masked]>
Date: Sat, 13 Aug 2011 23:13:39 -0400

On 8/13/2011 3:19 PM, Stefan Monnier wrote:
>> Probably.  That would keep me from having to get involved in the details of
>> how _heapinfo works, and it would probably be less error prone.  So I would
>> do something like the following:
> 
>> 1. Temporarily restore the pre-dump malloc state.
>> 2. Call xrealloc, getting storage of the new size in the old heap.
>> 3. Copy the contents into a temporary buffer, and then free the recently
>> allocated storage in the old heap.
>> 4. Restore the malloc state, call malloc, and then copy the contents of the
>> temporary buffer into the just-acquired storage in the new heap.
> 
>> Is that what you had in mind?
> 
> No, I meant, during initialization of Emacs, make sure we get the old
> _heapinfo state, so that malloc/realloc/... just works.

No, that wouldn't be good because the old _heapinfo state uses the static heap, which is very small.  It's big enough for temacs, but not for a usable emacs.  In any case, Eli's suggestion turned out to be very easy to implement.  My revised patch is appended below, and it seems to solve all the problems I encountered.  I still want to test it further, but I think it's OK.  Eli, thanks for your help.

Ken

=== modified file 'src/gmalloc.c'
--- src/gmalloc.c       2011-08-04 17:04:39 +0000
+++ src/gmalloc.c       2011-08-13 22:32:45 +0000
@@ -351,11 +351,19 @@
 #endif
 #include <errno.h>

-/* How to really get more memory.  */
-#if defined(CYGWIN)
+/* On Cygwin there are two heaps.  temacs uses the static heap
+   (defined in sheap.c and managed with bss_sbrk), and the dumped
+   emacs uses the Cygwin heap (managed with sbrk).  When emacs starts
+   on Cygwin, it reinitializes malloc, and we save the old info for
+   use by free and realloc if they're called with a pointer into the
+   static heap. */
+#ifdef CYGWIN
 extern __ptr_t bss_sbrk PP ((ptrdiff_t __size));
 extern int bss_sbrk_did_unexec;
+char *bss_sbrk_heapbase;
+malloc_info *bss_sbrk_heapinfo;
 #endif
+
 __ptr_t (*__morecore) PP ((__malloc_ptrdiff_t __size)) = __default_morecore;

 /* Debugging hook for `malloc'.  */
@@ -584,6 +592,16 @@
   mcheck (NULL);
 #endif

+#ifdef CYGWIN
+  if (bss_sbrk_did_unexec)
+    /* we're reinitializing the dumped emacs */
+    {
+      bss_sbrk_heapbase = _heapbase;
+      bss_sbrk_heapinfo = _heapinfo;
+      memset (_fraghead, 0, BLOCKLOG * sizeof (struct list));
+    }
+#endif
+
   if (__malloc_initialize_hook)
     (*__malloc_initialize_hook) ();

@@ -1054,6 +1072,12 @@
   if (ptr == NULL)
     return;

+#ifdef CYGWIN
+  if (ptr < _heapbase)
+    /* We're being asked to free something in the static heap. */
+    return;
+#endif
+
   PROTECT_MALLOC_STATE (0);

   LOCK_ALIGNED_BLOCKS ();
@@ -1346,9 +1370,33 @@
 #include <malloc.h>
 #endif

-
 #define min(A, B) ((A) < (B) ? (A) : (B))

+/* On Cygwin the dumped emacs may try to realloc storage allocated in
+   the static heap.  We just malloc space in the new heap and copy the
+   data.  */
+#ifdef CYGWIN
+__ptr_t
+special_realloc (ptr, size)
+     __ptr_t ptr;
+     __malloc_size_t size;
+{
+  __ptr_t result;
+  int type;
+  __malloc_size_t block, oldsize;
+
+  block = ((char *) ptr - bss_sbrk_heapbase) / BLOCKSIZE + 1;
+  type = bss_sbrk_heapinfo[block].busy.type;
+  oldsize =
+    type == 0 ? bss_sbrk_heapinfo[block].busy.info.size * BLOCKSIZE
+    : (__malloc_size_t) 1 << type;
+  result = _malloc_internal_nolock (size);
+  if (result != NULL)
+    memcpy (result, ptr, min (oldsize, size));
+  return result;
+}
+#endif
+
 /* Debugging hook for realloc.  */
 __ptr_t (*__realloc_hook) PP ((__ptr_t __ptr, __malloc_size_t __size));

@@ -1375,6 +1423,12 @@
   else if (ptr == NULL)
     return _malloc_internal_nolock (size);

+#ifdef CYGWIN
+  if (ptr < _heapbase)
+    /* ptr points into the static heap */
+    return special_realloc (ptr, size);
+#endif
+
   block = BLOCK (ptr);

   PROTECT_MALLOC_STATE (0);

=== modified file 'src/unexcw.c'
--- src/unexcw.c        2011-03-17 20:18:59 +0000
+++ src/unexcw.c        2011-08-12 20:10:03 +0000
@@ -33,6 +33,8 @@

 extern int bss_sbrk_did_unexec;

+extern int __malloc_initialized;
+
 /* emacs symbols that indicate where bss and data end for emacs internals */
 extern char my_endbss[];
 extern char my_edata[];
@@ -210,9 +212,12 @@
            lseek (fd, (long) (exe_header->section_header[i].s_scnptr),
                   SEEK_SET);
          assert (ret != -1);
+         /* force the dumped emacs to reinitialize malloc */
+         __malloc_initialized = 0;
          ret =
            write (fd, (char *) start_address,
                   my_endbss - (char *) start_address);
+         __malloc_initialized = 1;
          assert (ret == (my_endbss - (char *) start_address));
          if (debug_unexcw)
            printf ("         .bss, mem start 0x%08x mem length %d\n",