Don't lose top specpdl entry when memory is exhausted.

* eval.c (grow_specpdl): Increment specpdl top by 1 and check for
specpdl overflow here, to simplify callers; all callers changed.
Always reserve an unused entry at the stack top; this avoids
losing the top entry's information when memory is exhausted.

2 files changed, 41 insertions, 23 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 6e3a82c7c13..60e7e376729 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,11 @@
+2013-07-13  Paul Eggert  <eggert@cs.ucla.edu>
+
+        Don't lose top specpdl entry when memory is exhausted.
+        * eval.c (grow_specpdl): Increment specpdl top by 1 and check for
+        specpdl overflow here, to simplify callers; all callers changed.
+        Always reserve an unused entry at the stack top; this avoids
+        losing the top entry's information when memory is exhausted.
+
 2013-07-12  Paul Eggert  <eggert@cs.ucla.edu>
 
         Clean up errno reporting and fix some errno-reporting bugs.
diff --git a/src/eval.c b/src/eval.c
index 31a774b9d27..0e231bdb285 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1996,38 +1996,52 @@ If LEXICAL is t, evaluate using lexical scoping.  */)
   return unbind_to (count, eval_sub (form));
 }
 
+/* Grow the specpdl stack by one entry.
+   The caller should have already initialized the entry.
+   Signal an error on stack overflow.
+
+   Make sure that there is always one unused entry past the top of the
+   stack, so that the just-initialized entry is safely unwound if
+   memory exhausted and an error is signaled here.  Also, allocate a
+   never-used entry just before the bottom of the stack; sometimes its
+   address is taken.  */
+
 static void
 grow_specpdl (void)
 {
-  ptrdiff_t count = SPECPDL_INDEX ();
-  ptrdiff_t max_size = min (max_specpdl_size, PTRDIFF_MAX - 1000);
-  union specbinding *pdlvec = specpdl - 1;
-  ptrdiff_t pdlvecsize = specpdl_size + 1;
-  if (max_size <= specpdl_size)
+  specpdl_ptr++;
+
+  if (specpdl_ptr == specpdl + specpdl_size)
     {
-      if (max_specpdl_size < 400)
-        max_size = max_specpdl_size = 400;
+      ptrdiff_t count = SPECPDL_INDEX ();
+      ptrdiff_t max_size = min (max_specpdl_size, PTRDIFF_MAX - 1000);
+      union specbinding *pdlvec = specpdl - 1;
+      ptrdiff_t pdlvecsize = specpdl_size + 1;
       if (max_size <= specpdl_size)
-        signal_error ("Variable binding depth exceeds max-specpdl-size", Qnil);
+        {
+          if (max_specpdl_size < 400)
+            max_size = max_specpdl_size = 400;
+          if (max_size <= specpdl_size)
+            signal_error ("Variable binding depth exceeds max-specpdl-size",
+                          Qnil);
+        }
+      pdlvec = xpalloc (pdlvec, &pdlvecsize, 1, max_size + 1, sizeof *specpdl);
+      specpdl = pdlvec + 1;
+      specpdl_size = pdlvecsize - 1;
+      specpdl_ptr = specpdl + count;
     }
-  pdlvec = xpalloc (pdlvec, &pdlvecsize, 1, max_size + 1, sizeof *specpdl);
-  specpdl = pdlvec + 1;
-  specpdl_size = pdlvecsize - 1;
-  specpdl_ptr = specpdl + count;
 }
 
 void
 record_in_backtrace (Lisp_Object function, Lisp_Object *args, ptrdiff_t nargs)
 {
   eassert (nargs >= UNEVALLED);
-  if (specpdl_ptr == specpdl + specpdl_size)
-    grow_specpdl ();
   specpdl_ptr->bt.kind = SPECPDL_BACKTRACE;
   specpdl_ptr->bt.debug_on_exit = false;
   specpdl_ptr->bt.function = function;
   specpdl_ptr->bt.args = args;
   specpdl_ptr->bt.nargs = nargs;
-  specpdl_ptr++;
+  grow_specpdl ();
 }
 
 /* Eval a sub-expression of the current expression (i.e. in the same
@@ -3113,8 +3127,6 @@ specbind (Lisp_Object symbol, Lisp_Object value)
 
   CHECK_SYMBOL (symbol);
   sym = XSYMBOL (symbol);
-  if (specpdl_ptr == specpdl + specpdl_size)
-    grow_specpdl ();
 
  start:
   switch (sym->redirect)
@@ -3127,7 +3139,7 @@ specbind (Lisp_Object symbol, Lisp_Object value)
       specpdl_ptr->let.kind = SPECPDL_LET;
       specpdl_ptr->let.symbol = symbol;
       specpdl_ptr->let.old_value = SYMBOL_VAL (sym);
-      ++specpdl_ptr;
+      grow_specpdl ();
       if (!sym->constant)
         SET_SYMBOL_VAL (sym, value);
       else
@@ -3162,7 +3174,7 @@ specbind (Lisp_Object symbol, Lisp_Object value)
             if (NILP (Flocal_variable_p (symbol, Qnil)))
               {
                 specpdl_ptr->let.kind = SPECPDL_LET_DEFAULT;
-                ++specpdl_ptr;
+                grow_specpdl ();
                 Fset_default (symbol, value);
                 return;
               }
@@ -3170,7 +3182,7 @@ specbind (Lisp_Object symbol, Lisp_Object value)
         else
           specpdl_ptr->let.kind = SPECPDL_LET;
 
-        specpdl_ptr++;
+        grow_specpdl ();
         set_internal (symbol, value, Qnil, 1);
         break;
       }
@@ -3181,12 +3193,10 @@ specbind (Lisp_Object symbol, Lisp_Object value)
 void
 record_unwind_protect (Lisp_Object (*function) (Lisp_Object), Lisp_Object arg)
 {
-  if (specpdl_ptr == specpdl + specpdl_size)
-    grow_specpdl ();
   specpdl_ptr->unwind.kind = SPECPDL_UNWIND;
   specpdl_ptr->unwind.func = function;
   specpdl_ptr->unwind.arg = arg;
-  specpdl_ptr++;
+  grow_specpdl ();
 }
 
 Lisp_Object