Do not allocate huge temporary memory areas and objects while encoding

for file I/O, thus reducing an enormous memory usage for large buffers.
See http://lists.gnu.org/archive/html/emacs-devel/2013-10/msg00180.html.
* coding.h (struct coding_system): New member raw_destination.
* coding.c (setup_coding_system): Initialize it to zero.
(encode_coding_object): If raw_destination is set, do not create
dst_object.  Add comment.
* fileio.c (toplevel): New constant E_WRITE_MAX.
(e_write): Do not encode more than E_WRITE_MAX characters per one loop
iteration.  Use raw_destination if E_WRITE_MAX characters is encoded.

src/ChangeLog

@@ -1,3 +1,16 @@
+2013-10-08  Dmitry Antipov  <dmantipov@yandex.ru>
+
+	Do not allocate huge temporary memory areas and objects while encoding
+	for file I/O, thus reducing an enormous memory usage for large buffers.
+	See http://lists.gnu.org/archive/html/emacs-devel/2013-10/msg00180.html.
+	* coding.h (struct coding_system): New member raw_destination.
+	* coding.c (setup_coding_system): Initialize it to zero.
+	(encode_coding_object): If raw_destination is set, do not create
+	dst_object.  Add comment.
+	* fileio.c (toplevel): New constant E_WRITE_MAX.
+	(e_write): Do not encode more than E_WRITE_MAX characters per one loop
+	iteration.  Use raw_destination if E_WRITE_MAX characters is encoded.
+
 2013-10-08  Jan Djärv  <jan.h.d@swipnet.se>
 
 	* nsterm.m (windowDidExitFullScreen:):

src/coding.c

@@ -5761,6 +5761,7 @@ setup_coding_system (Lisp_Object coding_system, struct coding_system *coding)
   coding->safe_charsets = SDATA (val);
   coding->default_char = XINT (CODING_ATTR_DEFAULT_CHAR (attrs));
   coding->carryover_bytes = 0;
+  coding->raw_destination = 0;
 
   coding_type = CODING_ATTR_TYPE (attrs);
   if (EQ (coding_type, Qundecided))
@@ -8352,6 +8353,11 @@ encode_coding_object (struct coding_system *coding,
     {
       if (BUFFERP (coding->dst_object))
 	coding->dst_object = Fbuffer_string ();
+      else if (coding->raw_destination)
+	/* This is used to avoid creating huge Lisp string.
+	   NOTE: caller who sets `raw_destination' is also
+	   responsible for freeing `destination' buffer.  */
+	coding->dst_object = Qnil;
       else
 	{
 	  coding->dst_object

src/coding.h

@@ -512,6 +512,10 @@ struct coding_system
      `charbuf', but at `src_object'.  */
   unsigned chars_at_source : 1;
 
+  /* Nonzero if the result of conversion is in `destination'
+     buffer rather than in `dst_object'.  */
+  unsigned raw_destination : 1;
+
   /* Set to 1 if charbuf contains an annotation.  */
   unsigned annotated : 1;
 

src/fileio.c

@@ -5263,6 +5263,10 @@ a_write (int desc, Lisp_Object string, ptrdiff_t pos,
   return 1;
 }
 
+/* Maximum number of characters that the next
+   function encodes per one loop iteration.  */
+
+enum { E_WRITE_MAX = 8 * 1024 * 1024 };
 
 /* Write text in the range START and END into descriptor DESC,
    encoding them with coding system CODING.  If STRING is nil, START
@@ -5289,9 +5293,16 @@ e_write (int desc, Lisp_Object string, ptrdiff_t start, ptrdiff_t end,
 	  coding->src_multibyte = SCHARS (string) < SBYTES (string);
 	  if (CODING_REQUIRE_ENCODING (coding))
 	    {
-	      encode_coding_object (coding, string,
+	      ptrdiff_t nchars = min (end - start, E_WRITE_MAX);
-				    start, string_char_to_byte (string, start),
+
-				    end, string_char_to_byte (string, end), Qt);
+	      /* Avoid creating huge Lisp string in encode_coding_object.  */
+	      if (nchars == E_WRITE_MAX)
+		coding->raw_destination = 1;
+
+	      encode_coding_object
+		(coding, string, start, string_char_to_byte (string, start),
+		 start + nchars, string_char_to_byte (string, start + nchars),
+		 Qt);
 	    }
 	  else
 	    {
@@ -5308,8 +5319,15 @@ e_write (int desc, Lisp_Object string, ptrdiff_t start, ptrdiff_t end,
 	  coding->src_multibyte = (end - start) < (end_byte - start_byte);
 	  if (CODING_REQUIRE_ENCODING (coding))
 	    {
-	      encode_coding_object (coding, Fcurrent_buffer (),
+	      ptrdiff_t nchars = min (end - start, E_WRITE_MAX);
-				    start, start_byte, end, end_byte, Qt);
+
+	      /* Likewise.  */
+	      if (nchars == E_WRITE_MAX)
+		coding->raw_destination = 1;
+
+	      encode_coding_object
+		(coding, Fcurrent_buffer (), start, start_byte,
+		 start + nchars, CHAR_TO_BYTE (start + nchars), Qt);
 	    }
 	  else
 	    {
@@ -5330,11 +5348,19 @@ e_write (int desc, Lisp_Object string, ptrdiff_t start, ptrdiff_t end,
 
       if (coding->produced > 0)
 	{
-	  char *buf = (STRINGP (coding->dst_object)
+	  char *buf = (coding->raw_destination ? (char *) coding->destination
-		       ? SSDATA (coding->dst_object)
+		       : (STRINGP (coding->dst_object)
-		       : (char *) BYTE_POS_ADDR (coding->dst_pos_byte));
+			  ? SSDATA (coding->dst_object)
+			  : (char *) BYTE_POS_ADDR (coding->dst_pos_byte)));
 	  coding->produced -= emacs_write_sig (desc, buf, coding->produced);
 
+	  if (coding->raw_destination)
+	    {
+	      /* We're responsible for freeing this, see
+		 encode_coding_object to check why.  */
+	      xfree (coding->destination);
+	      coding->raw_destination = 0;
+	    }
 	  if (coding->produced)
 	    return 0;
 	}