>From 9c60f6f603563c335737102fc54609b837f5d02a Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Wed, 6 Jul 2016 12:44:46 +0200
Subject: [PATCH] copy-file now uses GNU/Linux file cloning

>From a suggestion by Kieran Colford (see Bug#23904).
* configure.ac: Check for linux/fs.h.
* src/fileio.c [HAVE_LINUX_FS_H]: Include sys/ioctl.h and linux/fs.h.
(clone_file_range): New function.
(Fcopy_file): Use it.
---
 configure.ac |  1 +
 src/fileio.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++-------------
 2 files changed, 50 insertions(+), 13 deletions(-)

diff --git a/configure.ac b/configure.ac
index aaddfcd..1798fa3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1636,6 +1636,7 @@ AC_DEFUN
 
 dnl checks for header files
 AC_CHECK_HEADERS_ONCE(
+  linux/fs.h
   malloc.h
   sys/systeminfo.h
   sys/sysinfo.h
diff --git a/src/fileio.c b/src/fileio.c
index b1f9d3c..f980247 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -52,6 +52,11 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "region-cache.h"
 #include "frame.h"
 
+#ifdef HAVE_LINUX_FS_H
+# include <sys/ioctl.h>
+# include <linux/fs.h>
+#endif
+
 #ifdef WINDOWSNT
 #define NOMINMAX 1
 #include <windows.h>
@@ -1828,6 +1833,23 @@ barf_or_query_if_file_exists (Lisp_Object absname, bool known_to_exist,
     }
 }
 
+/* Copy data to DEST from SOURCE if possible, starting at byte offset
+   OFF and continuing for LENGTH bytes.  Return true if OK.  */
+static bool
+clone_file_range (int dest, int source, off_t off, off_t length)
+{
+#ifdef FICLONERANGE
+  /* FIXME: Is the ioctl interruptible?  If so, what happens if the
+     ioctl is interrupted by a signal?  Might it partially copy the
+     range?  */
+  struct file_clone_range range = { src_fd: source, src_offset: off,
+				    src_length: length, dest_offset: off };
+  if (ioctl (dest, FICLONERANGE, &range) == 0)
+    return true;
+#endif
+  return false;
+}
+
 DEFUN ("copy-file", Fcopy_file, Scopy_file, 2, 6,
        "fCopy file: \nGCopy %s to file: \np\nP",
        doc: /* Copy FILE to NEWNAME.  Both args must be strings.
@@ -1974,7 +1996,7 @@ permissions.  */)
 
   record_unwind_protect_int (close_file_unwind, ofd);
 
-  off_t oldsize = 0, newsize = 0;
+  off_t oldsize = 0, newsize;
 
   if (already_exists)
     {
@@ -1990,18 +2012,32 @@ permissions.  */)
 
   immediate_quit = 1;
   QUIT;
-  while (true)
-    {
-      char buf[MAX_ALLOCA];
-      ptrdiff_t n = emacs_read (ifd, buf, sizeof buf);
-      if (n < 0)
-	report_file_error ("Read error", file);
-      if (n == 0)
-	break;
-      if (emacs_write_sig (ofd, buf, n) != n)
-	report_file_error ("Write error", newname);
-      newsize += n;
-    }
+
+  off_t insize = st.st_size;
+  /* FIXME: is 1 GiB a good number?  */
+  off_t max_clone_chunk_size = 1 << 30;
+  off_t chunk_size = min (insize, max_clone_chunk_size);
+
+  if (clone_file_range (ofd, ifd, 0, chunk_size))
+    for (newsize = chunk_size; newsize < insize; newsize += chunk_size)
+      {
+	chunk_size = min (insize - newsize, max_clone_chunk_size);
+	if (! clone_file_range (ofd, ifd, newsize, chunk_size))
+	  report_file_error ("Clone error", file);
+      }
+  else
+    for (newsize = 0; ; newsize += chunk_size)
+      {
+	char buf[MAX_ALLOCA];
+	ptrdiff_t n = emacs_read (ifd, buf, sizeof buf);
+	if (n < 0)
+	  report_file_error ("Read error", file);
+	if (n == 0)
+	  break;
+	if (emacs_write_sig (ofd, buf, n) != n)
+	  report_file_error ("Write error", newname);
+	chunk_size = n;
+      }
 
   /* Truncate any existing output file after writing the data.  This
      is more likely to work than truncation before writing, if the
-- 
2.5.5