bug#23904: Btrfs clone support in copy operations

[Paul Eggert <eggert@cs.ucla.edu>]

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Tags: patch

The attached proposed patch to Emacs master builds on a suggestion by 
Kieran Colford. Kieran, can you please comment on its two FIXMEs?

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From 4b2a532a87ec2a21afc205b83db4a779664ccd20 Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Wed, 6 Jul 2016 02:42:58 +0200
Subject: [PATCH] copy-file now uses GNU/Linux file cloning

From a suggestion by Kieran Colford in:
http://lists.gnu.org/archive/html/emacs-devel/2016-07/msg00191.html
* configure.ac: Check for linux/fs.h.
* src/fileio.c [HAVE_LINUX_FS_H]: Include sys/ioctl.h and linux/fs.h.
(clone_file): New function.
(Fcopy_file): Use it.
---
 configure.ac |  1 +
 src/fileio.c | 64 ++++++++++++++++++++++++++++++++++++++++++------------------
 2 files changed, 46 insertions(+), 19 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..c2e3a18 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 +1833,24 @@ barf_or_query_if_file_exists (Lisp_Object absname, bool known_to_exist,
     }
 }
 
+/* Copy all data to DEST from SOURCE if possible.  Return true if OK.  */
+static bool
+clone_file (int dest, int source)
+{
+#ifdef FICLONE
+  /* FIXME: Might this ioctl take a long time if the file is very
+     large?  Is the ioctl interruptible?  If so, what happens if the
+     ioctl is interrupted by a signal?  Might it partially copy the
+     file?  */
+  /* FIXME: Does this ioctl truncate the output file to be the same
+     size as the input file, if the output file already exists and is
+     larger than the input file?  */
+  if (ioctl (dest, FICLONE, source) == 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.
@@ -1988,28 +2011,31 @@ permissions.  */)
 	oldsize = out_st.st_size;
     }
 
-  immediate_quit = 1;
-  QUIT;
-  while (true)
+  if (! clone_file (ofd, ifd))
     {
-      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;
-    }
+      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;
+	}
 
-  /* Truncate any existing output file after writing the data.  This
-     is more likely to work than truncation before writing, if the
-     file system is out of space or the user is over disk quota.  */
-  if (newsize < oldsize && ftruncate (ofd, newsize) != 0)
-    report_file_error ("Truncating output file", newname);
+      /* Truncate any existing output file after writing the data.  This
+	 is more likely to work than truncation before writing, if the
+	 file system is out of space or the user is over disk quota.  */
+      if (newsize < oldsize && ftruncate (ofd, newsize) != 0)
+	report_file_error ("Truncating output file", newname);
 
-  immediate_quit = 0;
+      immediate_quit = 0;
+    }
 
 #ifndef MSDOS
   /* Preserve the original file permissions, and if requested, also its
-- 
2.5.5