Use copy_file_range to copy files

The copy_file_range syscall (introduced in Linux kernel
version 4.5) can copy files more efficiently via server-side
copy etc.
* admin/merge-gnulib (GNULIB_MODULES): Add copy-file-range.
* lib/copy-file-range.c, m4/copy-file-range.m4:
New files, copied from Gnulib.
* lib/gnulib.mk.in, m4/gnulib-comp.m4: Regenerate.
* src/fileio.c (Fcopy_file): Try copy_file_range first,
falling back on read+write only if copy_file_range failed or
if the input is empty and so could be a /proc file.

6 files changed, 123 insertions, 9 deletions

diff --git a/admin/merge-gnulib b/admin/merge-gnulib
index 4a69310d83c..c2d9291b843 100755
--- a/admin/merge-gnulib
+++ b/admin/merge-gnulib
@@ -27,7 +27,7 @@ GNULIB_URL=git://git.savannah.gnu.org/gnulib.git
 
 GNULIB_MODULES='
   alloca-opt binary-io byteswap c-ctype c-strcase
-  careadlinkat close-stream
+  careadlinkat close-stream copy-file-range
   count-leading-zeros count-one-bits count-trailing-zeros
   crypto/md5-buffer crypto/sha1-buffer crypto/sha256-buffer crypto/sha512-buffer
   d-type diffseq dosname dtoastr dtotimespec dup2
diff --git a/lib/copy-file-range.c b/lib/copy-file-range.c
new file mode 100644
index 00000000000..39b5efbc1cc
--- /dev/null
+++ b/lib/copy-file-range.c
@@ -0,0 +1,33 @@
+/* Stub for copy_file_range
+   Copyright 2019 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#include <config.h>
+
+#include <unistd.h>
+
+#include <errno.h>
+
+ssize_t
+copy_file_range (int infd, off_t *pinoff,
+                 int outfd, off_t *poutoff,
+                 size_t length, unsigned int flags)
+{
+  /* There is little need to emulate copy_file_range with read+write,
+     since programs that use copy_file_range must fall back on
+     read+write anyway.  */
+  errno = ENOSYS;
+  return -1;
+}
diff --git a/lib/gnulib.mk.in b/lib/gnulib.mk.in
index 403d83829cd..6e817fa6897 100644
--- a/lib/gnulib.mk.in
+++ b/lib/gnulib.mk.in
@@ -74,6 +74,7 @@
 #  c-strcase \
 #  careadlinkat \
 #  close-stream \
+#  copy-file-range \
 #  count-leading-zeros \
 #  count-one-bits \
 #  count-trailing-zeros \
@@ -1274,6 +1275,17 @@ EXTRA_DIST += close-stream.h
 endif
 ## end   gnulib module close-stream
 
+## begin gnulib module copy-file-range
+ifeq (,$(OMIT_GNULIB_MODULE_copy-file-range))
+
+
+EXTRA_DIST += copy-file-range.c
+
+EXTRA_libgnu_a_SOURCES += copy-file-range.c
+
+endif
+## end   gnulib module copy-file-range
+
 ## begin gnulib module count-leading-zeros
 ifeq (,$(OMIT_GNULIB_MODULE_count-leading-zeros))
 
diff --git a/m4/copy-file-range.m4 b/m4/copy-file-range.m4
new file mode 100644
index 00000000000..20fd05697fe
--- /dev/null
+++ b/m4/copy-file-range.m4
@@ -0,0 +1,36 @@
+# copy-file-range.m4
+dnl Copyright 2019 Free Software Foundation, Inc.
+dnl This file is free software; the Free Software Foundation
+dnl gives unlimited permission to copy and/or distribute it,
+dnl with or without modifications, as long as this notice is preserved.
+
+AC_DEFUN([gl_FUNC_COPY_FILE_RANGE],
+[
+  AC_REQUIRE([gl_UNISTD_H_DEFAULTS])
+
+  dnl Persuade glibc <unistd.h> to declare copy_file_range.
+  AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
+
+  dnl Use AC_LINK_IFELSE, rather than AC_CHECK_FUNCS or a variant,
+  dnl since we don't want AC_CHECK_FUNCS's checks for glibc stubs.
+  dnl Programs that use copy_file_range must fall back on read+write
+  dnl anyway, and there's little point to substituting the Gnulib stub
+  dnl for a glibc stub.
+  AC_CACHE_CHECK([for copy_file_range], [gl_cv_func_copy_file_range],
+    [AC_LINK_IFELSE(
+       [AC_LANG_PROGRAM(
+          [[#include <unistd.h>
+          ]],
+          [[ssize_t (*func) (int, off_t *, int, off_t, size_t, unsigned)
+              = copy_file_range;
+            return func (0, 0, 0, 0, 0, 0) & 127;
+          ]])
+       ],
+       [gl_cv_func_copy_file_range=yes],
+       [gl_cv_func_copy_file_range=no])
+    ])
+
+  if test "$gl_cv_func_copy_file_range" != yes; then
+    HAVE_COPY_FILE_RANGE=0
+  fi
+])
diff --git a/m4/gnulib-comp.m4 b/m4/gnulib-comp.m4
index 0a7a30e5ae2..4627575bf63 100644
--- a/m4/gnulib-comp.m4
+++ b/m4/gnulib-comp.m4
@@ -56,6 +56,7 @@ AC_DEFUN([gl_EARLY],
   # Code from module clock-time:
   # Code from module cloexec:
   # Code from module close-stream:
+  # Code from module copy-file-range:
   # Code from module count-leading-zeros:
   # Code from module count-one-bits:
   # Code from module count-trailing-zeros:
@@ -201,6 +202,11 @@ AC_DEFUN([gl_INIT],
   AC_CHECK_FUNCS_ONCE([readlinkat])
   gl_CLOCK_TIME
   gl_MODULE_INDICATOR([close-stream])
+  gl_FUNC_COPY_FILE_RANGE
+  if test $HAVE_COPY_FILE_RANGE = 0; then
+    AC_LIBOBJ([copy-file-range])
+  fi
+  gl_UNISTD_MODULE_INDICATOR([copy-file-range])
   gl_COUNT_LEADING_ZEROS
   gl_COUNT_ONE_BITS
   gl_COUNT_TRAILING_ZEROS
@@ -846,6 +852,7 @@ AC_DEFUN([gl_FILE_LIST], [
   lib/cloexec.h
   lib/close-stream.c
   lib/close-stream.h
+  lib/copy-file-range.c
   lib/count-leading-zeros.c
   lib/count-leading-zeros.h
   lib/count-one-bits.c
@@ -995,6 +1002,7 @@ AC_DEFUN([gl_FILE_LIST], [
   m4/builtin-expect.m4
   m4/byteswap.m4
   m4/clock_time.m4
+  m4/copy-file-range.m4
   m4/count-leading-zeros.m4
   m4/count-one-bits.m4
   m4/count-trailing-zeros.m4
diff --git a/src/fileio.c b/src/fileio.c
index 9e9779967dd..f7376ce74fe 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -2117,14 +2117,39 @@ permissions.  */)
     newsize = st.st_size;
   else
     {
-      char buf[MAX_ALLOCA];
-      ptrdiff_t n;
-      for (newsize = 0; 0 < (n = emacs_read_quit (ifd, buf, sizeof buf));
-           newsize += n)
-        if (emacs_write_quit (ofd, buf, n) != n)
-          report_file_error ("Write error", newname);
-      if (n < 0)
-        report_file_error ("Read error", file);
+      off_t insize = st.st_size;
+      ssize_t copied;
+
+      for (newsize = 0; newsize < insize; newsize += copied)
+        {
+          /* Copy at most COPY_MAX bytes at a time; this is min
+             (PTRDIFF_MAX, SIZE_MAX) truncated to a value that is
+             surely aligned well.  */
+          ptrdiff_t copy_max = min (PTRDIFF_MAX, SIZE_MAX) >> 30 << 30;
+          off_t intail = insize - newsize;
+          ptrdiff_t len = min (intail, copy_max);
+          copied = copy_file_range (ifd, NULL, ofd, NULL, len, 0);
+          if (copied <= 0)
+            break;
+          maybe_quit ();
+        }
+
+      /* Fall back on read+write if copy_file_range failed, or if the
+         input is empty and so could be a /proc file.  read+write will
+         either succeed, or report an error more precisely than
+         copy_file_range would.  */
+      if (newsize != insize || insize == 0)
+        {
+          char buf[MAX_ALLOCA];
+          for (; (copied = emacs_read_quit (ifd, buf, sizeof buf));
+               newsize += copied)
+            {
+              if (copied < 0)
+                report_file_error ("Read error", file);
+              if (emacs_write_quit (ofd, buf, copied) != copied)
+                report_file_error ("Write error", newname);
+            }
+        }
     }
 
   /* Truncate any existing output file after writing the data.  This