[PATCH 4/8] add copy_file_range syscall support

[David Disseldorp]

From: Luis Henriques <lhenriques@suse.de>

This helper function uses the copy_file_range(2) Linux syscall for
in-kernel transfer between two file descriptors. Copy-on-write
enabled filesystems may optimize I/O by using reflinks.

Signed-off-by: David Disseldorp <ddiss@suse.de>
---
 configure.ac |  6 ++++++
 src/extern.h |  4 ++++
 src/util.c   | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 60 insertions(+)

diff --git a/configure.ac b/configure.ac
index 0a35e4c..ed6e3d2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -51,6 +51,12 @@ AC_COMPILE_CHECK_RETTYPE([minor], [0])
 AC_CHECK_FUNCS([fchmod fchown])
 # This is needed for mingw build
 AC_CHECK_FUNCS([setmode getpwuid getpwnam getgrgid getgrnam pipe fork getuid 
geteuid])
+AC_CHECK_FUNC([copy_file_range],
+              [AC_CHECK_SIZEOF(off_t)
+               AC_CHECK_SIZEOF(loff_t)
+               if (test "$ac_cv_sizeof_off_t" -eq "$ac_cv_sizeof_loff_t"); then
+                   AC_DEFINE(HAVE_CPIO_REFLINK, [1], [cpio --reflink support])
+               fi])
 
 # gnulib modules
 gl_INIT
diff --git a/src/extern.h b/src/extern.h
index 3b59053..6be95cf 100644
--- a/src/extern.h
+++ b/src/extern.h
@@ -171,6 +171,10 @@ void tape_toss_input (int in_des, off_t num_bytes);
 void copy_files_tape_to_disk (int in_des, int out_des, off_t num_bytes);
 void copy_files_disk_to_tape (int in_des, int out_des, off_t num_bytes, char 
*filename);
 void copy_files_disk_to_disk (int in_des, int out_des, off_t num_bytes, char 
*filename);
+#ifdef HAVE_CPIO_REFLINK
+ssize_t copy_files_range (int in_des, int out_des, off_t num_bytes);
+#endif
+
 void warn_if_file_changed (char *file_name, off_t old_file_size,
                            time_t old_file_mtime);
 void create_all_directories (char const *name);
diff --git a/src/util.c b/src/util.c
index d81ef30..4c0d75f 100644
--- a/src/util.c
+++ b/src/util.c
@@ -516,6 +516,56 @@ copy_files_disk_to_tape (int in_des, int out_des, off_t 
num_bytes,
       in_buff += size;
     }
 }
+
+#ifdef HAVE_CPIO_REFLINK
+/*
+ * Use the copy_file_range(2) syscall for in-kernel transfer between two file
+ * descriptors. Copy-on-write enabled filesystems may optimize I/O by using
+ * reflinks.
+ */
+ssize_t
+copy_files_range (int in_des, int out_des, off_t num_bytes)
+{
+  loff_t in_off = 0, out_off = 0;
+  ssize_t total = 0;
+  ssize_t ret;
+
+  in_off = lseek (in_des, 0, SEEK_CUR);
+  if (in_off < 0)
+    return -errno;
+
+  out_off = lseek (out_des, 0, SEEK_CUR);
+  if (out_off < 0)
+    return -errno;
+
+  while (total < num_bytes)
+    {
+      ret = copy_file_range (in_des, &in_off, out_des, &out_off,
+                            num_bytes - total, 0);
+      /* simplify read/write fallback: don't partially seek on error */
+      if (ret == 0)
+        return -ENODATA;
+      if (ret < 0)
+       return -errno;
+      total += ret;
+    }
+
+  /* copy complete. seek to new offset */
+  in_off = lseek (in_des, in_off, SEEK_SET);
+  if (in_off < 0)
+    error(1, errno, "failed final src seek");
+
+  out_off = lseek (out_des, out_off, SEEK_SET);
+  if (out_off < 0)
+    error(1, errno, "failed final dst seek");
+
+  output_bytes += total;
+  input_bytes += total;
+
+  return total;
+}
+#endif /* HAVE_CPIO_REFLINK */
+
 /* Copy a file using the input and output buffers, which may start out
    partly full.  After the copy, the files are not closed nor the last
    block flushed to output, and the input buffer may still be partly
-- 
2.26.2