cp: accept the --reflink option

* NEWS: Mention it.
* doc/coreutils.texi (cp invocation): Describe it.
* src/copy.h (struct cp_options) [reflink]: New member.
* src/copy.c (usage): Describe it.
(copy_reg): If reflink is true try to clone the file.
(main): Check for --reflink.
(cp_option_init): Initialize the new member.
* src/install.c (cp_option_init): Initialize the new member.
* src/mv.c (cp_option_init): Likewise.
* tests/cp/sparse: Add a new test case.

8 files changed, 46 insertions, 6 deletions

diff --git a/NEWS b/NEWS
index c61f6667b..6df0d65a6 100644
--- a/NEWS
+++ b/NEWS
@@ -41,6 +41,10 @@ GNU coreutils NEWS                                    -*- outline -*-
 
   chroot now accepts the options --userspec and --groups.
 
+  cp accepts a new option, --reflink: create a lightweight copy
+  using copy-on-write (COW).  This is currently supported only on
+  btrfs file systems.
+
   cp now preserves time stamps on symbolic links, when possible
 
   cp, install, mv: take advantage of btrfs' O(1) copy-on-write feature
diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 90a54b268..bb1d87a87 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -7543,6 +7543,15 @@ Also, it is not portable to use @option{-R} to copy symbolic links
 unless you also specify @option{-P}, as @acronym{POSIX} allows
 implementations that dereference symbolic links by default.
 
+@item --reflink
+@opindex --reflink
+Perform a lightweight, copy-on-write (COW) copy.
+Copying with this option can succeed only on some relatively new file systems.
+Once it has succeeded, beware that the source and destination files
+share the same disk data blocks as long as they remain unmodified.
+Thus, if a disk I/O error affects data blocks of one of the files,
+the other suffers the exact same fate.
+
 @item --remove-destination
 @opindex --remove-destination
 Remove each existing destination file before attempting to open it
diff --git a/src/copy.c b/src/copy.c
index 24b5f6b46..bed90c459 100644
--- a/src/copy.c
+++ b/src/copy.c
@@ -624,13 +624,16 @@ copy_reg (char const *src_name, char const *dst_name,
       goto close_src_and_dst_desc;
     }
 
-  /* If --sparse=auto is in effect, attempt a btrfs clone operation.
-     If the operation is not supported or it fails then copy the file
-     in the usual way.  */
-  bool copied = (x->sparse_mode == SPARSE_AUTO
-                 && clone_file (dest_desc, source_desc) == 0);
+  if (x->reflink)
+    {
+      if (clone_file (dest_desc, source_desc))
+        {
+          error (0, errno, _("failed to clone %s"), quote (dst_name));
+          return_val = false;
+        }
+      goto close_src_and_dst_desc;
+    }
 
-  if (!copied)
   {
     typedef uintptr_t word;
     off_t n_read_total = 0;
@@ -2232,6 +2235,7 @@ valid_options (const struct cp_options *co)
   assert (VALID_BACKUP_TYPE (co->backup_type));
   assert (VALID_SPARSE_MODE (co->sparse_mode));
   assert (!(co->hard_link && co->symbolic_link));
+  assert (!(co->reflink && co->sparse_mode != SPARSE_AUTO));
   return true;
 }
 
diff --git a/src/copy.h b/src/copy.h
index 8e0b4080e..ddf4f4e2c 100644
--- a/src/copy.h
+++ b/src/copy.h
@@ -219,6 +219,9 @@ struct cp_options
      such a symlink) and returns false.  */
   bool open_dangling_dest_symlink;
 
+  /* If true, attempt to clone the file instead of copying it.  */
+  bool reflink;
+
   /* This is a set of destination name/inode/dev triples.  Each such triple
      represents a file we have created corresponding to a source file name
      that was specified on the command line.  Use it to avoid clobbering
diff --git a/src/cp.c b/src/cp.c
index 878507616..c99aff311 100644
--- a/src/cp.c
+++ b/src/cp.c
@@ -76,6 +76,7 @@ enum
   NO_PRESERVE_ATTRIBUTES_OPTION,
   PARENTS_OPTION,
   PRESERVE_ATTRIBUTES_OPTION,
+  REFLINK_OPTION,
   SPARSE_OPTION,
   STRIP_TRAILING_SLASHES_OPTION,
   UNLINK_DEST_BEFORE_OPENING
@@ -121,6 +122,7 @@ static struct option const long_opts[] =
   {"recursive", no_argument, NULL, 'R'},
   {"remove-destination", no_argument, NULL, UNLINK_DEST_BEFORE_OPENING},
   {"sparse", required_argument, NULL, SPARSE_OPTION},
+  {"reflink", no_argument, NULL, REFLINK_OPTION},
   {"strip-trailing-slashes", no_argument, NULL, STRIP_TRAILING_SLASHES_OPTION},
   {"suffix", required_argument, NULL, 'S'},
   {"symbolic-link", no_argument, NULL, 's'},
@@ -190,6 +192,7 @@ Mandatory arguments to long options are mandatory for short options too.\n\
 "), stdout);
       fputs (_("\
   -R, -r, --recursive          copy directories recursively\n\
+      --reflink                perform a lightweight (CoW/clone) copy\n\
       --remove-destination     remove each existing destination file before\n\
                                  attempting to open it (contrast with --force)\n\
 "), stdout);
@@ -752,6 +755,7 @@ cp_option_init (struct cp_options *x)
   x->interactive = I_UNSPECIFIED;
   x->move_mode = false;
   x->one_file_system = false;
+  x->reflink = false;
 
   x->preserve_ownership = false;
   x->preserve_links = false;
@@ -916,6 +920,10 @@ main (int argc, char **argv)
 				     sparse_type_string, sparse_type);
 	  break;
 
+	case REFLINK_OPTION:
+	  x.reflink = true;
+	  break;
+
 	case 'a':		/* Like -dR --preserve=all with reduced failure diagnostics. */
 	  x.dereference = DEREF_NEVER;
 	  x.preserve_links = true;
@@ -1076,6 +1084,12 @@ main (int argc, char **argv)
       usage (EXIT_FAILURE);
     }
 
+  if (x.reflink && x.sparse_mode != SPARSE_AUTO)
+    {
+      error (0, 0, _("--reflink can be used only with --sparse=auto"));
+      usage (EXIT_FAILURE);
+    }
+
   if (backup_suffix_string)
     simple_backup_suffix = xstrdup (backup_suffix_string);
 
diff --git a/src/install.c b/src/install.c
index fd8f71ef3..73b3981ef 100644
--- a/src/install.c
+++ b/src/install.c
@@ -269,6 +269,7 @@ cp_option_init (struct cp_options *x)
 {
   cp_options_default (x);
   x->copy_as_regular = true;
+  x->reflink = false;
   x->dereference = DEREF_ALWAYS;
   x->unlink_dest_before_opening = true;
   x->unlink_dest_after_failed_open = false;
diff --git a/src/mv.c b/src/mv.c
index 8b9b6a111..8d77380ac 100644
--- a/src/mv.c
+++ b/src/mv.c
@@ -105,6 +105,7 @@ cp_option_init (struct cp_options *x)
 
   cp_options_default (x);
   x->copy_as_regular = false;  /* FIXME: maybe make this an option */
+  x->reflink = false;
   x->dereference = DEREF_NEVER;
   x->unlink_dest_before_opening = false;
   x->unlink_dest_after_failed_open = false;
diff --git a/tests/cp/sparse b/tests/cp/sparse
index bd1a84f40..9683a9e33 100755
--- a/tests/cp/sparse
+++ b/tests/cp/sparse
@@ -38,4 +38,8 @@ cp --sparse=always sparse copy || fail=1
 # Ensure that the copy has the same block count as the original.
 test `stat --printf %b copy` -le `stat --printf %b sparse` || fail=1
 
+# Ensure that --sparse={always,never} with --reflink fail.
+cp --sparse=always --reflink sparse copy && fail=1
+cp --sparse=never --reflink sparse copy && fail=1
+
 Exit $fail