Subject: bug#68504: [PATCH v2] Add copy-on-write support to scm_copy_file.

From: Tomas Volf <[email-masked]>
Date: Wed, 24 Jan 2024 20:10:32 +0100

On modern file-systems (BTRFS, ZFS) it is possible to copy a file using
copy-on-write method.  For large files it has the advantage of being
much faster and saving disk space (since identical extents are not
duplicated).  This feature is stable and for example coreutils' `cp'
does use it automatically (see --reflink).

This commit adds support for this feature into our
copy-file (scm_copy_file) procedure.  Same as `cp', it defaults to
'auto, meaning the copy-on-write is attempted, and in case of failure
the regular copy is performed.

No tests are provided, because the behavior depends on the system,
underlying file-system and its configuration.  That makes it challenging
to write a test for it.  Manual testing was performed instead:

    $ btrfs filesystem du /tmp/cow*
         Total   Exclusive  Set shared  Filename
      36.00KiB    36.00KiB       0.00B  /tmp/cow

    $ cat cow-test.scm
    (copy-file "/tmp/cow" "/tmp/cow-unspecified")
    (copy-file "/tmp/cow" "/tmp/cow-always" #:copy-on-write 'always)
    (copy-file "/tmp/cow" "/tmp/cow-auto" #:copy-on-write 'auto)
    (copy-file "/tmp/cow" "/tmp/cow-never" #:copy-on-write 'never)
    (copy-file "/tmp/cow" "/dev/shm/cow-unspecified")
    (copy-file "/tmp/cow" "/dev/shm/cow-auto" #:copy-on-write 'auto)
    (copy-file "/tmp/cow" "/dev/shm/cow-never" #:copy-on-write 'never)
    $ ./meta/guile -s cow-test.scm

    $ btrfs filesystem du /tmp/cow*
         Total   Exclusive  Set shared  Filename
      36.00KiB       0.00B    36.00KiB  /tmp/cow
      36.00KiB       0.00B    36.00KiB  /tmp/cow-always
      36.00KiB       0.00B    36.00KiB  /tmp/cow-auto
      36.00KiB    36.00KiB       0.00B  /tmp/cow-never
      36.00KiB       0.00B    36.00KiB  /tmp/cow-unspecified

    $ sha1sum /tmp/cow* /dev/shm/cow*
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /tmp/cow
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /tmp/cow-always
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /tmp/cow-auto
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /tmp/cow-never
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /tmp/cow-unspecified
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /dev/shm/cow-auto
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /dev/shm/cow-never
    4c665f87b5dc2e7d26279c4b48968d085e1ace32  /dev/shm/cow-unspecified

This commit also adds to new failure modes for (copy-file).

Failure to copy-on-write when 'always was passed in:

    scheme@(guile-user)> (copy-file "/tmp/cow" "/dev/shm/cow" #:copy-on-write 'always)
    ice-9/boot-9.scm:1676:22: In procedure raise-exception:
    In procedure copy-file: copy-on-write failed: Invalid cross-device link

Passing in invalid value for the #:copy-on-write keyword argument:

    scheme@(guile-user)> (copy-file "/tmp/cow" "/dev/shm/cow" #:copy-on-write 'nevr)
    ice-9/boot-9.scm:1676:22: In procedure raise-exception:
    In procedure copy-file: invalid value for #:copy-on-write: nevr

* NEWS: Add note for copy-file supporting copy-on-write.
* configure.ac: Check for linux/fs.h.
* doc/ref/posix.texi (File System)[copy-file]: Document the new
signature.
* libguile/filesys.c (clone_file): New function cloning a file using
FICLONE, if supported.
(k_copy_on_write): New keyword.
(sym_always, sym_auto, sym_never): New symbols.
(scm_copy_file2): Renamed from scm_copy_file.  New #:copy-on-write
keyword argument.  Attempt copy-on-write copy by default.
(scm_copy_file): Call scm_copy_file2.
* libguile/filesys.h: Add scm_copy_file2 as SCM_INTERNAL.
---
v2: Introduce scm_copy_file2 in order to preserve backwards compatibility.

 NEWS               |  9 +++++
 configure.ac       |  1 +
 doc/ref/posix.texi |  9 ++++-
 libguile/filesys.c | 82 +++++++++++++++++++++++++++++++++++++++-------
 libguile/filesys.h |  1 +
 5 files changed, 89 insertions(+), 13 deletions(-)

diff --git a/NEWS b/NEWS
index b319404d7..9147098c9 100644
--- a/NEWS
+++ b/NEWS
@@ -21,6 +21,15 @@ definitely unused---this is notably the case for modules that are only
 used at macro-expansion time, such as (srfi srfi-26).  In those cases,
 the compiler reports it as "possibly unused".

+** copy-file now supports copy-on-write
+
+The copy-file procedure now takes an additional keyword argument,
+#:copy-on-write, specifying whether copy-on-write should be done, if the
+underlying file-system supports it.  Possible values are 'always, 'auto
+and 'never, with 'auto being the default.
+
+This speeds up copying large files a lot while saving the disk space.
+
 * Bug fixes

 ** (ice-9 suspendable-ports) incorrect UTF-8 decoding
diff --git a/configure.ac b/configure.ac
index d0a2dc79b..c46586e9b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -418,6 +418,7 @@ AC_SUBST([SCM_I_GSC_HAVE_STRUCT_DIRENT64])
 #   sys/sendfile.h - non-POSIX, found in glibc
 #
 AC_CHECK_HEADERS([complex.h fenv.h io.h memory.h process.h \
+linux/fs.h \
 sys/dir.h sys/ioctl.h sys/select.h \
 sys/time.h sys/timeb.h sys/times.h sys/stdtypes.h sys/types.h \
 sys/utime.h unistd.h utime.h pwd.h grp.h sys/utsname.h \
diff --git a/doc/ref/posix.texi b/doc/ref/posix.texi
index fec42d061..d26808d91 100644
--- a/doc/ref/posix.texi
+++ b/doc/ref/posix.texi
@@ -896,10 +896,17 @@ of @code{delete-file}.  Why doesn't POSIX have a @code{rmdirat} function
 for this instead?  No idea!
 @end deffn

-@deffn {Scheme Procedure} copy-file oldfile newfile
+@deffn {Scheme Procedure} copy-file @var{oldfile} @var{newfile} @
+       [#:copy-on-write='auto]
 @deffnx {C Function} scm_copy_file (oldfile, newfile)
 Copy the file specified by @var{oldfile} to @var{newfile}.
 The return value is unspecified.
+
+@code{#:copy-on-write} keyword argument determines whether copy-on-write
+copy should be attempted and the behavior in case of failure.  Possible
+values are @code{'always} (attempt the copy-on-write, return error if it
+fails), @code{'auto} (attempt the copy-on-write, fallback to regular
+copy if it fails) and @code{'never} (perform the regular copy).
 @end deffn

 @deffn {Scheme Procedure} sendfile out in count [offset]
diff --git a/libguile/filesys.c b/libguile/filesys.c
index 1f0bba556..5be42b825 100644
--- a/libguile/filesys.c
+++ b/libguile/filesys.c
@@ -67,6 +67,11 @@
 # include <sys/sendfile.h>
 #endif

+#if defined(HAVE_SYS_IOCTL_H) && defined(HAVE_LINUX_FS_H)
+# include <linux/fs.h>
+# include <sys/ioctl.h>
+#endif
+
 #include "async.h"
 #include "boolean.h"
 #include "dynwind.h"
@@ -75,6 +80,7 @@
 #include "fports.h"
 #include "gsubr.h"
 #include "iselect.h"
+#include "keywords.h"
 #include "list.h"
 #include "load.h"	/* for scm_i_mirror_backslashes */
 #include "modules.h"
@@ -1255,20 +1261,49 @@ SCM_DEFINE (scm_readlink, "readlink", 1, 0, 0,
 }
 #undef FUNC_NAME

-SCM_DEFINE (scm_copy_file, "copy-file", 2, 0, 0,
-            (SCM oldfile, SCM newfile),
+static int
+clone_file (int oldfd, int newfd)
+{
+#ifdef FICLONE
+  return ioctl (newfd, FICLONE, oldfd);
+#else
+  (void)oldfd;
+  (void)newfd;
+  errno = EOPNOTSUPP;
+  return -1;
+#endif
+}
+
+SCM_KEYWORD (k_copy_on_write, "copy-on-write");
+SCM_SYMBOL (sym_always, "always");
+SCM_SYMBOL (sym_auto, "auto");
+SCM_SYMBOL (sym_never, "never");
+
+SCM_DEFINE (scm_copy_file2, "copy-file", 2, 0, 1,
+            (SCM oldfile, SCM newfile, SCM rest),
 	    "Copy the file specified by @var{oldfile} to @var{newfile}.\n"
-	    "The return value is unspecified.")
-#define FUNC_NAME s_scm_copy_file
+	    "The return value is unspecified.\n"
+            "\n"
+            "@code{#:copy-on-write} keyword argument determines whether "
+            "copy-on-write copy should be attempted and the "
+            "behavior in case of failure.  Possible values are "
+            "@code{'always} (attempt the copy-on-write, return error if "
+            "it fails), @code{'auto} (attempt the copy-on-write, "
+            "fallback to regular copy if it fails) and @code{'never} "
+            "(perform the regular copy)."
+            )
+#define FUNC_NAME s_scm_copy_file2
 {
   char *c_oldfile, *c_newfile;
   int oldfd, newfd;
   int n, rv;
+  SCM cow = sym_auto;
+  int clone_res;
   char buf[BUFSIZ];
   struct stat_or_stat64 oldstat;

   scm_dynwind_begin (0);
-
+
   c_oldfile = scm_to_locale_string (oldfile);
   scm_dynwind_free (c_oldfile);
   c_newfile = scm_to_locale_string (newfile);
@@ -1292,13 +1327,30 @@ SCM_DEFINE (scm_copy_file, "copy-file", 2, 0, 0,
       SCM_SYSERROR;
     }

-  while ((n = read (oldfd, buf, sizeof buf)) > 0)
-    if (write (newfd, buf, n) != n)
-      {
-	close (oldfd);
-	close (newfd);
-	SCM_SYSERROR;
-      }
+  scm_c_bind_keyword_arguments ("copy-file", rest, 0,
+                                k_copy_on_write, &cow,
+                                SCM_UNDEFINED);
+
+  if (scm_is_eq (cow, sym_always) || scm_is_eq (cow, sym_auto))
+    clone_res = clone_file(oldfd, newfd);
+  else if (scm_is_eq (cow, sym_never))
+    clone_res = -1;
+  else
+    scm_misc_error ("copy-file",
+                    "invalid value for #:copy-on-write: ~S",
+                    scm_list_1 (cow));
+
+  if (scm_is_eq (cow, sym_always) && clone_res)
+    scm_syserror ("copy-file: copy-on-write failed");
+
+  if (clone_res)
+    while ((n = read (oldfd, buf, sizeof buf)) > 0)
+      if (write (newfd, buf, n) != n)
+        {
+          close (oldfd);
+          close (newfd);
+          SCM_SYSERROR;
+        }
   close (oldfd);
   if (close (newfd) == -1)
     SCM_SYSERROR;
@@ -1308,6 +1360,12 @@ SCM_DEFINE (scm_copy_file, "copy-file", 2, 0, 0,
 }
 #undef FUNC_NAME

+SCM
+scm_copy_file (SCM oldfile, SCM newfile)
+{
+  return scm_copy_file2 (oldfile, newfile, SCM_UNSPECIFIED);
+}
+
 SCM_DEFINE (scm_sendfile, "sendfile", 3, 1, 0,
 	    (SCM out, SCM in, SCM count, SCM offset),
 	    "Send @var{count} bytes from @var{in} to @var{out}, both of which "
diff --git a/libguile/filesys.h b/libguile/filesys.h
index 1ce50d30e..8e849fe7a 100644
--- a/libguile/filesys.h
+++ b/libguile/filesys.h
@@ -74,6 +74,7 @@ SCM_API SCM scm_symlinkat (SCM dir, SCM oldpath, SCM newpath);
 SCM_API SCM scm_readlink (SCM path);
 SCM_API SCM scm_lstat (SCM str);
 SCM_API SCM scm_copy_file (SCM oldfile, SCM newfile);
+SCM_INTERNAL SCM scm_copy_file2 (SCM oldfile, SCM newfile, SCM rest);
 SCM_API SCM scm_mkstemp (SCM tmpl);
 SCM_API SCM scm_mkdtemp (SCM tmpl);
 SCM_API SCM scm_dirname (SCM filename);
--
2.41.0