sync: support syncing specified arguments

* m4/jm-macros.m4 (coreutils_MACROS): Check for syncfs().
* man/sync.x: Add references to syncfs, fsync and fdatasync.
* doc/coreutils.texi (sync invocation): Document the new feature.
* src/sync.c: Include "quote.h".
(AUTHORS): Include myself.
(MODE_FILE, MODE_DATA, MODE_FILE_SYSTEM, MODE_SYNC): New enum values.
(long_options): Define.
(sync_arg): New function.
(usage): Describe that arguments are now accepted.
(main): Add arguments parsing and add support for fsync(2),
fdatasync(2) and syncfs(2).
* tests/misc/sync.sh: New (and only) test for sync.
* tests/local.mk: Reference the new test.
* AUTHORS: Add myself to sync's authors.
* NEWS: Mention the new feature.

AUTHORS

@@ -83,7 +83,7 @@ stat: Michael Meskes
 stdbuf: Pádraig Brady
 stty: David MacKenzie
 sum: Kayvan Aghaiepour, David MacKenzie
-sync: Jim Meyering
+sync: Jim Meyering, Giuseppe Scrivano
 tac: Jay Lepreau, David MacKenzie
 tail: Paul Rubin, David MacKenzie, Ian Lance Taylor, Jim Meyering
 tee: Mike Parker, Richard M. Stallman, David MacKenzie

NEWS

@@ -51,6 +51,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   stty allows setting the "extproc" option where supported, which is
   a useful setting with high latency links.
 
+  sync no longer ignores arguments, and syncs each specified file, or with the
+  --file-system option, the file systems associated with each specified file.
+
 ** Changes in behavior
 
   df no longer suppresses separate exports of the same remote device, as

doc/coreutils.texi

@@ -114,7 +114,7 @@
 * stdbuf: (coreutils)stdbuf invocation.         Modify stdio buffering.
 * stty: (coreutils)stty invocation.             Print/change terminal settings.
 * sum: (coreutils)sum invocation.               Print traditional checksum.
-* sync: (coreutils)sync invocation.             Synchronize memory and disk.
+* sync: (coreutils)sync invocation.             Synchronize memory to disk.
 * tac: (coreutils)tac invocation.               Reverse files.
 * tail: (coreutils)tail invocation.             Output the last part of files.
 * tee: (coreutils)tee invocation.               Redirect to multiple files.
@@ -346,7 +346,7 @@ Disk usage
 * df invocation::                Report file system disk space usage
 * du invocation::                Estimate file space usage
 * stat invocation::              Report file or file system status
-* sync invocation::              Synchronize data on disk with memory
+* sync invocation::              Synchronize cached writes to persistent storage
 * truncate invocation::          Shrink or extend the size of a file
 
 Printing text
@@ -11191,7 +11191,7 @@ file status information, and write buffers to disk.
 * df invocation::               Report file system disk space usage.
 * du invocation::               Estimate file space usage.
 * stat invocation::             Report file or file system status.
-* sync invocation::             Synchronize memory and disk.
+* sync invocation::             Synchronize cached writes to persistent storage.
 * truncate invocation::         Shrink or extend the size of a file.
 @end menu
 
@@ -12033,28 +12033,60 @@ with @env{TZ}, libc, The GNU C Library Reference Manual}.
 
 
 @node sync invocation
-@section @command{sync}: Synchronize data on disk with memory
+@section @command{sync}: Synchronize cached writes to persistent storage
 
 @pindex sync
 @cindex synchronize disk and memory
+@cindex Synchronize cached writes to persistent storage
+
+@command{sync} synchronizes in memory files or file systems to persistent
+storage.  Synopsis:
+
+@example
+sync [@var{option}] [@var{file}]@dots{}
+@end example
 
 @cindex superblock, writing
 @cindex inodes, written buffered
 @command{sync} writes any data buffered in memory out to disk.  This can
 include (but is not limited to) modified superblocks, modified inodes,
 and delayed reads and writes.  This must be implemented by the kernel;
-The @command{sync} program does nothing but exercise the @code{sync} system
-call.
+The @command{sync} program does nothing but exercise the @code{sync},
+@code{syncfs}, @code{fsync}, and @code{fdatasync} system calls.
 
 @cindex crashes and corruption
 The kernel keeps data in memory to avoid doing (relatively slow) disk
 reads and writes.  This improves performance, but if the computer
 crashes, data may be lost or the file system corrupted as a
-result.  The @command{sync} command ensures everything in memory
-is written to disk.
+result.  The @command{sync} command instructs the kernel to write
+data in memory to persistent storage.
 
-Any arguments are ignored, except for a lone @option{--help} or
-@option{--version} (@pxref{Common options}).
+If any argument is specified then only those files will be
+synchronized using the fsync(2) syscall by default.
+
+If at least one file is specified, it is possible to change the
+synchronization method with the following options.  Also see
+@ref{Common options}.
+
+@table @samp
+@item -d
+@itemx --data
+@opindex --data
+Use fdatasync(2) to sync only the data for the file,
+and any metadata required to maintain file system consistency.
+
+@item -f
+@itemx --file-system
+@opindex --file-system
+Synchronize all the I/O waiting for the file systems that contain the file,
+using the syscall syncfs(2).  Note you would usually @emph{not} specify
+this option if passing a device node like @samp{/dev/sda} for example,
+as that would sync the containing file system rather than the referenced one.
+Note also that depending on the system, passing individual device nodes or files
+may have different sync characteristics than using no arguments.
+I.E. arguments passed to fsync(2) may provide greater guarantees through
+write barriers, than a global sync(2) used when no arguments are provided.
+@end table
 
 @exitstatus
 

m4/jm-macros.m4

@@ -89,6 +89,7 @@ AC_DEFUN([coreutils_MACROS],
     sethostname
     siginterrupt
     sync
+    syncfs
     sysctl
     sysinfo
     tcgetpgrp

man/sync.x

@@ -1,6 +1,6 @@
 [NAME]
-sync \- flush file system buffers
+sync \- Synchronize cached writes to persistent storage
 [DESCRIPTION]
 .\" Add any additional description here
 [SEE ALSO]
-sync(2)
+fdatasync(2), fsync(2), sync(2), syncfs(2)

src/sync.c

@@ -17,18 +17,42 @@
 /* Written by Jim Meyering */
 
 #include <config.h>
+#include <assert.h>
 #include <getopt.h>
 #include <stdio.h>
 #include <sys/types.h>
 
 #include "system.h"
 #include "error.h"
-#include "long-options.h"
+#include "quote.h"
 
 /* The official name of this program (e.g., no 'g' prefix).  */
 #define PROGRAM_NAME "sync"
 
-#define AUTHORS proper_name ("Jim Meyering")
+#define AUTHORS                                 \
+  proper_name ("Jim Meyering"),                 \
+  proper_name ("Giuseppe Scrivano")
+
+#ifndef HAVE_SYNCFS
+# define HAVE_SYNCFS 0
+#endif
+
+enum sync_mode
+{
+  MODE_FILE,
+  MODE_DATA,
+  MODE_FILE_SYSTEM,
+  MODE_SYNC
+};
+
+static struct option const long_options[] =
+{
+  {"data", no_argument, NULL, 'd'},
+  {"file-system", no_argument, NULL, 'f'},
+  {GETOPT_HELP_OPTION_DECL},
+  {GETOPT_VERSION_OPTION_DECL},
+  {NULL, 0, NULL, 0}
+};
 
 void
 usage (int status)
@@ -37,11 +61,22 @@ usage (int status)
     emit_try_help ();
   else
     {
-      printf (_("Usage: %s [OPTION]\n"), program_name);
+      printf (_("Usage: %s [OPTION] [FILE]...\n"), program_name);
       fputs (_("\
-Force changed blocks to disk, update the super block.\n\
+Synchronize cached writes to persistent storage\n\
+\n\
+If one or more files are specified, sync only them,\n\
+or their containing file systems.\n\
 \n\
 "), stdout);
+
+      fputs (_("\
+  -d, --data             sync only file data, no unneeded metadata\n\
+"), stdout);
+      fputs (_("\
+  -f, --file-system      sync the file systems that contain the files\n\
+"), stdout);
+
       fputs (HELP_OPTION_DESCRIPTION, stdout);
       fputs (VERSION_OPTION_DESCRIPTION, stdout);
       emit_ancillary_info (PROGRAM_NAME);
@@ -49,9 +84,86 @@ Force changed blocks to disk, update the super block.\n\
   exit (status);
 }
 
+/* Sync the specified FILE, or file systems associated with FILE.
+   Return 1 on success.  */
+
+static bool
+sync_arg (enum sync_mode mode, char const *file)
+{
+  bool ret = true;
+  int fd;
+
+  /* Note O_PATH might be supported with syncfs(),
+     though as of Linux 3.18 is not.  */
+  if ((fd = open (file, O_RDONLY | O_NONBLOCK)) < 0)
+    {
+      /* Use the O_RDONLY errno, which is significant
+         with directories for example.  */
+      int rd_errno = errno;
+      if ((fd = open (file, O_WRONLY | O_NONBLOCK)) < 0)
+        error (0, rd_errno, _("error opening %s"), quote (file));
+      return false;
+    }
+
+  /* We used O_NONBLOCK above to not hang with fifos,
+     so reset that here.  */
+  int fdflags;
+  if ((fdflags = fcntl (fd, F_GETFL)) == -1
+      || fcntl (fd, F_SETFL, fdflags & ~O_NONBLOCK) < 0)
+    {
+      error (0, errno, _("couldn't reset non-blocking mode %s"), quote (file));
+      ret = false;
+    }
+
+  if (ret == true)
+    {
+      int sync_status;
+
+      switch (mode)
+        {
+        case MODE_DATA:
+          sync_status = fdatasync (fd);
+          break;
+
+        case MODE_FILE:
+          sync_status = fsync (fd);
+          break;
+
+#if HAVE_SYNCFS
+        case MODE_FILE_SYSTEM:
+          sync_status = syncfs (fd);
+          break;
+#endif
+
+        default:
+          assert ("invalid sync_mode");
+        }
+
+      if (sync_status < 0)
+        {
+          error (0, errno, _("error syncing %s"), quote (file));
+          ret = false;
+        }
+    }
+
+  if (close (fd) < 0)
+    {
+      error (0, errno, _("failed to close %s"), quote (file));
+      ret = false;
+    }
+
+  return ret;
+}
+
 int
 main (int argc, char **argv)
 {
+  int c;
+  bool args_specified;
+  bool arg_data = false, arg_file_system = false;
+  enum sync_mode mode;
+  bool ok = true;
+
   initialize_main (&argc, &argv);
   set_program_name (argv[0]);
   setlocale (LC_ALL, "");
@@ -60,14 +172,55 @@ main (int argc, char **argv)
 
   atexit (close_stdout);
 
-  parse_long_options (argc, argv, PROGRAM_NAME, PACKAGE, Version,
-                      usage, AUTHORS, (char const *) NULL);
-  if (getopt_long (argc, argv, "", NULL, NULL) != -1)
-    usage (EXIT_FAILURE);
+  while ((c = getopt_long (argc, argv, "df", long_options, NULL))
+         != -1)
+    {
+      switch (c)
+        {
+        case 'd':
+          arg_data = true;
+          break;
 
-  if (optind < argc)
-    error (0, 0, _("ignoring all arguments"));
+        case 'f':
+          arg_file_system = true;
+          break;
 
-  sync ();
-  return EXIT_SUCCESS;
+        case_GETOPT_HELP_CHAR;
+
+        case_GETOPT_VERSION_CHAR (PROGRAM_NAME, AUTHORS);
+
+        default:
+          usage (EXIT_FAILURE);
+        }
+    }
+
+  args_specified = optind < argc;
+
+  if (arg_data && arg_file_system)
+    {
+      error (EXIT_FAILURE, 0,
+             _("cannot specify both --data and --file-system"));
+    }
+
+  if (!args_specified && arg_data)
+    error (EXIT_FAILURE, 0, _("--data needs at least one argument"));
+
+  if (! args_specified || (arg_file_system && ! HAVE_SYNCFS))
+    mode = MODE_SYNC;
+  else if (arg_file_system)
+    mode = MODE_FILE_SYSTEM;
+  else if (! arg_data)
+    mode = MODE_FILE;
+  else
+    mode = MODE_DATA;
+
+  if (mode == MODE_SYNC)
+    sync ();
+  else
+    {
+      for (; optind < argc; optind++)
+        ok &= sync_arg (mode, argv[optind]);
+    }
+
+  return ok ? EXIT_SUCCESS : EXIT_FAILURE;
 }

tests/local.mk

@@ -375,6 +375,7 @@ all_tests =					\
   tests/misc/stty-row-col.sh			\
   tests/misc/sum.pl				\
   tests/misc/sum-sysv.sh			\
+  tests/misc/sync.sh				\
   tests/misc/tac.pl				\
   tests/misc/tac-continue.sh			\
   tests/misc/tac-2-nonseekable.sh		\

tests/misc/sync.sh

@@ -0,0 +1,50 @@
+#!/bin/sh
+# Test various sync(1) operations
+
+# Copyright (C) 2015 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 <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ sync
+
+touch file
+
+# fdatasync+syncfs is nonsensical
+returns_ 1 sync --data --file-system || fail=1
+
+# fdatasync needs an operand
+returns_ 1 sync -d || fail=1
+
+# Test syncing of file (fsync) (little side effects)
+sync file || fail=1
+
+# Ensure multiple args are processed and diagnosed
+returns_ 1 sync file nofile || fail=1
+
+# Ensure inaccessible dirs give an appropriate error
+mkdir norw || framework_failure_
+chmod 0 norw || framework_failure_
+sync norw 2>err
+printf "sync: error opening 'norw': Permission denied\n" >exp
+compare exp err || fail=1
+
+if test "$fail" != '1'; then
+  # Ensure a fifo doesn't block
+  mkfifo_or_skip_ fifo
+  timeout 10 sync fifo
+  test $? = 124 && fail=1
+fi
+
+Exit $fail