stat: use e.g. %.3X instead of %X.%3:X for sub-second precision

* NEWS: Document this.
* doc/coreutils.texi (stat invocation): Likewise.
* gl/lib/fstimeprec.c, gl/lib/fstimeprec.h, gl/modules/fstimeprec:
* gl/modules/fstimeprec-tests, gl/tests/test-fstimeprec.c:
New files.
* bootstrap.conf (gnulib_modules): Add fstimeprec.
* src/stat.c: Include fstimeprec.h.  Don't include xstrtol.h.
(decimal_point, decimal_point_len): New static vars.
(main): Initialize them.
(epoch_sec, out_ns): Remove.
(out_int, out_uint): Now returns whatever printf returned.
(out_minus_zero, out_epoch_secs): New functions.
(print_stat): Use out_epoch_sec instead of out_ns and epoch_sec.
(print_stat, print_it, usage): Remove the %:X-style formats.
* tests/misc/stat-nanoseconds: Set TZ=UTC0 to avoid problems
with weird time zones.  Use a time stamp near the Epoch so that we
don't have to worry about leap seconds.  Redo test cases to match
new behavior.
* tests/touch/60-seconds: Change %Y.%:Y to %.9Y, to adjust to
new behavior.

NEWS

@@ -20,15 +20,12 @@ GNU coreutils NEWS                                    -*- outline -*-
   stat's %X, %Y, and %Z directives once again print only the integer
   part of seconds since the epoch.  This reverts a change from
   coreutils-8.6, that was deemed unnecessarily disruptive.  To obtain
-  the nanoseconds portion corresponding to %X, you may now use %:X.
-  I.e., to print the floating point number of seconds using maximum
-  precision, use this format string: %X.%:X.  Likewise for %Y, %Z and %W.
+  a full resolution time stamp for %X, use %.X; if you want (say) just
+  3 fractional digits, use %.3X.  Likewise for %Y and %Z.
 
   stat's new %W format directive would print floating point seconds.
   However, with the above change to %X, %Y and %Z, we've made %W work
-  the same way:  %W now expands to seconds since the epoch (or 0 when
-  not supported), and %:W expands to the nanoseconds portion, or to
-  0 if not supported.
+  the same way as the others.
 
 
 * Noteworthy changes in release 8.6 (2010-10-15) [stable]

bootstrap.conf

@@ -85,6 +85,7 @@ gnulib_modules="
   freopen
   freopen-safer
   fseeko
+  fstimeprec
   fsusage
   fsync
   ftello

doc/coreutils.texi

@@ -10707,37 +10707,36 @@ The valid @var{format} directives for files with @option{--format} and
 @item %U - User name of owner
 @item %w - Time of file birth, or @samp{-} if unknown
 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
-@item %:W - Time of file birth: nanoseconds remainder, or @samp{0}
 @item %x - Time of last access
 @item %X - Time of last access as seconds since Epoch
-@item %:X - Time of last access: nanoseconds remainder
 @item %y - Time of last modification
 @item %Y - Time of last modification as seconds since Epoch
-@item %:Y - Time of last modification: nanoseconds remainder
 @item %z - Time of last change
 @item %Z - Time of last change as seconds since Epoch
-@item %:Z - Time of last change: nanoseconds remainder
 @end itemize
 
-Note that each of @samp{%:W}, @samp{%:X}, @samp{%:Y} and @samp{%:Z}
-prints its zero-padded number of nanoseconds on a field of width 9.
-However, if you specify anything between the @samp{%} and @samp{:},
-e.g., @samp{%10:X}, your modifier replaces the default of @samp{09}.
-
-Here are some examples:
+The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
+precision preceded by a period to specify the number of digits to
+print after the decimal point. For example, @samp{%.9X} outputs the
+last access time to nanosecond precision.  If a period is given but no
+precision, @command{stat} uses the estimated precision of the file
+system. When discarding excess precision, time stamps are truncated
+toward minus infinity.
 
 @example
 zero pad:
-  $ stat -c '[%015:Y]' /usr
-  [000000031367045]
+  $ stat -c '[%015Y]' /usr
+  [000001288929712]
 space align:
-  $ stat -c '[%15:Y]' /usr
-  [       31367045]
-  $ stat -c '[%-15:Y]' /usr
-  [31367045       ]
-truncate:
-  $ stat -c '[%.3:Y]' /usr
-  [031]
+  $ stat -c '[%15Y]' /usr
+  [     1288929712]
+  $ stat -c '[%-15Y]' /usr
+  [1288929712     ]
+precision:
+  $ stat -c '[%.3Y]' /usr
+  [1288929712.114]
+  $ stat -c '[%.Y]' /usr
+  [1288929712.114951]
 @end example
 
 The mount point printed by @samp{%m} is similar to that output

gl/lib/fstimeprec.c

@@ -0,0 +1,178 @@
+/* Determine a file system's time stamp precision.
+
+   Copyright 2010 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/>.  */
+
+/* written by Paul Eggert */
+
+#include <config.h>
+#include "fstimeprec.h"
+
+#include "hash.h"
+#include "stat-time.h"
+#include <limits.h>
+#include <stdlib.h>
+
+/* A hash table that maps device numbers to precisions.  */
+struct fstimeprec
+{
+  /* Map device numbers to precisions.  */
+  struct hash_table *map;
+
+  /* Cache of the most recently allocated and otherwise-unused storage
+     for probing this table.  */
+  struct ent *probe;
+};
+
+/* A pair that maps a device number to a precision.  */
+struct ent
+{
+  dev_t dev;
+  int prec;
+};
+
+/* Hash an entry.  */
+static size_t
+ent_hash (void const *x, size_t table_size)
+{
+  struct ent const *p = x;
+  dev_t dev = p->dev;
+
+  /* FIXME: This code is duplicated from di-set.c.  */
+  /* When DEV is wider than size_t, exclusive-OR the words of DEV into H.
+     This avoids loss of info, without applying % to the wider type,
+     which could be quite slow on some systems.  */
+  size_t h = dev;
+  unsigned int i;
+  unsigned int n_words = sizeof dev / sizeof h + (sizeof dev % sizeof h != 0);
+  for (i = 1; i < n_words; i++)
+    h ^= dev >> CHAR_BIT * sizeof h * i;
+
+  return h % table_size;
+}
+
+/* Return true if two entries are the same.  */
+static bool
+ent_compare (void const *x, void const *y)
+{
+  struct ent const *a = x;
+  struct ent const *b = y;
+  return a->dev == b->dev;
+}
+
+/* Using the SET table, map a device to an entry that represents
+   the file system precision.  Return NULL on error.  */
+static struct ent *
+map_device (struct fstimeprec *tab, dev_t dev)
+{
+  /* Find space for the probe, reusing the cache if available.  */
+  struct ent *ent;
+  struct ent *probe = tab->probe;
+  if (probe)
+    {
+      /* If repeating a recent query, return the cached result.   */
+      if (probe->dev == dev)
+        return probe;
+    }
+  else
+    {
+      tab->probe = probe = malloc (sizeof *probe);
+      if (! probe)
+        return NULL;
+      probe->prec = 0;
+    }
+
+  /* Probe for the device.  */
+  probe->dev = dev;
+  ent = hash_insert (tab->map, probe);
+  if (ent == probe)
+    tab->probe = NULL;
+  return ent;
+}
+
+/* Return a newly allocated table of file system time stamp
+   resolutions, or NULL if out of memory.  */
+struct fstimeprec *
+fstimeprec_alloc (void)
+{
+  struct fstimeprec *tab = malloc (sizeof *tab);
+  if (tab)
+    {
+      enum { INITIAL_DEV_MAP_SIZE = 11 };
+      tab->map = hash_initialize (INITIAL_DEV_MAP_SIZE, NULL,
+                                  ent_hash, ent_compare, free);
+      if (! tab->map)
+        {
+          free (tab);
+          return NULL;
+        }
+      tab->probe = NULL;
+    }
+  return tab;
+}
+
+/* Free TAB.  */
+void
+fstimeprec_free (struct fstimeprec *tab)
+{
+  hash_free (tab->map);
+  free (tab->probe);
+  free (tab);
+}
+
+/* Using the cached information in TAB, return the estimated precision
+   of time stamps for the file system containing the file whose status
+   info is in ST.  The returned value counts the number of decimal
+   fraction digits.  Return 0 on failure.
+
+   If TAB is null, the guess is based purely on ST.  */
+int
+fstimeprec (struct fstimeprec *tab, struct stat const *st)
+{
+  /* Map the device number to an entry.  */
+  struct ent *ent = (tab ? map_device (tab, st->st_dev) : NULL);
+
+  /* Guess the precision based on what has been seen so far.  */
+  int prec = (ent ? ent->prec : 0);
+
+  /* If the current file's timestamp resolution is higher than the
+     guess, increase the guess.  */
+  if (prec < 9)
+    {
+      struct timespec ats = get_stat_atime (st);
+      struct timespec mts = get_stat_mtime (st);
+      struct timespec cts = get_stat_ctime (st);
+      struct timespec bts = get_stat_birthtime (st);
+      unsigned int ans = ats.tv_nsec;
+      unsigned int mns = mts.tv_nsec;
+      unsigned int cns = cts.tv_nsec;
+      unsigned int bns = bts.tv_nsec < 0 ? 0 : bts.tv_nsec;
+      unsigned int power = 10;
+      int p;
+      for (p = prec + 1; p < 9; p++)
+        power *= 10;
+
+      while (ans % power | mns % power | cns % power | bns % power)
+        {
+          power /= 10;
+          prec++;
+        }
+
+      if (ent)
+        ent->prec = prec;
+    }
+
+  return prec;
+}

gl/lib/fstimeprec.h

@@ -0,0 +1,22 @@
+#include <sys/stat.h>
+
+#ifndef ATTRIBUTE_MALLOC
+# if __GNUC__ >= 3
+#  define ATTRIBUTE_MALLOC __attribute__ ((__malloc__))
+# else
+#  define ATTRIBUTE_MALLOC
+# endif
+#endif
+
+#ifndef _GL_ARG_NONNULL
+# if (__GNUC__ == 3 && __GNUC_MINOR__ >= 3) || __GNUC__ > 3
+#  define _GL_ARG_NONNULL(params) __attribute__ ((__nonnull__ params))
+# else
+#  define _GL_ARG_NONNULL(params)
+# endif
+#endif
+
+struct fstimeprec *fstimeprec_alloc (void) ATTRIBUTE_MALLOC;
+void fstimeprec_free (struct fstimeprec *) _GL_ARG_NONNULL ((1));
+int fstimeprec (struct fstimeprec *, struct stat const *)
+  _GL_ARG_NONNULL ((2));

gl/modules/fstimeprec

@@ -0,0 +1,24 @@
+Description:
+estimate precision of time stamps in file systems
+
+Files:
+lib/fstimeprec.c
+lib/fstimeprec.h
+
+Depends-on:
+hash
+stat-time
+
+configure.ac:
+
+Makefile.am:
+lib_SOURCES += fstimeprec.c fstimeprec.h
+
+Include:
+"fstimeprec.h"
+
+License
+GPL
+
+Maintainer:
+Paul Eggert and Jim Meyering

gl/modules/fstimeprec-tests

@@ -0,0 +1,10 @@
+Files:
+tests/test-fstimeprec.c
+
+Depends-on:
+
+configure.ac:
+
+Makefile.am:
+TESTS += test-fstimeprec
+check_PROGRAMS += test-fstimeprec

gl/tests/test-fstimeprec.c

@@ -0,0 +1,74 @@
+/* Test the fstimeprec module.
+   Copyright (C) 2010 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/>.  */
+
+/* written by Paul Eggert */
+
+#include <config.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/stat.h>
+
+#define ASSERT(expr) \
+  do                                                                         \
+    {                                                                        \
+      if (!(expr))                                                           \
+        {                                                                    \
+          fprintf (stderr, "%s:%d: assertion failed\n", __FILE__, __LINE__); \
+          fflush (stderr);                                                   \
+          abort ();                                                          \
+        }                                                                    \
+    }                                                                        \
+  while (0)
+
+#include "fstimeprec.h"
+
+static int
+fstimeprec_file (struct fstimeprec *tab, char const *file)
+{
+  struct stat st;
+  if (stat (file, &st) != 0)
+    return 0;
+  return fstimeprec (tab, &st);
+}
+
+int
+main (void)
+{
+  struct fstimeprec *tab = fstimeprec_alloc ();
+  ASSERT (tab);
+
+  int m1 = fstimeprec_file (tab, "/");
+  int m2 = fstimeprec_file (tab, ".");
+  int m3 = fstimeprec_file (tab, "..");
+  ASSERT (0 <= m1 && m1 <= 9);
+  ASSERT (0 <= m2 && m2 <= 9);
+  ASSERT (0 <= m3 && m3 <= 9);
+
+  int n1 = fstimeprec_file (tab, "/");
+  int n2 = fstimeprec_file (tab, ".");
+  int n3 = fstimeprec_file (tab, "..");
+  ASSERT (0 <= n1 && n1 <= 9);
+  ASSERT (0 <= n2 && n2 <= 9);
+  ASSERT (0 <= n3 && n3 <= 9);
+
+  ASSERT (m1 <= n1);
+  ASSERT (m2 <= n2);
+  ASSERT (m3 <= n3);
+
+  fstimeprec_free (tab);
+
+  return 0;
+}