Re: bug#7529: Bug#605639: deal better with different filesystem timestamp resolutions

[Paul Eggert]

Good eye!  Thanks for the bug report and example.  I installed
the following one-byte patch into gnulib; please give it a try.
It should propagate into coreutils the next time coreutils
updates from gnulib.

A test case for this would require two file systems, one with
finer-grained time stamps than the other, where we can create
files in the latter.  I suspect this goes beyond what coreutils's
test cases can easily do.

>From 409c6b774c25afce33f8b67fbf7af3eb3304f6cf Mon Sep 17 00:00:00 2001
From: Paul Eggert <address@hidden>
Date: Wed, 1 Dec 2010 21:25:56 -0800
Subject: [PATCH] utimecmp: fine-grained src to nearby coarse-grained dest

* lib/utimecmp.c (utimecmp): When UTIMECMP_TRUNCATE_SOURCE is set,
and the source is on a file system with higher-resolution time
stamps, than the destination, and _PC_TIMESTAMP_RESOLUTION does
not work, and the time stamps are close together, the algorithm to
determine the exact resolution from the read-back mtime was buggy:
it had a "!=" where it should have had an "==".  This bug has been
in the code ever since it was introduced to gnulib.
Problem reported by Dan Jacobson in
<http://debbugs.gnu.org/cgi/bugreport.cgi?bug=7529>.
---
 ChangeLog      |   14 ++++++++++++++
 lib/utimecmp.c |    2 +-
 2 files changed, 15 insertions(+), 1 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index d4eb684..67e2977 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,17 @@
+2010-12-01  Paul Eggert  <address@hidden>
+
+       utimecmp: fine-grained src to nearby coarse-grained dest
+
+       * lib/utimecmp.c (utimecmp): When UTIMECMP_TRUNCATE_SOURCE is set,
+       and the source is on a file system with higher-resolution time
+       stamps, than the destination, and _PC_TIMESTAMP_RESOLUTION does
+       not work, and the time stamps are close together, the algorithm to
+       determine the exact resolution from the read-back mtime was buggy:
+       it had a "!=" where it should have had an "==".  This bug has been
+       in the code ever since it was introduced to gnulib.
+       Problem reported by Dan Jacobson in
+       <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=7529>.
+
 2010-11-30  Bruno Haible  <address@hidden>
 
        strerror_r-posix: Fix autoconf test.
diff --git a/lib/utimecmp.c b/lib/utimecmp.c
index 63a0c9a..8c3ca65 100644
--- a/lib/utimecmp.c
+++ b/lib/utimecmp.c
@@ -325,7 +325,7 @@ utimecmp (char const *dst_name,
 
                 res = SYSCALL_RESOLUTION;
 
-                for (a /= res; a % 10 != 0; a /= 10)
+                for (a /= res; a % 10 == 0; a /= 10)
                   {
                     if (res == BILLION)
                       {
-- 
1.7.2