New module: mreadlink-with-size

[Jim Meyering]

I need the following new module for coreutils.  The problem is that in
using xreadlink_with_size, programs like cp, ls and stat can mistakenly
exit too early (before processing all command line arguments) due to
xreadlink_with_size's exit.  However, in general, library functions will
be more usable if they don't exit at all.  The solution is to use the
new mreadlink_with_size instead.  It is identical, except for the
fact that it doesn't use xmalloc or xrealloc, and returns NULL rather
than calling exit.

At first I named the new function simply readlink_with_size, but that
won't generalize to xreadlink's case.  We can't create an analogous-named
function for xreadlink, since the name "readlink" is already taken.
Hence the addition of the "m" (for malloc) prefix.

        New module: mreadlink-with-size.
        * MODULES.html.sh: Add mreadlink-with-size.
        * modules/mreadlink-with-size: New module
        * modules/canonicalize (Depends-on): Depend on mreadlink-with-size,
        not xreadlink-with-size.
        * lib/mreadlink-with-size.c, lib/mreadlink.h: New files.

So, if you have code (esp. if it's part of a library) that uses xreadlink
or xreadlink_with_size, I encourage you to switch to mreadlink_with_size.
If there's interest, I'll add an "mreadlink" module, but at least
for coreutils, I have no need for that one.

canonicalize_filename_mode has the same problem,
and I've changed it already.  Patch coming up.

Jim

 MODULES.html.sh             |    1
 lib/mreadlink-with-size.c   |  110 ++++++++++++++++++++++++++++++++++++++++++++
 lib/mreadlink.h             |   24 +++++++++
 modules/canonicalize        |    2
 modules/mreadlink-with-size |   25 ++++++++++
 5 files changed, 161 insertions(+), 1 deletion(-)

        New module: mreadlink-with-size.
        * MODULES.html.sh: Add mreadlink-with-size.
        * modules/mreadlink-with-size: New module
        * modules/canonicalize (Depends-on): Depend on mreadlink-with-size,
        not xreadlink-with-size.
        * lib/mreadlink-with-size.c, lib/mreadlink.h: New files.

Index: MODULES.html.sh
===================================================================
RCS file: /cvsroot/gnulib/gnulib/MODULES.html.sh,v
retrieving revision 1.216
diff -u -p -r1.216 MODULES.html.sh
--- MODULES.html.sh     11 Jun 2007 00:29:31 -0000      1.216
+++ MODULES.html.sh     18 Jun 2007 12:14:12 -0000
@@ -2149,6 +2149,7 @@ func_all_modules ()
   func_module mkdir-p
   func_module modechange
   func_module mountlist
+  func_module mreadlink-with-size
   func_module openat
   func_module pathmax
   func_module same
Index: modules/mreadlink-with-size
===================================================================
RCS file: modules/mreadlink-with-size
diff -N modules/mreadlink-with-size
--- /dev/null   1 Jan 1970 00:00:00 -0000
+++ modules/mreadlink-with-size 18 Jun 2007 12:14:12 -0000
@@ -0,0 +1,25 @@
+Description:
+Return the contents of a symbolic link, without size limitations.
+
+Files:
+lib/mreadlink.h
+lib/mreadlink-with-size.c
+
+Depends-on:
+readlink
+ssize_t
+unistd
+
+configure.ac:
+
+Makefile.am:
+lib_SOURCES += mreadlink-with-size.c
+
+Include:
+"mreadlink.h"
+
+License:
+GPL
+
+Maintainer:
+Jim Meyering
Index: modules/canonicalize
===================================================================
RCS file: /cvsroot/gnulib/gnulib/modules/canonicalize,v
retrieving revision 1.14
diff -u -p -r1.14 canonicalize
--- modules/canonicalize        1 Mar 2007 00:39:40 -0000       1.14
+++ modules/canonicalize        18 Jun 2007 12:14:12 -0000
@@ -13,7 +13,7 @@ filenamecat
 sys_stat
 xalloc
 xgetcwd
-xreadlink-with-size
+mreadlink-with-size

 configure.ac:
 AC_FUNC_CANONICALIZE_FILE_NAME
Index: lib/mreadlink-with-size.c
===================================================================
RCS file: lib/mreadlink-with-size.c
diff -N lib/mreadlink-with-size.c
--- /dev/null   1 Jan 1970 00:00:00 -0000
+++ lib/mreadlink-with-size.c   18 Jun 2007 12:14:12 -0000
@@ -0,0 +1,110 @@
+/* readlink wrapper to return the link name in malloc'd storage.
+   Unlike xreadlink and xreadlink_with_size, don't ever call exit.
+
+   Copyright (C) 2001, 2003-2007 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 2, 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; see the file COPYING.
+   If not, write to the Free Software Foundation,
+   51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+/* Written by Jim Meyering <address@hidden>  */
+
+#include <config.h>
+
+#include "mreadlink.h"
+
+#include <stdio.h>
+#include <errno.h>
+#include <limits.h>
+#include <sys/types.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+#ifndef SIZE_MAX
+# define SIZE_MAX ((size_t) -1)
+#endif
+#ifndef SSIZE_MAX
+# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
+#endif
+
+/* SYMLINK_MAX is used only for an initial memory-allocation sanity
+   check, so it's OK to guess too small on hosts where there is no
+   arbitrary limit to symbolic link length.  */
+#ifndef SYMLINK_MAX
+# define SYMLINK_MAX 1024
+#endif
+
+#define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX)
+
+/* Call readlink to get the symbolic link value of FILE.
+   SIZE is a hint as to how long the link is expected to be;
+   typically it is taken from st_size.  It need not be correct.
+   Return a pointer to that NUL-terminated string in malloc'd storage.
+   If readlink fails, malloc fails, or if the link value is longer
+   than SSIZE_MAX, return NULL (caller may use errno to diagnose).  */
+
+char *
+mreadlink_with_size (char const *file, size_t size)
+{
+  /* Some buggy file systems report garbage in st_size.  Defend
+     against them by ignoring outlandish st_size values in the initial
+     memory allocation.  */
+  size_t symlink_max = SYMLINK_MAX;
+  size_t INITIAL_LIMIT_BOUND = 8 * 1024;
+  size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND
+                         ? symlink_max + 1
+                         : INITIAL_LIMIT_BOUND);
+
+  /* The initial buffer size for the link value.  */
+  size_t buf_size = size < initial_limit ? size + 1 : initial_limit;
+
+  while (1)
+    {
+      ssize_t r;
+      size_t link_length;
+      char *buffer = malloc (buf_size);
+
+      if (buffer == NULL)
+       return NULL;
+      r = readlink (file, buffer, buf_size);
+      link_length = r;
+
+      /* On AIX 5L v5.3 and HP-UX 11i v2 04/09, readlink returns -1
+        with errno == ERANGE if the buffer is too small.  */
+      if (r < 0 && errno != ERANGE)
+       {
+         int saved_errno = errno;
+         free (buffer);
+         errno = saved_errno;
+         return NULL;
+       }
+
+      if (link_length < buf_size)
+       {
+         buffer[link_length] = 0;
+         return buffer;
+       }
+
+      free (buffer);
+      if (buf_size <= MAXSIZE / 2)
+       buf_size *= 2;
+      else if (buf_size < MAXSIZE)
+       buf_size = MAXSIZE;
+      else
+       {
+         errno = ENOMEM;
+         return NULL;
+       }
+    }
+}
Index: lib/mreadlink.h
===================================================================
RCS file: lib/mreadlink.h
diff -N lib/mreadlink.h
--- /dev/null   1 Jan 1970 00:00:00 -0000
+++ lib/mreadlink.h     18 Jun 2007 12:14:12 -0000
@@ -0,0 +1,24 @@
+/* Read symbolic links without size limitation.
+
+   Copyright (C) 2001, 2003, 2004, 2007 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 2, 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; see the file COPYING.
+   If not, write to the Free Software Foundation,
+   51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+/* Written by Jim Meyering <address@hidden>  */
+
+#include <stddef.h>
+
+extern char *mreadlink_with_size (char const *filename, size_t size_hint);