ensure edited ini file contents are always complete

Use method 9 from http://www.pixelbeat.org/docs/unix_file_replacement.html
to fulfill the AC properties of ACID, useful for
asynchronous readers and writers.
I.E. that readers will never get a partially written file,
and writers will not intersperse their output.
Caveats are noted in the source comments.

We also better handle any external issues like permissions etc.
when writing the ini file, so that only the error is printed
without a backtrace, which should also avoid any auto backtrace
reporting that may be enabled on the system.

Author: pixelb

crudini

@@ -8,12 +8,17 @@
 # under the terms of the GPLv2, the GNU General Public License version 2, as
 # published by the Free Software Foundation. http://gnu.org/licenses/gpl.html
 
+import os
 import sys
+import errno
+import contextlib
 import ConfigParser
 import getopt
 import iniparse
 import pipes
+import shutil
 import string
+import tempfile
 from cStringIO import StringIO
 
 try:
@@ -195,7 +200,7 @@ def _parse_file(filename, add_default=False):
         conf = iniparse.RawConfigParser()
         conf.readfp(fp)
         return conf
-    except IOError as e:
+    except EnvironmentError as e:
         error(str(e))
         sys.exit(1)
 
@@ -355,18 +360,84 @@ except ConfigParser.NoOptionError:
     error('Parameter not found: %s' % param)
     sys.exit(1)
 
+def delete_if_exists(path):
+    """Delete a file, but ignore file not found error.
+    """
+    try:
+        os.unlink(path)
+    except EnvironmentError as e:
+        if e.errno != errno.ENOENT:
+            print str(e)
+            raise
+
+@contextlib.contextmanager
+def remove_file_on_error(path):
+    """Protect code that wants to operate on PATH atomically.
+    Any exception will cause PATH to be removed.
+    """
+    try:
+        yield
+    except Exception:
+        t, v, tb = sys.exc_info()
+        delete_if_exists(path)
+        raise t, v, tb
+
+def file_replace(name, data):
+    """Replace file as atomically as possible,
+    fulfilling and AC properties of ACID.
+    This is essentially using method 9 from:
+    http://www.pixelbeat.org/docs/unix_file_replacement.html
+
+    Caveats:
+     - Changes ownership of the file being edited
+       by non root users (due to POSIX interface limitations).
+     - Loses any extended attributes of the original file
+       (due to the simplicity of this implementation).
+     - Existing hardlinks will be separated from the
+       newly replaced file.
+     - Ignores the write permissions of the original file.
+     - Requires write permission on the directory as well as the file.
+     - With python2 on windows we don't fulfill the A ACID property.
+
+    To avoid the above caveats see the --inplace option.
+    """
+    (f, tmp) = tempfile.mkstemp(".tmp", prefix=name+".", dir=".")
+
+    with remove_file_on_error(tmp):
+        shutil.copystat(name, tmp)
+
+        if hasattr(os, 'fchown') and os.geteuid() == 0:
+            st = os.stat(name)
+            os.fchown(f, st.st_uid, st.st_gid)
+
+        os.write(f, data)
+        os.close(f)
+
+        if hasattr(os,'replace'): # >= python 3.3
+            os.replace(tmp, name) # atomic even on windos
+        elif os.name == 'posix':
+            os.rename(tmp, name) # atomic on POSIX
+        else:
+            backup = tmp+'.backup'
+            os.rename(name, backup)
+            os.rename(tmp, name)
+            delete_if_exists(backup)
+
 if mode != '--get':
-    with open(cfgfile, 'w') as f:
-        # XXX: Ideally we should just do conf.write(f) here,
-        # but to avoid iniparse issues, we massage the data a little here
-        str_data = str(conf.data)
-        if len(str_data) and str_data[-1] != '\n':
-            str_data += '\n'
-
-        if (
-            (added_default_section and not (section_explicit_default and mode in ('--set', '--merge')))
-            or (mode == '--del' and section == iniparse.DEFAULTSECT and param is None)
-           ):
-            str_data = str_data.replace('[%s]\n' % iniparse.DEFAULTSECT, '', 1)
-
-        f.write(str_data)
+    # XXX: Ideally we should just do conf.write(f) here,
+    # but to avoid iniparse issues, we massage the data a little here
+    str_data = str(conf.data)
+    if len(str_data) and str_data[-1] != '\n':
+        str_data += '\n'
+
+    if (
+        (added_default_section and not (section_explicit_default and mode in ('--set', '--merge')))
+        or (mode == '--del' and section == iniparse.DEFAULTSECT and param is None)
+        ):
+        str_data = str_data.replace('[%s]\n' % iniparse.DEFAULTSECT, '', 1)
+
+    try:
+        file_replace(cfgfile, str_data)
+    except EnvironmentError as e:
+        error(str(e))
+        sys.exit(1)