core: fix semantics of nm_utils_do_sysctl()

This function gets used for both /proc/sys (ie, sysctl) and for
sysfs attributes.  There are two issues with it:

1) most sysctl values don't care about a trailing LF, but some
sysfs attributes (infiniband) do; so we always have to add the
trailing LF.  Just move that into the function to ensure that
callers don't forget to add it.

2) neither sysfs or sysctl support partial writes, while the
existing function did partial writes.  Practically, both the
write handlers for sysfs and sysctl should always handle all
the data, but if they don't, partial writes are wrong.  So
instead, try three times to write all the data.

src/NetworkManagerUtils.c

@@ -610,27 +610,48 @@ value_hash_add_object_property (GHashTable *hash,
 gboolean
 nm_utils_do_sysctl (const char *path, const char *value)
 {
-	int fd, len, nwrote, total;
+	int fd, len, nwrote, tries;
+	char *actual;
+
+	g_return_val_if_fail (path != NULL, FALSE);
+	g_return_val_if_fail (value != NULL, FALSE);
+	g_return_val_if_fail (value[0], FALSE);
 
 	fd = open (path, O_WRONLY | O_TRUNC);
-	if (fd == -1)
+	if (fd == -1) {
+		nm_log_warn (LOGD_CORE, "sysctl: failed to open '%s': (%d) %s",
+		             path, errno, strerror (errno));
 		return FALSE;
+	}
 
-	len = strlen (value);
-	total = 0;
-	do {
-		nwrote = write (fd, value + total, len - total);
+	nm_log_dbg (LOGD_CORE, "sysctl: setting '%s' to '%s'", path, value);
+
+	/* Most sysfs and sysctl options don't care about a trailing CR, while some
+	 * (like infiniband) do.  So always add the CR.  Also, neither sysfs nor
+	 * sysctl support partial writes so the CR must be added to the string we're
+	 * about to write.
+	 */
+	actual = g_strdup_printf ("%s\n", value);
+
+	/* Try to write the entire value three times if a partial write occurs */
+	len = strlen (actual);
+	for (tries = 0, nwrote = 0; tries < 3 && nwrote != len; tries++) {
+		nwrote = write (fd, actual, len);
 		if (nwrote == -1) {
 			if (errno == EINTR)
 				continue;
-			close (fd);
-			return FALSE;
+			break;
 		}
-		total += nwrote;
-	} while (total < len);
+	}
+	g_free (actual);
+
+	if (nwrote != len) {
+		nm_log_warn (LOGD_CORE, "sysctl: failed to set '%s' to '%s': (%d) %s",
+		             path, value, errno, strerror (errno));
+	}
 
 	close (fd);
-	return TRUE;
+	return (nwrote == len);
 }
 
 gboolean