lua: Attach __tostring functions exceptions.

This includes a test.

lua/Makefile.am

@@ -50,7 +50,7 @@ guestfs.so: libluaguestfs.la
 	ln -sf .libs/libluaguestfs.so $@
 
 # Tests.
-TESTS_ENVIRONMENT = $(top_builddir)/run --test
+TESTS_ENVIRONMENT = LC_ALL=C $(top_builddir)/run --test
 TESTS = \
 	run-bindtests \
 	tests/010-load.lua \
@@ -60,7 +60,8 @@ TESTS = \
 	tests/027-create-multiple.lua \
 	tests/030-config.lua \
 	tests/070-optargs.lua \
-	tests/400-events.lua
+	tests/400-events.lua \
+	tests/900-errors.lua
 
 if ENABLE_APPLIANCE
 TESTS += \
@@ -80,7 +81,8 @@ EXTRA_DIST += \
 	tests/060-readdir.lua \
 	tests/070-optargs.lua \
 	tests/400-events.lua \
-	tests/400-progress.lua
+	tests/400-progress.lua \
+	tests/900-errors.lua
 
 # Custom install rule.
 install-data-hook:

lua/examples/guestfs-lua.pod

@@ -90,8 +90,12 @@ The C<errno> (corresponding to L<guestfs(3)/guestfs_last_errno>).
 
 =back
 
-Note that some errors can also be thrown as plain strings.  You
-need to check the type.
+These objects also have C<__tostring> functions attached to them
+so you can use C<tostring> (or implicit conversion) to convert them
+into printable strings.
+
+Note that the library also throws some errors as plain strings.  You
+may need to check the type.
 
 =head2 EVENTS
 

lua/tests/900-errors.lua

@@ -0,0 +1,37 @@
+#!/usr/bin/lua
+-- libguestfs Lua bindings -*- lua -*-
+-- Copyright (C) 2012 Red Hat 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 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, write to the Free Software
+-- Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+local G = require "guestfs"
+
+local g = G.create ()
+
+code, err = pcall (function (fn) g:add_drive (fn) end, "/NOTSUCHFILE")
+assert (not code)
+
+-- The default __tostring function will convert the error to either:
+--
+--   "%s", msg
+-- or:
+--   "%s: %s", msg, strerror (err)
+--
+-- We are expecting the second case here, but since the libguestfs
+-- code calls perrorf, the string version of ENOENT is already
+-- included in 'msg' and so appears twice here.
+local str = tostring (err)
+assert (str == "/NOTSUCHFILE: No such file or directory: No such file or directory",
+        string.format ("unexpected error string: %s", str))