tmpfile: improve error message

* spot/misc/tmpfile.cc: Display strerror(errno) plus some suggestions
that depend on the error.  Based on a report from Shengping Shaw.
* THANKS: Add reporter.
* tests/core/ltlcross5.test: New file.
* tests/Makefile.am: Add it.

THANKS

@@ -42,6 +42,7 @@ Reuben Rowe
 Rüdiger Ehlers
 Silien Hong
 Simon Jantsch
+Shengping Shaw
 Shufang Zhu
 Sonali Dutta
 Tereza Šťastná

spot/misc/tmpfile.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2013, 2015, 2017, 2018 Laboratoire de Recherche et
+// Copyright (C) 2013, 2015, 2017-2019 Laboratoire de Recherche et
 // Développement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -75,7 +75,29 @@ namespace spot
           fd = mkstemp(*name);
         }
       if (fd < 0)
-        throw std::runtime_error("failed to create "s + *name);
+        {
+          std::string err = ("failed to create temporary file "s + *name
+                             + ": " + strerror(errno));
+          if (errno == EACCES)
+            {
+              if (tmpdir)
+                err += ("\nConsider setting the SPOT_TMPDIR environment "
+                        "variable to a writable directory.");
+              else
+                err += ("\nConsider executing this from a writable "
+                        "directory, or setting\nthe SPOT_TMPDIR environment "
+                        "variable to such a directory.");
+            }
+          else if (tmpdir)
+            {
+              const char* dir =
+                secure_getenv("SPOT_TMPDIR") ? "SPOT_TMPDIR" : "TMPDIR";
+              err += ("\nNote that the directory comes from the "s
+                      + dir
+                      + " environment variable.");
+            }
+          throw std::runtime_error(err);
+        }
       return fd;
     }
   }

tests/Makefile.am

@@ -214,6 +214,7 @@ TESTS_twa = \
   core/bdddict.test \
   core/alternating.test \
   core/ltlcross3.test \
+  core/ltlcross5.test \
   core/taatgba.test \
   core/renault.test \
   core/nondet.test \

tests/core/ltlcross5.test

@@ -0,0 +1,65 @@
+#!/bin/sh
+# -*- coding: utf-8 -*-
+# Copyright (C) 2019 Laboratoire de Recherche et Développement de
+# l'Epita (LRDE).
+#
+# This file is part of Spot, a model checking library.
+#
+# Spot 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 3 of the License, or
+# (at your option) any later version.
+#
+# Spot 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, see <http://www.gnu.org/licenses/>.
+
+. ./defs
+set -e
+
+unset TMPDIR
+unset SPOT_TMPDIR
+
+mkdir foo
+chmod a-w foo
+cd foo
+err=0
+if touch bar; then
+    # We are likely running as root, so we cannot detect permission
+    # issues.
+    cd ..
+    rm -rf foo
+    exit 77
+fi
+
+ltlcross ltl2tgba -f GFa 2>../err && err=1
+cd ..
+cat err
+grep 'failed to create temporary file' err || err=1
+grep 'executing this from a writable' err || err=1
+grep 'SPOT_TMPDIR' err || err=1
+
+SPOT_TMPDIR=foo ltlcross ltl2tgba -f GFa 2>err && err=2
+cat err
+grep 'failed to create temporary file' err || err=2
+grep 'executing this from a writable' err && err=2
+grep 'SPOT_TMPDIR' err
+
+chmod a+w foo
+rmdir foo
+
+SPOT_TMPDIR=bar ltlcross ltl2tgba -f GFa 2>err && err=3
+cat err
+grep 'failed to create temporary file' err
+grep 'Note that the directory.*SPOT_TMPDIR ' err
+
+TMPDIR=bar ltlcross ltl2tgba -f GFa 2>err && err=4
+cat err
+grep 'failed to create temporary file' err
+grep 'Note that the directory.* TMPDIR ' err
+
+exit $err