TT#17401 Fail if destination file matches an existing directory

If a template named
/etc/ngcp-config/templates/etc/apt/apt.conf.d/71_no_recommended.tt2 exists
the resulting destination file would be
/etc/apt/apt.conf.d/71_no_recommended.  Though if
/etc/apt/apt.conf.d/71_no_recommended exists as directory ngcpcfg should
abort and inform the user about the existing situation with a useful error
message.

Change-Id: I2c6e1e3a4ec485183674c1fe72251631ad9867ac

helper/build_config

@@ -44,6 +44,12 @@ else
 fi
 # }}}
 
+# ensure we don't try to generate a file where a directory with same name exists already
+if [ -d "${output_file}" ] ; then
+  log_error "Generating file ${output_file} not possible, it's an existing directory." >&2
+  exit 1
+fi
+
 # pre-execution script in template store:
 if [ -r "${NGCPCTL_MAIN}/templates/${output_file}.prebuild" ] ; then
   log_info "Executing prebuild for ${output_file}"

t/test_ngcpcfg.py

@@ -1,5 +1,6 @@
 #!/usr/bin/env py.test-3
 
+import os
 import pytest
 import re
 import tempfile
@@ -35,3 +36,20 @@ def test_simple_build_template_ok(ngcpcfgcli):
                        tmpdir +
                        r"//etc/apt/apt.conf.d/71_no_recommended: OK")
     assert re.search(regex, out.stdout)
+
+
+@pytest.mark.tt_17401
+def test_fail_on_existing_dir_matching_output_filename(ngcpcfgcli, tmpdir):
+    tmpdir = tempfile.mkdtemp(prefix='ngcp-', suffix='-pytest-output')
+    output = "/etc/apt/apt.conf.d/71_no_recommended"
+    os.makedirs(tmpdir + output)
+    out = ngcpcfgcli("build", "--ignore-branch-check", output,
+                     env={
+                         'NGCP_PORTFILE': '/tmp/ngcpcfg.port',
+                         'OUTPUT_DIRECTORY': tmpdir,
+                         })
+    regex = re.compile("Error: Generating file " +
+                       tmpdir + "/" + output +
+                       r" not possible, it\'s an existing directory.")
+    assert re.search(regex, out.stderr)
+    assert out.returncode == 1