Normalize the Ndiff exit codes.

0 if the scans are equal,
	1 if they differ, and
	2 for runtime errors.
Add tests and man page documentation.

ndiff/ndifftest.py

@@ -2,6 +2,7 @@
 
 # Unit tests for Ndiff.
 
+import subprocess
 import unittest
 import xml.dom.minidom
 import StringIO
@@ -688,4 +689,43 @@ def host_apply_diff(host, diff):
             host.script_results[host.script_results.index(sr_a)] = sr_b
     host.script_results.sort()
 
+def call_quiet(args, **kwargs):
+    """Run a command with subprocess.call and hide its output."""
+    return subprocess.call(args, stdout = subprocess.PIPE,
+        stderr = subprocess.STDOUT, **kwargs)
+
+class exit_code_test(unittest.TestCase):
+    NDIFF = "./ndiff"
+
+    def test_exit_equal(self):
+        """Test that the exit code is 0 when the diff is empty."""
+        for format in ("--text", "--xml"):
+            code = call_quiet([self.NDIFF, format, 
+                "test-scans/simple.xml", "test-scans/simple.xml"])
+            self.assertEqual(code, 0)
+        # Should be independent of verbosity.
+        for format in ("--text", "--xml"):
+            code = call_quiet([self.NDIFF, "-v", format, 
+                "test-scans/simple.xml", "test-scans/simple.xml"])
+            self.assertEqual(code, 0)
+
+    def test_exit_different(self):
+        """Test that the exit code is 1 when the diff is not empty."""
+        for format in ("--text", "--xml"):
+            code = call_quiet([self.NDIFF, format, 
+                "test-scans/simple.xml", "test-scans/complex.xml"])
+            self.assertEqual(code, 1)
+
+    def test_exit_error(self):
+        """Test that the exit code is 2 when there is an error."""
+        code = call_quiet([self.NDIFF])
+        self.assertEqual(code, 2)
+        code = call_quiet([self.NDIFF, "test-scans/simple.xml"])
+        self.assertEqual(code, 2)
+        code = call_quiet([self.NDIFF, "test-scans/simple.xml",
+            "test-scans/nonexistent.xml"])
+        self.assertEqual(code, 2)
+        code = call_quiet([self.NDIFF, "--nothing"])
+        self.assertEqual(code, 2)
+
 unittest.main()