Fix UnicodeDecodeError when decode API input

Convert UnicodeDecodeError to HTTPBadRequest in
FaultWrapper.

**NOTE**: Cinder will raise 500 error when failed to decode invalid
unicode character at anyplace where try to collect url
parematers as below:

```
params = req.params
```

This patch converts this kind of exception into BadRequest, plus
an explicit error message.

Change-Id: I816f05084b0a0ef670ef293d381868409b96ed7d
Closes-Bug: #1746202

cinder/api/middleware/fault.py

@@ -44,6 +44,11 @@ class FaultWrapper(base_wsgi.Middleware):
             status, webob.exc.HTTPInternalServerError)()
 
     def _error(self, inner, req):
+        if isinstance(inner, UnicodeDecodeError):
+            msg = _("Error decoding your request. Either the URL or the "
+                    "request body contained characters that could not be "
+                    "decoded by Cinder.")
+            return wsgi.Fault(webob.exc.HTTPBadRequest(explanation=msg))
         if not isinstance(inner, exception.QuotaError):
             LOG.exception("Caught error: %(type)s %(error)s",
                           {'type': type(inner),

cinder/tests/unit/api/middleware/test_faults.py

@@ -17,6 +17,7 @@ from oslo_serialization import jsonutils
 from six.moves import http_client
 import webob.dec
 
+from cinder.api.middleware import fault
 from cinder.api.openstack import wsgi
 from cinder import test
 
@@ -79,3 +80,18 @@ class TestFaults(test.TestCase):
         """Ensure the status_int is set correctly on faults."""
         fault = wsgi.Fault(webob.exc.HTTPBadRequest(explanation='what?'))
         self.assertEqual(http_client.BAD_REQUEST, fault.status_int)
+
+
+class ExceptionTest(test.TestCase):
+
+    def _wsgi_app(self, inner_app):
+        return fault.FaultWrapper(inner_app)
+
+    def test_unicode_decode_error(self):
+        @webob.dec.wsgify
+        def unicode_error(req):
+            raise UnicodeDecodeError("ascii", b"", 0, 1, "bad")
+
+        api = self._wsgi_app(unicode_error)
+        resp = webob.Request.blank('/').get_response(api)
+        self.assertEqual(400, resp.status_int)