Add text and json fallback error handlers (#1937)

* Add text and json fallback error handlers

* Add tests and auto-detect error fallback type

tests/test_app.py

@@ -126,7 +126,7 @@ def test_app_handle_request_handler_is_none(app, monkeypatch):
     def handler(request):
         return text("test")
 
-    request, response = app.test_client.get("/test")
+    _, response = app.test_client.get("/test")
 
     assert (
         "'None' was returned while requesting a handler from the router"

tests/test_errorpages.py

@@ -0,0 +1,86 @@
+import pytest
+
+from sanic import Sanic
+from sanic.errorpages import exception_response
+from sanic.exceptions import NotFound
+from sanic.request import Request
+from sanic.response import HTTPResponse
+
+
+@pytest.fixture
+def app():
+    app = Sanic("error_page_testing")
+
+    @app.route("/error", methods=["GET", "POST"])
+    def err(request):
+        raise Exception("something went wrong")
+
+    return app
+
+
+@pytest.fixture
+def fake_request(app):
+    return Request(b"/foobar", {}, "1.1", "GET", None, app)
+
+
+@pytest.mark.parametrize(
+    "fallback,content_type, exception, status",
+    (
+        (None, "text/html; charset=utf-8", Exception, 500),
+        ("html", "text/html; charset=utf-8", Exception, 500),
+        ("auto", "text/html; charset=utf-8", Exception, 500),
+        ("text", "text/plain; charset=utf-8", Exception, 500),
+        ("json", "application/json", Exception, 500),
+        (None, "text/html; charset=utf-8", NotFound, 404),
+        ("html", "text/html; charset=utf-8", NotFound, 404),
+        ("auto", "text/html; charset=utf-8", NotFound, 404),
+        ("text", "text/plain; charset=utf-8", NotFound, 404),
+        ("json", "application/json", NotFound, 404),
+    ),
+)
+def test_should_return_html_valid_setting(
+    fake_request, fallback, content_type, exception, status
+):
+    if fallback:
+        fake_request.app.config.FALLBACK_ERROR_FORMAT = fallback
+
+    try:
+        raise exception("bad stuff")
+    except Exception as e:
+        response = exception_response(fake_request, e, True)
+
+    assert isinstance(response, HTTPResponse)
+    assert response.status == status
+    assert response.content_type == content_type
+
+
+def test_auto_fallback_with_data(app):
+    app.config.FALLBACK_ERROR_FORMAT = "auto"
+
+    _, response = app.test_client.get("/error")
+    assert response.status == 500
+    assert response.content_type == "text/html; charset=utf-8"
+
+    _, response = app.test_client.post("/error", json={"foo": "bar"})
+    assert response.status == 500
+    assert response.content_type == "application/json"
+
+    _, response = app.test_client.post("/error", data={"foo": "bar"})
+    assert response.status == 500
+    assert response.content_type == "text/html; charset=utf-8"
+
+
+def test_auto_fallback_with_content_type(app):
+    app.config.FALLBACK_ERROR_FORMAT = "auto"
+
+    _, response = app.test_client.get(
+        "/error", headers={"content-type": "application/json"}
+    )
+    assert response.status == 500
+    assert response.content_type == "application/json"
+
+    _, response = app.test_client.get(
+        "/error", headers={"content-type": "text/plain"}
+    )
+    assert response.status == 500
+    assert response.content_type == "text/plain; charset=utf-8"