begin rewriting path to resource resolver

this code was getting too messy and scattered, and I realized that Flask
wasn't doing as much as I thought it was here, so now we need more
safety and sanity checks

tests/instance/unreachable.md

@@ -0,0 +1 @@
+this file exists but the app should not serve it.

tests/test_pages.py

@@ -1,7 +1,8 @@
 """Unit test helper methods."""
+import pytest
 from werkzeug.http import dump_cookie
 
-from incorporealcms.pages import generate_parent_navs, is_file_path_actually_dir_path, render, resolve_page_file
+from incorporealcms.pages import generate_parent_navs, render, request_path_to_instance_resource,  resolve_page_file
 
 
 def test_resolve_page_file_dir_to_index():
@@ -54,30 +55,6 @@ def test_generate_page_navs_subdir_with_title_parsing_real_page(app):
         ]
 
 
-def test_is_file_path_actually_dir_path_valid_file_is_yes(app):
-    """Test that a file request for what's actually a directory is detected as such."""
-    with app.app_context():
-        assert is_file_path_actually_dir_path('/subdir')
-
-
-def test_is_file_path_actually_dir_path_valid_dir_is_no(app):
-    """Test that a directory request is still a directory request."""
-    with app.app_context():
-        assert not is_file_path_actually_dir_path('/subdir/')
-
-
-def test_is_file_path_actually_dir_path_nonsense_file_is_no(app):
-    """Test that requests to nonsense file-looking paths aren't treated as dirs."""
-    with app.app_context():
-        assert not is_file_path_actually_dir_path('/antphnathpnthapnthsnthax')
-
-
-def test_is_file_path_actually_dir_path_nonsense_dir_is_no(app):
-    """Test that a directory request is a directory request even if the dir doesn't exist."""
-    with app.app_context():
-        assert not is_file_path_actually_dir_path('/antphnathpnthapnthsnthax/')
-
-
 def test_render_with_user_dark_theme(app):
     """Test that a request with the dark theme selected renders the dark theme."""
     cookie = dump_cookie("user-style", 'dark')
@@ -98,3 +75,73 @@ def test_render_with_no_user_theme(app):
     with app.test_request_context():
         assert b'light.css' in render('base.html').data
         assert b'dark.css' not in render('base.html').data
+
+
+def test_request_path_to_instance_resource(app):
+    """Test a normal URL request is transformed into the file path."""
+    with app.test_request_context():
+        assert request_path_to_instance_resource('index') == 'pages/index.md'
+
+
+def test_request_path_to_instance_resource_direct_file(app):
+    """Test a normal URL request is transformed into the file path."""
+    with app.test_request_context():
+        assert request_path_to_instance_resource('no-title') == 'pages/no-title.md'
+
+
+def test_request_path_to_instance_resource_in_subdir(app):
+    """Test a normal URL request is transformed into the file path."""
+    with app.test_request_context():
+        assert request_path_to_instance_resource('subdir/page') == 'pages/subdir/page.md'
+
+
+def test_request_path_to_instance_resource_subdir_index(app):
+    """Test a normal URL request is transformed into the file path."""
+    with app.test_request_context():
+        assert request_path_to_instance_resource('subdir/') == 'pages/subdir/index.md'
+
+
+def test_request_path_to_instance_resource_relatives_walked(app):
+    """Test a normal URL request is transformed into the file path."""
+    with app.test_request_context():
+        assert request_path_to_instance_resource('subdir/more-subdir/../../more-metadata') == 'pages/more-metadata.md'
+
+
+def test_request_path_to_instance_resource_relatives_walked_indexes_work_too(app):
+    """Test a normal URL request is transformed into the file path."""
+    with app.test_request_context():
+        assert request_path_to_instance_resource('subdir/more-subdir/../../') == 'pages/index.md'
+
+
+def test_request_path_to_instance_resource_relatives_walked_into_subdirs_also_fine(app):
+    """Test a normal URL request is transformed into the file path."""
+    with app.test_request_context():
+        assert request_path_to_instance_resource('subdir/more-subdir/../../subdir/page') == 'pages/subdir/page.md'
+
+
+def test_request_path_to_instance_resource_permission_error_on_ref_above_pages(app):
+    """Test that attempts to get above the base dir ("/../../foo") fail."""
+    with app.test_request_context():
+        with pytest.raises(PermissionError):
+            assert request_path_to_instance_resource('../unreachable')
+
+
+def test_request_path_to_instance_resource_isadirectory_on_file_like_req_for_dir(app):
+    """Test that a request for e.g. '/foo' when foo is a dir indicate to redirect."""
+    with app.test_request_context():
+        with pytest.raises(IsADirectoryError):
+            assert request_path_to_instance_resource('subdir')
+
+
+def test_request_path_to_instance_resource_nonexistant_file_errors(app):
+    """Test that a request for something not on disk errors."""
+    with app.test_request_context():
+        with pytest.raises(FileNotFoundError):
+            assert request_path_to_instance_resource('nthanpthpnh')
+
+
+def test_request_path_to_instance_resource_absolute_file_errors(app):
+    """Test that a request for something not on disk errors."""
+    with app.test_request_context():
+        with pytest.raises(PermissionError):
+            assert request_path_to_instance_resource('/etc/hosts')