Refactor the 'find an entry based on a URL' behaviour into a utility function

entries/from_url.py

@@ -0,0 +1,35 @@
+from typing import Union
+from urllib.parse import urlparse
+
+from django.contrib.sites.models import Site
+from django.http import HttpResponse
+from django.urls import resolve, Resolver404
+from micropub.views import error
+
+from .models import Entry
+
+
+def from_url(url: str) -> Union[Entry, HttpResponse]:
+    domain = Site.objects.get_current().domain
+    if not url:
+        return error.bad_req('url parameter required')
+    if '//' not in url:
+        url = '//' + url
+    parts = urlparse(url, scheme='https')
+    if parts.scheme not in ('http', 'https') or parts.netloc != domain:
+        return error.bad_req('url does not point to this site')
+
+    try:
+        match = resolve(parts.path)
+    except Resolver404:
+        return error.bad_req('url does not point to a valid page on this site')
+
+    if match.view_name != 'entries:entry':
+        return error.bad_req('url does not point to an entry on this site')
+
+    try:
+        entry = Entry.objects.get(pk=match.kwargs['id'])
+    except Entry.DoesNotExist:
+        return error.bad_req('url does not point to an existing entry')
+
+    return entry