Feature: Add additional caching support to suggestions and metadata (#5414)

* Adds ETag and Last-Modified headers to suggestions, metadata and previews

* Slight update to the suggestions etag

* Small user message for why classifier didn't train again

src/documents/conditionals.py

@@ -0,0 +1,87 @@
+import pickle
+from datetime import datetime
+from typing import Optional
+
+from django.conf import settings
+
+from documents.classifier import DocumentClassifier
+from documents.models import Document
+
+
+def suggestions_etag(request, pk: int) -> Optional[str]:
+    """
+    Returns an optional string for the ETag, allowing browser caching of
+    suggestions if the classifier has not been changed and the suggested dates
+    setting is also unchanged
+
+    TODO: It would be nice to not duplicate the partial loading and the loading
+    between here and the actual classifier
+    """
+    if not settings.MODEL_FILE.exists():
+        return None
+    with open(settings.MODEL_FILE, "rb") as f:
+        schema_version = pickle.load(f)
+        if schema_version != DocumentClassifier.FORMAT_VERSION:
+            return None
+        _ = pickle.load(f)
+        last_auto_type_hash: bytes = pickle.load(f)
+        return f"{last_auto_type_hash}:{settings.NUMBER_OF_SUGGESTED_DATES}"
+
+
+def suggestions_last_modified(request, pk: int) -> Optional[datetime]:
+    """
+    Returns the datetime of classifier last modification.  This is slightly off,
+    as there is not way to track the suggested date setting modification, but it seems
+    unlikely that changes too often
+    """
+    if not settings.MODEL_FILE.exists():
+        return None
+    with open(settings.MODEL_FILE, "rb") as f:
+        schema_version = pickle.load(f)
+        if schema_version != DocumentClassifier.FORMAT_VERSION:
+            return None
+        last_doc_change_time = pickle.load(f)
+        return last_doc_change_time
+
+
+def metadata_etag(request, pk: int) -> Optional[str]:
+    """
+    Metadata is extracted from the original file, so use its checksum as the
+    ETag
+    """
+    try:
+        doc = Document.objects.get(pk=pk)
+        return doc.checksum
+    except Document.DoesNotExist:
+        return None
+    return None
+
+
+def metadata_last_modified(request, pk: int) -> Optional[datetime]:
+    """
+    Metadata is extracted from the original file, so use its modified.  Strictly speaking, this is
+    not the modification of the original file, but of the database object, but might as well
+    error on the side of more cautious
+    """
+    try:
+        doc = Document.objects.get(pk=pk)
+        return doc.modified
+    except Document.DoesNotExist:
+        return None
+    return None
+
+
+def preview_etag(request, pk: int) -> Optional[str]:
+    """
+    ETag for the document preview, using the original or archive checksum, depending on the request
+    """
+    try:
+        doc = Document.objects.get(pk=pk)
+        use_original = (
+            "original" in request.query_params
+            and request.query_params["original"] == "true"
+        )
+        return doc.checksum if use_original else doc.archive_checksum
+    except Document.DoesNotExist:
+        return None
+    return None