✨(backend) add legacy conversion policy helpers

Legacy Office formats need an explicit conversion policy before they can be
opened for editing.

Keep the mapping small and configuration-driven so the backend only converts
formats intentionally forced by the WOPI client.

src/backend/drive/settings.py

@@ -1398,14 +1398,6 @@ class Base(Configuration):
     WOPI_LOCK_TIMEOUT = values.IntegerValue(
         30 * 60, environ_name="WOPI_LOCK_TIMEOUT", environ_prefix=None
     )
-<<<<<<< HEAD
-||||||| parent of f8297c68 (✨(backend) add OnlyOffice conversion backend)
-    WOPI_LEGACY_CONVERSION_TARGETS = {
-        "doc": "docx",
-        "xls": "xlsx",
-        "ppt": "pptx",
-    }
-=======
     WOPI_LEGACY_CONVERSION_TARGETS = {
         "doc": "docx",
         "xls": "xlsx",
@@ -1423,7 +1415,6 @@ class Base(Configuration):
     WOPI_ONLYOFFICE_CONVERT_DOWNLOAD_READ_TIMEOUT = values.IntegerValue(
         30, environ_name="WOPI_ONLYOFFICE_CONVERT_DOWNLOAD_READ_TIMEOUT", environ_prefix=None
     )
->>>>>>> f8297c68 (✨(backend) add OnlyOffice conversion backend)
     WOPI_DISABLE_CHAT = values.IntegerValue(
         0, environ_name="WOPI_DISABLE_CHAT", environ_prefix=None
     )

src/backend/wopi/conversion/__init__.py

@@ -0,0 +1 @@
+"""Server-to-server conversion of legacy Office files via WOPI providers."""

src/backend/wopi/conversion/policy.py

@@ -0,0 +1,42 @@
+"""Forced-conversion policy for WOPI legacy formats."""
+
+from django.conf import settings
+
+
+def _normalize(value):
+    return value.lower() if isinstance(value, str) else value
+
+
+def target_extension_for(source_extension):
+    """Return the converted extension for a legacy source extension.
+
+    Item.extension preserves filename case (REPORT.DOC -> "DOC"), so the
+    lookup must be case-insensitive.
+    """
+    if not source_extension:
+        return None
+
+    return settings.WOPI_LEGACY_CONVERSION_TARGETS.get(source_extension.lower())
+
+
+def is_forced_conversion(item, client_options):
+    """Return True when the active WOPI client forces a server-side conversion.
+
+    The decision is product policy, not discovery-derived: a client is allowed
+    to advertise an "edit" action for a legacy format and still require the
+    Drive backend to convert it first.
+    """
+    if not client_options:
+        return False
+
+    extension = _normalize(item.extension)
+    forced_extensions = {_normalize(e) for e in client_options.get("ForceConvertExtensions") or []}
+    if extension and extension in forced_extensions:
+        return True
+
+    mimetype = _normalize(item.mimetype)
+    forced_mimetypes = {_normalize(m) for m in client_options.get("ForceConvertMimetypes") or []}
+    if mimetype and mimetype in forced_mimetypes:
+        return True
+
+    return False

src/backend/wopi/tests/conversion/test_policy.py

@@ -0,0 +1,74 @@
+"""Tests for the conversion policy helpers."""
+
+from django.conf import settings
+
+from core import factories
+from wopi.conversion.policy import is_forced_conversion, target_extension_for
+
+
+def _item(filename="document.doc", mimetype="application/msword"):
+    """Build a minimal in-memory Item for policy tests."""
+    return factories.ItemFactory.build(filename=filename, mimetype=mimetype)
+
+
+def test_target_extension_for_known_legacy_formats():
+    """Map known legacy Office extensions to their modern equivalents."""
+    assert target_extension_for("doc") == "docx"
+    assert target_extension_for("xls") == "xlsx"
+    assert target_extension_for("ppt") == "pptx"
+
+
+def test_target_extension_for_unknown_format_returns_none():
+    """Return None for unsupported or empty source extensions."""
+    assert target_extension_for("docx") is None
+    assert target_extension_for("") is None
+    assert target_extension_for(None) is None
+
+
+def test_target_extension_for_is_case_insensitive():
+    """Match the target extension regardless of source extension case."""
+    assert target_extension_for("DOC") == "docx"
+    assert target_extension_for("Xls") == "xlsx"
+
+
+def test_legacy_conversion_targets_only_lists_legacy_formats():
+    """Restrict the conversion policy to legacy Office formats."""
+    assert set(settings.WOPI_LEGACY_CONVERSION_TARGETS) == {"doc", "xls", "ppt"}
+
+
+def test_is_forced_conversion_true_when_extension_listed():
+    """Force conversion when the item extension is listed."""
+    options = {"ForceConvertExtensions": ["doc", "xls", "ppt"]}
+    assert is_forced_conversion(_item(filename="document.doc"), options) is True
+
+
+def test_is_forced_conversion_true_when_mimetype_listed():
+    """Force conversion when the item mimetype is listed."""
+    options = {"ForceConvertMimetypes": ["application/msword"]}
+    item = _item(filename="document.unknown", mimetype="application/msword")
+    assert is_forced_conversion(item, options) is True
+
+
+def test_is_forced_conversion_false_when_neither_matches():
+    """Skip conversion when neither extension nor mimetype matches."""
+    options = {
+        "ForceConvertExtensions": ["doc"],
+        "ForceConvertMimetypes": ["application/msword"],
+    }
+    item = _item(filename="document.docx", mimetype="application/vnd.openxmlformats")
+    assert is_forced_conversion(item, options) is False
+
+
+def test_is_forced_conversion_false_when_options_missing():
+    """Skip conversion when client options are missing."""
+    assert is_forced_conversion(_item(), {}) is False
+    assert is_forced_conversion(_item(), None) is False
+
+
+def test_is_forced_conversion_extension_check_is_case_insensitive():
+    """Match forced extensions regardless of case."""
+    options = {"ForceConvertExtensions": ["doc"]}
+    assert is_forced_conversion(_item(filename="REPORT.DOC"), options) is True
+
+    options = {"ForceConvertExtensions": ["DOC"]}
+    assert is_forced_conversion(_item(filename="document.doc"), options) is True