filer: fail writes when the existing-entry lookup fails (#11699)

* filer: fail writes when the existing-entry lookup fails

CreateEntry only returned a FindEntry error for exclusive creates; a
regular write treated a transient store error as "entry not found" and,
on upsert stores, replaced the entry without loading its old chunks —
leaving them out of the cleanup process. Return any lookup error except
ErrNotFound for both paths.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* filer: test that a lookup failure fails a regular write

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/filer/filer.go

@@ -339,9 +339,9 @@ func (f *Filer) CreateEntry(ctx context.Context, entry *Entry, existing *Entry,
 	if oldEntry == nil {
 		var findErr error
 		oldEntry, findErr = f.FindEntry(ctx, entry.FullPath)
-		if o_excl && findErr != nil && !errors.Is(findErr, filer_pb.ErrNotFound) {
-			// An exclusive create cannot decide whether the path exists when
-			// the lookup itself failed; proceeding would upsert over it.
+		if findErr != nil && !errors.Is(findErr, filer_pb.ErrNotFound) {
+			// A create cannot decide whether the path exists when the lookup
+			// itself failed; proceeding would upsert over it.
 			return fmt.Errorf("find entry %s: %w", entry.FullPath, findErr)
 		}
 	}

weed/filer/filer_inode_test.go

@@ -265,3 +265,28 @@ func TestUpdateEntryBackfillsMissingLegacyInode(t *testing.T) {
 	assert.NotZero(t, stored.Attr.Inode)
 	assert.NotEqual(t, uint64(1), stored.Attr.Inode)
 }
+
+// A transient store error must fail the write instead of being mistaken for a
+// missing entry: on upsert stores proceeding would replace the entry without
+// loading the old chunks, orphaning them from cleanup.
+func TestCreateEntryFailsWhenLookupErrors(t *testing.T) {
+	f, store := newTestFilerWithStubStore()
+	path := util.FullPath("/dir/file.txt")
+	require.NoError(t, store.InsertEntry(context.Background(), &Entry{
+		FullPath: path,
+		Content:  []byte("existing"),
+	}))
+	store.findErr = errors.New("transient store failure")
+
+	entry := &Entry{
+		FullPath: path,
+		Attr:     Attr{Mode: 0o644},
+		Content:  []byte("overwrite"),
+	}
+	require.Error(t, f.CreateEntry(context.Background(), entry, nil, false, false, nil, false, f.MaxFilenameLength))
+
+	store.findErr = nil
+	stored, findErr := store.FindEntry(context.Background(), path)
+	require.NoError(t, findErr)
+	assert.Equal(t, "existing", string(stored.Content))
+}