fix: retain maintenance read compatibility with write safety fixes

Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/maintenance-read-compatibility_test.go

@@ -0,0 +1,83 @@
+// Copyright (c) 2026 Feng Ruohang
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+package cmd
+
+import (
+	"bytes"
+	"context"
+	"io"
+	"testing"
+)
+
+type compatibilityFailedOverwrite struct {
+	StorageAPI
+	commit bool
+}
+
+func (d compatibilityFailedOverwrite) RenameData(ctx context.Context, sv, sp string, fi FileInfo, dv, dp string, opts RenameOptions) (RenameDataResp, error) {
+	if d.commit {
+		if _, err := d.StorageAPI.RenameData(ctx, sv, sp, fi, dv, dp, opts); err != nil {
+			return RenameDataResp{}, err
+		}
+	}
+	return RenameDataResp{}, errDiskNotFound
+}
+
+// Maintenance compatibility requirement: after a completely joined failed
+// overwrite with one residue, keep the previously acknowledged quorum readable.
+// This tests a known history; it does not claim the resulting snapshot proves it.
+func TestMaintenancePartialFailedOverwriteAvailability(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+	dirs, err := getRandomDisks(16)
+	if err != nil {
+		t.Fatal(err)
+	}
+	obj, _, err := initObjectLayer(ctx, mustGetPoolEndpoints(0, dirs...))
+	if err != nil {
+		removeRoots(dirs)
+		t.Fatal(err)
+	}
+	z := obj.(*erasureServerPools)
+	previous := newObjectLayerFn()
+	setObjectLayer(z)
+	defer func() { cancel(); z.Shutdown(context.Background()); removeRoots(dirs); setObjectLayer(previous) }()
+	bucket := "maintenance-compatibility"
+	key := "partial-overwrite"
+	if err := z.MakeBucket(ctx, bucket, MakeBucketOptions{}); err != nil {
+		t.Fatal(err)
+	}
+	oldBody := bytes.Repeat([]byte("old-A"), 65536)
+	newBody := bytes.Repeat([]byte("new-B"), 65536)
+	put := func(body []byte) error {
+		_, err := z.PutObject(ctx, bucket, key, mustGetPutObjReader(t, bytes.NewReader(body), int64(len(body)), "", ""), ObjectOptions{})
+		return err
+	}
+	if err := put(oldBody); err != nil {
+		t.Fatal(err)
+	}
+	set := z.serverPools[0].getHashedSet(key)
+	getDisks := set.getDisks
+	disks := getDisks()
+	wrapped := append([]StorageAPI(nil), disks...)
+	for i, d := range disks {
+		wrapped[i] = compatibilityFailedOverwrite{d, i == 0}
+	}
+	set.getDisks = func() []StorageAPI { return wrapped }
+	err = put(newBody)
+	set.getDisks = getDisks
+	if err == nil {
+		t.Fatal("partial overwrite must fail")
+	}
+	t.Logf("Overwrite completed with failure; one new residue, fifteen original shards: %v", err)
+	r, err := z.GetObjectNInfo(ctx, bucket, key, nil, nil, ObjectOptions{})
+	if err != nil {
+		t.Fatalf("MAINTENANCE COMPATIBILITY: previously readable acknowledged value now refused: %v", err)
+	}
+	data, err := io.ReadAll(r)
+	r.Close()
+	if err != nil || !bytes.Equal(data, oldBody) {
+		t.Fatalf("old acknowledged payload mismatch: bytes=%d err=%v", len(data), err)
+	}
+}