fix(storage): evaluate multipart preconditions across pools

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

CHANGELOG.md

@@ -1,11 +1,11 @@
 # Changelog
 
-## Unreleased — main as of 2026-09-13
+## Unreleased
 
-The coordinated source is merged through `5d955b5b7444f8a3ab550ce92713607998f89c0d`.
+The entries below describe source changes on main since the latest published Server.
 **The latest published Server remains 20260903.** These changes are not in its
 binaries, packages or images. See the [component matrix](https://silo.pgsty.com/compatibility/versions/)
-and [complete commit range](https://github.com/pgsty/silo/compare/RELEASE.2026-09-03T13-18-01Z...5d955b5b7444f8a3ab550ce92713607998f89c0d).
+and [complete commit range](https://github.com/pgsty/silo/compare/RELEASE.2026-09-03T13-18-01Z...main).
 
 ### Authorization and security
 
@@ -22,6 +22,17 @@ and [complete commit range](https://github.com/pgsty/silo/compare/RELEASE.2026-0
 
 ### Object storage and replication
 
+- Evaluate conditional multipart completion against the logical current object
+  across all pools while holding the existing object lock. A stale `If-Match`
+  can no longer replace newer data in another pool, and the current ETag is no
+  longer rejected because the upload resides next to an older copy. Conditions
+  are evaluated once; a current delete marker counts as an absent object.
+  **Availability change:** if metadata cannot be read from any pool, conditional
+  completion fails even when another pool can still serve GET/HEAD. This also
+  applies when the unreadable pool may not hold the object: absence cannot be
+  verified. Retry after the pool recovers. Unconditional completion and the
+  single-pool path retain their existing behavior.
+
 - Reconcile ordinary single-object version DELETE across all pools, including
   null versions, delete markers and unqualified directory-marker DELETE. This
   applies the deletion to every resolved pool copy under existing quorum

cmd/erasure-server-pool-multipart-cond_test.go

@@ -0,0 +1,172 @@
+// Copyright (c) 2026 Feng Ruohang
+//
+// This file is part of Silo Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package cmd
+
+import (
+	"bytes"
+	"testing"
+	"time"
+)
+
+// A multipart completion's If-Match must be evaluated against the logical
+// latest object across pools, not against the copy local to a pool.
+//
+// Multi-pool write placement is not sticky (getPoolIdx picks by available
+// space even for existing objects), so an upload and a newer overwrite of
+// the same name routinely end up in different pools. Uploads are pinned to
+// their pools directly: routing through z.NewMultipartUpload would make the
+// placement depend on the space-weighted random choice.
+func TestPoolsMultipartConditionalUsesLogicalLatest(t *testing.T) {
+	z, bucket := consistencyPools(t)
+	ctx := t.Context()
+
+	ifMatch := func(etag string) (opts ObjectOptions) {
+		return ObjectOptions{
+			HasIfMatch: true,
+			CheckPrecondFn: func(oi ObjectInfo) bool {
+				return oi.ETag != etag
+			},
+		}
+	}
+
+	uploadPart := func(t *testing.T, bucket, object, uploadID string) []CompletePart {
+		t.Helper()
+		pi, err := z.PutObjectPart(ctx, bucket, object, uploadID, 1,
+			mustGetPutObjReader(t, bytes.NewBufferString("part"), 4, "", ""), ObjectOptions{})
+		if err != nil {
+			t.Fatal(err)
+		}
+		return []CompletePart{{PartNumber: 1, ETag: pi.ETag}}
+	}
+
+	// Scenario A: the uploaded If-Match carries the stale ETag of the pool-0
+	// copy while the logical latest object lives in pool 1. The completion
+	// must fail with 412 instead of shadowing the newer logical state.
+	objectA := "cond-mp-stale-etag"
+	base := time.Now()
+	oldA := putConsistencyObject(t, z, bucket, objectA, 0, "old", ObjectOptions{MTime: base.Add(-2 * time.Minute)})
+	mpA, err := z.serverPools[0].NewMultipartUpload(ctx, bucket, objectA, ObjectOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	newerA := putConsistencyObject(t, z, bucket, objectA, 1, "new", ObjectOptions{
+		MTime: base.Add(-time.Minute),
+	})
+
+	latest, _, err := z.getLatestObjectInfoWithIdx(ctx, bucket, objectA, ObjectOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if latest.ETag != newerA.ETag {
+		t.Fatalf("logical latest should be the pool-1 copy: got %s want %s", latest.ETag, newerA.ETag)
+	}
+
+	if _, err = z.CompleteMultipartUpload(ctx, bucket, objectA, mpA.UploadID,
+		uploadPart(t, bucket, objectA, mpA.UploadID), ifMatch(oldA.ETag)); err == nil {
+		t.Fatal("If-Match with the stale pool-0 ETag must not complete over the newer pool-1 object")
+	} else if _, ok := err.(PreConditionFailed); !ok {
+		t.Fatalf("expected PreconditionFailed, got %v", err)
+	}
+
+	if latest, _, err = z.getLatestObjectInfoWithIdx(ctx, bucket, objectA, ObjectOptions{}); err != nil {
+		t.Fatal(err)
+	}
+	if latest.ETag != newerA.ETag {
+		t.Fatalf("the newer pool-1 object must remain the logical latest, got %s", latest.ETag)
+	}
+
+	// Scenario B: the uploaded If-Match carries the logical latest ETag (the
+	// pool-1 copy) while the upload sits next to the stale pool-0 copy. The
+	// precondition is satisfied, so completion must succeed and its result
+	// must become the logical latest.
+	objectB := "cond-mp-latest-etag"
+	putConsistencyObject(t, z, bucket, objectB, 0, "old", ObjectOptions{MTime: base.Add(-2 * time.Minute)})
+	mpB, err := z.serverPools[0].NewMultipartUpload(ctx, bucket, objectB, ObjectOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	newerB := putConsistencyObject(t, z, bucket, objectB, 1, "new", ObjectOptions{
+		MTime: base.Add(-time.Minute),
+	})
+	oiB, err := z.CompleteMultipartUpload(ctx, bucket, objectB, mpB.UploadID,
+		uploadPart(t, bucket, objectB, mpB.UploadID), ifMatch(newerB.ETag))
+	if err != nil {
+		t.Fatalf("If-Match with the logical latest ETag must complete, got %v", err)
+	}
+	if oiB.ETag == "" {
+		t.Fatal("completion returned an empty ETag")
+	}
+	if latest, _, err = z.getLatestObjectInfoWithIdx(ctx, bucket, objectB, ObjectOptions{}); err != nil {
+		t.Fatal(err)
+	}
+	if latest.ETag != oiB.ETag {
+		t.Fatalf("the completed object must be the logical latest: got %s want %s", latest.ETag, oiB.ETag)
+	}
+
+	// Scenario C: the upload lives in pool 1 with the newer copy while pool 0
+	// holds the stale one. A set-local evaluation order would let pool 0's
+	// stale copy fail the request before pool 1 is reached; the logical
+	// latest ETag must complete.
+	objectC := "cond-mp-upload-other-pool"
+	putConsistencyObject(t, z, bucket, objectC, 0, "old", ObjectOptions{MTime: base.Add(-2 * time.Minute)})
+	mpC, err := z.serverPools[1].NewMultipartUpload(ctx, bucket, objectC, ObjectOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	newerC := putConsistencyObject(t, z, bucket, objectC, 1, "new", ObjectOptions{
+		MTime: base.Add(-time.Minute),
+	})
+	if _, err = z.CompleteMultipartUpload(ctx, bucket, objectC, mpC.UploadID,
+		uploadPart(t, bucket, objectC, mpC.UploadID), ifMatch(newerC.ETag)); err != nil {
+		t.Fatalf("If-Match with the logical latest ETag must complete regardless of upload pool, got %v", err)
+	}
+
+	// Scenario D: pool 1 holds the newer copy but cannot be read. An
+	// unreadable pool may contain the newest state, so the unverifiable
+	// condition must fail the request rather than pass it against pool 0's
+	// stale ETag.
+	objectD := "cond-mp-unreadable-pool"
+	oldD := putConsistencyObject(t, z, bucket, objectD, 0, "old", ObjectOptions{MTime: base.Add(-2 * time.Minute)})
+	mpD, err := z.serverPools[0].NewMultipartUpload(ctx, bucket, objectD, ObjectOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	newerD := putConsistencyObject(t, z, bucket, objectD, 1, "new", ObjectOptions{
+		MTime: base.Add(-time.Minute),
+	})
+	if latest, _, err = z.getLatestObjectInfoWithIdx(ctx, bucket, objectD, ObjectOptions{}); err != nil {
+		t.Fatal(err)
+	}
+	if latest.ETag != newerD.ETag {
+		t.Fatalf("logical latest before faulting pool 1 should be its copy: got %s want %s", latest.ETag, newerD.ETag)
+	}
+	set := z.serverPools[1].getHashedSet(objectD)
+	getDisks := set.getDisks
+	faulty := append([]StorageAPI(nil), getDisks()...)
+	for i := range faulty {
+		faulty[i] = consistencyReadFaultDisk{StorageAPI: faulty[i], bucket: bucket, object: objectD}
+	}
+	set.getDisks = func() []StorageAPI { return faulty }
+	defer func() { set.getDisks = getDisks }()
+
+	_, err = z.CompleteMultipartUpload(ctx, bucket, objectD, mpD.UploadID,
+		uploadPart(t, bucket, objectD, mpD.UploadID), ifMatch(oldD.ETag))
+	if !isErrReadQuorum(err) {
+		t.Fatalf("expected an insufficient read quorum error, got %v", err)
+	}
+}

cmd/erasure-server-pool-multipart-http_test.go

@@ -0,0 +1,341 @@
+// Copyright (c) 2026 Feng Ruohang
+//
+// This file is part of Silo Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package cmd
+
+import (
+	"bytes"
+	"encoding/xml"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	xhttp "github.com/minio/minio/internal/http"
+)
+
+func multipartConditionRequest(t *testing.T, router http.Handler, method, target, body string, headers map[string]string) *httptest.ResponseRecorder {
+	t.Helper()
+	req, err := newTestSignedRequestV4(method, target, int64(len(body)), strings.NewReader(body), globalActiveCred.AccessKey, globalActiveCred.SecretKey, headers)
+	if err != nil {
+		t.Fatal(err)
+	}
+	rec := httptest.NewRecorder()
+	router.ServeHTTP(rec, req)
+	return rec
+}
+
+// The server writes the wire spelling ETag directly into Header; unlike a
+// network response, httptest's map has not canonicalized it to Etag.
+func multipartConditionResponseETag(rec *httptest.ResponseRecorder) string {
+	for key, values := range rec.Header() {
+		if strings.EqualFold(key, xhttp.ETag) && len(values) > 0 {
+			return strings.Trim(values[0], "\"")
+		}
+	}
+	return ""
+}
+
+func multipartConditionUpload(t *testing.T, z *erasureServerPools, bucket, object string, owner int, opts ObjectOptions) (string, []CompletePart) {
+	t.Helper()
+	mp, err := z.serverPools[owner].NewMultipartUpload(t.Context(), bucket, object, opts)
+	if err != nil {
+		t.Fatal(err)
+	}
+	part, err := z.serverPools[owner].PutObjectPart(t.Context(), bucket, object, mp.UploadID, 1, mustGetPutObjReader(t, bytes.NewBufferString("replacement"), 11, "", ""), ObjectOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	return mp.UploadID, []CompletePart{{PartNumber: 1, ETag: part.ETag}}
+}
+
+func multipartConditionCompleteBody(parts []CompletePart) string {
+	data, err := xml.Marshal(CompleteMultipartUpload{Parts: parts})
+	if err != nil {
+		panic(err)
+	}
+	return string(data)
+}
+
+func multipartConditionError(t *testing.T, rec *httptest.ResponseRecorder, code string) {
+	t.Helper()
+	decoder := xml.NewDecoder(strings.NewReader(rec.Body.String()))
+	var response APIErrorResponse
+	if err := decoder.Decode(&response); err != nil || response.Code != code {
+		t.Fatalf("expected %s error, got %q: %v", code, rec.Body.String(), err)
+	}
+	if err := decoder.Decode(&response); err != io.EOF {
+		t.Fatalf("expected exactly one error response, got %q: %v", rec.Body.String(), err)
+	}
+}
+
+// State is deliberately placed per pool; the final request uses signed HTTP
+// and the real handler, precondition callback, erasure metadata and rename.
+func TestPoolsMultipartConditionalHTTPMatrix(t *testing.T) {
+	z, _ := consistencyPools(t)
+	bucket, router, err := initAPIHandlerTest(t.Context(), z, nil, MakeBucketOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for owner := range 2 {
+		for _, localOld := range []bool{false, true} {
+			for _, condition := range []string{"match-old", "match-current", "none-match"} {
+				t.Run(fmt.Sprintf("owner=%d/old=%t/%s", owner, localOld, condition), func(t *testing.T) {
+					object := fmt.Sprintf("http-%d-%t-%s", owner, localOld, condition)
+					oldETag := "old-does-not-exist"
+					if localOld {
+						oldETag = putConsistencyObject(t, z, bucket, object, owner, "old", ObjectOptions{MTime: UTCNow().Add(-time.Hour)}).ETag
+					}
+					id, parts := multipartConditionUpload(t, z, bucket, object, owner, ObjectOptions{})
+					current := putConsistencyObject(t, z, bucket, object, 1-owner, "current", ObjectOptions{MTime: UTCNow().Add(-time.Minute)})
+					head := multipartConditionRequest(t, router, http.MethodHead, getGetObjectURL("", bucket, object), "", nil)
+					if head.Code != 200 || multipartConditionResponseETag(head) != current.ETag {
+						t.Fatalf("bad HEAD: %d %v", head.Code, head.Header())
+					}
+					h := map[string]string{xhttp.IfMatch: "\"" + oldETag + "\""}
+					want := 412
+					if condition == "match-current" {
+						h[xhttp.IfMatch] = "\"" + current.ETag + "\""
+						want = 200
+					}
+					if condition == "none-match" {
+						h = map[string]string{xhttp.IfNoneMatch: "*"}
+					}
+					rec := multipartConditionRequest(t, router, http.MethodPost, getCompleteMultipartUploadURL("", bucket, object, id), multipartConditionCompleteBody(parts), h)
+					t.Logf("HEAD current=%s, requested=%v, complete HTTP=%d", current.ETag, h, rec.Code)
+					if rec.Code != want {
+						t.Errorf("want HTTP %d, got %d: %s", want, rec.Code, rec.Body.String())
+					}
+					if want == 412 {
+						multipartConditionError(t, rec, "PreconditionFailed")
+						got := multipartConditionRequest(t, router, http.MethodGet, getGetObjectURL("", bucket, object), "", nil)
+						if got.Code != 200 || got.Body.String() != "current" {
+							t.Errorf("rejected request must preserve current data: %d %q", got.Code, got.Body.String())
+						}
+						if _, err := z.serverPools[owner].ListObjectParts(t.Context(), bucket, object, id, 0, 10, ObjectOptions{}); err != nil {
+							t.Errorf("rejected request consumed upload: %v", err)
+						}
+					}
+				})
+			}
+		}
+	}
+}
+
+func TestPoolsMultipartConditionalHTTPAbsentObject(t *testing.T) {
+	for _, deleted := range []bool{false, true} {
+		for _, match := range []bool{false, true} {
+			t.Run(fmt.Sprintf("delete-marker=%t/if-match=%t", deleted, match), func(t *testing.T) {
+				z, _ := consistencyPools(t)
+				bucket, router, err := initAPIHandlerTest(t.Context(), z, nil, MakeBucketOptions{VersioningEnabled: deleted})
+				if err != nil {
+					t.Fatal(err)
+				}
+				object := "http-absent"
+				if deleted {
+					putConsistencyObject(t, z, bucket, object, 0, "old", ObjectOptions{Versioned: true, MTime: UTCNow().Add(-time.Hour)})
+					_, err = z.serverPools[1].DeleteObject(t.Context(), bucket, object, ObjectOptions{Versioned: true, VersionID: mustGetUUID(), DeleteMarker: true, MTime: UTCNow().Add(-time.Minute)})
+					if err != nil {
+						t.Fatal(err)
+					}
+				}
+				id, parts := multipartConditionUpload(t, z, bucket, object, 0, ObjectOptions{Versioned: deleted})
+				headers := map[string]string{xhttp.IfNoneMatch: "*"}
+				want := http.StatusOK
+				if match {
+					headers = map[string]string{xhttp.IfMatch: "\"missing\""}
+					want = http.StatusNotFound
+				}
+				rec := multipartConditionRequest(t, router, http.MethodPost, getCompleteMultipartUploadURL("", bucket, object, id), multipartConditionCompleteBody(parts), headers)
+				if rec.Code != want {
+					t.Fatalf("expected HTTP %d, got %d: %s", want, rec.Code, rec.Body.String())
+				}
+				if match {
+					multipartConditionError(t, rec, "NoSuchKey")
+					if _, err := z.serverPools[0].ListObjectParts(t.Context(), bucket, object, id, 0, 10, ObjectOptions{}); err != nil {
+						t.Errorf("rejected request consumed upload: %v", err)
+					}
+				}
+			})
+		}
+	}
+}
+
+// All writes below use ordinary signed S3 requests. The result must be correct
+// for every placement; the matrix above deterministically covers split pools.
+func TestPoolsMultipartConditionalHTTPNormalRouting(t *testing.T) {
+	z, _ := consistencyPools(t)
+	bucket, router, err := initAPIHandlerTest(t.Context(), z, nil, MakeBucketOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	object := "normal-routing"
+	url := getPutObjectURL("", bucket, object)
+	old := multipartConditionRequest(t, router, http.MethodPut, url, "old-data", nil)
+	if old.Code != http.StatusOK {
+		t.Fatalf("initial PUT %d: %s", old.Code, old.Body.String())
+	}
+	init := multipartConditionRequest(t, router, http.MethodPost, url+"?uploads", "", nil)
+	if init.Code != http.StatusOK {
+		t.Fatalf("init %d: %s", init.Code, init.Body.String())
+	}
+	var mp InitiateMultipartUploadResponse
+	if err := xml.Unmarshal(init.Body.Bytes(), &mp); err != nil {
+		t.Fatal(err)
+	}
+	part := multipartConditionRequest(t, router, http.MethodPut, getPutObjectPartURL("", bucket, object, mp.UploadID, "1"), "replacement", nil)
+	if part.Code != http.StatusOK {
+		t.Fatalf("part %d: %s", part.Code, part.Body.String())
+	}
+	parts := []CompletePart{{PartNumber: 1, ETag: multipartConditionResponseETag(part)}}
+	newer := multipartConditionRequest(t, router, http.MethodPut, url, "newer-data", nil)
+	if newer.Code != http.StatusOK {
+		t.Fatalf("new PUT %d: %s", newer.Code, newer.Body.String())
+	}
+	head := multipartConditionRequest(t, router, http.MethodHead, url, "", nil)
+	if head.Code != http.StatusOK || multipartConditionResponseETag(head) != multipartConditionResponseETag(newer) {
+		t.Fatalf("HEAD did not pick new object: %d %v", head.Code, head.Header())
+	}
+	rec := multipartConditionRequest(t, router, http.MethodPost, getCompleteMultipartUploadURL("", bucket, object, mp.UploadID), multipartConditionCompleteBody(parts), map[string]string{xhttp.IfMatch: "\"" + multipartConditionResponseETag(old) + "\""})
+	if rec.Code != http.StatusPreconditionFailed {
+		t.Errorf("stale If-Match should be HTTP 412, got %d: %s", rec.Code, rec.Body.String())
+	}
+	get := multipartConditionRequest(t, router, http.MethodGet, url, "", nil)
+	if get.Code != http.StatusOK || get.Body.String() != "newer-data" {
+		t.Errorf("conditional completion changed newer data: %d %q", get.Code, get.Body.String())
+	}
+}
+
+func TestPoolsMultipartConditionalUnreadablePool(t *testing.T) {
+	z, bucket := consistencyPools(t)
+	object := "quorum-with-readable-copy"
+	old := putConsistencyObject(t, z, bucket, object, 0, "readable", ObjectOptions{MTime: UTCNow().Add(-time.Hour)})
+	putConsistencyObject(t, z, bucket, object, 1, "hidden-newer", ObjectOptions{MTime: UTCNow().Add(-time.Minute)})
+	id, parts := multipartConditionUpload(t, z, bucket, object, 0, ObjectOptions{})
+	set := z.serverPools[1].getHashedSet(object)
+	original := set.getDisks
+	disks := append([]StorageAPI(nil), original()...)
+	for i := range disks {
+		disks[i] = consistencyReadFaultDisk{StorageAPI: disks[i], bucket: bucket, object: object}
+	}
+	set.getDisks = func() []StorageAPI { return disks }
+	defer func() { set.getDisks = original }()
+	read, readErr := z.GetObjectInfo(t.Context(), bucket, object, ObjectOptions{})
+	t.Logf("ordinary GET lookup: etag=%s err=%v", read.ETag, readErr)
+	called := 0
+	_, err := z.CompleteMultipartUpload(t.Context(), bucket, object, id, parts, ObjectOptions{HasIfMatch: true, CheckPrecondFn: func(oi ObjectInfo) bool { called++; return oi.ETag != old.ETag }})
+	if !isErrReadQuorum(err) {
+		t.Errorf("conditional write must fail on unreadable pool, got %v", err)
+	}
+	if called != 0 {
+		t.Errorf("callback evaluated without complete state: %d calls", called)
+	}
+}
+
+func TestPoolsMultipartConditionalLatestVersionAndCallbackOnce(t *testing.T) {
+	for _, explicit := range []bool{false, true} {
+		t.Run(fmt.Sprintf("explicit=%t", explicit), func(t *testing.T) {
+			z, bucket := consistencyPools(t)
+			object := "tie-version"
+			opts := ObjectOptions{MTime: UTCNow().Add(-time.Hour), Versioned: explicit}
+			if explicit {
+				opts.VersionID = mustGetUUID()
+			}
+			current := putConsistencyObject(t, z, bucket, object, 0, "first", opts)
+			putConsistencyObject(t, z, bucket, object, 1, "second", opts)
+			if explicit {
+				current = putConsistencyObject(t, z, bucket, object, 1, "latest-other-version", ObjectOptions{Versioned: true, MTime: UTCNow().Add(-time.Minute)})
+			}
+			id, parts := multipartConditionUpload(t, z, bucket, object, 1, opts)
+			called := 0
+			opts.CheckPrecondFn = func(oi ObjectInfo) bool { called++; return oi.ETag != current.ETag }
+			opts.MTime = time.Time{}
+			opts.HasIfMatch = true
+			_, err := z.CompleteMultipartUpload(t.Context(), bucket, object, id, parts, opts)
+			if err != nil {
+				t.Errorf("logical current object should match: %v", err)
+			}
+			if called != 1 {
+				t.Errorf("condition evaluated %d times; want exactly once", called)
+			}
+		})
+	}
+}
+
+func TestPoolsMultipartConditionalConcurrentCompletes(t *testing.T) {
+	z, bucket := consistencyPools(t)
+	object := "concurrent-completes"
+	old := putConsistencyObject(t, z, bucket, object, 0, "old", ObjectOptions{MTime: UTCNow().Add(-time.Hour)})
+	putConsistencyObject(t, z, bucket, object, 1, "old", ObjectOptions{MTime: old.ModTime})
+	ids := make([]string, 2)
+	parts := make([][]CompletePart, 2)
+	for i := range 2 {
+		ids[i], parts[i] = multipartConditionUpload(t, z, bucket, object, i, ObjectOptions{})
+	}
+	entered := make(chan struct{})
+	release := make(chan struct{})
+	var gate, releaseOnce sync.Once
+	defer releaseOnce.Do(func() { close(release) })
+	errs := make([]error, 2)
+	var wg sync.WaitGroup
+	// Let pool 1's completion hold the object lock before pool 0's starts.
+	// Once pool 1 commits, a set-local read in pool 0 would still see the
+	// old ETag and incorrectly accept the second completion.
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		_, errs[1] = z.CompleteMultipartUpload(t.Context(), bucket, object, ids[1], parts[1], ObjectOptions{HasIfMatch: true, CheckPrecondFn: func(oi ObjectInfo) bool {
+			gate.Do(func() { close(entered); <-release })
+			return oi.ETag != old.ETag
+		}})
+	}()
+	select {
+	case <-entered:
+	case <-time.After(10 * time.Second):
+		t.Fatal("first completion did not enter its condition callback")
+	}
+	started := make(chan struct{})
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		close(started)
+		_, errs[0] = z.CompleteMultipartUpload(t.Context(), bucket, object, ids[0], parts[0], ObjectOptions{HasIfMatch: true, CheckPrecondFn: func(oi ObjectInfo) bool { return oi.ETag != old.ETag }})
+	}()
+	<-started
+	releaseOnce.Do(func() { close(release) })
+	wg.Wait()
+	success, failed := 0, 0
+	for _, err := range errs {
+		var p PreConditionFailed
+		switch {
+		case err == nil:
+			success++
+		case errors.As(err, &p):
+			failed++
+		default:
+			t.Errorf("unexpected completion error %v", err)
+		}
+	}
+	if success != 1 || failed != 1 {
+		t.Errorf("CAS writers: success=%d conditional failures=%d errors=%v", success, failed, errs)
+	}
+}

cmd/erasure-server-pool-multipart-preconditions_test.go

@@ -0,0 +1,135 @@
+// Copyright (c) 2026 Feng Ruohang
+//
+// This file is part of Silo Object Storage stack
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package cmd
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"testing"
+	"time"
+)
+
+func TestPoolsMultipartConditionMatrix(t *testing.T) {
+	for owner := range 2 {
+		for _, withOld := range []bool{false, true} {
+			for _, condition := range []string{"match-current", "match-old", "none-match-any"} {
+				t.Run(fmt.Sprintf("upload-pool=%d/old-copy=%t/%s", owner, withOld, condition), func(t *testing.T) {
+					z, bucket := consistencyPools(t)
+					object := "conditional-multipart"
+					oldETag := "arbitrary-old"
+					if withOld {
+						old := putConsistencyObject(t, z, bucket, object, owner, "old", ObjectOptions{MTime: UTCNow().Add(-time.Hour)})
+						oldETag = old.ETag
+					}
+					mp, err := z.serverPools[owner].NewMultipartUpload(t.Context(), bucket, object, ObjectOptions{})
+					if err != nil {
+						t.Fatal(err)
+					}
+					part, err := z.serverPools[owner].PutObjectPart(t.Context(), bucket, object, mp.UploadID, 1, mustGetPutObjReader(t, bytes.NewBufferString("replacement"), 11, "", ""), ObjectOptions{})
+					if err != nil {
+						t.Fatal(err)
+					}
+					current := putConsistencyObject(t, z, bucket, object, 1-owner, "current", ObjectOptions{MTime: UTCNow().Add(-time.Minute)})
+					visible, err := z.GetObjectInfo(t.Context(), bucket, object, ObjectOptions{})
+					if err != nil || visible.ETag != current.ETag {
+						t.Fatalf("invalid current state: %v", err)
+					}
+					opts := ObjectOptions{HasIfMatch: condition != "none-match-any", CheckPrecondFn: func(oi ObjectInfo) bool {
+						switch condition {
+						case "match-current":
+							return oi.ETag != current.ETag
+						case "match-old":
+							return oi.ETag != oldETag
+						default:
+							return true
+						}
+					}}
+					_, err = z.CompleteMultipartUpload(t.Context(), bucket, object, mp.UploadID, []CompletePart{{PartNumber: 1, ETag: part.ETag}}, opts)
+					if condition == "match-current" {
+						if err != nil {
+							t.Errorf("correct logical If-Match rejected: %v", err)
+						}
+					} else {
+						var expected PreConditionFailed
+						if !errors.As(err, &expected) {
+							t.Errorf("logical condition must fail, got %v", err)
+						}
+					}
+				})
+			}
+		}
+	}
+}
+
+func TestPoolsMultipartConditionBoundaries(t *testing.T) {
+	for _, state := range []string{"missing", "latest-delete-marker", "unreadable-other-pool"} {
+		for _, match := range []bool{false, true} {
+			t.Run(fmt.Sprintf("%s/if-match=%t", state, match), func(t *testing.T) {
+				z, bucket := consistencyPools(t)
+				object := "conditional-boundary"
+				versioned := state == "latest-delete-marker"
+				if versioned {
+					putConsistencyObject(t, z, bucket, object, 0, "old", ObjectOptions{Versioned: true, MTime: UTCNow().Add(-time.Hour)})
+					if _, err := z.serverPools[1].DeleteObject(t.Context(), bucket, object, ObjectOptions{Versioned: true, VersionID: mustGetUUID(), DeleteMarker: true, MTime: UTCNow().Add(-time.Minute)}); err != nil {
+						t.Fatal(err)
+					}
+				}
+				mp, err := z.serverPools[0].NewMultipartUpload(t.Context(), bucket, object, ObjectOptions{Versioned: versioned})
+				if err != nil {
+					t.Fatal(err)
+				}
+				part, err := z.serverPools[0].PutObjectPart(t.Context(), bucket, object, mp.UploadID, 1, mustGetPutObjReader(t, bytes.NewBufferString("new"), 3, "", ""), ObjectOptions{})
+				if err != nil {
+					t.Fatal(err)
+				}
+				if state == "unreadable-other-pool" {
+					set := z.serverPools[1].getHashedSet(object)
+					getDisks := set.getDisks
+					faulty := append([]StorageAPI(nil), getDisks()...)
+					for i := range faulty {
+						faulty[i] = consistencyReadFaultDisk{StorageAPI: faulty[i], bucket: bucket, object: object}
+					}
+					set.getDisks = func() []StorageAPI { return faulty }
+					defer func() { set.getDisks = getDisks }()
+				}
+				opts := ObjectOptions{Versioned: versioned, HasIfMatch: match, CheckPrecondFn: func(ObjectInfo) bool { return true }}
+				_, err = z.CompleteMultipartUpload(t.Context(), bucket, object, mp.UploadID, []CompletePart{{PartNumber: 1, ETag: part.ETag}}, opts)
+				switch {
+				case state == "unreadable-other-pool":
+					if !isErrReadQuorum(err) {
+						t.Errorf("unreadable pool must not mean absence: %v", err)
+					}
+				case match:
+					if !isErrObjectNotFound(err) {
+						t.Errorf("If-Match against logical absence should report absence: %v", err)
+					}
+				default:
+					if err != nil {
+						t.Errorf("If-None-Match against logical absence must succeed: %v", err)
+					}
+				}
+				if err != nil {
+					if _, lerr := z.serverPools[0].ListObjectParts(t.Context(), bucket, object, mp.UploadID, 0, 10, ObjectOptions{}); lerr != nil {
+						t.Errorf("failed condition consumed upload: %v", lerr)
+					}
+				}
+			})
+		}
+	}
+}

cmd/erasure-server-pool.go

@@ -2154,6 +2154,47 @@ func (z *erasureServerPools) CompleteMultipartUpload(ctx context.Context, bucket
 			defer lk.Unlock(lkctx)
 		}
 		opts.NoLock = true
+
+		// A conditional completion must be evaluated against the logical
+		// latest object across pools, under the object write lock held for
+		// this operation. The pool hosting the upload may only hold a stale
+		// duplicate, so its set-local check would both accept an outdated
+		// ETag and reject the current one. An unreadable pool is not
+		// absence: it may hold the newest copy, so a read that cannot be
+		// verified fails the request instead of passing the condition.
+		// Once satisfied, the callback is cleared so the set layer does not
+		// re-evaluate it against its local copy.
+		if opts.CheckPrecondFn != nil {
+			copies, lerr := z.objectPoolInfos(ctx, bucket, encodeDirObject(object), ObjectOptions{
+				// Conditions always compare the logical current object,
+				// independently of the completion's destination version.
+				VersionID:        "",
+				Versioned:        opts.Versioned,
+				VersionSuspended: opts.VersionSuspended,
+				NoAuditLog:       true,
+			})
+			var latest ObjectInfo
+			if lerr == nil {
+				latest = copies[0].ObjInfo
+				if latest.DeleteMarker {
+					// A delete-marker latest reads as an absent key, matching
+					// the set layer's getObjectInfo.
+					lerr = toObjectErr(errFileNotFound, bucket, object)
+				}
+			}
+			if lerr == nil && opts.CheckPrecondFn(latest) {
+				return ObjectInfo{}, PreConditionFailed{}
+			}
+			if lerr != nil && !isErrVersionNotFound(lerr) && !isErrObjectNotFound(lerr) {
+				return ObjectInfo{}, lerr
+			}
+			// if object doesn't exist return error for If-Match conditional requests
+			// If-None-Match should be allowed to proceed for non-existent objects
+			if lerr != nil && opts.HasIfMatch && (isErrObjectNotFound(lerr) || isErrVersionNotFound(lerr)) {
+				return ObjectInfo{}, lerr
+			}
+			opts.CheckPrecondFn = nil
+		}
 	}
 
 	// Hold write locks to verify uploaded parts, also disallows any

docs/investigations/multipart-conditional-completion.md

@@ -0,0 +1,57 @@
+# Conditional multipart completion across pools
+
+## Defect and scope
+
+An unfinished multipart upload can remain in one pool while another pool holds
+the logical current object. Evaluating `If-Match` against the upload pool's
+local copy can accept a stale ETag or reject the current ETag. The pools object
+lock serializes writes, but a local read still does not identify the logical
+current object.
+
+This layout does not require rebalance. Commit
+`83b2ad418b15ff0fa78175e2014d78d02046edd8` (upstream #21115) made `getPoolIdx`
+choose an available pool even when `pinfo.Err == nil`: normal overwrites and
+upload initiation can select different pools. This change predates the SILO
+multi-pool consistency work. The deterministic regression fixtures place copies
+and uploads directly in real erasure pools; they do not claim to run rebalance.
+
+## Minimal correction
+
+For multi-pool conditional completion, retain the existing object lock and use
+`objectPoolInfos` to read the logical current object before completing the
+upload. An unreadable pool is an error, not proof of absence. The first sorted
+copy supplies the ETag and encryption metadata used by the existing callback.
+A current delete marker is treated as an absent key. `If-Match` then fails for
+an absent object; `If-None-Match: *` may proceed.
+
+Use explicit read options with an empty `VersionID` and `NoAuditLog: true`.
+The precondition concerns the logical current object, independently of an
+internal completion's destination version. After a successful check, clear
+the callback before entering the set layer, so it is evaluated only once.
+No new lock, storage format, replica cleanup algorithm or distributed protocol
+is introduced. Single-pool and unconditional completion retain their existing
+paths.
+
+## Availability and validation
+
+If any pool cannot supply the required metadata, conditional completion fails,
+even when GET/HEAD can still read a copy from another pool. The unreadable pool
+might hold a newer object, a delete marker, or no copy at all; none of these
+possibilities can be assumed. Retry after recovery. This behavior is recorded
+in the unreleased changelog.
+
+The regression suite covers both upload-pool directions, stale/current ETags,
+`If-None-Match: *`, absent objects and delete markers, read-quorum errors,
+explicit destination versions, tied modification times, callback counts,
+upload preservation, signed HTTP error bodies and concurrent completions.
+The ordinary HTTP routing control accepts every valid placement; deterministic
+fixtures provide the cross-pool regression gate.
+
+## Separate follow-up scope
+
+The pool-placement change and conditional checks in `PutObject` and
+`NewMultipartUpload` require separate assessment. This completion fix does not
+repair those paths. In particular, PUT has live destination-version and
+preserved-ETag semantics, so its repair must not copy this completion-specific
+empty-VersionID rule without examining that contract. Parallelizing the shared
+pool metadata reader is also outside this correctness fix.