fix(pools): preserve delete markers and recheck migration cleanup

Move solitary and null delete markers instead of dropping them, including
idempotent UUID marker retries. Check that a reader's source version still
exists before moving it, and preserve the established read winner for
conflicting timestamp ties.

Retire only the versions that were successfully moved while holding the
pooled object lock. Recheck the source and retained copies before cleanup,
so concurrent deletes and uncertain destination reads cannot revive data.
Count remaining markers when verifying decommission completion.

Add two/three-pool walker, history, purge, concurrent-delete, read-fault,
and timestamp-tie regressions.

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

cmd/erasure-server-pool-migration_test.go

@@ -116,15 +116,18 @@ func migrationTestPools(t *testing.T) (*erasureServerPools, string) {
 	t.Helper()
 	z, bucket := consistencyPools(t)
 	for _, pool := range z.serverPools {
-		for _, set := range pool.sets {
-			original := set.getDisks
-			disks := append([]StorageAPI(nil), original()...)
-			for i := range disks {
-				disks[i] = tagTestCapacityDisk{StorageAPI: disks[i]}
+		pool.erasureDisksMu.Lock()
+		previous := pool.erasureDisks
+		wrapped := make([][]StorageAPI, len(previous))
+		for i, disks := range previous {
+			wrapped[i] = append([]StorageAPI(nil), disks...)
+			for j := range wrapped[i] {
+				wrapped[i][j] = tagTestCapacityDisk{StorageAPI: wrapped[i][j]}
 			}
-			set.getDisks = func() []StorageAPI { return disks }
-			t.Cleanup(func() { set.getDisks = original })
 		}
+		pool.erasureDisks = wrapped
+		pool.erasureDisksMu.Unlock()
+		t.Cleanup(func() { pool.erasureDisksMu.Lock(); pool.erasureDisks = previous; pool.erasureDisksMu.Unlock() })
 	}
 	return z, bucket
 }
@@ -705,6 +708,7 @@ func TestPoolsMigrationKeepsCurrentCopy(t *testing.T) {
 							want, other := "source", "other"
 							if offset == 0 {
 								other = "source"
+								opts.PreserveETag = oi.ETag // interrupted copies preserve the original multipart ETag
 							} else if offset < 0 {
 								want = "other"
 							}