fix(replication): verify-before-destroy in VolumeCopy, check.disk, and over-replication trim (#9943)

* volume: verify before destroy in VolumeCopy and replication repair

Four data-safety fixes around copy/repair paths that could destroy or
resurrect data before verifying the source or survivors.

(a) VolumeCopy no longer deletes a pre-existing local replica up front.
The delete is deferred until ReadVolumeFileStatus on the source succeeds,
so a transient source outage (or a retry after one) can no longer wipe a
healthy destination replica. Gated on source readability only; size/count
comparisons are intentionally not used because they invert legitimately
after divergent vacuum/compaction. Mirrored in the Rust volume server.

(b) volume.check.disk no longer resurrects vacuumed-deleted needles. A
key present-and-live on the source but entirely absent on the target is
ambiguous: it may be a genuine missing write, or a needle deleted on the
target and then vacuumed (its index entry and any tombstone are gone). An
individual needle AppendAtNs has no monotonic relation to a vacuum
watermark, so the old cutoff heuristic could not tell them apart. Without
positive proof the absence is a missing write, the safe default is to NOT
push it back. Tradeoff: a real missing write may go unrepaired until a
tombstone-aware path exists, but we never raise back deleted data.

(c) Over-replication trim no longer resurrects needles or removes the
wrong replica. The pre-delete sync now runs read-only (divergence check
only) instead of writing the doomed replica's needles into the survivor.
pickOneReplicaToDelete only ever removes the smallest of multiple healthy
writable replicas; it refuses the trim when doing so would leave only
read-only/integrity-flagged survivors, since file_count>0 alone cannot
prove the survivor's .dat is readable.

(d) Incomplete-volume (.note) cleanup keeps the shared .vif when an .ecx
for the same vid coexists on the disk, so removing an interrupted regular
copy cannot strip a coexisting EC volume's info file. VolumeCopy now
surfaces .note write/remove errors instead of ignoring them. In the Rust
volume server (where a persisting note is actually reachable) the .note
check moves below the empty-stub sweep and EC validation, keeps the .vif
on EC coexistence, and the mount path fails when a .note still persists.

* shell: scope the over-replication writable-survivor guard to the trim path only

The writable-survivor guard (never trim down to a read-only survivor) lived
inside the shared pickOneReplicaToDelete, so it also gated the misplaced-volume
relocation via pickOneMisplacedVolume -- a misplaced read-only volume (e.g. a
full one) would silently stop being rebalanced. Extract pickSmallestReplica
for the relocation path (which deletes-and-recreates and must act on read-only
replicas), and keep the writable-survivor guard only in pickOneReplicaToDelete
used by the over-replication trim.

* seaweed-volume: recompute keep_vif after invalid-EC cleanup in the .note path

keep_vif used the pre-validation ecx_exists snapshot, so when the EC-validation
step above removed the invalid .ecx/shards, the .note cleanup still preserved a
now-orphaned .vif. Re-check .ecx existence at cleanup time, matching the Go
hasEcxFile re-check.

* shell: keep placement when picking an over-replication victim to delete

The trim picked the smallest writable replica without regard to placement, so
it could delete the only replica in a required failure domain (e.g. with "100"
and replicas dc1 + two in dc2, deleting dc1 leaves both survivors in dc2).
Prefer a writable replica whose removal still satisfies placement, falling back
to the smallest writable only when none does.

weed/shell/command_volume_check_disk_test.go

@@ -6,8 +6,66 @@ import (
 	"time"
 
 	"github.com/seaweedfs/seaweedfs/weed/pb/master_pb"
+	"github.com/seaweedfs/seaweedfs/weed/storage/needle_map"
+	"github.com/seaweedfs/seaweedfs/weed/storage/types"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
 )
 
+// TestDoVolumeCheckDiskDoesNotResurrectAbsentNeedle verifies that a needle
+// present-and-live on the source but entirely absent on the target is NOT
+// pushed back by default. Such an absence is indistinguishable from a needle
+// that was deleted on the target and then vacuumed away, so resurrecting it
+// would raise back data the operator deleted.
+func TestDoVolumeCheckDiskDoesNotResurrectAbsentNeedle(t *testing.T) {
+	sourceDB, targetDB := needle_map.NewMemDb(), needle_map.NewMemDb()
+	defer sourceDB.Close()
+	defer targetDB.Close()
+
+	// Source has a live needle; target lacks it (e.g. deleted+vacuumed there).
+	if err := sourceDB.Set(types.NeedleId(1001), types.ToOffset(8), types.Size(123)); err != nil {
+		t.Fatalf("seed source: %v", err)
+	}
+
+	var buf bytes.Buffer
+	vcd := &volumeCheckDisk{
+		commandEnv: &CommandEnv{
+			option: &ShellOptions{
+				GrpcDialOption: grpc.WithTransportCredentials(insecure.NewCredentials()),
+			},
+		},
+		writer:             &buf,
+		now:                time.Now(),
+		applyChanges:       false, // simulation: should not even read the source blob
+		nonRepairThreshold: 1,
+		// resurrectMissingNeedles left false: the safe default.
+	}
+
+	// Source points at an unreachable address: with the fix no blob read is
+	// attempted, so this is never contacted. The pre-fix code queues the
+	// absent needle and tries to read it from here, surfacing as an error.
+	source := &VolumeReplica{
+		location: &location{"dc1", "r1", &master_pb.DataNodeInfo{Id: "127.0.0.1:1"}},
+		info:     &master_pb.VolumeInformationMessage{Id: 7},
+	}
+	target := &VolumeReplica{
+		location: &location{"dc1", "r2", &master_pb.DataNodeInfo{Id: "127.0.0.1:2"}},
+		info:     &master_pb.VolumeInformationMessage{Id: 7},
+	}
+
+	// With the safe default, the absent needle is skipped before any network
+	// read, so this returns cleanly with no changes. On the pre-fix code the
+	// needle is queued and a source blob read is attempted, which has no server
+	// to reach and surfaces as an error (or a resurrection) instead.
+	hasChanges, err := vcd.doVolumeCheckDisk(sourceDB, targetDB, source, target)
+	if err != nil {
+		t.Fatalf("doVolumeCheckDisk returned error: %v", err)
+	}
+	if hasChanges {
+		t.Fatalf("absent-on-target needle was resurrected; expected no changes")
+	}
+}
+
 type testCommandVolumeCheckDisk struct {
 	commandVolumeCheckDisk
 }