fix: honor --dstNodes in balance-segment

[czs007]

What

balance-segment accepts a destination-node flag, but the value was never sent to querycoord.

BalanceSegmentCommand parses the flag into p.DstNodes and then builds the request without it:

req := &querypb.LoadBalanceRequest{
    Base:             &commonpb.MsgBase{TargetID: s.session.ServerID},
    CollectionID:     p.CollectionID,
    SealedSegmentIDs: p.SegmentIDs,
    SourceNodeIDs:    p.SourceNodes,
}   // p.DstNodes is never used

On the server side, an empty DstNodeIDs means "any RW node of the replica" — querycoord inserts replica.GetRWNodes() into the destination set and lets the assign policy pick. So the flag is silently ignored and the caller gets a destination they did not ask for, with no error.

Repro

Against a local Milvus 2.6.19 cluster (3 querynodes: 8, 10, 11):

$ balance-segment --collection 467844892567274894 \
      --segment 467844892568376490 --srcNodes 10 --dstNode 8
(success, no error)

$ show segment-loaded-grpc
ServerID 8     (segment not here)
ServerID 10    (segment not here)
ServerID 11
SegmentID: 467844892568376490 ... NumOfRows 120000     <- landed on 11, not the requested 8

Fix

Populate DstNodeIDs from the flag.

Per review feedback, DstNodes is []int64 with flag --dstNodes, mirroring SourceNodes / --srcNodes and the repeated DstNodeIDs field in the proto. An empty value keeps the previous behaviour (let querycoord pick), so existing usage is unaffected — and nothing could have depended on the old --dstNode spelling, since the value was never sent.

Test

Rebuilt and re-ran the same command against the same cluster; the segment now lands on the requested node. go build ./... and gofmt clean.

Backport to v1.1.x: #509.

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[congqixia]

can we use []int64 like SourceNodes for DstNodes?

[czs007]

Good catch, done in 1d01527.

DstNodes is now []int64 with flag --dstNodes, mirroring SourceNodes / --srcNodes and the repeated DstNodeIDs field in the proto. This also lets the 0 sentinel and the if p.DstNodes != 0 branch go away — an empty slice already means "let querycoord pick".

I renamed the flag from --dstNode to --dstNodes for symmetry; nothing depended on the old spelling since the value was never sent to querycoord in the first place. Happy to keep --dstNode as the flag name if you would rather not change it.

Backport #509 is updated the same way.

[congqixia]

/lgtm

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