Fix delegator state inconsistency on put/delete pairs

[DracoLi]

Why this should be merged

Defensive fix for an internal consistency issue in baseStakers and diffStakers. The pending writes don't cancel out matching PutDelegator / DeleteDelegator pairs, so both addedDelegators and deletedDelegators can end up populated for the same TxID, inconsistent with the in-memory btree.

There's no way to trigger this normally. The bug requires the same delegator (same staker.TxID) to be both added and removed within a single diff, which no current code path does.

How this works

Cancel out matching put/delete pairs in two places:

• baseStakers.PutDelegator / DeleteDelegator: when a call is the inverse of an earlier call in the pending writes, undo the prior call instead of recording the new one.
• diffStakers.PutDelegator / DeleteDelegator: same shape applied to the in-flight diff. Without this, GetDelegatorIterator on a delete-then-readd diff would filter the re-added delegator out.

How this was tested

• TestDiffStakersDelegatorCancelOut — diff-level read correctness via GetDelegatorIterator / GetStakerIterator for both delete-then-put and put-then-delete. The delete-then-put subtest fails without the fix: deletedDelegators still contains the staker, so the iterator filters out the parent-supplied entry and returns nil.

• TestBaseStakersDelegatorCancelOutPersistence — writes the diff through writeCurrentDelegatorDiff into a memdb-backed linkeddb and checks the persisted row for both orderings. The delete-then-put subtest fails without the fix: writeCurrentDelegatorDiff sees the staker in deletedDelegators and removes the DB row.

Need to be documented in RELEASES.md?

No

[Copilot]

Pull request overview

Defensive fix to keep delegator diffs internally consistent when the same delegator is both put and deleted within a single diff cycle, ensuring pending writes and iterators don’t disagree (and persistence doesn’t accidentally delete or re-add rows).

Changes:

• Cancel out inverse PutDelegator/DeleteDelegator pairs in baseStakers pending writes.
• Apply the same cancel-out behavior in diffStakers to keep GetDelegatorIterator/GetStakerIterator correct under delete-then-readd and put-then-delete sequences.
• Add regression tests covering iterator correctness and persisted DB row behavior; fix iterator lifecycle management in existing diff tests.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
vms/platformvm/state/stakers.go	Adds cancel-out logic for delegator put/delete pairs in both base and diff staker trackers to prevent inconsistent added/deleted sets.
vms/platformvm/state/stakers_test.go	Adds tests for delegator cancel-out behavior at diff and persistence layers (but currently has iterator double-release issues).
vms/platformvm/state/diff_test.go	Ensures iterators are properly released in delegator iterator tests.
vms/platformvm/state/BUILD.bazel	Adds the linkeddb dependency needed by the new persistence test.

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[yacovm]

I don't think this can be triggered in reality because we can't add and remove and vice versa the same delegator in a block, so it should be fine adding this as a sanity check for future P-chain development.

[rrazvan1]

if we want to support this, we need to cover the situation where we delete + put, but we put a delegator with updated fields, for example a different weight.

This test is failing:

func TestStateDiffIntegration_PutThenDeleteSameDelegatorTxIDWithDifferentWeight(t *testing.T) {
	state := newTestState(t, memdb.New())

	diff, err := NewDiffOn(state, StakerAdditionAfterDeletionAllowed)
	require.NoError(t, err)

	delegator := newTestStaker(ids.GenerateTestID(), ids.GenerateTestNodeID())
	delegator.Weight = 2
	require.NoError(t, diff.PutCurrentDelegator(delegator))
	require.NoError(t, diff.Apply(state))

	diff, err = NewDiffOn(state, StakerAdditionAfterDeletionAllowed)
	require.NoError(t, err)

	lowerWeightDelegator := *delegator
	lowerWeightDelegator.Weight = delegator.Weight - 1

	require.NoError(t, diff.DeleteCurrentDelegator(delegator))
	require.NoError(t, diff.PutCurrentDelegator(&lowerWeightDelegator))
	require.NoError(t, diff.Apply(state))

	delegatorIterator, err := state.GetCurrentDelegatorIterator(delegator.SubnetID, delegator.NodeID)
	require.NoError(t, err)
	defer delegatorIterator.Release()

	require.Equal(t, []*Staker{&lowerWeightDelegator}, iterator.ToSlice(delegatorIterator))
}

[yacovm]

when I revert this change, the tests still pass 🫠

.