release-26.2: mmaprototype: fix nil-pointer panic when StorePool sees stores MMA does not

pkg/kv/kvserver/allocator/allocatorimpl/allocator.go

@@ -1914,7 +1914,7 @@ func (a Allocator) RebalanceTarget(
 	// continues to correspond to the original candidate set and source store,
 	// even as candidates are removed.
 	for {
-		target, existingCandidate, bestIdx = bestRebalanceTarget(a.randGen, results, a.as)
+		target, existingCandidate, bestIdx = bestRebalanceTarget(ctx, a.randGen, results, a.as)
 		if target == nil {
 			return zero, zero, "", false
 		}
@@ -2555,7 +2555,7 @@ func (a *Allocator) TransferLeaseTarget(
 		for _, s := range sl.Stores {
 			targetStores = append(targetStores, s.StoreID)
 		}
-		handle := a.as.BuildMMARebalanceAdvisor(source.StoreID, targetStores)
+		handle := a.as.BuildMMARebalanceAdvisor(ctx, source.StoreID, targetStores)
 		var bestOption roachpb.ReplicaDescriptor
 		candidates := make([]roachpb.ReplicaDescriptor, 0, len(validTargets))
 		bestOptionLeaseCount := int32(math.MaxInt32)

pkg/kv/kvserver/allocator/allocatorimpl/allocator_scorer.go

@@ -1952,7 +1952,10 @@ func rankedCandidateListForRebalancing(
 // Contract: responsible for making sure that the returned bestIdx has the
 // corresponding MMA advisor in advisors.
 func bestRebalanceTarget(
-	randGen allocatorRand, options []rebalanceOptions, as *mmaintegration.AllocatorSync,
+	ctx context.Context,
+	randGen allocatorRand,
+	options []rebalanceOptions,
+	as *mmaintegration.AllocatorSync,
 ) (target, existingCandidate *candidate, bestIdx int) {
 	bestIdx = -1
 	var bestTarget *candidate
@@ -1982,7 +1985,7 @@ func bestRebalanceTarget(
 		for _, cand := range options[bestIdx].candidates {
 			stores = append(stores, cand.store.StoreID)
 		}
-		options[bestIdx].advisor = as.BuildMMARebalanceAdvisor(options[bestIdx].existing.store.StoreID, stores)
+		options[bestIdx].advisor = as.BuildMMARebalanceAdvisor(ctx, options[bestIdx].existing.store.StoreID, stores)
 	}
 
 	// Copy the selected target out of the candidates slice before modifying

pkg/kv/kvserver/allocator/allocatorimpl/allocator_scorer_test.go

@@ -448,7 +448,7 @@ func TestBestRebalanceTarget(t *testing.T) {
 	var i int
 	for {
 		i++
-		target, existing, _ := bestRebalanceTarget(allocRand, candidates, a.as)
+		target, existing, _ := bestRebalanceTarget(ctx, allocRand, candidates, a.as)
 		if len(expectedTargets) == 0 {
 			if target == nil {
 				break

pkg/kv/kvserver/allocator/mmaprototype/BUILD.bazel

@@ -61,6 +61,7 @@ go_test(
         "logging_test.go",
         "memo_helper_test.go",
         "mma_metrics_test.go",
+        "rebalance_advisor_test.go",
     ],
     data = glob(["testdata/**"]),
     embed = [":mmaprototype"],

pkg/kv/kvserver/allocator/mmaprototype/allocator.go

@@ -193,7 +193,9 @@ type Allocator interface {
 	//
 	// TODO(sumeer): merge the above comment with the comment in the
 	// implementation.
-	BuildMMARebalanceAdvisor(existing roachpb.StoreID, cands []roachpb.StoreID) *MMARebalanceAdvisor
+	BuildMMARebalanceAdvisor(
+		ctx context.Context, existing roachpb.StoreID, cands []roachpb.StoreID,
+	) *MMARebalanceAdvisor
 
 	// IsInConflictWithMMA is called by the allocator sync to determine if the
 	// given candidate is in conflict with the existing store.

pkg/kv/kvserver/allocator/mmaprototype/cluster_state.go

@@ -2409,6 +2409,22 @@ func (cs *clusterState) getStoreReportedLoad(storeID roachpb.StoreID) (roachpb.N
 	return 0, nil
 }
 
+// hasStore reports whether storeID is known to mma's clusterState.
+// Used by BuildMMARebalanceAdvisor to filter candidate slices that come
+// from the legacy allocator's StorePool view, which can include stores
+// that gossip has not yet announced to MMA.
+func (cs *clusterState) hasStore(storeID roachpb.StoreID) bool {
+	_, ok := cs.stores[storeID]
+	return ok
+}
+
+// notHasStore is the negation of hasStore, exposed so callers can pass
+// it as a method value to slices.IndexFunc / slices.DeleteFunc without
+// an inline closure.
+func (cs *clusterState) notHasStore(storeID roachpb.StoreID) bool {
+	return !cs.hasStore(storeID)
+}
+
 func (cs *clusterState) getNodeReportedLoad(nodeID roachpb.NodeID) *NodeLoad {
 	if nodeState, ok := cs.nodes[nodeID]; ok {
 		return &nodeState.NodeLoad

pkg/kv/kvserver/allocator/mmaprototype/load.go

@@ -484,6 +484,15 @@ func (mm *meansMemo) getStoreLoadSummary(
 // stores may contain duplicate storeIDs, in which case computeMeansForStoreSet
 // should deduplicate processing of the stores. stores should be immutable.
 //
+// Precondition: every storeID in stores must be known to loadProvider
+// (loadProvider.getStoreReportedLoad must return a non-nil *storeLoad).
+// Callers receiving storeIDs from outside MMA (notably the legacy
+// allocator's StorePool view, which can run ahead of MMA's gossip
+// updates) must filter unknown stores out first. BuildMMARebalanceAdvisor
+// is the canonical filtering point; clusterState.hasStore is the helper
+// to use. A violation is asserted in test builds and logged-and-skipped
+// in production rather than nil-dereferenced (see #170703).
+//
 // If stores is empty, computeMeansForStoreSet returns the zero meansLoad
 // and ok=false. Callers must check ok before consuming the returned means;
 // the zero value would otherwise misclassify stores as overloaded due to
@@ -510,6 +519,17 @@ func computeMeansForStoreSet(
 		if _, ok := scratchStores[storeID]; ok {
 			continue
 		}
+		if sload == nil {
+			// Precondition violation: a caller passed an unknown storeID. In
+			// test builds this panics, surfacing the bug; in production it
+			// logs and skips, avoiding the nil-pointer crash from #170703.
+			// context.Background is used because plumbing ctx into this
+			// internal helper would touch every memo path; the assertion is a
+			// last-resort safety net and is not expected to fire.
+			assertTruef(context.Background(), false,
+				"computeMeansForStoreSet: storeID %d not known to loadProvider", storeID)
+			continue
+		}
 		n++
 		scratchStores[storeID] = struct{}{}
 		for j := range sload.reportedLoad {
@@ -530,6 +550,11 @@ func computeMeansForStoreSet(
 			scratchNodes[nodeID] = loadProvider.getNodeReportedLoad(nodeID)
 		}
 	}
+	if n == 0 {
+		// Every storeID hit the nil-sload guard above. Bail out before the
+		// divisions below would panic with division by zero.
+		return meansLoad{}, false
+	}
 	for i := range means.storeLoad.load {
 		if means.storeLoad.capacity[i] != UnknownCapacity {
 			// NB: capacity can be 0 for CPURate when nodeCPURateUsage >>

pkg/kv/kvserver/allocator/mmaprototype/rebalance_advisor.go

@@ -7,6 +7,7 @@ package mmaprototype
 
 import (
 	"context"
+	"slices"
 
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
 	"github.com/cockroachdb/cockroach/pkg/util/log"
@@ -71,29 +72,59 @@ func NoopMMARebalanceAdvisor() *MMARebalanceAdvisor {
 // duplicate storeIDs. It is up to computeMeansForStoreSet to handle
 // de-duplication of storeIDs from the cands list.
 //
+// Callers may pass storeIDs (in `existing` or `cands`) that MMA's
+// clusterState has not yet seen. This happens during startup, when the
+// legacy allocator's StorePool reflects gossiped store descriptors that
+// MMA has not yet been notified of via SetStore. Such stores are filtered
+// out of `cands` before computing means; if `existing` itself is unknown,
+// a NoopMMARebalanceAdvisor is returned because MMA has no load history
+// against which to judge candidates. The same asymmetry is acknowledged
+// in updateStoreStatuses, which logs and skips unknown stores rather
+// than panicking. See #170703.
+//
 // The returned advisor should be passed to IsInConflictWithMMA as a helper to
 // determine if a candidate is vetoed by the multi-metric allocator due to
 // running counter to its goals.
 func (a *allocatorState) BuildMMARebalanceAdvisor(
-	existing roachpb.StoreID, cands []roachpb.StoreID,
+	ctx context.Context, existing roachpb.StoreID, cands []roachpb.StoreID,
 ) *MMARebalanceAdvisor {
 	// a.cs is mutated by gossip-driven callbacks (e.g. ProcessStoreLoadMsg) and
 	// must only be accessed under a.mu. The other public methods on
 	// allocatorState follow this discipline.
 	a.mu.Lock()
 	defer a.mu.Unlock()
+	if !a.cs.hasStore(existing) {
+		// MMA has no load history for the source store, so it cannot judge
+		// whether any candidate is more overloaded than existing. Fall back to
+		// a no-op advisor rather than risk a misclassification (or, before the
+		// hasStore filter on cands below was added, a nil-pointer panic).
+		log.KvDistribution.VEventf(ctx, 2,
+			"mma skipping advisor: existing store s%d not yet known to mma", existing)
+		return NoopMMARebalanceAdvisor()
+	}
+	// Drop any cand the integration layer learned about (via gossip / StorePool)
+	// before MMA did. computeMeansForStoreSet would otherwise nil-deref on the
+	// missing storeLoad. In the steady state every cand is known, so the
+	// IndexFunc walk completes without allocating; only when an unknown cand
+	// is present do we copy and compact.
+	if slices.IndexFunc(cands, a.cs.notHasStore) != -1 {
+		cp := make([]roachpb.StoreID, len(cands))
+		copy(cp, cands)
+		cands = slices.DeleteFunc(cp, a.cs.notHasStore)
+	}
 	// TODO(wenyihu6): for simplicity, we create a new scratchNodes every call.
 	// We should reuse the scratchNodes instead.
 	scratchNodes := map[roachpb.NodeID]*NodeLoad{}
 	scratchStores := map[roachpb.StoreID]struct{}{}
 	cands = append(cands, existing)
 	means, ok := computeMeansForStoreSet(a.cs, cands, scratchNodes, scratchStores)
 	if !ok {
-		// Unreachable: cands always contains at least `existing`. Assert in
-		// test builds; in production, fall back to a no-op advisor rather than
-		// return a zero-valued means that would misclassify stores. Gating on
-		// !ok avoids variadic arg boxing on the success path.
-		assertTruef(context.Background(), false, "computeMeansForStoreSet returned !ok for non-empty cands=%v", cands)
+		// Unreachable: cands always contains at least `existing`, which we
+		// just verified is known to MMA. Assert in test builds; in production,
+		// fall back to a no-op advisor rather than return a zero-valued means
+		// that would misclassify stores. Gating on !ok avoids variadic arg
+		// boxing on the success path.
+		assertTruef(ctx, false, "computeMeansForStoreSet returned !ok for non-empty cands=%v", cands)
 		return NoopMMARebalanceAdvisor()
 	}
 	return &MMARebalanceAdvisor{

pkg/kv/kvserver/allocator/mmaprototype/rebalance_advisor_test.go

@@ -0,0 +1,70 @@
+// Copyright 2026 The Cockroach Authors.
+//
+// Use of this software is governed by the CockroachDB Software License
+// included in the /LICENSE file.
+
+package mmaprototype
+
+import (
+	"context"
+	"math/rand"
+	"testing"
+
+	"github.com/cockroachdb/cockroach/pkg/roachpb"
+	"github.com/cockroachdb/cockroach/pkg/util/leaktest"
+	"github.com/cockroachdb/cockroach/pkg/util/log"
+	"github.com/cockroachdb/cockroach/pkg/util/timeutil"
+	"github.com/stretchr/testify/require"
+)
+
+// TestBuildMMARebalanceAdvisorUnknownStores covers the crash described in
+// #170703: before the fix in this commit, BuildMMARebalanceAdvisor would
+// nil-pointer-deref if any storeID it was given (either `existing` or
+// anything in `cands`) was not yet known to MMA's clusterState. This
+// happens during startup, when the caller (the legacy allocator) builds
+// candidate lists from StorePool's gossip-driven view before MMA has been
+// notified of the corresponding stores via SetStore.
+//
+// After the fix, unknown cands are silently dropped, and an unknown
+// existing falls back to a no-op advisor (which always reports no
+// conflict).
+func TestBuildMMARebalanceAdvisorUnknownStores(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	defer log.Scope(t).Close(t)
+
+	ctx := context.Background()
+
+	const (
+		knownStore   roachpb.StoreID = 1
+		unknownStore roachpb.StoreID = 99
+	)
+
+	makeAllocator := func() *allocatorState {
+		a := NewAllocatorState(timeutil.DefaultTimeSource{}, rand.New(rand.NewSource(0)))
+		a.SetStore(StoreAttributesAndLocality{
+			StoreID: knownStore,
+			NodeID:  roachpb.NodeID(knownStore),
+		})
+		return a
+	}
+
+	t.Run("unknown cand", func(t *testing.T) {
+		a := makeAllocator()
+		// Unknown cand is silently dropped; the advisor is built over just
+		// the known existing store.
+		advisor := a.BuildMMARebalanceAdvisor(ctx, knownStore, []roachpb.StoreID{unknownStore})
+		require.NotNil(t, advisor)
+		require.False(t, advisor.disabled)
+		require.Equal(t, knownStore, advisor.existingStoreID)
+	})
+
+	t.Run("unknown existing", func(t *testing.T) {
+		a := makeAllocator()
+		// MMA cannot judge candidates against a source it does not know;
+		// the no-op advisor short-circuits IsInConflictWithMMA to false.
+		advisor := a.BuildMMARebalanceAdvisor(ctx, unknownStore, nil)
+		require.NotNil(t, advisor)
+		require.True(t, advisor.disabled)
+		require.False(t, a.IsInConflictWithMMA(ctx, knownStore, advisor, false /* cpuOnly */))
+	})
+}

pkg/kv/kvserver/mmaintegration/allocator_sync.go

@@ -58,7 +58,9 @@ type mmaState interface {
 	// MMARebalanceAdvisor for the given existing store and candidates. The
 	// advisor should be later passed to IsInConflictWithMMA to determine if a
 	// given candidate is in conflict with the existing store.
-	BuildMMARebalanceAdvisor(existing roachpb.StoreID, cands []roachpb.StoreID) *mmaprototype.MMARebalanceAdvisor
+	BuildMMARebalanceAdvisor(
+		ctx context.Context, existing roachpb.StoreID, cands []roachpb.StoreID,
+	) *mmaprototype.MMARebalanceAdvisor
 	// IsInConflictWithMMA is called by the allocator sync to determine if the
 	// given candidate is in conflict with the existing store.
 	IsInConflictWithMMA(ctx context.Context, cand roachpb.StoreID, advisor *mmaprototype.MMARebalanceAdvisor, cpuOnly bool) bool

pkg/kv/kvserver/mmaintegration/thrashing.go

@@ -106,12 +106,12 @@ import (
 // after calling this function, so the caller is responsible for keeping track
 // of the returned advisor and associating it.
 func (as *AllocatorSync) BuildMMARebalanceAdvisor(
-	existing roachpb.StoreID, cands []roachpb.StoreID,
+	ctx context.Context, existing roachpb.StoreID, cands []roachpb.StoreID,
 ) *mmaprototype.MMARebalanceAdvisor {
 	if kvserverbase.GetLoadBasedRebalancingMode(&as.st.SV) != kvserverbase.LBRebalancingMultiMetricAndCount {
 		return mmaprototype.NoopMMARebalanceAdvisor()
 	}
-	return as.mmaAllocator.BuildMMARebalanceAdvisor(existing, cands)
+	return as.mmaAllocator.BuildMMARebalanceAdvisor(ctx, existing, cands)
 }
 
 // IsInConflictWithMMA determines if a candidate conflicts with MMA's goals.