mmaprototype: make conf read only in makeNormalizedSpanConfig

Previously, when fixing how makeNormalizedSpanConfig uses conf.NumVoters to
determine the number of voters, we incorrectly changed conf.NumVoters directly
to conf.NumReplicas when its value was 0. This commit corrects the behavior by
ensuring that the input conf is not modified when falling back to NumReplicas.

Author: wenyihu6

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

@@ -252,14 +252,12 @@ type internedLeasePreference struct {
 func makeNormalizedSpanConfig(
 	conf *roachpb.SpanConfig, interner *stringInterner,
 ) (*normalizedSpanConfig, error) {
-	if conf.NumVoters == 0 {
-		conf.NumVoters = conf.NumReplicas
-	}
+	numVoters := conf.GetNumVoters()
 	var normalizedConstraints, normalizedVoterConstraints []internedConstraintsConjunction
 	var err error
 	if conf.VoterConstraints != nil {
 		normalizedVoterConstraints, err = normalizeConstraints(
-			conf.VoterConstraints, conf.NumVoters, interner)
+			conf.VoterConstraints, numVoters, interner)
 		if err != nil {
 			return nil, err
 		}
@@ -269,7 +267,7 @@ func makeNormalizedSpanConfig(
 		if err != nil {
 			return nil, err
 		}
-	} else if (conf.NumReplicas-conf.NumVoters > 0) || len(normalizedVoterConstraints) == 0 {
+	} else if (conf.NumReplicas-numVoters > 0) || len(normalizedVoterConstraints) == 0 {
 		// - No constraints, but have some non-voters.
 		// - No voter constraints either.
 		// Need an empty constraints conjunction so that non-voters or voters have
@@ -287,7 +285,7 @@ func makeNormalizedSpanConfig(
 			constraints: interner.internConstraintsConj(conf.LeasePreferences[i].Constraints)})
 	}
 	nConf := &normalizedSpanConfig{
-		numVoters:        conf.NumVoters,
+		numVoters:        numVoters,
 		numReplicas:      conf.NumReplicas,
 		constraints:      normalizedConstraints,
 		voterConstraints: normalizedVoterConstraints,

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

@@ -97,7 +97,7 @@ func parseSpanConfig(t *testing.T, d *datadriven.TestData) roachpb.SpanConfig {
 }
 
 func printSpanConfig(b *strings.Builder, conf roachpb.SpanConfig) {
-	fmt.Fprintf(b, " num-replicas=%d num-voters=%d\n", conf.NumReplicas, conf.NumVoters)
+	fmt.Fprintf(b, " num-replicas=%d num-voters=%d\n", conf.NumReplicas, conf.GetNumVoters())
 	if len(conf.Constraints) > 0 {
 		fmt.Fprintf(b, " constraints:\n")
 		for _, cc := range conf.Constraints {