server: introduce COCKROACH_FALLBACK_SPANCONFIG_NUM_REPLICAS_OVERRIDE

It controls what replication factor is used for ranges with no explicit
span configs set. This is a backportable form of the
spanconfig.store.fallback_config_override we added in cockroachdb#92466.

Release note: None

pkg/kv/kvserver/BUILD.bazel

@@ -415,6 +415,7 @@ go_test(
         "//pkg/util/contextutil",
         "//pkg/util/ctxgroup",
         "//pkg/util/encoding",
+        "//pkg/util/envutil",
         "//pkg/util/hlc",
         "//pkg/util/humanizeutil",
         "//pkg/util/leaktest",

pkg/kv/kvserver/client_spanconfigs_test.go

@@ -26,6 +26,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/sql/sqlutil"
 	"github.com/cockroachdb/cockroach/pkg/testutils"
 	"github.com/cockroachdb/cockroach/pkg/testutils/serverutils"
+	"github.com/cockroachdb/cockroach/pkg/util/envutil"
 	"github.com/cockroachdb/cockroach/pkg/util/hlc"
 	"github.com/cockroachdb/cockroach/pkg/util/leaktest"
 	"github.com/cockroachdb/errors"
@@ -100,6 +101,23 @@ func TestSpanConfigUpdateAppliedToReplica(t *testing.T) {
 	})
 }
 
+// TestFallbackSpanConfigOverride ensures that
+// COCKROACH_FALLBACK_SPANCONFIG_NUM_REPLICAS_OVERRIDE works as expected.
+func TestFallbackSpanConfigNumReplicasOverride(t *testing.T) {
+	defer leaktest.AfterTest(t)()
+	defer envutil.TestSetEnv(t,
+		"COCKROACH_FALLBACK_SPANCONFIG_NUM_REPLICAS_OVERRIDE", "42")()
+
+	ctx := context.Background()
+	s, _, _ := serverutils.StartServer(t, base.TestServerArgs{})
+	defer s.Stopper().Stop(context.Background())
+	scKVSubscriber := s.SpanConfigKVSubscriber().(spanconfig.KVSubscriber)
+
+	conf, err := scKVSubscriber.GetSpanConfigForKey(ctx, roachpb.RKey("non-existent"))
+	require.NoError(t, err)
+	require.Equal(t, int32(42), conf.NumReplicas)
+}
+
 type mockSpanConfigSubscriber struct {
 	callback func(ctx context.Context, config roachpb.Span)
 	spanconfig.Store

pkg/server/server.go

@@ -79,6 +79,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/ts"
 	"github.com/cockroachdb/cockroach/pkg/util"
 	"github.com/cockroachdb/cockroach/pkg/util/admission"
+	"github.com/cockroachdb/cockroach/pkg/util/envutil"
 	"github.com/cockroachdb/cockroach/pkg/util/goschedstats"
 	"github.com/cockroachdb/cockroach/pkg/util/hlc"
 	"github.com/cockroachdb/cockroach/pkg/util/log"
@@ -600,6 +601,12 @@ func NewServer(cfg Config, stopper *stop.Stopper) (*Server, error) {
 			// really only applies to user table ranges
 			fallbackConf.GCPolicy.IgnoreStrictEnforcement = true
 
+			// fallbackSpanConfigNumReplicasOverride controls what replication
+			// factor is used for ranges with no explicit span configs set.
+			var fallbackSpanConfigNumReplicasOverride = envutil.EnvOrDefaultInt(
+				"COCKROACH_FALLBACK_SPANCONFIG_NUM_REPLICAS_OVERRIDE", int(fallbackConf.NumReplicas))
+			fallbackConf.NumReplicas = int32(fallbackSpanConfigNumReplicasOverride)
+
 			spanConfig.subscriber = spanconfigkvsubscriber.New(
 				clock,
 				rangeFeedFactory,