spanconfig/store: intern span configs

- Avoid the expensive proto.Equal() when computing split keys;
- Reduce memory overhead of the data structure

    $ dev bench pkg/spanconfig/spanconfigstore -f=BenchmarkStoreComputeSplitKey -v

    BenchmarkStoreComputeSplitKey
    BenchmarkStoreComputeSplitKey/num-entries=10000
    BenchmarkStoreComputeSplitKey/num-entries=10000-10                 90323 ns/op
    BenchmarkStoreComputeSplitKey/num-entries=100000
    BenchmarkStoreComputeSplitKey/num-entries=100000-10               915936 ns/op
    BenchmarkStoreComputeSplitKey/num-entries=1000000
    BenchmarkStoreComputeSplitKey/num-entries=1000000-10             9575781 ns/op

    $ benchstat old.txt new.txt # from previous commit

    name                                         old time/op  new time/op  delta
    StoreComputeSplitKey/num-entries=10000-10     431µs ± 0%    90µs ± 0%   ~     (p=1.000 n=1+1)
    StoreComputeSplitKey/num-entries=100000-10   4.31ms ± 0%  0.92ms ± 0%   ~     (p=1.000 n=1+1)
    StoreComputeSplitKey/num-entries=1000000-10  43.8ms ± 0%   9.6ms ± 0%   ~     (p=1.000 n=1+1)

Release note: None

pkg/spanconfig/spanconfigstore/BUILD.bazel

@@ -4,6 +4,7 @@ load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
 go_library(
     name = "spanconfigstore",
     srcs = [
+        "interner.go",
         "span_store.go",
         "store.go",
         "system_store.go",
@@ -19,6 +20,7 @@ go_library(
         "//pkg/spanconfig",
         "//pkg/util/iterutil",
         "//pkg/util/log",
+        "//pkg/util/protoutil",
         "//pkg/util/syncutil",
         "//pkg/util/timeutil",  # keep
         "@com_github_cockroachdb_errors//:errors",

pkg/spanconfig/spanconfigstore/interner.go

@@ -0,0 +1,88 @@
+// Copyright 2022 The Cockroach Authors.
+//
+// Use of this software is governed by the Business Source License
+// included in the file licenses/BSL.txt.
+//
+// As of the Change Date specified in that file, in accordance with
+// the Business Source License, use of this software will be governed
+// by the Apache License, Version 2.0, included in the file
+// licenses/APL.txt.
+
+package spanconfigstore
+
+import (
+	"context"
+
+	"github.com/cockroachdb/cockroach/pkg/roachpb"
+	"github.com/cockroachdb/cockroach/pkg/util/log"
+	"github.com/cockroachdb/cockroach/pkg/util/protoutil"
+)
+
+type internerID uint64
+
+// interner interns span config protos. It's a ref-counted data structure that
+// maintains a single copy of every unique config that that's been added to
+// it. Configs that are maintained are identified/retrivable using an
+// internerID. Configs can also be removed, and if there are no more references
+// to it, we no longer hold onto it in memory.
+type interner struct {
+	internIDAlloc internerID
+
+	configToID   map[string]internerID
+	idToConfig   map[internerID]roachpb.SpanConfig
+	idToRefCount map[internerID]uint64
+}
+
+func newInterner() *interner {
+	return &interner{
+		configToID:   make(map[string]internerID),
+		idToConfig:   make(map[internerID]roachpb.SpanConfig),
+		idToRefCount: make(map[internerID]uint64),
+	}
+}
+
+func (i *interner) add(ctx context.Context, conf roachpb.SpanConfig) internerID {
+	marshalled, err := protoutil.Marshal(&conf)
+	if err != nil {
+		log.Fatalf(ctx, "%v", err)
+	}
+
+	if id, found := i.configToID[string(marshalled)]; found {
+		i.idToRefCount[id]++
+		return id
+	}
+
+	i.internIDAlloc++
+
+	id := i.internIDAlloc
+	i.configToID[string(marshalled)] = id
+	i.idToConfig[i.internIDAlloc] = conf
+	i.idToRefCount[id] = 1
+	return id
+}
+
+func (i *interner) get(id internerID) (roachpb.SpanConfig, bool) {
+	conf, found := i.idToConfig[id]
+	return conf, found
+}
+
+func (i *interner) remove(ctx context.Context, id internerID) {
+	conf, found := i.idToConfig[id]
+	if !found {
+		return // nothing to do
+	}
+
+	i.idToRefCount[id]--
+	if i.idToRefCount[id] != 0 {
+		return // nothing to do
+	}
+
+	marshalled, err := protoutil.Marshal(&conf)
+	if err != nil {
+		log.Infof(ctx, "%v", err)
+	}
+
+	delete(i.idToConfig, id)
+	delete(i.idToRefCount, id)
+	delete(i.configToID, string(marshalled))
+}