[#48] pool: add Close method

Fix occasional panic in tests:
```
> for i in (seq 1 100); go test -race -count=1 ./pool/... ; end
...
{"level":"warn","ts":1635251466.567485,"caller":"pool/pool.go:122","msg":"failed to create neofs session token for client","address":"peer0","error":"error session"}
panic: Fail in goroutine after TestTwoNodes has completed

goroutine 6 [running]:
testing.(*common).Fail(0xc0002e1380)
        /usr/lib/go/src/testing/testing.go:710 +0x1b4
testing.(*common).FailNow(0xc0002e1380)
        /usr/lib/go/src/testing/testing.go:732 +0x2f
testing.(*common).Fatalf(0xc000074070, {0xd9d816, 0x2e}, {0xc000094050, 0x5, 0x5})
        /usr/lib/go/src/testing/testing.go:830 +0x85
github.com/golang/mock/gomock.(*Controller).Call.func1(0xc0002f4120, {0xd68380, 0xc0002dac30}, {0xd8847f, 0xc}, {0xc000074020, 0x1, 0x1})
        /home/dzeta/go/pkg/mod/github.com/golang/mock@v1.6.0/gomock/controller.go:231 +0x44d
github.com/golang/mock/gomock.(*Controller).Call(0xc0002f4120, {0xd68380, 0xc0002dac30}, {0xd8847f, 0xc}, {0xc000074020, 0x1, 0x1})
        /home/dzeta/go/pkg/mod/github.com/golang/mock@v1.6.0/gomock/controller.go:247 +0xce
github.com/nspcc-dev/neofs-sdk-go/pool.(*MockClient).EndpointInfo(0xc0002dac30, {0xe85528, 0xc00008a120}, {0x0, 0x0, 0x0})
        /home/dzeta/repo/neofs-sdk-go/pool/mock_test.go:186 +0x298
github.com/nspcc-dev/neofs-sdk-go/pool.updateNodesHealth.func1(0x1, {0xe950d8, 0xc0002dac30})
        /home/dzeta/repo/neofs-sdk-go/pool/pool.go:183 +0x188
created by github.com/nspcc-dev/neofs-sdk-go/pool.updateNodesHealth
        /home/dzeta/repo/neofs-sdk-go/pool/pool.go:174 +0x233
```

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pool/pool.go

@@ -88,6 +88,7 @@ type Pool interface {
 	Connection() (client.Client, *session.Token, error)
 	OwnerID() *owner.ID
 	WaitForContainerPresence(context.Context, *cid.ID, *ContainerPollingParams) error
+	Close()
 }
 
 type clientPack struct {
@@ -103,6 +104,8 @@ type pool struct {
 	sampler     *Sampler
 	owner       *owner.ID
 	clientPacks []*clientPack
+	cancel      context.CancelFunc
+	closedCh    chan struct{}
 }
 
 func newPool(ctx context.Context, options *BuilderOptions) (Pool, error) {
@@ -139,7 +142,8 @@ func newPool(ctx context.Context, options *BuilderOptions) (Pool, error) {
 	}
 	ownerID := owner.NewIDFromNeo3Wallet(wallet)
 
-	pool := &pool{sampler: sampler, owner: ownerID, clientPacks: clientPacks}
+	ctx, cancel := context.WithCancel(ctx)
+	pool := &pool{sampler: sampler, owner: ownerID, clientPacks: clientPacks, cancel: cancel, closedCh: make(chan struct{})}
 	go startRebalance(ctx, pool, options)
 	return pool, nil
 }
@@ -151,6 +155,7 @@ func startRebalance(ctx context.Context, p *pool, options *BuilderOptions) {
 	for {
 		select {
 		case <-ctx.Done():
+			close(p.closedCh)
 			return
 		case <-ticker.C:
 			updateNodesHealth(ctx, p, options, buffer)
@@ -406,3 +411,9 @@ func (p *pool) WaitForContainerPresence(ctx context.Context, cid *cid.ID, pollPa
 		}
 	}
 }
+
+// Cloce closes the pool and releases all the associated resources.
+func (p *pool) Close() {
+	p.cancel()
+	<-p.closedCh
+}

pool/pool_test.go

@@ -117,6 +117,7 @@ func TestBuildPoolOneNodeFailed(t *testing.T) {
 
 	clientPool, err := pb.Build(context.TODO(), opts)
 	require.NoError(t, err)
+	t.Cleanup(clientPool.Close)
 
 	condition := func() bool {
 		_, st, err := clientPool.Connection()
@@ -163,11 +164,9 @@ func TestOneNode(t *testing.T) {
 		clientBuilder: clientBuilder,
 	}
 
-	ctx, cancel := context.WithCancel(context.Background())
+	pool, err := pb.Build(context.Background(), opts)
-	defer cancel()
-
-	pool, err := pb.Build(ctx, opts)
 	require.NoError(t, err)
+	t.Cleanup(pool.Close)
 
 	_, st, err := pool.Connection()
 	require.NoError(t, err)
@@ -203,11 +202,9 @@ func TestTwoNodes(t *testing.T) {
 		clientBuilder: clientBuilder,
 	}
 
-	ctx, cancel := context.WithCancel(context.Background())
+	pool, err := pb.Build(context.Background(), opts)
-	defer cancel()
-
-	pool, err := pb.Build(ctx, opts)
 	require.NoError(t, err)
+	t.Cleanup(pool.Close)
 
 	_, st, err := pool.Connection()
 	require.NoError(t, err)
@@ -259,11 +256,9 @@ func TestOneOfTwoFailed(t *testing.T) {
 		ClientRebalanceInterval: 200 * time.Millisecond,
 	}
 
-	ctx, cancel := context.WithCancel(context.Background())
+	pool, err := pb.Build(context.Background(), opts)
-	defer cancel()
-
-	pool, err := pb.Build(ctx, opts)
 	require.NoError(t, err)
+	t.Cleanup(pool.Close)
 
 	time.Sleep(2 * time.Second)
 
@@ -297,11 +292,9 @@ func TestTwoFailed(t *testing.T) {
 		ClientRebalanceInterval: 200 * time.Millisecond,
 	}
 
-	ctx, cancel := context.WithCancel(context.Background())
+	pool, err := pb.Build(context.Background(), opts)
-	defer cancel()
-
-	pool, err := pb.Build(ctx, opts)
 	require.NoError(t, err)
+	t.Cleanup(pool.Close)
 
 	time.Sleep(2 * time.Second)