Simplify custom type autoloading with pgxpool

Provide some backwards-compatible configuration options for pgxpool
which streamlines the use of the bulk loading of custom types:
- AutoLoadTypes: a list of type (or class) names to automatically
  load for each connection, automatically also loading any other
  types these depend on.
- ReuseTypeMaps: if enabled, pgxpool will cache the typemap information,
  avoiding the need to perform any further queries as new connections
  are created.

ReuseTypeMaps is disabled by default as in some situations, a
connection string might resolve to a pool of servers which do not share
the same type name -> OID mapping.

pgxpool/pool.go

@@ -12,14 +12,17 @@ import (
 
 	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgconn"
+	"github.com/jackc/pgx/v5/pgtype"
 	"github.com/jackc/puddle/v2"
 )
 
-var defaultMaxConns = int32(4)
-var defaultMinConns = int32(0)
-var defaultMaxConnLifetime = time.Hour
-var defaultMaxConnIdleTime = time.Minute * 30
-var defaultHealthCheckPeriod = time.Minute
+var (
+	defaultMaxConns          = int32(4)
+	defaultMinConns          = int32(0)
+	defaultMaxConnLifetime   = time.Hour
+	defaultMaxConnIdleTime   = time.Minute * 30
+	defaultHealthCheckPeriod = time.Minute
+)
 
 type connResource struct {
 	conn       *pgx.Conn
@@ -100,6 +103,11 @@ type Pool struct {
 
 	closeOnce sync.Once
 	closeChan chan struct{}
+
+	autoLoadTypes     []string
+	reuseTypeMap      bool
+	autoLoadMutex     *sync.Mutex
+	autoLoadTypeInfos []*pgtype.DerivedTypeInfo
 }
 
 // Config is the configuration struct for creating a pool. It must be created by [ParseConfig] and then it can be
@@ -147,6 +155,15 @@ type Config struct {
 	// HealthCheckPeriod is the duration between checks of the health of idle connections.
 	HealthCheckPeriod time.Duration
 
+	// AutoLoadTypes is a list of user-defined types which should automatically be loaded
+	// as each new connection is created. This will also load any related types, directly
+	// or indirectly required to handle these types.
+	AutoLoadTypes []string
+
+	// ReuseTypeMaps, if enabled, will reuse the typemap information being used by AutoLoadTypes.
+	// This removes the need to query the database each time a new connection is created.
+	ReuseTypeMaps bool
+
 	createdByParseConfig bool // Used to enforce created by ParseConfig rule.
 }
 
@@ -185,6 +202,8 @@ func NewWithConfig(ctx context.Context, config *Config) (*Pool, error) {
 		config:                config,
 		beforeConnect:         config.BeforeConnect,
 		afterConnect:          config.AfterConnect,
+		autoLoadTypes:         config.AutoLoadTypes,
+		reuseTypeMap:          config.ReuseTypeMaps,
 		beforeAcquire:         config.BeforeAcquire,
 		afterRelease:          config.AfterRelease,
 		beforeClose:           config.BeforeClose,
@@ -196,6 +215,7 @@ func NewWithConfig(ctx context.Context, config *Config) (*Pool, error) {
 		healthCheckPeriod:     config.HealthCheckPeriod,
 		healthCheckChan:       make(chan struct{}, 1),
 		closeChan:             make(chan struct{}),
+		autoLoadMutex:         new(sync.Mutex),
 	}
 
 	if t, ok := config.ConnConfig.Tracer.(AcquireTracer); ok {
@@ -237,6 +257,19 @@ func NewWithConfig(ctx context.Context, config *Config) (*Pool, error) {
 					}
 				}
 
+				if len(p.autoLoadTypes) > 0 {
+					types, err := p.loadTypes(ctx, conn, p.autoLoadTypes)
+					if err != nil {
+						conn.Close(ctx)
+						panic(err)
+					}
+					if err = conn.TypeMap().RegisterDerivedTypes(types); err != nil {
+						conn.Close(ctx)
+
+						panic(err)
+					}
+				}
+
 				jitterSecs := rand.Float64() * config.MaxConnLifetimeJitter.Seconds()
 				maxAgeTime := time.Now().Add(config.MaxConnLifetime).Add(time.Duration(jitterSecs) * time.Second)
 
@@ -388,6 +421,27 @@ func (p *Pool) Close() {
 	})
 }
 
+// loadTypes is used internally to autoload the custom types for a connection,
+// potentially reusing previously-loaded typemap information.
+func (p *Pool) loadTypes(ctx context.Context, conn *pgx.Conn, typeNames []string) ([]*pgtype.DerivedTypeInfo, error) {
+	if p.reuseTypeMap {
+		p.autoLoadMutex.Lock()
+		defer p.autoLoadMutex.Unlock()
+		if p.autoLoadTypeInfos != nil {
+			return p.autoLoadTypeInfos, nil
+		}
+		types, err := pgx.LoadDerivedTypes(ctx, conn, typeNames)
+		if err != nil {
+			return nil, err
+		}
+		p.autoLoadTypeInfos = types
+		return types, err
+	}
+	// Avoid needing to acquire the mutex and allow connections to initialise in parallel
+	// if we have chosen to not reuse the type mapping
+	return pgx.LoadDerivedTypes(ctx, conn, typeNames)
+}
+
 func (p *Pool) isExpired(res *puddle.Resource[*connResource]) bool {
 	return time.Now().After(res.Value().maxAgeTime)
 }
@@ -482,7 +536,6 @@ func (p *Pool) checkMinConns() error {
 func (p *Pool) createIdleResources(parentCtx context.Context, targetResources int) error {
 	ctx, cancel := context.WithCancel(parentCtx)
 	defer cancel()
-
 	errs := make(chan error, targetResources)
 
 	for i := 0; i < targetResources; i++ {
@@ -495,7 +548,6 @@ func (p *Pool) createIdleResources(parentCtx context.Context, targetResources in
 			errs <- err
 		}()
 	}
-
 	var firstError error
 	for i := 0; i < targetResources; i++ {
 		err := <-errs

pgxpool/pool_test.go

@@ -261,6 +261,31 @@ func TestPoolBeforeConnect(t *testing.T) {
 	assert.EqualValues(t, "pgx", str)
 }
 
+func TestAutoLoadTypes(t *testing.T) {
+	t.Parallel()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
+	defer cancel()
+
+	config, err := pgxpool.ParseConfig(os.Getenv("PGX_TEST_DATABASE"))
+	require.NoError(t, err)
+
+	db1, err := pgxpool.NewWithConfig(ctx, config)
+	require.NoError(t, err)
+	defer db1.Close()
+	db1.Exec(ctx, "DROP DOMAIN IF EXISTS autoload_uint64; CREATE DOMAIN autoload_uint64 as numeric(20,0)")
+	defer db1.Exec(ctx, "DROP DOMAIN autoload_uint64")
+
+	config.AutoLoadTypes = []string{"autoload_uint64"}
+	db2, err := pgxpool.NewWithConfig(ctx, config)
+	require.NoError(t, err)
+
+	var n uint64
+	err = db2.QueryRow(ctx, "select 12::autoload_uint64").Scan(&n)
+	require.NoError(t, err)
+	assert.EqualValues(t, uint64(12), n)
+}
+
 func TestPoolAfterConnect(t *testing.T) {
 	t.Parallel()
 
@@ -676,7 +701,6 @@ func TestPoolQuery(t *testing.T) {
 	stats = pool.Stat()
 	assert.EqualValues(t, 0, stats.AcquiredConns())
 	assert.EqualValues(t, 1, stats.TotalConns())
-
 }
 
 func TestPoolQueryRow(t *testing.T) {
@@ -1104,7 +1128,6 @@ func TestConnectEagerlyReachesMinPoolSize(t *testing.T) {
 	}
 
 	t.Fatal("did not reach min pool size")
-
 }
 
 func TestPoolSendBatchBatchCloseTwice(t *testing.T) {