Add pool AfterRelease hook

Also, just close returned connections that are in a transaction rather
than automatically rolling back.

pool/conn.go

@@ -2,7 +2,6 @@ package pool
 
 import (
 	"context"
-	"time"
 
 	"github.com/jackc/pgconn"
 	"github.com/jackc/pgx/v4"
@@ -12,6 +11,7 @@ import (
 // Conn is an acquired *pgx.Conn from a Pool.
 type Conn struct {
 	res *puddle.Resource
+	p   *Pool
 }
 
 // Release returns c to the pool it was acquired from. Once Release has been called, other methods must not be called.
@@ -26,22 +26,12 @@ func (c *Conn) Release() {
 	c.res = nil
 
 	go func() {
-		if !conn.IsAlive() {
+		if !conn.IsAlive() || conn.PgConn().TxStatus != 'I' {
 			res.Destroy()
 			return
 		}
 
-		if conn.PgConn().TxStatus != 'I' {
+		if c.p.afterRelease == nil || c.p.afterRelease(conn) {
-			ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
-			_, err := conn.Exec(ctx, "rollback")
-			cancel()
-			if err != nil {
-				res.Destroy()
-				return
-			}
-		}
-
-		if conn.IsAlive() {
 			res.Release()
 		} else {
 			res.Destroy()

pool/pool.go

@@ -16,6 +16,7 @@ const defaultMaxConns = 5
 type Pool struct {
 	p             *puddle.Pool
 	beforeAcquire func(*pgx.Conn) bool
+	afterRelease  func(*pgx.Conn) bool
 }
 
 type Config struct {
@@ -26,6 +27,10 @@ type Config struct {
 	// acquired.
 	BeforeAcquire func(*pgx.Conn) bool
 
+	// AfterRelease is called after a connection is released, but before it is returned to the pool. It must return true to
+	// return the connection to the pool or false to destroy the connection.
+	AfterRelease func(*pgx.Conn) bool
+
 	MaxConns int32
 }
 
@@ -45,6 +50,7 @@ func Connect(ctx context.Context, connString string) (*Pool, error) {
 func ConnectConfig(ctx context.Context, config *Config) (*Pool, error) {
 	p := &Pool{
 		beforeAcquire: config.BeforeAcquire,
+		afterRelease:  config.AfterRelease,
 	}
 
 	p.p = puddle.NewPool(
@@ -102,7 +108,7 @@ func (p *Pool) Acquire(ctx context.Context) (*Conn, error) {
 		}
 
 		if p.beforeAcquire == nil || p.beforeAcquire(res.Value().(*pgx.Conn)) {
-			return &Conn{res: res}, nil
+			return &Conn{res: res, p: p}, nil
 		}
 
 		res.Destroy()
@@ -116,7 +122,7 @@ func (p *Pool) AcquireAllIdle() []*Conn {
 	conns := make([]*Conn, 0, len(resources))
 	for _, res := range resources {
 		if p.beforeAcquire == nil || p.beforeAcquire(res.Value().(*pgx.Conn)) {
-			conns = append(conns, &Conn{res: res})
+			conns = append(conns, &Conn{res: res, p: p})
 		} else {
 			res.Destroy()
 		}

pool/pool_test.go

@@ -92,6 +92,36 @@ func TestPoolBeforeAcquire(t *testing.T) {
 	assert.EqualValues(t, 12, acquireAttempts)
 }
 
+func TestPoolAfterRelease(t *testing.T) {
+	t.Parallel()
+
+	config, err := pool.ParseConfig(os.Getenv("PGX_TEST_DATABASE"))
+	require.NoError(t, err)
+
+	afterReleaseCount := 0
+
+	config.AfterRelease = func(c *pgx.Conn) bool {
+		afterReleaseCount += 1
+		return afterReleaseCount%2 == 1
+	}
+
+	db, err := pool.ConnectConfig(context.Background(), config)
+	require.NoError(t, err)
+	defer db.Close()
+
+	connPIDs := map[uint32]struct{}{}
+
+	for i := 0; i < 10; i++ {
+		conn, err := db.Acquire(context.Background())
+		assert.NoError(t, err)
+		connPIDs[conn.Conn().PgConn().PID()] = struct{}{}
+		conn.Release()
+		waitForReleaseToComplete()
+	}
+
+	assert.EqualValues(t, 5, len(connPIDs))
+}
+
 func TestPoolAcquireAllIdle(t *testing.T) {
 	t.Parallel()
 
@@ -243,7 +273,7 @@ func TestPoolCopyFrom(t *testing.T) {
 	assert.Equal(t, inputRows, outputRows)
 }
 
-func TestConnReleaseRollsBackFailedTransaction(t *testing.T) {
+func TestConnReleaseClosesConnInFailedTransaction(t *testing.T) {
 	t.Parallel()
 
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
@@ -275,13 +305,13 @@ func TestConnReleaseRollsBackFailedTransaction(t *testing.T) {
 	c, err = pool.Acquire(ctx)
 	require.NoError(t, err)
 
-	assert.Equal(t, pid, c.Conn().PgConn().PID())
+	assert.NotEqual(t, pid, c.Conn().PgConn().PID())
 	assert.Equal(t, byte('I'), c.Conn().PgConn().TxStatus)
 
 	c.Release()
 }
 
-func TestConnReleaseRollsBackInTransaction(t *testing.T) {
+func TestConnReleaseClosesConnInTransaction(t *testing.T) {
 	t.Parallel()
 
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
@@ -308,7 +338,7 @@ func TestConnReleaseRollsBackInTransaction(t *testing.T) {
 	c, err = pool.Acquire(ctx)
 	require.NoError(t, err)
 
-	assert.Equal(t, pid, c.Conn().PgConn().PID())
+	assert.NotEqual(t, pid, c.Conn().PgConn().PID())
 	assert.Equal(t, byte('I'), c.Conn().PgConn().TxStatus)
 
 	c.Release()