pgx.Conn: Fix memory leak: Delete items from preparedStatements

Previously, items were never removed from the preparedStatements map.
This means workloads that send a large number of unique queries could
run out of memory. Delete items from the map when sending the
deallocate command to Postgres. Add a test to verify this works.

Fixes #1456

conn.go

@@ -1326,6 +1326,7 @@ func (c *Conn) deallocateInvalidatedCachedStatements(ctx context.Context) error
 
 	for _, sd := range invalidatedStatements {
 		pipeline.SendDeallocate(sd.Name)
+		delete(c.preparedStatements, sd.Name)
 	}
 
 	err := pipeline.Sync()

conn_internal_test.go

@@ -0,0 +1,55 @@
+package pgx
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func mustParseConfig(t testing.TB, connString string) *ConnConfig {
+	config, err := ParseConfig(connString)
+	require.Nil(t, err)
+	return config
+}
+
+func mustConnect(t testing.TB, config *ConnConfig) *Conn {
+	conn, err := ConnectConfig(context.Background(), config)
+	if err != nil {
+		t.Fatalf("Unable to establish connection: %v", err)
+	}
+	return conn
+}
+
+// Ensures the connection limits the size of its cached objects.
+// This test examines the internals of *Conn so must be in the same package.
+func TestStmtCacheSizeLimit(t *testing.T) {
+	const cacheLimit = 16
+
+	connConfig := mustParseConfig(t, os.Getenv("PGX_TEST_DATABASE"))
+	connConfig.StatementCacheCapacity = cacheLimit
+	conn := mustConnect(t, connConfig)
+	defer func() {
+		err := conn.Close(context.Background())
+		if err != nil {
+			t.Fatal(err)
+		}
+	}()
+
+	// run a set of unique queries that should overflow the cache
+	ctx := context.Background()
+	for i := 0; i < cacheLimit*2; i++ {
+		uniqueString := fmt.Sprintf("unique %d", i)
+		uniqueSQL := fmt.Sprintf("select '%s'", uniqueString)
+		var output string
+		err := conn.QueryRow(ctx, uniqueSQL).Scan(&output)
+		require.NoError(t, err)
+		require.Equal(t, uniqueString, output)
+	}
+	// preparedStatements contains cacheLimit+1 because deallocation happens before the query
+	assert.Len(t, conn.preparedStatements, cacheLimit+1)
+	assert.Equal(t, cacheLimit, conn.statementCache.Len())
+}