Ignore cancellation in puddle constructor

Fixes #1259

Author: jameshartig

pgxpool/pool.go

@@ -70,6 +70,16 @@ func (cr *connResource) getPoolRows(c *Conn, r pgx.Rows) *poolRows {
 	return pr
 }
 
+// detachedCtx wraps a context and will never be canceled, regardless of if
+// the wrapped one is cancelled. The Err() method will never return any errors.
+type detachedCtx struct {
+	context.Context
+}
+
+func (detachedCtx) Done() <-chan struct{}       { return nil }
+func (detachedCtx) Deadline() (time.Time, bool) { return time.Time{}, false }
+func (detachedCtx) Err() error                  { return nil }
+
 // Pool allows for connection reuse.
 type Pool struct {
 	p                     *puddle.Pool
@@ -195,6 +205,14 @@ func ConnectConfig(ctx context.Context, config *Config) (*Pool, error) {
 
 	p.p = puddle.NewPool(
 		func(ctx context.Context) (interface{}, error) {
+			// we ignore cancellation on the original context because its either from
+			// the health check or its from a query and we don't want to cancel creating
+			// a connection just because the original query was cancelled since that
+			// could end up stampeding the server
+			// this will keep any Values in the original context and will just ignore
+			// cancellation
+			// see https://github.com/jackc/pgx/issues/1259
+			ctx = detachedCtx{ctx}
 			connConfig := p.config.ConnConfig
 
 			if p.beforeConnect != nil {

pgxpool/pool_test.go

@@ -73,6 +73,33 @@ func TestLazyConnect(t *testing.T) {
 	assert.Equal(t, context.Canceled, err)
 }
 
+func TestConstructorIgnoresContext(t *testing.T) {
+	t.Parallel()
+
+	config, err := pgxpool.ParseConfig(os.Getenv("PGX_TEST_DATABASE"))
+	assert.NoError(t, err)
+	config.LazyConnect = true
+	var cancel func()
+	config.BeforeConnect = func(context.Context, *pgx.ConnConfig) error {
+		// cancel the query's context before we actually Dial to ensure the Dial's
+		// context isn't cancelled
+		cancel()
+		return nil
+	}
+
+	pool, err := pgxpool.ConnectConfig(context.Background(), config)
+	require.NoError(t, err)
+
+	assert.EqualValues(t, 0, pool.Stat().TotalConns())
+
+	var ctx context.Context
+	ctx, cancel = context.WithCancel(context.Background())
+	defer cancel()
+	_, err = pool.Exec(ctx, "SELECT 1")
+	assert.ErrorIs(t, err, context.Canceled)
+	assert.EqualValues(t, 1, pool.Stat().TotalConns())
+}
+
 func TestConnectConfigRequiresConnConfigFromParseConfig(t *testing.T) {
 	t.Parallel()