pgconn.CommandTag is now an opaque type

It now makes a copy instead of retaining driver memory. This is in
preparation to reuse the driver read buffer.

pgxpool/batch_results.go

@@ -10,7 +10,7 @@ type errBatchResults struct {
 }
 
 func (br errBatchResults) Exec() (pgconn.CommandTag, error) {
-	return nil, br.err
+	return pgconn.CommandTag{}, br.err
 }
 
 func (br errBatchResults) Query() (pgx.Rows, error) {
@@ -18,7 +18,7 @@ func (br errBatchResults) Query() (pgx.Rows, error) {
 }
 
 func (br errBatchResults) QueryFunc(scans []interface{}, f func(pgx.QueryFuncRow) error) (pgconn.CommandTag, error) {
-	return nil, br.err
+	return pgconn.CommandTag{}, br.err
 }
 
 func (br errBatchResults) QueryRow() pgx.Row {

pgxpool/common_test.go

@@ -27,7 +27,7 @@ type execer interface {
 func testExec(t *testing.T, db execer) {
 	results, err := db.Exec(context.Background(), "set time zone 'America/Chicago'")
 	require.NoError(t, err)
-	assert.EqualValues(t, "SET", results)
+	assert.EqualValues(t, "SET", results.String())
 }
 
 type queryer interface {

pgxpool/pool.go

@@ -470,7 +470,7 @@ func (p *Pool) Stat() *Stat {
 func (p *Pool) Exec(ctx context.Context, sql string, arguments ...interface{}) (pgconn.CommandTag, error) {
 	c, err := p.Acquire(ctx)
 	if err != nil {
-		return nil, err
+		return pgconn.CommandTag{}, err
 	}
 	defer c.Release()
 
@@ -527,7 +527,7 @@ func (p *Pool) QueryRow(ctx context.Context, sql string, args ...interface{}) pg
 func (p *Pool) QueryFunc(ctx context.Context, sql string, args []interface{}, scans []interface{}, f func(pgx.QueryFuncRow) error) (pgconn.CommandTag, error) {
 	c, err := p.Acquire(ctx)
 	if err != nil {
-		return nil, err
+		return pgconn.CommandTag{}, err
 	}
 	defer c.Release()
 

pgxpool/rows.go

@@ -12,7 +12,7 @@ type errRows struct {
 
 func (errRows) Close()                                         {}
 func (e errRows) Err() error                                   { return e.err }
-func (errRows) CommandTag() pgconn.CommandTag                  { return nil }
+func (errRows) CommandTag() pgconn.CommandTag                  { return pgconn.CommandTag{} }
 func (errRows) FieldDescriptions() []pgproto3.FieldDescription { return nil }
 func (errRows) Next() bool                                     { return false }
 func (e errRows) Scan(dest ...interface{}) error               { return e.err }