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.

batch.go

@@ -64,10 +64,10 @@ type batchResults struct {
 // Exec reads the results from the next query in the batch as if the query has been sent with Exec.
 func (br *batchResults) Exec() (pgconn.CommandTag, error) {
 	if br.err != nil {
-		return nil, br.err
+		return pgconn.CommandTag{}, br.err
 	}
 	if br.closed {
-		return nil, fmt.Errorf("batch already closed")
+		return pgconn.CommandTag{}, fmt.Errorf("batch already closed")
 	}
 
 	query, arguments, _ := br.nextQueryAndArgs()
@@ -84,7 +84,7 @@ func (br *batchResults) Exec() (pgconn.CommandTag, error) {
 				"err":  err,
 			})
 		}
-		return nil, err
+		return pgconn.CommandTag{}, err
 	}
 
 	commandTag, err := br.mrr.ResultReader().Close()
@@ -151,29 +151,29 @@ func (br *batchResults) Query() (Rows, error) {
 // QueryFunc reads the results from the next query in the batch as if the query has been sent with Conn.QueryFunc.
 func (br *batchResults) QueryFunc(scans []interface{}, f func(QueryFuncRow) error) (pgconn.CommandTag, error) {
 	if br.closed {
-		return nil, fmt.Errorf("batch already closed")
+		return pgconn.CommandTag{}, fmt.Errorf("batch already closed")
 	}
 
 	rows, err := br.Query()
 	if err != nil {
-		return nil, err
+		return pgconn.CommandTag{}, err
 	}
 	defer rows.Close()
 
 	for rows.Next() {
 		err = rows.Scan(scans...)
 		if err != nil {
-			return nil, err
+			return pgconn.CommandTag{}, err
 		}
 
 		err = f(rows)
 		if err != nil {
-			return nil, err
+			return pgconn.CommandTag{}, err
 		}
 	}
 
 	if err := rows.Err(); err != nil {
-		return nil, err
+		return pgconn.CommandTag{}, err
 	}
 
 	return rows.CommandTag(), nil

conn.go

@@ -432,7 +432,7 @@ optionLoop:
 	if c.stmtcache != nil {
 		sd, err := c.stmtcache.Get(ctx, sql)
 		if err != nil {
-			return nil, err
+			return pgconn.CommandTag{}, err
 		}
 
 		if c.stmtcache.Mode() == stmtcache.ModeDescribe {
@@ -443,7 +443,7 @@ optionLoop:
 
 	sd, err := c.Prepare(ctx, "", sql)
 	if err != nil {
-		return nil, err
+		return pgconn.CommandTag{}, err
 	}
 	return c.execPrepared(ctx, sd, arguments)
 }
@@ -452,7 +452,7 @@ func (c *Conn) execSimpleProtocol(ctx context.Context, sql string, arguments []i
 	if len(arguments) > 0 {
 		sql, err = c.sanitizeForSimpleQuery(sql, arguments...)
 		if err != nil {
-			return nil, err
+			return pgconn.CommandTag{}, err
 		}
 	}
 
@@ -493,7 +493,7 @@ func (c *Conn) execParamsAndPreparedPrefix(sd *pgconn.StatementDescription, argu
 func (c *Conn) execParams(ctx context.Context, sd *pgconn.StatementDescription, arguments []interface{}) (pgconn.CommandTag, error) {
 	err := c.execParamsAndPreparedPrefix(sd, arguments)
 	if err != nil {
-		return nil, err
+		return pgconn.CommandTag{}, err
 	}
 
 	result := c.pgConn.ExecParams(ctx, sd.SQL, c.eqb.paramValues, sd.ParamOIDs, c.eqb.paramFormats, c.eqb.resultFormats).Read()
@@ -504,7 +504,7 @@ func (c *Conn) execParams(ctx context.Context, sd *pgconn.StatementDescription,
 func (c *Conn) execPrepared(ctx context.Context, sd *pgconn.StatementDescription, arguments []interface{}) (pgconn.CommandTag, error) {
 	err := c.execParamsAndPreparedPrefix(sd, arguments)
 	if err != nil {
-		return nil, err
+		return pgconn.CommandTag{}, err
 	}
 
 	result := c.pgConn.ExecPrepared(ctx, sd.Name, c.eqb.paramValues, c.eqb.paramFormats, c.eqb.resultFormats).Read()
@@ -688,24 +688,24 @@ type QueryFuncRow interface {
 func (c *Conn) QueryFunc(ctx context.Context, sql string, args []interface{}, scans []interface{}, f func(QueryFuncRow) error) (pgconn.CommandTag, error) {
 	rows, err := c.Query(ctx, sql, args...)
 	if err != nil {
-		return nil, err
+		return pgconn.CommandTag{}, err
 	}
 	defer rows.Close()
 
 	for rows.Next() {
 		err = rows.Scan(scans...)
 		if err != nil {
-			return nil, err
+			return pgconn.CommandTag{}, err
 		}
 
 		err = f(rows)
 		if err != nil {
-			return nil, err
+			return pgconn.CommandTag{}, err
 		}
 	}
 
 	if err := rows.Err(); err != nil {
-		return nil, err
+		return pgconn.CommandTag{}, err
 	}
 
 	return rows.CommandTag(), nil

conn_test.go

@@ -188,31 +188,31 @@ func TestExec(t *testing.T) {
 	t.Parallel()
 
 	testWithAndWithoutPreferSimpleProtocol(t, func(t *testing.T, conn *pgx.Conn) {
-		if results := mustExec(t, conn, "create temporary table foo(id integer primary key);"); string(results) != "CREATE TABLE" {
+		if results := mustExec(t, conn, "create temporary table foo(id integer primary key);"); results.String() != "CREATE TABLE" {
 			t.Error("Unexpected results from Exec")
 		}
 
 		// Accept parameters
-		if results := mustExec(t, conn, "insert into foo(id) values($1)", 1); string(results) != "INSERT 0 1" {
+		if results := mustExec(t, conn, "insert into foo(id) values($1)", 1); results.String() != "INSERT 0 1" {
 			t.Errorf("Unexpected results from Exec: %v", results)
 		}
 
-		if results := mustExec(t, conn, "drop table foo;"); string(results) != "DROP TABLE" {
+		if results := mustExec(t, conn, "drop table foo;"); results.String() != "DROP TABLE" {
 			t.Error("Unexpected results from Exec")
 		}
 
 		// Multiple statements can be executed -- last command tag is returned
-		if results := mustExec(t, conn, "create temporary table foo(id serial primary key); drop table foo;"); string(results) != "DROP TABLE" {
+		if results := mustExec(t, conn, "create temporary table foo(id serial primary key); drop table foo;"); results.String() != "DROP TABLE" {
 			t.Error("Unexpected results from Exec")
 		}
 
 		// Can execute longer SQL strings than sharedBufferSize
-		if results := mustExec(t, conn, strings.Repeat("select 42; ", 1000)); string(results) != "SELECT 1" {
+		if results := mustExec(t, conn, strings.Repeat("select 42; ", 1000)); results.String() != "SELECT 1" {
 			t.Errorf("Unexpected results from Exec: %v", results)
 		}
 
 		// Exec no-op which does not return a command tag
-		if results := mustExec(t, conn, "--;"); string(results) != "" {
+		if results := mustExec(t, conn, "--;"); results.String() != "" {
 			t.Errorf("Unexpected results from Exec: %v", results)
 		}
 	})
@@ -260,7 +260,7 @@ func TestExecContextWithoutCancelation(t *testing.T) {
 		if err != nil {
 			t.Fatal(err)
 		}
-		if string(commandTag) != "CREATE TABLE" {
+		if commandTag.String() != "CREATE TABLE" {
 			t.Fatalf("Unexpected results from Exec: %v", commandTag)
 		}
 		assert.False(t, pgconn.SafeToRetry(err))
@@ -350,15 +350,15 @@ func TestExecStatementCacheModes(t *testing.T) {
 
 			commandTag, err := conn.Exec(context.Background(), "select 1")
 			assert.NoError(t, err, tt.name)
-			assert.Equal(t, "SELECT 1", string(commandTag), tt.name)
+			assert.Equal(t, "SELECT 1", commandTag.String(), tt.name)
 
 			commandTag, err = conn.Exec(context.Background(), "select 1 union all select 1")
 			assert.NoError(t, err, tt.name)
-			assert.Equal(t, "SELECT 2", string(commandTag), tt.name)
+			assert.Equal(t, "SELECT 2", commandTag.String(), tt.name)
 
 			commandTag, err = conn.Exec(context.Background(), "select 1")
 			assert.NoError(t, err, tt.name)
-			assert.Equal(t, "SELECT 1", string(commandTag), tt.name)
+			assert.Equal(t, "SELECT 1", commandTag.String(), tt.name)
 
 			ensureConnValid(t, conn)
 		}()
@@ -378,7 +378,7 @@ func TestExecPerQuerySimpleProtocol(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if string(commandTag) != "CREATE TABLE" {
+	if commandTag.String() != "CREATE TABLE" {
 		t.Fatalf("Unexpected results from Exec: %v", commandTag)
 	}
 
@@ -390,7 +390,7 @@ func TestExecPerQuerySimpleProtocol(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if string(commandTag) != "INSERT 0 1" {
+	if commandTag.String() != "INSERT 0 1" {
 		t.Fatalf("Unexpected results from Exec: %v", commandTag)
 	}
 
@@ -720,12 +720,12 @@ func TestInsertBoolArray(t *testing.T) {
 	t.Parallel()
 
 	testWithAndWithoutPreferSimpleProtocol(t, func(t *testing.T, conn *pgx.Conn) {
-		if results := mustExec(t, conn, "create temporary table foo(spice bool[]);"); string(results) != "CREATE TABLE" {
+		if results := mustExec(t, conn, "create temporary table foo(spice bool[]);"); results.String() != "CREATE TABLE" {
 			t.Error("Unexpected results from Exec")
 		}
 
 		// Accept parameters
-		if results := mustExec(t, conn, "insert into foo(spice) values($1)", []bool{true, false, true}); string(results) != "INSERT 0 1" {
+		if results := mustExec(t, conn, "insert into foo(spice) values($1)", []bool{true, false, true}); results.String() != "INSERT 0 1" {
 			t.Errorf("Unexpected results from Exec: %v", results)
 		}
 	})
@@ -735,12 +735,12 @@ func TestInsertTimestampArray(t *testing.T) {
 	t.Parallel()
 
 	testWithAndWithoutPreferSimpleProtocol(t, func(t *testing.T, conn *pgx.Conn) {
-		if results := mustExec(t, conn, "create temporary table foo(spice timestamp[]);"); string(results) != "CREATE TABLE" {
+		if results := mustExec(t, conn, "create temporary table foo(spice timestamp[]);"); results.String() != "CREATE TABLE" {
 			t.Error("Unexpected results from Exec")
 		}
 
 		// Accept parameters
-		if results := mustExec(t, conn, "insert into foo(spice) values($1)", []time.Time{time.Unix(1419143667, 0), time.Unix(1419143672, 0)}); string(results) != "INSERT 0 1" {
+		if results := mustExec(t, conn, "insert into foo(spice) values($1)", []time.Time{time.Unix(1419143667, 0), time.Unix(1419143672, 0)}); results.String() != "INSERT 0 1" {
 			t.Errorf("Unexpected results from Exec: %v", results)
 		}
 	})

pgconn/benchmark_private_test.go

@@ -0,0 +1,73 @@
+package pgconn
+
+import (
+	"strings"
+	"testing"
+)
+
+func BenchmarkCommandTagRowsAffected(b *testing.B) {
+	benchmarks := []struct {
+		commandTag   string
+		rowsAffected int64
+	}{
+		{"UPDATE 1", 1},
+		{"UPDATE 123456789", 123456789},
+		{"INSERT 0 1", 1},
+		{"INSERT 0 123456789", 123456789},
+	}
+
+	for _, bm := range benchmarks {
+		ct := CommandTag{buf: []byte(bm.commandTag)}
+		b.Run(bm.commandTag, func(b *testing.B) {
+			var n int64
+			for i := 0; i < b.N; i++ {
+				n = ct.RowsAffected()
+			}
+			if n != bm.rowsAffected {
+				b.Errorf("expected %d got %d", bm.rowsAffected, n)
+			}
+		})
+	}
+}
+
+func BenchmarkCommandTagTypeFromString(b *testing.B) {
+	ct := CommandTag{buf: []byte("UPDATE 1")}
+
+	var update bool
+	for i := 0; i < b.N; i++ {
+		update = strings.HasPrefix(ct.String(), "UPDATE")
+	}
+	if !update {
+		b.Error("expected update")
+	}
+}
+
+func BenchmarkCommandTagInsert(b *testing.B) {
+	benchmarks := []struct {
+		commandTag string
+		is         bool
+	}{
+		{"INSERT 1", true},
+		{"INSERT 1234567890", true},
+		{"UPDATE 1", false},
+		{"UPDATE 1234567890", false},
+		{"DELETE 1", false},
+		{"DELETE 1234567890", false},
+		{"SELECT 1", false},
+		{"SELECT 1234567890", false},
+		{"UNKNOWN 1234567890", false},
+	}
+
+	for _, bm := range benchmarks {
+		ct := CommandTag{buf: []byte(bm.commandTag)}
+		b.Run(bm.commandTag, func(b *testing.B) {
+			var is bool
+			for i := 0; i < b.N; i++ {
+				is = ct.Insert()
+			}
+			if is != bm.is {
+				b.Errorf("expected %v got %v", bm.is, is)
+			}
+		})
+	}
+}

pgconn/benchmark_test.go

@@ -4,7 +4,6 @@ import (
 	"bytes"
 	"context"
 	"os"
-	"strings"
 	"testing"
 
 	"github.com/jackc/pgx/v5/pgconn"
@@ -253,70 +252,3 @@ func BenchmarkExecPreparedPossibleToCancel(b *testing.B) {
 // 		conn.ChanToSetDeadline().Ignore()
 // 	}
 // }
-
-func BenchmarkCommandTagRowsAffected(b *testing.B) {
-	benchmarks := []struct {
-		commandTag   string
-		rowsAffected int64
-	}{
-		{"UPDATE 1", 1},
-		{"UPDATE 123456789", 123456789},
-		{"INSERT 0 1", 1},
-		{"INSERT 0 123456789", 123456789},
-	}
-
-	for _, bm := range benchmarks {
-		ct := pgconn.CommandTag(bm.commandTag)
-		b.Run(bm.commandTag, func(b *testing.B) {
-			var n int64
-			for i := 0; i < b.N; i++ {
-				n = ct.RowsAffected()
-			}
-			if n != bm.rowsAffected {
-				b.Errorf("expected %d got %d", bm.rowsAffected, n)
-			}
-		})
-	}
-}
-
-func BenchmarkCommandTagTypeFromString(b *testing.B) {
-	ct := pgconn.CommandTag("UPDATE 1")
-
-	var update bool
-	for i := 0; i < b.N; i++ {
-		update = strings.HasPrefix(ct.String(), "UPDATE")
-	}
-	if !update {
-		b.Error("expected update")
-	}
-}
-
-func BenchmarkCommandTagInsert(b *testing.B) {
-	benchmarks := []struct {
-		commandTag string
-		is         bool
-	}{
-		{"INSERT 1", true},
-		{"INSERT 1234567890", true},
-		{"UPDATE 1", false},
-		{"UPDATE 1234567890", false},
-		{"DELETE 1", false},
-		{"DELETE 1234567890", false},
-		{"SELECT 1", false},
-		{"SELECT 1234567890", false},
-		{"UNKNOWN 1234567890", false},
-	}
-
-	for _, bm := range benchmarks {
-		ct := pgconn.CommandTag(bm.commandTag)
-		b.Run(bm.commandTag, func(b *testing.B) {
-			var is bool
-			for i := 0; i < b.N; i++ {
-				is = ct.Insert()
-			}
-			if is != bm.is {
-				b.Errorf("expected %v got %v", bm.is, is)
-			}
-		})
-	}
-}

pgconn/pgconn.go

@@ -685,15 +685,17 @@ func (pgConn *PgConn) ParameterStatus(key string) string {
 }
 
 // CommandTag is the result of an Exec function
-type CommandTag []byte
+type CommandTag struct {
+	buf []byte
+}
 
 // RowsAffected returns the number of rows affected. If the CommandTag was not
 // for a row affecting command (e.g. "CREATE TABLE") then it returns 0.
 func (ct CommandTag) RowsAffected() int64 {
 	// Find last non-digit
 	idx := -1
-	for i := len(ct) - 1; i >= 0; i-- {
+	for i := len(ct.buf) - 1; i >= 0; i-- {
-		if ct[i] >= '0' && ct[i] <= '9' {
+		if ct.buf[i] >= '0' && ct.buf[i] <= '9' {
 			idx = i
 		} else {
 			break
@@ -705,7 +707,7 @@ func (ct CommandTag) RowsAffected() int64 {
 	}
 
 	var n int64
-	for _, b := range ct[idx:] {
+	for _, b := range ct.buf[idx:] {
 		n = n*10 + int64(b-'0')
 	}
 
@@ -713,51 +715,51 @@ func (ct CommandTag) RowsAffected() int64 {
 }
 
 func (ct CommandTag) String() string {
-	return string(ct)
+	return string(ct.buf)
 }
 
 // Insert is true if the command tag starts with "INSERT".
 func (ct CommandTag) Insert() bool {
-	return len(ct) >= 6 &&
+	return len(ct.buf) >= 6 &&
-		ct[0] == 'I' &&
+		ct.buf[0] == 'I' &&
-		ct[1] == 'N' &&
+		ct.buf[1] == 'N' &&
-		ct[2] == 'S' &&
+		ct.buf[2] == 'S' &&
-		ct[3] == 'E' &&
+		ct.buf[3] == 'E' &&
-		ct[4] == 'R' &&
+		ct.buf[4] == 'R' &&
-		ct[5] == 'T'
+		ct.buf[5] == 'T'
 }
 
 // Update is true if the command tag starts with "UPDATE".
 func (ct CommandTag) Update() bool {
-	return len(ct) >= 6 &&
+	return len(ct.buf) >= 6 &&
-		ct[0] == 'U' &&
+		ct.buf[0] == 'U' &&
-		ct[1] == 'P' &&
+		ct.buf[1] == 'P' &&
-		ct[2] == 'D' &&
+		ct.buf[2] == 'D' &&
-		ct[3] == 'A' &&
+		ct.buf[3] == 'A' &&
-		ct[4] == 'T' &&
+		ct.buf[4] == 'T' &&
-		ct[5] == 'E'
+		ct.buf[5] == 'E'
 }
 
 // Delete is true if the command tag starts with "DELETE".
 func (ct CommandTag) Delete() bool {
-	return len(ct) >= 6 &&
+	return len(ct.buf) >= 6 &&
-		ct[0] == 'D' &&
+		ct.buf[0] == 'D' &&
-		ct[1] == 'E' &&
+		ct.buf[1] == 'E' &&
-		ct[2] == 'L' &&
+		ct.buf[2] == 'L' &&
-		ct[3] == 'E' &&
+		ct.buf[3] == 'E' &&
-		ct[4] == 'T' &&
+		ct.buf[4] == 'T' &&
-		ct[5] == 'E'
+		ct.buf[5] == 'E'
 }
 
 // Select is true if the command tag starts with "SELECT".
 func (ct CommandTag) Select() bool {
-	return len(ct) >= 6 &&
+	return len(ct.buf) >= 6 &&
-		ct[0] == 'S' &&
+		ct.buf[0] == 'S' &&
-		ct[1] == 'E' &&
+		ct.buf[1] == 'E' &&
-		ct[2] == 'L' &&
+		ct.buf[2] == 'L' &&
-		ct[3] == 'E' &&
+		ct.buf[3] == 'E' &&
-		ct[4] == 'C' &&
+		ct.buf[4] == 'C' &&
-		ct[5] == 'T'
+		ct.buf[5] == 'T'
 }
 
 type StatementDescription struct {
@@ -1076,13 +1078,13 @@ func (pgConn *PgConn) execExtendedPrefix(ctx context.Context, paramValues [][]by
 	result := &pgConn.resultReader
 
 	if err := pgConn.lock(); err != nil {
-		result.concludeCommand(nil, err)
+		result.concludeCommand(CommandTag{}, err)
 		result.closed = true
 		return result
 	}
 
 	if len(paramValues) > math.MaxUint16 {
-		result.concludeCommand(nil, fmt.Errorf("extended protocol limited to %v parameters", math.MaxUint16))
+		result.concludeCommand(CommandTag{}, fmt.Errorf("extended protocol limited to %v parameters", math.MaxUint16))
 		result.closed = true
 		pgConn.unlock()
 		return result
@@ -1091,7 +1093,7 @@ func (pgConn *PgConn) execExtendedPrefix(ctx context.Context, paramValues [][]by
 	if ctx != context.Background() {
 		select {
 		case <-ctx.Done():
-			result.concludeCommand(nil, newContextAlreadyDoneError(ctx))
+			result.concludeCommand(CommandTag{}, newContextAlreadyDoneError(ctx))
 			result.closed = true
 			pgConn.unlock()
 			return result
@@ -1111,7 +1113,7 @@ func (pgConn *PgConn) execExtendedSuffix(buf []byte, result *ResultReader) {
 	n, err := pgConn.conn.Write(buf)
 	if err != nil {
 		pgConn.asyncClose()
-		result.concludeCommand(nil, &writeError{err: err, safeToRetry: n == 0})
+		result.concludeCommand(CommandTag{}, &writeError{err: err, safeToRetry: n == 0})
 		pgConn.contextWatcher.Unwatch()
 		result.closed = true
 		pgConn.unlock()
@@ -1124,14 +1126,14 @@ func (pgConn *PgConn) execExtendedSuffix(buf []byte, result *ResultReader) {
 // CopyTo executes the copy command sql and copies the results to w.
 func (pgConn *PgConn) CopyTo(ctx context.Context, w io.Writer, sql string) (CommandTag, error) {
 	if err := pgConn.lock(); err != nil {
-		return nil, err
+		return CommandTag{}, err
 	}
 
 	if ctx != context.Background() {
 		select {
 		case <-ctx.Done():
 			pgConn.unlock()
-			return nil, newContextAlreadyDoneError(ctx)
+			return CommandTag{}, newContextAlreadyDoneError(ctx)
 		default:
 		}
 		pgConn.contextWatcher.Watch(ctx)
@@ -1146,7 +1148,7 @@ func (pgConn *PgConn) CopyTo(ctx context.Context, w io.Writer, sql string) (Comm
 	if err != nil {
 		pgConn.asyncClose()
 		pgConn.unlock()
-		return nil, &writeError{err: err, safeToRetry: n == 0}
+		return CommandTag{}, &writeError{err: err, safeToRetry: n == 0}
 	}
 
 	// Read results
@@ -1156,7 +1158,7 @@ func (pgConn *PgConn) CopyTo(ctx context.Context, w io.Writer, sql string) (Comm
 		msg, err := pgConn.receiveMessage()
 		if err != nil {
 			pgConn.asyncClose()
-			return nil, preferContextOverNetTimeoutError(ctx, err)
+			return CommandTag{}, preferContextOverNetTimeoutError(ctx, err)
 		}
 
 		switch msg := msg.(type) {
@@ -1165,13 +1167,13 @@ func (pgConn *PgConn) CopyTo(ctx context.Context, w io.Writer, sql string) (Comm
 			_, err := w.Write(msg.Data)
 			if err != nil {
 				pgConn.asyncClose()
-				return nil, err
+				return CommandTag{}, err
 			}
 		case *pgproto3.ReadyForQuery:
 			pgConn.unlock()
 			return commandTag, pgErr
 		case *pgproto3.CommandComplete:
-			commandTag = CommandTag(msg.CommandTag)
+			commandTag = pgConn.makeCommandTag(msg.CommandTag)
 		case *pgproto3.ErrorResponse:
 			pgErr = ErrorResponseToPgError(msg)
 		}
@@ -1184,14 +1186,14 @@ func (pgConn *PgConn) CopyTo(ctx context.Context, w io.Writer, sql string) (Comm
 // could still block.
 func (pgConn *PgConn) CopyFrom(ctx context.Context, r io.Reader, sql string) (CommandTag, error) {
 	if err := pgConn.lock(); err != nil {
-		return nil, err
+		return CommandTag{}, err
 	}
 	defer pgConn.unlock()
 
 	if ctx != context.Background() {
 		select {
 		case <-ctx.Done():
-			return nil, newContextAlreadyDoneError(ctx)
+			return CommandTag{}, newContextAlreadyDoneError(ctx)
 		default:
 		}
 		pgConn.contextWatcher.Watch(ctx)
@@ -1205,7 +1207,7 @@ func (pgConn *PgConn) CopyFrom(ctx context.Context, r io.Reader, sql string) (Co
 	n, err := pgConn.conn.Write(buf)
 	if err != nil {
 		pgConn.asyncClose()
-		return nil, &writeError{err: err, safeToRetry: n == 0}
+		return CommandTag{}, &writeError{err: err, safeToRetry: n == 0}
 	}
 
 	// Send copy data
@@ -1255,7 +1257,7 @@ func (pgConn *PgConn) CopyFrom(ctx context.Context, r io.Reader, sql string) (Co
 			msg, err := pgConn.receiveMessage()
 			if err != nil {
 				pgConn.asyncClose()
-				return nil, preferContextOverNetTimeoutError(ctx, err)
+				return CommandTag{}, preferContextOverNetTimeoutError(ctx, err)
 			}
 
 			switch msg := msg.(type) {
@@ -1279,7 +1281,7 @@ func (pgConn *PgConn) CopyFrom(ctx context.Context, r io.Reader, sql string) (Co
 	_, err = pgConn.conn.Write(buf)
 	if err != nil {
 		pgConn.asyncClose()
-		return nil, err
+		return CommandTag{}, err
 	}
 
 	// Read results
@@ -1288,14 +1290,14 @@ func (pgConn *PgConn) CopyFrom(ctx context.Context, r io.Reader, sql string) (Co
 		msg, err := pgConn.receiveMessage()
 		if err != nil {
 			pgConn.asyncClose()
-			return nil, preferContextOverNetTimeoutError(ctx, err)
+			return CommandTag{}, preferContextOverNetTimeoutError(ctx, err)
 		}
 
 		switch msg := msg.(type) {
 		case *pgproto3.ReadyForQuery:
 			return commandTag, pgErr
 		case *pgproto3.CommandComplete:
-			commandTag = CommandTag(msg.CommandTag)
+			commandTag = pgConn.makeCommandTag(msg.CommandTag)
 		case *pgproto3.ErrorResponse:
 			pgErr = ErrorResponseToPgError(msg)
 		}
@@ -1368,7 +1370,7 @@ func (mrr *MultiResultReader) NextResult() bool {
 			return true
 		case *pgproto3.CommandComplete:
 			mrr.pgConn.resultReader = ResultReader{
-				commandTag:       CommandTag(msg.CommandTag),
+				commandTag:       mrr.pgConn.makeCommandTag(msg.CommandTag),
 				commandConcluded: true,
 				closed:           true,
 			}
@@ -1483,7 +1485,7 @@ func (rr *ResultReader) Close() (CommandTag, error) {
 	for !rr.commandConcluded {
 		_, err := rr.receiveMessage()
 		if err != nil {
-			return nil, rr.err
+			return CommandTag{}, rr.err
 		}
 	}
 
@@ -1491,7 +1493,7 @@ func (rr *ResultReader) Close() (CommandTag, error) {
 		for {
 			msg, err := rr.receiveMessage()
 			if err != nil {
-				return nil, rr.err
+				return CommandTag{}, rr.err
 			}
 
 			switch msg := msg.(type) {
@@ -1538,7 +1540,7 @@ func (rr *ResultReader) receiveMessage() (msg pgproto3.BackendMessage, err error
 
 	if err != nil {
 		err = preferContextOverNetTimeoutError(rr.ctx, err)
-		rr.concludeCommand(nil, err)
+		rr.concludeCommand(CommandTag{}, err)
 		rr.pgConn.contextWatcher.Unwatch()
 		rr.closed = true
 		if rr.multiResultReader == nil {
@@ -1552,11 +1554,11 @@ func (rr *ResultReader) receiveMessage() (msg pgproto3.BackendMessage, err error
 	case *pgproto3.RowDescription:
 		rr.fieldDescriptions = msg.Fields
 	case *pgproto3.CommandComplete:
-		rr.concludeCommand(CommandTag(msg.CommandTag), nil)
+		rr.concludeCommand(rr.pgConn.makeCommandTag(msg.CommandTag), nil)
 	case *pgproto3.EmptyQueryResponse:
-		rr.concludeCommand(nil, nil)
+		rr.concludeCommand(CommandTag{}, nil)
 	case *pgproto3.ErrorResponse:
-		rr.concludeCommand(nil, ErrorResponseToPgError(msg))
+		rr.concludeCommand(CommandTag{}, ErrorResponseToPgError(msg))
 	}
 
 	return msg, nil
@@ -1659,6 +1661,13 @@ func (pgConn *PgConn) EscapeString(s string) (string, error) {
 	return strings.Replace(s, "'", "''", -1), nil
 }
 
+// makeCommandTag makes a CommandTag. It does not retain a reference to buf or buf's underlying memory.
+func (pgConn *PgConn) makeCommandTag(buf []byte) CommandTag {
+	ct := make([]byte, len(buf))
+	copy(ct, buf)
+	return CommandTag{buf: ct}
+}
+
 // HijackedConn is the result of hijacking a connection.
 //
 // Due to the necessary exposure of internal implementation details, it is not covered by the semantic versioning

pgconn/pgconn_private_test.go

@@ -0,0 +1,41 @@
+package pgconn
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCommandTag(t *testing.T) {
+	t.Parallel()
+
+	var tests = []struct {
+		commandTag   CommandTag
+		rowsAffected int64
+		isInsert     bool
+		isUpdate     bool
+		isDelete     bool
+		isSelect     bool
+	}{
+		{commandTag: CommandTag{buf: []byte("INSERT 0 5")}, rowsAffected: 5, isInsert: true},
+		{commandTag: CommandTag{buf: []byte("UPDATE 0")}, rowsAffected: 0, isUpdate: true},
+		{commandTag: CommandTag{buf: []byte("UPDATE 1")}, rowsAffected: 1, isUpdate: true},
+		{commandTag: CommandTag{buf: []byte("DELETE 0")}, rowsAffected: 0, isDelete: true},
+		{commandTag: CommandTag{buf: []byte("DELETE 1")}, rowsAffected: 1, isDelete: true},
+		{commandTag: CommandTag{buf: []byte("DELETE 1234567890")}, rowsAffected: 1234567890, isDelete: true},
+		{commandTag: CommandTag{buf: []byte("SELECT 1")}, rowsAffected: 1, isSelect: true},
+		{commandTag: CommandTag{buf: []byte("SELECT 99999999999")}, rowsAffected: 99999999999, isSelect: true},
+		{commandTag: CommandTag{buf: []byte("CREATE TABLE")}, rowsAffected: 0},
+		{commandTag: CommandTag{buf: []byte("ALTER TABLE")}, rowsAffected: 0},
+		{commandTag: CommandTag{buf: []byte("DROP TABLE")}, rowsAffected: 0},
+	}
+
+	for i, tt := range tests {
+		ct := tt.commandTag
+		assert.Equalf(t, tt.rowsAffected, ct.RowsAffected(), "%d. %v", i, tt.commandTag)
+		assert.Equalf(t, tt.isInsert, ct.Insert(), "%d. %v", i, tt.commandTag)
+		assert.Equalf(t, tt.isUpdate, ct.Update(), "%d. %v", i, tt.commandTag)
+		assert.Equalf(t, tt.isDelete, ct.Delete(), "%d. %v", i, tt.commandTag)
+		assert.Equalf(t, tt.isSelect, ct.Select(), "%d. %v", i, tt.commandTag)
+	}
+}

pgconn/pgconn_test.go

@@ -538,7 +538,7 @@ func TestConnExec(t *testing.T) {
 
 	assert.Len(t, results, 1)
 	assert.Nil(t, results[0].Err)
-	assert.Equal(t, "SELECT 1", string(results[0].CommandTag))
+	assert.Equal(t, "SELECT 1", results[0].CommandTag.String())
 	assert.Len(t, results[0].Rows, 1)
 	assert.Equal(t, "Hello, world", string(results[0].Rows[0][0]))
 
@@ -579,12 +579,12 @@ func TestConnExecMultipleQueries(t *testing.T) {
 	assert.Len(t, results, 2)
 
 	assert.Nil(t, results[0].Err)
-	assert.Equal(t, "SELECT 1", string(results[0].CommandTag))
+	assert.Equal(t, "SELECT 1", results[0].CommandTag.String())
 	assert.Len(t, results[0].Rows, 1)
 	assert.Equal(t, "Hello, world", string(results[0].Rows[0][0]))
 
 	assert.Nil(t, results[1].Err)
-	assert.Equal(t, "SELECT 1", string(results[1].CommandTag))
+	assert.Equal(t, "SELECT 1", results[1].CommandTag.String())
 	assert.Len(t, results[1].Rows, 1)
 	assert.Equal(t, "1", string(results[1].Rows[0][0]))
 
@@ -741,7 +741,7 @@ func TestConnExecParams(t *testing.T) {
 	}
 	assert.Equal(t, 1, rowCount)
 	commandTag, err := result.Close()
-	assert.Equal(t, "SELECT 1", string(commandTag))
+	assert.Equal(t, "SELECT 1", commandTag.String())
 	assert.NoError(t, err)
 
 	ensureConnValid(t, pgConn)
@@ -840,7 +840,7 @@ func TestConnExecParamsCanceled(t *testing.T) {
 	}
 	assert.Equal(t, 0, rowCount)
 	commandTag, err := result.Close()
-	assert.Equal(t, pgconn.CommandTag(nil), commandTag)
+	assert.Equal(t, pgconn.CommandTag{}, commandTag)
 	assert.True(t, pgconn.Timeout(err))
 	assert.ErrorIs(t, err, context.DeadlineExceeded)
 
@@ -880,7 +880,7 @@ func TestConnExecParamsEmptySQL(t *testing.T) {
 	defer closeConn(t, pgConn)
 
 	result := pgConn.ExecParams(ctx, "", nil, nil, nil, nil).Read()
-	assert.Nil(t, result.CommandTag)
+	assert.Equal(t, pgconn.CommandTag{}, result.CommandTag)
 	assert.Len(t, result.Rows, 0)
 	assert.NoError(t, result.Err)
 
@@ -907,7 +907,7 @@ func TestResultReaderValuesHaveSameCapacityAsLength(t *testing.T) {
 	}
 	assert.Equal(t, 1, rowCount)
 	commandTag, err := result.Close()
-	assert.Equal(t, "SELECT 1", string(commandTag))
+	assert.Equal(t, "SELECT 1", commandTag.String())
 	assert.NoError(t, err)
 
 	ensureConnValid(t, pgConn)
@@ -937,7 +937,7 @@ func TestConnExecPrepared(t *testing.T) {
 	}
 	assert.Equal(t, 1, rowCount)
 	commandTag, err := result.Close()
-	assert.Equal(t, "SELECT 1", string(commandTag))
+	assert.Equal(t, "SELECT 1", commandTag.String())
 	assert.NoError(t, err)
 
 	ensureConnValid(t, pgConn)
@@ -1025,7 +1025,7 @@ func TestConnExecPreparedCanceled(t *testing.T) {
 	}
 	assert.Equal(t, 0, rowCount)
 	commandTag, err := result.Close()
-	assert.Equal(t, pgconn.CommandTag(nil), commandTag)
+	assert.Equal(t, pgconn.CommandTag{}, commandTag)
 	assert.True(t, pgconn.Timeout(err))
 	assert.True(t, pgConn.IsClosed())
 	select {
@@ -1069,7 +1069,7 @@ func TestConnExecPreparedEmptySQL(t *testing.T) {
 	require.NoError(t, err)
 
 	result := pgConn.ExecPrepared(ctx, "ps1", nil, nil, nil).Read()
-	assert.Nil(t, result.CommandTag)
+	assert.Equal(t, pgconn.CommandTag{}, result.CommandTag)
 	assert.Len(t, result.Rows, 0)
 	assert.NoError(t, result.Err)
 
@@ -1097,15 +1097,15 @@ func TestConnExecBatch(t *testing.T) {
 
 	require.Len(t, results[0].Rows, 1)
 	require.Equal(t, "ExecParams 1", string(results[0].Rows[0][0]))
-	assert.Equal(t, "SELECT 1", string(results[0].CommandTag))
+	assert.Equal(t, "SELECT 1", results[0].CommandTag.String())
 
 	require.Len(t, results[1].Rows, 1)
 	require.Equal(t, "ExecPrepared 1", string(results[1].Rows[0][0]))
-	assert.Equal(t, "SELECT 1", string(results[1].CommandTag))
+	assert.Equal(t, "SELECT 1", results[1].CommandTag.String())
 
 	require.Len(t, results[2].Rows, 1)
 	require.Equal(t, "ExecParams 2", string(results[2].Rows[0][0]))
-	assert.Equal(t, "SELECT 1", string(results[2].CommandTag))
+	assert.Equal(t, "SELECT 1", results[2].CommandTag.String())
 }
 
 func TestConnExecBatchDeferredError(t *testing.T) {
@@ -1199,7 +1199,7 @@ func TestConnExecBatchHuge(t *testing.T) {
 	for i := range args {
 		require.Len(t, results[i].Rows, 1)
 		require.Equal(t, args[i], string(results[i].Rows[0][0]))
-		assert.Equal(t, "SELECT 1", string(results[i].CommandTag))
+		assert.Equal(t, "SELECT 1", results[i].CommandTag.String())
 	}
 }
 
@@ -1247,47 +1247,13 @@ func TestConnLocking(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Len(t, results, 1)
 	assert.Nil(t, results[0].Err)
-	assert.Equal(t, "SELECT 1", string(results[0].CommandTag))
+	assert.Equal(t, "SELECT 1", results[0].CommandTag.String())
 	assert.Len(t, results[0].Rows, 1)
 	assert.Equal(t, "Hello, world", string(results[0].Rows[0][0]))
 
 	ensureConnValid(t, pgConn)
 }
 
-func TestCommandTag(t *testing.T) {
-	t.Parallel()
-
-	var tests = []struct {
-		commandTag   pgconn.CommandTag
-		rowsAffected int64
-		isInsert     bool
-		isUpdate     bool
-		isDelete     bool
-		isSelect     bool
-	}{
-		{commandTag: pgconn.CommandTag("INSERT 0 5"), rowsAffected: 5, isInsert: true},
-		{commandTag: pgconn.CommandTag("UPDATE 0"), rowsAffected: 0, isUpdate: true},
-		{commandTag: pgconn.CommandTag("UPDATE 1"), rowsAffected: 1, isUpdate: true},
-		{commandTag: pgconn.CommandTag("DELETE 0"), rowsAffected: 0, isDelete: true},
-		{commandTag: pgconn.CommandTag("DELETE 1"), rowsAffected: 1, isDelete: true},
-		{commandTag: pgconn.CommandTag("DELETE 1234567890"), rowsAffected: 1234567890, isDelete: true},
-		{commandTag: pgconn.CommandTag("SELECT 1"), rowsAffected: 1, isSelect: true},
-		{commandTag: pgconn.CommandTag("SELECT 99999999999"), rowsAffected: 99999999999, isSelect: true},
-		{commandTag: pgconn.CommandTag("CREATE TABLE"), rowsAffected: 0},
-		{commandTag: pgconn.CommandTag("ALTER TABLE"), rowsAffected: 0},
-		{commandTag: pgconn.CommandTag("DROP TABLE"), rowsAffected: 0},
-	}
-
-	for i, tt := range tests {
-		ct := tt.commandTag
-		assert.Equalf(t, tt.rowsAffected, ct.RowsAffected(), "%d. %v", i, tt.commandTag)
-		assert.Equalf(t, tt.isInsert, ct.Insert(), "%d. %v", i, tt.commandTag)
-		assert.Equalf(t, tt.isUpdate, ct.Update(), "%d. %v", i, tt.commandTag)
-		assert.Equalf(t, tt.isDelete, ct.Delete(), "%d. %v", i, tt.commandTag)
-		assert.Equalf(t, tt.isSelect, ct.Select(), "%d. %v", i, tt.commandTag)
-	}
-}
-
 func TestConnOnNotice(t *testing.T) {
 	t.Parallel()
 
@@ -1546,7 +1512,7 @@ func TestConnCopyToCanceled(t *testing.T) {
 	defer cancel()
 	res, err := pgConn.CopyTo(ctx, outputWriter, "copy (select *, pg_sleep(0.01) from generate_series(1,1000)) to stdout")
 	assert.Error(t, err)
-	assert.Equal(t, pgconn.CommandTag(nil), res)
+	assert.Equal(t, pgconn.CommandTag{}, res)
 
 	assert.True(t, pgConn.IsClosed())
 	select {
@@ -1571,7 +1537,7 @@ func TestConnCopyToPrecanceled(t *testing.T) {
 	require.Error(t, err)
 	assert.True(t, errors.Is(err, context.Canceled))
 	assert.True(t, pgconn.SafeToRetry(err))
-	assert.Equal(t, pgconn.CommandTag(nil), res)
+	assert.Equal(t, pgconn.CommandTag{}, res)
 
 	ensureConnValid(t, pgConn)
 }
@@ -1692,7 +1658,7 @@ func TestConnCopyFromPrecanceled(t *testing.T) {
 	require.Error(t, err)
 	assert.True(t, errors.Is(err, context.Canceled))
 	assert.True(t, pgconn.SafeToRetry(err))
-	assert.Equal(t, pgconn.CommandTag(nil), ct)
+	assert.Equal(t, pgconn.CommandTag{}, ct)
 
 	ensureConnValid(t, pgConn)
 }
@@ -2014,7 +1980,7 @@ func TestHijackAndConstruct(t *testing.T) {
 
 	assert.Len(t, results, 1)
 	assert.Nil(t, results[0].Err)
-	assert.Equal(t, "SELECT 1", string(results[0].CommandTag))
+	assert.Equal(t, "SELECT 1", results[0].CommandTag.String())
 	assert.Len(t, results[0].Rows, 1)
 	assert.Equal(t, "Hello, world", string(results[0].Rows[0][0]))
 

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 }

query_test.go

@@ -45,7 +45,7 @@ func TestConnQueryScan(t *testing.T) {
 		t.Fatalf("conn.Query failed: %v", err)
 	}
 
-	assert.Equal(t, "SELECT 10", string(rows.CommandTag()))
+	assert.Equal(t, "SELECT 10", rows.CommandTag().String())
 
 	if rowCount != 10 {
 		t.Error("Select called onDataRow wrong number of times")
@@ -79,7 +79,7 @@ func TestConnQueryWithoutResultSetCommandTag(t *testing.T) {
 	assert.NoError(t, err)
 	rows.Close()
 	assert.NoError(t, rows.Err())
-	assert.Equal(t, "CREATE TABLE", string(rows.CommandTag()))
+	assert.Equal(t, "CREATE TABLE", rows.CommandTag().String())
 }
 
 func TestConnQueryScanWithManyColumns(t *testing.T) {
@@ -1139,7 +1139,7 @@ func TestConnQueryDatabaseSQLDriverValuerWithAutoGeneratedPointerReceiver(t *tes
 	if err != nil {
 		t.Fatal(err)
 	}
-	if string(commandTag) != "INSERT 0 1" {
+	if commandTag.String() != "INSERT 0 1" {
 		t.Fatalf("want %s, got %s", "INSERT 0 1", commandTag)
 	}
 
@@ -1976,7 +1976,7 @@ func TestConnQueryFuncAbort(t *testing.T) {
 			},
 		)
 		require.EqualError(t, err, "abort")
-		require.Nil(t, ct)
+		require.Equal(t, pgconn.CommandTag{}, ct)
 	})
 }
 

tx.go

@@ -235,7 +235,7 @@ func (tx *dbTx) Commit(ctx context.Context) error {
 		}
 		return err
 	}
-	if string(commandTag) == "ROLLBACK" {
+	if commandTag.String() == "ROLLBACK" {
 		return ErrTxCommitRollback
 	}
 
@@ -296,7 +296,7 @@ func (tx *dbTx) QueryRow(ctx context.Context, sql string, args ...interface{}) R
 // QueryFunc delegates to the underlying *Conn.
 func (tx *dbTx) QueryFunc(ctx context.Context, sql string, args []interface{}, scans []interface{}, f func(QueryFuncRow) error) (pgconn.CommandTag, error) {
 	if tx.closed {
-		return nil, ErrTxClosed
+		return pgconn.CommandTag{}, ErrTxClosed
 	}
 
 	return tx.conn.QueryFunc(ctx, sql, args, scans, f)
@@ -380,7 +380,7 @@ func (sp *dbSavepoint) Rollback(ctx context.Context) error {
 // Exec delegates to the underlying Tx
 func (sp *dbSavepoint) Exec(ctx context.Context, sql string, arguments ...interface{}) (commandTag pgconn.CommandTag, err error) {
 	if sp.closed {
-		return nil, ErrTxClosed
+		return pgconn.CommandTag{}, ErrTxClosed
 	}
 
 	return sp.tx.Exec(ctx, sql, arguments...)
@@ -415,7 +415,7 @@ func (sp *dbSavepoint) QueryRow(ctx context.Context, sql string, args ...interfa
 // QueryFunc delegates to the underlying Tx.
 func (sp *dbSavepoint) QueryFunc(ctx context.Context, sql string, args []interface{}, scans []interface{}, f func(QueryFuncRow) error) (pgconn.CommandTag, error) {
 	if sp.closed {
-		return nil, ErrTxClosed
+		return pgconn.CommandTag{}, ErrTxClosed
 	}
 
 	return sp.tx.QueryFunc(ctx, sql, args, scans, f)