pgconn: support configuring the pgproto3 tracer before opening a connection

pgconn/config.go

@@ -83,6 +83,15 @@ type Config struct {
 	// that you close on FATAL errors by returning false.
 	OnPgError PgErrorHandler
 
+	// Tracer is an io.Writer to which the PostgreSQL frontend/backend protocol messages will be logged.
+	// The format roughly mimics the format produced by the libpq C function PQtrace. Messages are logged
+	// from the connection handshake onwards, providing visibility into authentication, parameter status
+	// messages, backend key data, and all subsequent protocol traffic.
+	Tracer io.Writer
+
+	// TracerOptions controls tracing behavior. Only relevant when Tracer is set.
+	TracerOptions pgproto3.TracerOptions
+
 	createdByParseConfig bool // Used to enforce created by ParseConfig rule.
 }
 

pgconn/pgconn.go

@@ -369,6 +369,10 @@ func connectOne(ctx context.Context, config *Config, connectConfig *connectOneCo
 	pgConn.bgReaderStarted = make(chan struct{})
 	pgConn.frontend = config.BuildFrontend(pgConn.bgReader, pgConn.conn)
 
+	if config.Tracer != nil {
+		pgConn.frontend.Trace(config.Tracer, config.TracerOptions)
+	}
+
 	startupMsg := pgproto3.StartupMessage{
 		ProtocolVersion: pgproto3.ProtocolVersionNumber,
 		Parameters:      make(map[string]string),

pgproto3/backend.go

@@ -1,7 +1,6 @@
 package pgproto3
 
 import (
-	"bytes"
 	"encoding/binary"
 	"fmt"
 	"io"
@@ -105,7 +104,6 @@ func (b *Backend) Flush() error {
 func (b *Backend) Trace(w io.Writer, options TracerOptions) {
 	b.tracer = &tracer{
 		w:             w,
-		buf:           &bytes.Buffer{},
 		TracerOptions: options,
 	}
 }

pgproto3/frontend.go

@@ -1,7 +1,6 @@
 package pgproto3
 
 import (
-	"bytes"
 	"encoding/binary"
 	"errors"
 	"fmt"
@@ -124,7 +123,6 @@ func (f *Frontend) Flush() error {
 func (f *Frontend) Trace(w io.Writer, options TracerOptions) {
 	f.tracer = &tracer{
 		w:             w,
-		buf:           &bytes.Buffer{},
 		TracerOptions: options,
 	}
 }