feat: add PawSQL docs examples and image CI

internal/pgwire/startup.go

@@ -0,0 +1,108 @@
+package pgwire
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+)
+
+const (
+	startupProtocolVersion uint32 = 196608
+	maxStartupMessageSize         = 64 << 10
+)
+
+var (
+	ErrInvalidStartupMessage = errors.New("invalid PostgreSQL StartupMessage")
+	ErrMissingDatabase       = errors.New("PostgreSQL StartupMessage has no database")
+)
+
+// StartupMessage is the PostgreSQL protocol preamble sent after TLS negotiation.
+// PawSQL reads it only when TLS SNI is unavailable for routing.
+type StartupMessage struct {
+	raw      []byte
+	Database string
+}
+
+// ReadStartupMessage reads the bounded protocol v3 StartupMessage and exposes
+// its database parameter without inspecting later PostgreSQL traffic.
+func ReadStartupMessage(reader io.Reader) (StartupMessage, error) {
+	var header [4]byte
+	if _, err := io.ReadFull(reader, header[:]); err != nil {
+		return StartupMessage{}, fmt.Errorf("read StartupMessage length: %w", err)
+	}
+	length := binary.BigEndian.Uint32(header[:])
+	if length < 8 || length > maxStartupMessageSize {
+		return StartupMessage{}, fmt.Errorf("%w: invalid length %d", ErrInvalidStartupMessage, length)
+	}
+
+	raw := make([]byte, length)
+	copy(raw, header[:])
+	if _, err := io.ReadFull(reader, raw[4:]); err != nil {
+		return StartupMessage{}, fmt.Errorf("read StartupMessage body: %w", err)
+	}
+	if version := binary.BigEndian.Uint32(raw[4:8]); version != startupProtocolVersion {
+		return StartupMessage{}, fmt.Errorf("%w: unsupported protocol version %d", ErrInvalidStartupMessage, version)
+	}
+
+	message := StartupMessage{raw: raw}
+	databaseSeen := false
+	terminated := false
+	for offset := 8; offset < len(raw); {
+		if raw[offset] == 0 {
+			if offset != len(raw)-1 {
+				return StartupMessage{}, fmt.Errorf("%w: unexpected parameter terminator", ErrInvalidStartupMessage)
+			}
+			terminated = true
+			break
+		}
+		keyStart := offset
+		keyEnd := bytes.IndexByte(raw[keyStart:], 0)
+		if keyEnd < 0 {
+			return StartupMessage{}, fmt.Errorf("%w: unterminated parameter name", ErrInvalidStartupMessage)
+		}
+		keyEnd += keyStart
+		valueStart := keyEnd + 1
+		valueEnd := bytes.IndexByte(raw[valueStart:], 0)
+		if valueEnd < 0 {
+			return StartupMessage{}, fmt.Errorf("%w: unterminated parameter value", ErrInvalidStartupMessage)
+		}
+		valueEnd += valueStart
+		if string(raw[keyStart:keyEnd]) == "database" {
+			if databaseSeen {
+				return StartupMessage{}, fmt.Errorf("%w: duplicate database parameter", ErrInvalidStartupMessage)
+			}
+			message.Database = string(raw[valueStart:valueEnd])
+			databaseSeen = true
+		}
+		offset = valueEnd + 1
+	}
+	if !terminated {
+		return StartupMessage{}, fmt.Errorf("%w: missing parameter terminator", ErrInvalidStartupMessage)
+	}
+	if !databaseSeen || message.Database == "" {
+		return StartupMessage{}, ErrMissingDatabase
+	}
+	return message, nil
+}
+
+// Bytes returns the exact StartupMessage bytes for replay to the backend.
+func (m StartupMessage) Bytes() []byte {
+	return m.raw
+}
+
+// Replay returns a connection that yields prefix before reading from conn.
+func Replay(conn net.Conn, prefix []byte) net.Conn {
+	return &replayConn{Conn: conn, reader: io.MultiReader(bytes.NewReader(prefix), conn)}
+}
+
+type replayConn struct {
+	net.Conn
+	reader io.Reader
+}
+
+func (c *replayConn) Read(buffer []byte) (int, error) {
+	return c.reader.Read(buffer)
+}