Draft of connection writable checking

Signed-off-by: Artemiy Ryabinkov <getlag@ya.ru>

conn.go

@@ -89,6 +89,13 @@ type ConnConfig struct {
 	// used by default. The same functionality can be controlled on a per query
 	// basis by setting QueryExOptions.SimpleProtocol.
 	PreferSimpleProtocol bool
+
+	// TargetSessionAttr allows to specify which servers are accepted for this connection.
+	//  "any", meaning that any kind of servers can be accepted. This is as well the default value.
+	//	"read-write", to disallow connections to read-only servers, hot standbys for example.
+	// @see https://www.postgresql.org/message-id/CAD__OuhqPRGpcsfwPHz_PDqAGkoqS1UvnUnOnAB-LBWBW=wu4A@mail.gmail.com
+	// @see https://paquier.xyz/postgresql-2/postgres-10-libpq-read-write/
+	TargetSessionAttrs string
 }
 
 func (cc *ConnConfig) networkAddress() (network, address string) {
@@ -262,8 +269,15 @@ func connect(config ConnConfig, connInfo *pgtype.ConnInfo) (c *Conn, err error)
 		}
 	}
 
+	if c.config.TargetSessionAttrs != "" &&
+		c.config.TargetSessionAttrs != "any" &&
+		c.config.TargetSessionAttrs != "read-write" {
+		return nil, errors.New("invalid value for target_session_attrs, expected \"any\" or \"read-write\"")
+	}
+
 	c.onNotice = config.OnNotice
 
+	// TODO: Parse multi-hosts
 	network, address := c.config.networkAddress()
 	if c.config.Dial == nil {
 		d := defaultDialer()
@@ -273,22 +287,58 @@ func connect(config ConnConfig, connInfo *pgtype.ConnInfo) (c *Conn, err error)
 	if c.shouldLog(LogLevelInfo) {
 		c.log(LogLevelInfo, "Dialing PostgreSQL server", map[string]interface{}{"network": network, "address": address})
 	}
-	err = c.connect(config, network, address, config.TLSConfig)
-	if err != nil && config.UseFallbackTLS {
-		if c.shouldLog(LogLevelInfo) {
-			c.log(LogLevelInfo, "connect with TLSConfig failed, trying FallbackTLSConfig", map[string]interface{}{"err": err})
+
+	// TODO: Start loop for all hosts [host0 .. hostN]
+	for {
+		err = c.connect(config, network, address, config.TLSConfig)
+		if err != nil && config.UseFallbackTLS {
+			if c.shouldLog(LogLevelInfo) {
+				c.log(LogLevelInfo, "connect with TLSConfig failed, trying FallbackTLSConfig", map[string]interface{}{"err": err})
+			}
+			err = c.connect(config, network, address, config.FallbackTLSConfig)
 		}
-		err = c.connect(config, network, address, config.FallbackTLSConfig)
+
+		if err != nil {
+			if c.shouldLog(LogLevelError) {
+				c.log(LogLevelError, "connect failed", map[string]interface{}{"err": err})
+			}
+			// TODO: Collect error
+			continue
+		}
+
+		err = c.writeable()
+		if err != nil {
+			// TODO: Log info about not writable host
+			// TODO: Collect error
+			continue
+		}
+
+		return c, nil
 	}
 
+
+	// TODO: Return collected errors
+	return nil, nil
+}
+
+func (c *Conn) writeable() error {
+	if c.config.TargetSessionAttrs == "" || c.config.TargetSessionAttrs == "any" {
+		return nil
+	}
+
+	var st string
+	err := c.QueryRowEx(context.Background(), "SHOW transaction_read_only", &QueryExOptions{SimpleProtocol: true}).
+		Scan(st)
+
 	if err != nil {
-		if c.shouldLog(LogLevelError) {
-			c.log(LogLevelError, "connect failed", map[string]interface{}{"err": err})
-		}
-		return nil, err
+		return errors.Wrap(err, "failed to fetch transaction_read_only state")
 	}
 
-	return c, nil
+	if st == "on" {
+		return errors.New("writable connection disabled by server")
+	}
+
+	return nil
 }
 
 func (c *Conn) connect(config ConnConfig, network, address string, tlsConfig *tls.Config) (err error) {
@@ -709,6 +759,10 @@ func (old ConnConfig) Merge(other ConnConfig) ConnConfig {
 
 	cc.PreferSimpleProtocol = old.PreferSimpleProtocol || other.PreferSimpleProtocol
 
+	if other.TargetSessionAttrs != "" {
+		cc.TargetSessionAttrs = other.TargetSessionAttrs
+	}
+
 	cc.RuntimeParams = make(map[string]string)
 	for k, v := range old.RuntimeParams {
 		cc.RuntimeParams[k] = v