Update max_protocol_version and min_protocol_version defaults

pgconn/config.go

@@ -454,23 +454,37 @@ func ParseConfigWithOptions(connString string, options ParseConfigOptions) (*Con
 
 	minProto, err := parseProtocolVersion(settings["min_protocol_version"])
 	if err != nil {
-		return nil, &ParseConfigError{ConnString: connString, msg: "invalid min_protocol_version", err: err}
+		return nil, &ParseConfigError{ConnString: connString, msg: fmt.Sprintf("invalid min_protocol_version: %q", settings["min_protocol_version"]), err: err}
 	}
 	maxProto, err := parseProtocolVersion(settings["max_protocol_version"])
 	if err != nil {
-		return nil, &ParseConfigError{ConnString: connString, msg: "invalid max_protocol_version", err: err}
-	}
-	if minProto > maxProto {
-		return nil, &ParseConfigError{ConnString: connString, msg: "min_protocol_version cannot be greater than max_protocol_version"}
+		return nil, &ParseConfigError{ConnString: connString, msg: fmt.Sprintf("invalid max_protocol_version: %q", settings["max_protocol_version"]), err: err}
 	}
 
 	config.MinProtocolVersion = settings["min_protocol_version"]
 	config.MaxProtocolVersion = settings["max_protocol_version"]
+
 	if config.MinProtocolVersion == "" {
 		config.MinProtocolVersion = "3.0"
 	}
+
+	// When max_protocol_version is not explicitly set, default based on
+	// min_protocol_version. This matches libpq behavior: if min > 3.0,
+	// default max to latest; otherwise default to 3.0 for compatibility
+	// with older servers/poolers that don't support NegotiateProtocolVersion.
 	if config.MaxProtocolVersion == "" {
-		config.MaxProtocolVersion = "3.0"
+		if minProto > pgproto3.ProtocolVersion30 {
+			config.MaxProtocolVersion = "latest"
+		} else {
+			config.MaxProtocolVersion = "3.0"
+		}
+	}
+
+	// Only error when max_protocol_version was explicitly set and conflicts
+	// with min_protocol_version. When max_protocol_version is not explicitly
+	// set, the auto-raise logic above already ensures a valid default.
+	if minProto > maxProto && settings["max_protocol_version"] != "" {
+		return nil, &ParseConfigError{ConnString: connString, msg: "min_protocol_version cannot be greater than max_protocol_version"}
 	}
 
 	switch channelBinding := settings["channel_binding"]; channelBinding {

pgconn/config_test.go

@@ -1228,14 +1228,20 @@ func TestParseConfigProtocolVersion(t *testing.T) {
 			expectedMin: "3.2",
 			expectedMax: "3.2",
 		},
+		{
+			name:        "min_protocol_version=latest auto-raises max",
+			connString:  "postgres://localhost/test?min_protocol_version=latest",
+			expectedMin: "latest",
+			expectedMax: "latest",
+		},
 		{
 			name:        "max_protocol_version=latest",
 			connString:  "postgres://localhost/test?max_protocol_version=latest",
 			expectedMin: "3.0",
 			expectedMax: "latest",
 		},
 		{
-			name:        "min and max = latest",
+			name:        "min_protocol_version and max_protocol_version = latest",
 			connString:  "postgres://localhost/test?min_protocol_version=latest&max_protocol_version=latest",
 			expectedMin: "latest",
 			expectedMax: "latest",
@@ -1244,27 +1250,39 @@ func TestParseConfigProtocolVersion(t *testing.T) {
 			name:               "invalid min_protocol_version",
 			connString:         "postgres://localhost/test?min_protocol_version=2.0",
 			expectError:        true,
-			expectedErrContain: "invalid min_protocol_version",
+			expectedErrContain: `invalid min_protocol_version: "2.0"`,
 		},
 		{
 			name:               "invalid max_protocol_version",
 			connString:         "postgres://localhost/test?max_protocol_version=4.0",
 			expectError:        true,
-			expectedErrContain: "invalid max_protocol_version",
+			expectedErrContain: `invalid max_protocol_version: "4.0"`,
 		},
 		{
-			name:               "min > max",
+			name:               "min_protocol_version > max_protocol_version",
 			connString:         "postgres://localhost/test?min_protocol_version=3.2&max_protocol_version=3.0",
 			expectError:        true,
 			expectedErrContain: "min_protocol_version cannot be greater than max_protocol_version",
 		},
 		{
-			name:               "environment variable PGMINPROTOCOLVERSION without matching max fails",
-			connString:         "postgres://localhost/test",
-			envMin:             "3.2",
+			name:               "min_protocol_version (latest) > max_protocol_version",
+			connString:         "postgres://localhost/test?min_protocol_version=latest&max_protocol_version=3.0",
 			expectError:        true,
 			expectedErrContain: "min_protocol_version cannot be greater than max_protocol_version",
 		},
+		{
+			name:        "min_protocol_version=3.2 auto-raises max",
+			connString:  "postgres://localhost/test?min_protocol_version=3.2",
+			expectedMin: "3.2",
+			expectedMax: "latest",
+		},
+		{
+			name:        "environment variable PGMINPROTOCOLVERSION auto-raises max",
+			connString:  "postgres://localhost/test",
+			envMin:      "3.2",
+			expectedMin: "3.2",
+			expectedMax: "latest",
+		},
 		{
 			name:        "environment variables PGMINPROTOCOLVERSION and PGMAXPROTOCOLVERSION together",
 			connString:  "postgres://localhost/test",

pgproto3/startup_message.go

@@ -13,6 +13,7 @@ import (
 const (
 	ProtocolVersion30     = 196608            // 3.0
 	ProtocolVersion32     = 196610            // 3.2
+	ProtocolVersionLatest = ProtocolVersion32 // Latest is 3.2
 	ProtocolVersionNumber = ProtocolVersion30 // Default is still 3.0
 )