Update max_protocol_version and min_protocol_version defaults

When `max_protocol_version` is not explicitly set, the default value is
set to "3.0". This is reasonable and is inline with the same behavior as
libpq[1]. Though, what libpq attempts to do that was not being done here
is to reasonably upgrade or auto-raise the value of
`max_protocol_version` when it has not been explicitly set.

Here, we're providing the same functionality. In libpq, this
determination is based on the parsed config values. We do the same here,
parsing the values in ParseConfig to validate and determine the
appropriate default for max_protocol_version. The string fields on
Config are then set accordingly, and re-parsed at connection time as
before.

After parsing, when `max_protocol_version` has not been explicitly set,
if `min_protocol_version` is greater than 3.0, `max_protocol_version`
defaults to `latest`; otherwise it defaults to 3.0 for compatibility
with older servers/poolers that don't support NegotiateProtocolVersion.

[1] postgres/postgres@285613c

Author: abrightwell

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
 )