Export defaultQueryParser struct for custom query parsers (grpc-eco…

…system#2651)

* Export `defaultQueryParser` struct for custom query parsers

* Update v2 migration guide for query parameter parsing change

docs/docs/development/grpc-gateway_v2_migration_guide.md

@@ -162,3 +162,7 @@ There is no workaround for this, as we considered it a correct interpretation of
 ## Error handling configuration has been overhauled
 
 `runtime.HTTPError`, `runtime.OtherErrorHandler`, `runtime.GlobalHTTPErrorHandler`, `runtime.WithProtoErrorHandler` are all gone. Error handling is rewritten around the use of gRPCs Status types. If you wish to configure how the gateway handles errors, please use `runtime.WithErrorHandler` and `runtime.WithStreamErrorHandler`. To handle routing errors (similar to the removed `runtime.OtherErrorHandler`) please use `runtime.WithRoutingErrorHandler`.
+
+## Default query parameter parsing behaviour change
+
+The default behaviour for query parameter parsing has changed to return an `InvalidArgument` (`400 Bad Request`) error when more than one of the same matching query parameters is parsed. Previously, it would log but not return an error, using the first query parameter that matched and ignoring any others. See [the original issue](https://github.com/grpc-ecosystem/grpc-gateway/issues/2632) for more information.

runtime/query.go

@@ -24,7 +24,7 @@ import (
 
 var valuesKeyRegexp = regexp.MustCompile(`^(.*)\[(.*)\]$`)
 
-var currentQueryParser QueryParameterParser = &defaultQueryParser{}
+var currentQueryParser QueryParameterParser = &DefaultQueryParser{}
 
 // QueryParameterParser defines interface for all query parameter parsers
 type QueryParameterParser interface {
@@ -37,11 +37,15 @@ func PopulateQueryParameters(msg proto.Message, values url.Values, filter *utili
 	return currentQueryParser.Parse(msg, values, filter)
 }
 
-type defaultQueryParser struct{}
+// DefaultQueryParser is a QueryParameterParser which implements the default
+// query parameters parsing behavior.
+//
+// See https://github.com/grpc-ecosystem/grpc-gateway/issues/2632 for more context.
+type DefaultQueryParser struct{}
 
 // Parse populates "values" into "msg".
 // A value is ignored if its key starts with one of the elements in "filter".
-func (*defaultQueryParser) Parse(msg proto.Message, values url.Values, filter *utilities.DoubleArray) error {
+func (*DefaultQueryParser) Parse(msg proto.Message, values url.Values, filter *utilities.DoubleArray) error {
 	for key, values := range values {
 		match := valuesKeyRegexp.FindStringSubmatch(key)
 		if len(match) == 3 {