Warn when using a non-empty spec.Servers (#883)

It's a fairly common mistake when using the OpenAPI request validation
filter from kin-openapi to receive `no matching operation was found`
errors if using the `servers` directive in your OpenAPI specification.

This is a valid usescase, but you may not always want to be enforcing
the validation that the `Host` header matches one of the `servers`
mentioned in your specification.

This makes it clearer to operators that there may be a configuration
issue here, as a way to avoid as many folks falling into the same trap
that many have before.

This also provides an option to silence the warning, as logging out can
be annoying and costly.

Closes #882.

Author: jamietanna

oapi_validate.go

@@ -18,6 +18,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"log"
 	"net/http"
 	"os"
 	"strings"
@@ -70,10 +71,16 @@ type Options struct {
 	ParamDecoder      openapi3filter.ContentParameterDecoder
 	UserData          interface{}
 	MultiErrorHandler MultiErrorHandler
+	// SilenceServersWarning allows silencing a warning for https://github.com/deepmap/oapi-codegen/issues/882 that reports when an OpenAPI spec has `spec.Servers != nil`
+	SilenceServersWarning bool
 }
 
 // OapiRequestValidatorWithOptions creates a validator from a swagger object, with validation options
 func OapiRequestValidatorWithOptions(swagger *openapi3.T, options *Options) gin.HandlerFunc {
+	if swagger.Servers != nil && (options == nil || options.SilenceServersWarning) {
+		log.Println("WARN: OapiRequestValidatorWithOptions called with an OpenAPI spec that has `Servers` set. This may lead to an HTTP 400 with `no matching operation was found` when sending a valid request, as the validator performs `Host` header validation. If you're expecting `Host` header validation, you can silence this warning by setting `Options.SilenceServersWarning = true`. See https://github.com/deepmap/oapi-codegen/issues/882 for more information.")
+	}
+
 	router, err := gorillamux.NewRouter(swagger)
 	if err != nil {
 		panic(err)