openapiv2: panic when SecurityRequirement is missing a value

[krishicks]

🐛 Bug Report

protoc-gen-openapiv2 panics when specifying an OpenAPI security option with a security requirement that is missing a value.

This was discovered when using buf to generate protos. @bufdev hinted that:

buf is correctly not producing a message value, while protoc is incorrectly producing a default value here

(Note: the issue was found in v1.9.6, and has been adapted to master, which includes the rename to openapiv2)

To Reproduce

Given the following proto:

syntax = "proto3";

import "protoc-gen-openapiv2/options/annotations.proto";

package mypackage;

option (grpc.gateway.protoc_gen_swagger.options.openapiv2_swagger) = {
    security: {
        security_requirement: {
            key: "bearer";
        }
    }
};

Try to compile the proto with buf:

buf generate --template '{"version":"v1beta1","plugins":[{"name":"openapiv2","out":"out"}]}' --file proto/swagger-description.proto

Expected behavior

The plugin exits successfully.

Actual Behavior

The plugin panics:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x6302f7]

goroutine 1 [running]:
github.com/grpc-ecosystem/grpc-gateway/protoc-gen-swagger/genswagger.applyTemplate(0xc0001e8080, 0xc0000cc640, 0xc0001c2fe0, 0x0, 0x0)
	/home/hicks/go/pkg/mod/github.com/grpc-ecosystem/grpc-gateway@v1.9.6/protoc-gen-swagger/genswagger/template.go:1177 +0x1017
github.com/grpc-ecosystem/grpc-gateway/protoc-gen-swagger/genswagger.(*generator).Generate(0xc000010420, 0xc000010610, 0x1, 0x1, 0x1, 0xc000010610, 0x0, 0x1, 0x340)
	/home/hicks/go/pkg/mod/github.com/grpc-ecosystem/grpc-gateway@v1.9.6/protoc-gen-swagger/genswagger/generator.go:116 +0x6d9
main.main()
	/home/hicks/go/pkg/mod/github.com/grpc-ecosystem/grpc-gateway@v1.9.6/protoc-gen-swagger/main.go:117 +0x7eb
plugin swagger: exit status 2

The master line that equates to the v1.9.6 panic above is here: https://github.com/grpc-ecosystem/grpc-gateway/blob/master/protoc-gen-openapiv2/internal/genopenapi/template.go#L1358

Note that if the above security_requirement is given value: {}, generation with buf succeeds.

Your Environment

gen-go-swagger v1.9.6
GNU/Linux x86_64