Allow HTTP POST calls for any showcase PATCH RPCs

[lqiu96]

I'm trying to add in the showcase compliance test suite for Java and I'm running into an issue with the PATCH calls.
Error:

Caused by: com.google.api.client.http.HttpResponseException: 400 Bad Request
POST http://localhost:7469/v1beta1/repeat:bodypatch
{"error":{"code":400,"message":"unrecognized request: POST \"/v1beta1/repeat:bodypatch\"","details":null,"Body":"","Header":null,"Errors":null}}
	at com.google.api.client.http.HttpResponseException$Builder.build(HttpResponseException.java:293)
	at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:1118)
	at com.google.api.gax.httpjson.HttpRequestRunnable.run(HttpRequestRunnable.java:118)
	... 6 more

Java Gax-httpjson's module sends PATCH requests as a POST request with a x-http-method-override: PATCH header, which results in a 400 Bad Request. Looking at the historical reasons for this:
https://github.com/googleapis/gapic-generator-java/blob/c23f981e2ac3c573bed51e725dc7061551179400/gax-java/gax-httpjson/src/main/java/com/google/api/gax/httpjson/HttpRequestRunnable.java#L217-L226

    // A workaround to support PATCH request. This assumes support of "X-HTTP-Method-Override"
    // header on the server side, which GCP services usually do.
    //
    // Long story short, the problems is as follows: gax-httpjson depends on NetHttpTransport class
    // from google-http-client, which depends on JDK standard java.net.HttpUrlConnection, which does
    // not support PATCH http method.
    //
    // It is a won't fix for JDK8: https://bugs.openjdk.java.net/browse/JDK-8207840.
    // A corresponding google-http-client issue:
    // https://github.com/googleapis/google-http-java-client/issues/167

I believe the mux router setup for bodyPatch to expect a PATCH request

gapic-showcase/server/genrest/genrest.go

Line 43 in b94ecfc

router.HandleFunc("/v1beta1/repeat:bodypatch", rest.HandleRepeatDataBodyPatch).Methods("PATCH")

Possible fix (Thanks to Noah for pointing me here):

• Adding a conditional for PATCH here:

  gapic-showcase/util/genrest/goviewcreator.go

  Lines 292 to 294 in b94ecfc

  	for _, handler := range registered {
  	file.P(` router.HandleFunc(%q, rest.%s).Methods(%q)`, handler.pattern, handler.function, handler.verb)
  	}

  
  so it would output
  router.HandleFunc("/v1beta1/repeat:bodypatch", rest.HandleRepeatDataBodyPatch).Methods("PATCH", "POST")

Alternatives:

• Does Go Mux support x-http-method-override?

[noahdietz]

I don't think we need to worry about if gorilla/mux supports the specific header, we just need to extend the handler to/add a new handler that checks for the header on POST calls to that same path and sends it to the appropriate Go handler func (i.e. the update method)