Adding interplugin communication. (#12829)

* Adding interplugin communication.

* Naming changes and moving ResponseTransfer to own file.

* Fix.

* Tests and moving to buffering bytes.

* Switching API to passing plugin ID through path rather than a header.

* Review feedback.

app/response_transfer.go

@@ -0,0 +1,70 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+type PluginResponseWriter struct {
+	bytes.Buffer
+	headers    http.Header
+	statusCode int
+}
+
+func (rt *PluginResponseWriter) Header() http.Header {
+	if rt.headers == nil {
+		rt.headers = make(http.Header)
+	}
+	return rt.headers
+}
+
+func (rt *PluginResponseWriter) WriteHeader(statusCode int) {
+	rt.statusCode = statusCode
+}
+
+// From net/http/httptest/recorder.go
+func parseContentLength(cl string) int64 {
+	cl = strings.TrimSpace(cl)
+	if cl == "" {
+		return -1
+	}
+	n, err := strconv.ParseInt(cl, 10, 64)
+	if err != nil {
+		return -1
+	}
+	return n
+
+}
+
+func (rt *PluginResponseWriter) GenerateResponse() *http.Response {
+	res := &http.Response{
+		Proto:      "HTTP/1.1",
+		ProtoMajor: 1,
+		ProtoMinor: 1,
+		StatusCode: rt.statusCode,
+		Header:     rt.headers.Clone(),
+	}
+
+	if res.StatusCode == 0 {
+		res.StatusCode = http.StatusOK
+	}
+
+	res.Status = fmt.Sprintf("%03d %s", res.StatusCode, http.StatusText(res.StatusCode))
+
+	if rt.Len() > 0 {
+		res.Body = ioutil.NopCloser(rt)
+	} else {
+		res.Body = http.NoBody
+	}
+
+	res.ContentLength = parseContentLength(rt.headers.Get("Content-Length"))
+
+	return res
+}