MM-10702 Moving plugins to use hashicorp go-plugin. (#8978)

* Moving plugins to use hashicorp go-plugin.

* Tweaks from feedback.

app/plugin_requests.go

@@ -0,0 +1,75 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/gorilla/mux"
+	"github.com/mattermost/mattermost-server/mlog"
+	"github.com/mattermost/mattermost-server/model"
+)
+
+func (a *App) ServePluginRequest(w http.ResponseWriter, r *http.Request) {
+	if a.Plugins == nil || !*a.Config().PluginSettings.Enable {
+		err := model.NewAppError("ServePluginRequest", "app.plugin.disabled.app_error", nil, "Enable plugins to serve plugin requests", http.StatusNotImplemented)
+		a.Log.Error(err.Error())
+		w.WriteHeader(err.StatusCode)
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(err.ToJson()))
+		return
+	}
+
+	params := mux.Vars(r)
+	hooks, err := a.Plugins.HooksForPlugin(params["plugin_id"])
+	if err != nil {
+		a.Log.Error("Access to route for non-existant plugin", mlog.String("missing_plugin_id", params["plugin_id"]), mlog.Err(err))
+		http.NotFound(w, r)
+		return
+	}
+
+	a.servePluginRequest(w, r, hooks.ServeHTTP)
+}
+
+func (a *App) servePluginRequest(w http.ResponseWriter, r *http.Request, handler http.HandlerFunc) {
+	token := ""
+
+	authHeader := r.Header.Get(model.HEADER_AUTH)
+	if strings.HasPrefix(strings.ToUpper(authHeader), model.HEADER_BEARER+" ") {
+		token = authHeader[len(model.HEADER_BEARER)+1:]
+	} else if strings.HasPrefix(strings.ToLower(authHeader), model.HEADER_TOKEN+" ") {
+		token = authHeader[len(model.HEADER_TOKEN)+1:]
+	} else if cookie, _ := r.Cookie(model.SESSION_COOKIE_TOKEN); cookie != nil && (r.Method == "GET" || r.Header.Get(model.HEADER_REQUESTED_WITH) == model.HEADER_REQUESTED_WITH_XML) {
+		token = cookie.Value
+	} else {
+		token = r.URL.Query().Get("access_token")
+	}
+
+	r.Header.Del("Mattermost-User-Id")
+	if token != "" {
+		if session, err := a.GetSession(token); session != nil && err == nil {
+			r.Header.Set("Mattermost-User-Id", session.UserId)
+		}
+	}
+
+	cookies := r.Cookies()
+	r.Header.Del("Cookie")
+	for _, c := range cookies {
+		if c.Name != model.SESSION_COOKIE_TOKEN {
+			r.AddCookie(c)
+		}
+	}
+	r.Header.Del(model.HEADER_AUTH)
+	r.Header.Del("Referer")
+
+	params := mux.Vars(r)
+
+	newQuery := r.URL.Query()
+	newQuery.Del("access_token")
+	r.URL.RawQuery = newQuery.Encode()
+	r.URL.Path = strings.TrimPrefix(r.URL.Path, "/plugins/"+params["plugin_id"])
+
+	handler(w, r)
+}