Implement GET /rooms/{roomAlias} (#494)

* Query whether a room alias exists on app services

Signed-off-by: Andrew Morgan <andrewm@matrix.org>

* URL encode room alias before sending to AS

* Add /room/ to path

* Query AS /alias/ API at a lower level

* Don't verify self-signed AS certificates

* Don't skip cert validation on appservices, fix logging

* Separate req.WithContext

* Linting

* Do not warn when an AS room alias does not exist

src/github.com/matrix-org/dendrite/appservice/api/query.go

@@ -18,16 +18,41 @@
 package api
 
 import (
+	"context"
 	"net/http"
+
+	commonHTTP "github.com/matrix-org/dendrite/common/http"
+	opentracing "github.com/opentracing/opentracing-go"
 )
 
+// RoomAliasExistsRequest is a request to an application service
+// about whether a room alias exists
+type RoomAliasExistsRequest struct {
+	// Alias we want to lookup
+	Alias string `json:"alias"`
+}
+
+// RoomAliasExistsResponse is a response from an application service
+// about whether a room alias exists
+type RoomAliasExistsResponse struct {
+	AliasExists bool `json:"exists"`
+}
+
 // AppServiceQueryAPI is used to query user and room alias data from application
 // services
 type AppServiceQueryAPI interface {
-	// TODO: Check whether a room alias exists within any application service namespaces
+	// Check whether a room alias exists within any application service namespaces
+	RoomAliasExists(
+		ctx context.Context,
+		req *RoomAliasExistsRequest,
+		response *RoomAliasExistsResponse,
+	) error
 	// TODO: QueryUserIDExists
 }
 
+// AppServiceRoomAliasExistsPath is the HTTP path for the RoomAliasExists API
+const AppServiceRoomAliasExistsPath = "/api/appservice/RoomAliasExists"
+
 // httpAppServiceQueryAPI contains the URL to an appservice query API and a
 // reference to a httpClient used to reach it
 type httpAppServiceQueryAPI struct {
@@ -47,3 +72,16 @@ func NewAppServiceQueryAPIHTTP(
 	}
 	return &httpAppServiceQueryAPI{appserviceURL, httpClient}
 }
+
+// RoomAliasExists implements AppServiceQueryAPI
+func (h *httpAppServiceQueryAPI) RoomAliasExists(
+	ctx context.Context,
+	request *RoomAliasExistsRequest,
+	response *RoomAliasExistsResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "appserviceRoomAliasExists")
+	defer span.Finish()
+
+	apiURL := h.appserviceURL + AppServiceRoomAliasExistsPath
+	return commonHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+}