messenger: Add handler decorator that is resource aware

go/lib/infra/BUILD.bazel

@@ -1,4 +1,4 @@
-load("@io_bazel_rules_go//go:def.bzl", "go_library")
+load("@io_bazel_rules_go//go:def.bzl", "go_library", "go_test")
 
 go_library(
     name = "go_default_library",
@@ -18,9 +18,23 @@ go_library(
         "//go/lib/ctrl/ifid:go_default_library",
         "//go/lib/ctrl/path_mgmt:go_default_library",
         "//go/lib/ctrl/seg:go_default_library",
+        "//go/lib/log:go_default_library",
         "//go/lib/prom:go_default_library",
         "//go/lib/scrypto/cert:go_default_library",
         "//go/lib/scrypto/trc:go_default_library",
         "//go/proto:go_default_library",
     ],
 )
+
+go_test(
+    name = "go_default_test",
+    srcs = ["common_test.go"],
+    embed = [":go_default_library"],
+    deps = [
+        "//go/lib/ctrl/ack:go_default_library",
+        "//go/lib/infra/mock_infra:go_default_library",
+        "//go/proto:go_default_library",
+        "@com_github_golang_mock//gomock:go_default_library",
+        "@com_github_smartystreets_goconvey//convey:go_default_library",
+    ],
+)

go/lib/infra/common.go

@@ -27,6 +27,7 @@ import (
 	"github.com/scionproto/scion/go/lib/ctrl/ifid"
 	"github.com/scionproto/scion/go/lib/ctrl/path_mgmt"
 	"github.com/scionproto/scion/go/lib/ctrl/seg"
+	"github.com/scionproto/scion/go/lib/log"
 	"github.com/scionproto/scion/go/lib/scrypto/cert"
 	"github.com/scionproto/scion/go/lib/scrypto/trc"
 	"github.com/scionproto/scion/go/proto"
@@ -246,6 +247,43 @@ func (mt MessageType) MetricLabel() string {
 	}
 }
 
+// ResourceHealth indicates the health of a resource. A resource could for example be a database.
+// The resource health can be added to a handler, so that the handler only replies if all it's
+// resources are healthy.
+type ResourceHealth interface {
+	// Name returns the name of this resource.
+	Name() string
+	// IsHealthy returns whether the resource is considered healthy currently.
+	// This method must not be blocking and should have the result cached and return ~immediately.
+	IsHealthy() bool
+}
+
+// NewResourceAwareHandler creates a decorated handler that calls the underlying handler if all
+// resources are healthy, otherwise it replies with an error message.
+func NewResourceAwareHandler(handler Handler, resources ...ResourceHealth) Handler {
+	return HandlerFunc(func(r *Request) *HandlerResult {
+		ctx := r.Context()
+		for _, resource := range resources {
+			if !resource.IsHealthy() {
+				logger := log.FromCtx(ctx)
+				rwriter, ok := ResponseWriterFromContext(ctx)
+				if !ok {
+					logger.Error("No response writer found")
+					return MetricsErrInternal
+				}
+				logger.Warn("Resource not healthy, can't handle request",
+					"resource", resource.Name())
+				rwriter.SendAckReply(ctx, &ack.Ack{
+					Err:     proto.Ack_ErrCode_reject,
+					ErrDesc: fmt.Sprintf("Resource %s not healthy", resource.Name()),
+				})
+				return MetricsErrInternal
+			}
+		}
+		return handler.Handle(r)
+	})
+}
+
 type Messenger interface {
 	SendAck(ctx context.Context, msg *ack.Ack, a net.Addr, id uint64) error
 	// GetTRC sends a cert_mgmt.TRCReq request to address a, blocks until it receives a

go/lib/infra/common_test.go

@@ -0,0 +1,63 @@
+// Copyright 2019 Anapaya Systems
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package infra_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/golang/mock/gomock"
+	. "github.com/smartystreets/goconvey/convey"
+
+	"github.com/scionproto/scion/go/lib/ctrl/ack"
+	"github.com/scionproto/scion/go/lib/infra"
+	"github.com/scionproto/scion/go/lib/infra/mock_infra"
+	"github.com/scionproto/scion/go/proto"
+)
+
+var _ infra.ResourceHealth = (*mockResource)(nil)
+
+type mockResource struct {
+	name    string
+	healthy bool
+}
+
+func (r *mockResource) Name() string {
+	return r.name
+}
+
+func (r *mockResource) IsHealthy() bool {
+	return r.healthy
+}
+
+func TestResourceHealth(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+	Convey("Unhealthy resource results in error replied", t, func() {
+		handler := infra.HandlerFunc(func(r *infra.Request) *infra.HandlerResult {
+			return nil
+		})
+		rHandler := infra.NewResourceAwareHandler(handler,
+			&mockResource{name: "tstFail", healthy: false})
+		rwMock := mock_infra.NewMockResponseWriter(ctrl)
+		ctx := infra.NewContextWithResponseWriter(context.Background(), rwMock)
+		rwMock.EXPECT().SendAckReply(gomock.Eq(ctx), gomock.Eq(&ack.Ack{
+			Err:     proto.Ack_ErrCode_reject,
+			ErrDesc: "Resource tstFail not healthy",
+		}))
+		req := infra.NewRequest(ctx, nil, nil, nil, 1)
+		rHandler.Handle(req)
+	})
+}