feat(runway): add Merger extension interface, mock, and noop impl

Add the pluggable VCS extension for Runway with the Merger interface
(CheckMergeability + Merge), using the landed proto-based MergeRequest/
MergeResult types from api/runway/messagequeue. Includes noop impl with
synthetic output IDs, generated mock, and table-driven tests.

Addresses review feedback: renamed VCS -> Merger, Land -> Merge.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kevinlnew

runway/extension/BUILD.bazel

@@ -0,0 +1,8 @@
+load("@rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "extension",
+    srcs = ["extension.go"],
+    importpath = "github.com/uber/submitqueue/runway/extension",
+    visibility = ["//visibility:public"],
+)

runway/extension/extension.go

@@ -0,0 +1,16 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// 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 extension holds Runway-specific extension implementations.
+package extension

runway/extension/vcs/BUILD.bazel

@@ -0,0 +1,9 @@
+load("@rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "vcs",
+    srcs = ["vcs.go"],
+    importpath = "github.com/uber/submitqueue/runway/extension/vcs",
+    visibility = ["//visibility:public"],
+    deps = ["//api/runway/messagequeue"],
+)

runway/extension/vcs/mock/BUILD.bazel

@@ -0,0 +1,13 @@
+load("@rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "mock",
+    srcs = ["vcs_mock.go"],
+    importpath = "github.com/uber/submitqueue/runway/extension/vcs/mock",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//api/runway/messagequeue",
+        "//runway/extension/vcs",
+        "@org_uber_go_mock//gomock",
+    ],
+)

runway/extension/vcs/mock/vcs_mock.go

@@ -0,0 +1,27 @@
+load("@rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "noop",
+    srcs = ["noop.go"],
+    importpath = "github.com/uber/submitqueue/runway/extension/vcs/noop",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//api/runway/messagequeue",
+        "//api/runway/messagequeue/protopb",
+        "//runway/extension/vcs",
+    ],
+)
+
+go_test(
+    name = "noop_test",
+    srcs = ["noop_test.go"],
+    embed = [":noop"],
+    deps = [
+        "//api/base/change/protopb",
+        "//api/base/mergestrategy/protopb",
+        "//api/runway/messagequeue",
+        "//api/runway/messagequeue/protopb",
+        "@com_github_stretchr_testify//assert",
+        "@com_github_stretchr_testify//require",
+    ],
+)

runway/extension/vcs/noop/BUILD.bazel

@@ -0,0 +1,68 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// 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 noop provides a no-op Merger implementation for local development and
+// testing. CheckMergeability always reports success; Merge produces synthetic
+// output IDs from an atomic counter.
+package noop
+
+import (
+	"context"
+	"fmt"
+	"sync/atomic"
+
+	runwaymq "github.com/uber/submitqueue/api/runway/messagequeue"
+	runwaypb "github.com/uber/submitqueue/api/runway/messagequeue/protopb"
+	"github.com/uber/submitqueue/runway/extension/vcs"
+)
+
+var _ vcs.Merger = (*Merger)(nil)
+
+// Merger is a no-op implementation that always succeeds.
+type Merger struct {
+	seq atomic.Uint64
+}
+
+// New returns a new no-op Merger instance.
+func New() *Merger { return &Merger{} }
+
+func (v *Merger) CheckMergeability(_ context.Context, req *runwaymq.MergeRequest) (*runwaymq.MergeResult, error) {
+	steps := make([]*runwaymq.StepResult, len(req.GetSteps()))
+	for i, s := range req.GetSteps() {
+		steps[i] = &runwaymq.StepResult{StepId: s.GetStepId()}
+	}
+	return &runwaymq.MergeResult{
+		Id:      req.GetId(),
+		Outcome: runwaypb.Outcome_SUCCEEDED,
+		Steps:   steps,
+	}, nil
+}
+
+func (v *Merger) Merge(_ context.Context, req *runwaymq.MergeRequest) (*runwaymq.MergeResult, error) {
+	steps := make([]*runwaymq.StepResult, len(req.GetSteps()))
+	for i, s := range req.GetSteps() {
+		n := v.seq.Add(1)
+		steps[i] = &runwaymq.StepResult{
+			StepId: s.GetStepId(),
+			Outputs: []*runwaymq.StepOutput{
+				{Id: fmt.Sprintf("%040x", n)},
+			},
+		}
+	}
+	return &runwaymq.MergeResult{
+		Id:      req.GetId(),
+		Outcome: runwaypb.Outcome_SUCCEEDED,
+		Steps:   steps,
+	}, nil
+}

runway/extension/vcs/noop/noop.go

@@ -0,0 +1,92 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// 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 noop
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	changepb "github.com/uber/submitqueue/api/base/change/protopb"
+	strategypb "github.com/uber/submitqueue/api/base/mergestrategy/protopb"
+	runwaymq "github.com/uber/submitqueue/api/runway/messagequeue"
+	runwaypb "github.com/uber/submitqueue/api/runway/messagequeue/protopb"
+)
+
+func testRequest() *runwaymq.MergeRequest {
+	return &runwaymq.MergeRequest{
+		Id:        "queue-a/42",
+		QueueName: "queue-a",
+		Steps: []*runwaymq.MergeStep{
+			{
+				StepId:   "queue-a/1",
+				Changes:  []*changepb.Change{{Uris: []string{"github://uber/repo/pull/1/abcdef0123456789abcdef0123456789abcdef01"}}},
+				Strategy: strategypb.Strategy_REBASE,
+			},
+			{
+				StepId:   "queue-a/2",
+				Changes:  []*changepb.Change{{Uris: []string{"github://uber/repo/pull/2/89abcdef0123456789abcdef0123456789abcdef"}}},
+				Strategy: strategypb.Strategy_MERGE,
+			},
+		},
+	}
+}
+
+func TestCheckMergeability(t *testing.T) {
+	v := New()
+	req := testRequest()
+
+	res, err := v.CheckMergeability(context.Background(), req)
+	require.NoError(t, err)
+
+	assert.Equal(t, req.GetId(), res.GetId())
+	assert.Equal(t, runwaypb.Outcome_SUCCEEDED, res.GetOutcome())
+	require.Len(t, res.GetSteps(), 2)
+	assert.Equal(t, "queue-a/1", res.GetSteps()[0].GetStepId())
+	assert.Empty(t, res.GetSteps()[0].GetOutputs())
+	assert.Equal(t, "queue-a/2", res.GetSteps()[1].GetStepId())
+	assert.Empty(t, res.GetSteps()[1].GetOutputs())
+}
+
+func TestMerge(t *testing.T) {
+	v := New()
+	req := testRequest()
+
+	res, err := v.Merge(context.Background(), req)
+	require.NoError(t, err)
+
+	assert.Equal(t, req.GetId(), res.GetId())
+	assert.Equal(t, runwaypb.Outcome_SUCCEEDED, res.GetOutcome())
+	require.Len(t, res.GetSteps(), 2)
+	assert.Equal(t, "queue-a/1", res.GetSteps()[0].GetStepId())
+	require.Len(t, res.GetSteps()[0].GetOutputs(), 1)
+	assert.NotEmpty(t, res.GetSteps()[0].GetOutputs()[0].GetId())
+	assert.Equal(t, "queue-a/2", res.GetSteps()[1].GetStepId())
+	require.Len(t, res.GetSteps()[1].GetOutputs(), 1)
+	assert.NotEmpty(t, res.GetSteps()[1].GetOutputs()[0].GetId())
+}
+
+func TestMerge_UniqueOutputIDs(t *testing.T) {
+	v := New()
+	req := testRequest()
+
+	res1, err := v.Merge(context.Background(), req)
+	require.NoError(t, err)
+	res2, err := v.Merge(context.Background(), req)
+	require.NoError(t, err)
+
+	assert.NotEqual(t, res1.GetSteps()[0].GetOutputs()[0].GetId(), res2.GetSteps()[0].GetOutputs()[0].GetId())
+}