[2/N] feat(runway): add Merger extension interface, mock, and noop impl

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/merger/BUILD.bazel

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

runway/extension/merger/merger.go

@@ -0,0 +1,58 @@
+// 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 merger defines the pluggable interface for version-control merge
+// operations that Runway performs on behalf of its callers. Implementations
+// resolve change URIs, apply changes to a target branch, and (for a committing
+// merge) push the result and finalize the change lifecycle (e.g. close PRs).
+package merger
+
+//go:generate mockgen -source=merger.go -destination=mock/merger_mock.go -package=mock
+
+import (
+	"context"
+	"errors"
+
+	runwaymq "github.com/uber/submitqueue/api/runway/messagequeue"
+)
+
+// ErrConflict signals that the ordered steps could not be applied cleanly.
+// Controllers treat this as an expected outcome (ack + publish a failure
+// result), not an infrastructure error.
+var ErrConflict = errors.New("merge conflict")
+
+// Merger performs version-control operations against a single landing target.
+// Both methods accept the same MergeRequest payload; the behavioral difference
+// is whether the result is committed to the remote.
+type Merger interface {
+	// CheckMergeability performs a dry-run merge without committing. The
+	// returned MergeResult reports per-step mergeability; Outputs are empty.
+	CheckMergeability(ctx context.Context, req *runwaymq.MergeRequest) (*runwaymq.MergeResult, error)
+	// Merge applies the ordered steps, commits the result to the remote, and
+	// reports per-step Outputs (the VCS-neutral revision identifiers produced).
+	Merge(ctx context.Context, req *runwaymq.MergeRequest) (*runwaymq.MergeResult, error)
+}
+
+// Config identifies the landing target a Merger instance operates on. The factory
+// resolves deployment-specific details (remote URL, credentials) from this.
+type Config struct {
+	// QueueName is the caller-provided queue name from the MergeRequest.
+	QueueName string
+}
+
+// Factory creates Merger instances bound to a landing target.
+type Factory interface {
+	// For returns a Merger instance configured for the given landing target.
+	For(cfg Config) (Merger, error)
+}

runway/extension/merger/mock/BUILD.bazel

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

runway/extension/merger/noop/BUILD.bazel

@@ -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/merger/noop",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//api/runway/messagequeue",
+        "//api/runway/messagequeue/protopb",
+        "//runway/extension/merger",
+    ],
+)
+
+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/merger/noop/noop.go

@@ -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/merger"
+)
+
+var _ merger.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
+}