feat(pusher): add Pusher extension and wire it into the merge controller

Makefile

@@ -245,7 +245,7 @@ local-stop: ## Stop all services (keep data)
 
 mocks: ## Generate mock files using mockgen
 	@echo "Generating mocks..."
-	@$(BAZEL) run @rules_go//go -- generate ./extension/storage/... ./extension/counter/... ./extension/queue/... ./extension/mergechecker/... ./extension/scorer/... ./core/consumer/...
+	@$(BAZEL) run @rules_go//go -- generate ./extension/storage/... ./extension/counter/... ./extension/queue/... ./extension/mergechecker/... ./extension/pusher/... ./extension/scorer/... ./core/consumer/...
 	@echo "Mocks generated successfully!"
 
 proto: ## Generate protobuf files from .proto definitions

example/server/orchestrator/BUILD.bazel

@@ -20,6 +20,8 @@ go_library(
         "//extension/counter/mysql",
         "//extension/mergechecker",
         "//extension/mergechecker/github",
+        "//extension/pusher",
+        "//extension/pusher/git",
         "//extension/queue",
         "//extension/queue/mysql",
         "//extension/scorer/heuristic",

example/server/orchestrator/main.go

@@ -39,6 +39,8 @@ import (
 	mysqlcounter "github.com/uber/submitqueue/extension/counter/mysql"
 	"github.com/uber/submitqueue/extension/mergechecker"
 	githubchecker "github.com/uber/submitqueue/extension/mergechecker/github"
+	"github.com/uber/submitqueue/extension/pusher"
+	gitpusher "github.com/uber/submitqueue/extension/pusher/git"
 	extqueue "github.com/uber/submitqueue/extension/queue"
 	queueMySQL "github.com/uber/submitqueue/extension/queue/mysql"
 	"github.com/uber/submitqueue/extension/scorer/heuristic"
@@ -203,8 +205,14 @@ func run() error {
 		return fmt.Errorf("failed to create change provider: %w", err)
 	}
 
+	// Create pusher
+	psh, err := newPusher(logger, scope)
+	if err != nil {
+		return fmt.Errorf("failed to create pusher: %w", err)
+	}
+
 	// Register controllers
-	if err := registerControllers(c, logger.Sugar(), scope, registry, mc, cp, cnt, store); err != nil {
+	if err := registerControllers(c, logger.Sugar(), scope, registry, mc, cp, psh, cnt, store); err != nil {
 		return err
 	}
 
@@ -389,7 +397,7 @@ func newTopicRegistry(q extqueue.Queue, subscriberName string) (consumer.TopicRe
 //                                        │        │                        │
 //                                        └────────┴────────────────────────┘
 
-func registerControllers(c consumer.Consumer, logger *zap.SugaredLogger, scope tally.Scope, registry consumer.TopicRegistry, mc mergechecker.MergeChecker, cp changeprovider.ChangeProvider, cnt counter.Counter, store storage.Storage) error {
+func registerControllers(c consumer.Consumer, logger *zap.SugaredLogger, scope tally.Scope, registry consumer.TopicRegistry, mc mergechecker.MergeChecker, cp changeprovider.ChangeProvider, psh pusher.Pusher, cnt counter.Counter, store storage.Storage) error {
 	requestController := start.NewController(
 		logger,
 		scope,
@@ -495,6 +503,7 @@ func registerControllers(c consumer.Consumer, logger *zap.SugaredLogger, scope t
 		scope,
 		store,
 		registry,
+		psh,
 		consumer.TopicKeyMerge,
 		"orchestrator-merge",
 	)
@@ -595,3 +604,21 @@ func newChangeProvider(logger *zap.Logger, scope tally.Scope) (changeprovider.Ch
 		MetricsScope: scope.SubScope("changeprovider"),
 	}), nil
 }
+
+// newPusher creates a git-backed Pusher bound to the configured checkout
+// path, remote, and target branch. Configured via PUSHER_CHECKOUT_PATH
+// (required), PUSHER_REMOTE (default "origin"), and PUSHER_TARGET (default
+// "main").
+func newPusher(logger *zap.Logger, scope tally.Scope) (pusher.Pusher, error) {
+	checkout := os.Getenv("PUSHER_CHECKOUT_PATH")
+	if checkout == "" {
+		return nil, fmt.Errorf("PUSHER_CHECKOUT_PATH environment variable is required")
+	}
+	return gitpusher.NewPusher(gitpusher.Params{
+		CheckoutPath: checkout,
+		Remote:       getEnv("PUSHER_REMOTE", "origin"),
+		Target:       getEnv("PUSHER_TARGET", "main"),
+		Logger:       logger.Sugar(),
+		MetricsScope: scope.SubScope("pusher"),
+	}), nil
+}

extension/pusher/BUILD.bazel

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

extension/pusher/README.md

@@ -0,0 +1,44 @@
+# Pusher
+
+Pluggable abstraction for landing a list of `entity.Change` values onto a
+target branch and pushing the result to a source-control remote.
+
+## Interface
+
+`Pusher` exposes a single `Push` method that accepts a list of changes.
+Implementations are bound to a specific `(checkout, remote, target)` tuple
+at construction time, so the interface itself stays vendor- and
+configuration-agnostic.
+
+The interface enforces an **all-or-nothing atomicity contract**: when `Push`
+returns an error, no change has reached the remote — neither partially nor
+fully. Callers can treat a non-nil error as "the remote is exactly as it was
+before the call". The `ErrConflict` sentinel marks user-caused failures so
+callers can route them to a non-retry path.
+
+A successful `Push` returns one `ChangeOutcome` per input change in input
+order. Each outcome reports either:
+
+- `OutcomeStatusCommitted` with the list of `CommitSHAs` produced on the
+  target branch (one change can land as multiple commits, e.g. a stack of
+  PRs); or
+- `OutcomeStatusAlreadyExisted` with no commits, when the change is already
+  present on the target branch (previously landed via another path, or
+  subsumed by an earlier change in the same push). Git surfaces this as
+  "rebased out" during a cherry-pick.
+
+## Implementations
+
+- [`git/`](git/) — applies changes against a local checkout via `git
+  cherry-pick`, then `git push`. Construction takes the path to the
+  checkout, the remote name, and the target branch; the implementation
+  owns that working tree and serializes concurrent invocations.
+
+## Adding a new backend
+
+1. Create `extension/pusher/{backend}/` with a `Pusher` implementation.
+2. Bind the implementation to its checkout/remote/target at construction.
+3. Map each `entity.Change` to the backend's commit/push primitives.
+4. Honour the atomicity contract: never publish partial state. Return
+   `ErrConflict` (wrapped) for user-caused apply failures and a plain error
+   for transient infra failures.

extension/pusher/git/BUILD.bazel

@@ -0,0 +1,30 @@
+load("@rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "git",
+    srcs = ["git_pusher.go"],
+    importpath = "github.com/uber/submitqueue/extension/pusher/git",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//core/metrics",
+        "//entity",
+        "//entity/github",
+        "//extension/pusher",
+        "@com_github_uber_go_tally_v4//:tally",
+        "@org_uber_go_zap//:zap",
+    ],
+)
+
+go_test(
+    name = "git_test",
+    srcs = ["git_pusher_test.go"],
+    embed = [":git"],
+    deps = [
+        "//entity",
+        "//extension/pusher",
+        "@com_github_stretchr_testify//assert",
+        "@com_github_stretchr_testify//require",
+        "@com_github_uber_go_tally_v4//:tally",
+        "@org_uber_go_zap//zaptest",
+    ],
+)