refactor(mergechecker): accept entity.Request, resolve change internally (#217)

## Summary
Change MergeChecker.Check to take the orchestrator's request identity
(entity.Request) instead of a controller-pre-resolved entity.Change, per
the extension contract. The GitHub implementation and the fake read
request.Change themselves; the validate controller hands over the
request it already loaded.

Output is unchanged (mergechecker.Result). The factory and Config are
unchanged — no dependency injection is needed since the checker resolves
nothing beyond the change already on the request.

## Test Plan


## Issues


## Stack
1. @ #217
1. #218
1. #219
1. #221
1. #222
1. #223
1. #227

Author: behinddwalls

submitqueue/extension/mergechecker/fake/fake.go

@@ -52,9 +52,9 @@ func New() mergechecker.MergeChecker {
 }
 
 // Check reports the change as mergeable unless a recognized marker token is
-// present in one of its URIs.
-func (checker) Check(_ context.Context, change entity.Change) (mergechecker.Result, error) {
-	switch fakemarker.Token(change.URIs) {
+// present in one of the request's change URIs.
+func (checker) Check(_ context.Context, request entity.Request) (mergechecker.Result, error) {
+	switch fakemarker.Token(request.Change.URIs) {
 	case tokenUnmergeable:
 		return mergechecker.Result{Mergeable: false, Reason: "fake: marked unmergeable"}, nil
 	case tokenError:

submitqueue/extension/mergechecker/fake/fake_test.go

@@ -66,7 +66,7 @@ func TestChecker_Check(t *testing.T) {
 	c := New()
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			res, err := c.Check(context.Background(), entity.Change{URIs: tt.uris})
+			res, err := c.Check(context.Background(), entity.Request{Change: entity.Change{URIs: tt.uris}})
 			if tt.wantErr {
 				require.Error(t, err)
 				return

submitqueue/extension/mergechecker/github/checker.go

@@ -60,13 +60,15 @@ func NewMergeChecker(params Params) mergechecker.MergeChecker {
 	}
 }
 
-// Check assesses whether a change can merge cleanly using the GitHub GraphQL API.
-func (c *mergeChecker) Check(ctx context.Context, change entity.Change) (result mergechecker.Result, retErr error) {
+// Check assesses whether a request's change can merge cleanly using the GitHub GraphQL API.
+func (c *mergeChecker) Check(ctx context.Context, request entity.Request) (result mergechecker.Result, retErr error) {
 	const opName = "check"
 
 	op := metrics.Begin(c.metricsScope, opName)
 	defer func() { op.Complete(retErr) }()
 
+	change := request.Change
+
 	// Parse all change IDs
 	// TODO: classify parse errors as user errors (non-retryable) vs system errors.
 	changeIDs := make([]entitygithub.ChangeID, 0, len(change.URIs))

submitqueue/extension/mergechecker/github/checker_test.go

@@ -156,7 +156,7 @@ func TestMergeChecker_Check(t *testing.T) {
 			defer server.Close()
 
 			mc := newTestMergeChecker(t, server.URL)
-			result, err := mc.Check(context.Background(), tt.change)
+			result, err := mc.Check(context.Background(), entity.Request{Change: tt.change})
 			if tt.wantErr {
 				require.Error(t, err)
 				return

submitqueue/extension/mergechecker/mergechecker.go

@@ -22,12 +22,13 @@ import (
 	"github.com/uber/submitqueue/submitqueue/entity"
 )
 
-// MergeChecker predicts whether a set of changes can merge cleanly.
+// MergeChecker predicts whether a request's changes can merge cleanly.
 type MergeChecker interface {
 	// Check is a fail-fast mergeability check that optimistically assesses
-	// whether the changes can be merged. A positive result does not
+	// whether the request's changes can be merged. It is handed the request
+	// identity and reads request.Change itself. A positive result does not
 	// guarantee that the changes will apply cleanly at merge time.
-	Check(ctx context.Context, change entity.Change) (Result, error)
+	Check(ctx context.Context, request entity.Request) (Result, error)
 }
 
 // Result holds the outcome of a mergeability check.

submitqueue/extension/mergechecker/mock/mergechecker_mock.go

@@ -140,7 +140,7 @@ func (c *Controller) Process(ctx context.Context, delivery consumer.Delivery) (r
 		coremetrics.NamedCounter(c.metricsScope, "process", "merge_check_errors", 1)
 		return fmt.Errorf("failed to build merge checker for queue %s: %w", request.Queue, err)
 	}
-	mergeResult, err := mergeChecker.Check(ctx, request.Change)
+	mergeResult, err := mergeChecker.Check(ctx, request)
 	if err != nil {
 		coremetrics.NamedCounter(c.metricsScope, "process", "merge_check_errors", 1)
 		return fmt.Errorf("merge check failed: %w", err)