avoid multiple "GetAssignableUsers" calls when pushing a new stack

As described in #199, we were calling GetAssignableUsers (a graphql
API query) for each new PR created. This is a property of the
repository, though, and we don't expect it to be changing while we're
pushing a single stack.

Stash the slice of RepoAssignee the first time it's needed and
continue to use it for the rest of the calls. Added a test case to
verify the behaviour.

commit-id:dd9d9f40

spr/spr.go

@@ -77,8 +77,8 @@ func (sd *stackediff) AmendCommit(ctx context.Context) {
 	sd.mustgit("rebase -i --autosquash --autostash", nil)
 }
 
-func (sd *stackediff) addReviewers(ctx context.Context, pr *github.PullRequest, reviewers []string) {
-	assignable := sd.github.GetAssignableUsers(ctx)
+func (sd *stackediff) addReviewers(ctx context.Context,
+	pr *github.PullRequest, reviewers []string, assignable []github.RepoAssignee) {
 	userIDs := make([]string, 0, len(reviewers))
 	for _, r := range reviewers {
 		found := false
@@ -151,6 +151,7 @@ func (sd *stackediff) UpdatePullRequests(ctx context.Context, reviewers []string
 	}
 
 	updateQueue := make([]prUpdate, 0)
+	var assignable []github.RepoAssignee
 
 	// iterate through local_commits and update pull_requests
 	for commitIndex, c := range localCommits {
@@ -184,7 +185,10 @@ func (sd *stackediff) UpdatePullRequests(ctx context.Context, reviewers []string
 			githubInfo.PullRequests = append(githubInfo.PullRequests, pr)
 			updateQueue = append(updateQueue, prUpdate{pr, c, prevCommit})
 			if len(reviewers) != 0 {
-				sd.addReviewers(ctx, pr, reviewers)
+				if assignable == nil {
+					assignable = sd.github.GetAssignableUsers(ctx)
+				}
+				sd.addReviewers(ctx, pr, reviewers, assignable)
 			}
 		}
 	}

spr/spr_test.go

@@ -112,20 +112,32 @@ func TestSPRBasicFlowFourCommits(t *testing.T) {
 	gitmock.ExpectFetch()
 	gitmock.ExpectLogAndRespond([]*git.Commit{&c4, &c3, &c2, &c1})
 	gitmock.ExpectPushCommits([]*git.Commit{&c3, &c4})
+
+	// For the first "create" call we should call GetAssignableUsers
 	githubmock.ExpectCreatePullRequest(c3, &c2)
+	githubmock.ExpectGetAssignableUsers()
+	githubmock.ExpectAddReviewers([]string{mockclient.NobodyUserID})
+
+	// For the first "create" call we should *not* call GetAssignableUsers
 	githubmock.ExpectCreatePullRequest(c4, &c3)
+	githubmock.ExpectAddReviewers([]string{mockclient.NobodyUserID})
+
 	githubmock.ExpectUpdatePullRequest(c1, nil)
 	githubmock.ExpectUpdatePullRequest(c2, &c1)
 	githubmock.ExpectUpdatePullRequest(c3, &c2)
 	githubmock.ExpectUpdatePullRequest(c4, &c3)
 	githubmock.ExpectGetInfo()
-	s.UpdatePullRequests(ctx, nil)
+	s.UpdatePullRequests(ctx, []string{mockclient.NobodyLogin})
 	lines = strings.Split(output.String(), "\n")
 	fmt.Printf("OUT: %s\n", output.String())
-	assert.Equal("[✔✔✔✔]   1 : test commit 4", lines[0])
-	assert.Equal("[✔✔✔✔]   1 : test commit 3", lines[1])
-	assert.Equal("[✔✔✔✔]   1 : test commit 2", lines[2])
-	assert.Equal("[✔✔✔✔]   1 : test commit 1", lines[3])
+	assert.Equal([]string{
+		"warning: not updating reviewers for PR #1",
+		"warning: not updating reviewers for PR #1",
+		"[✔✔✔✔]   1 : test commit 4",
+		"[✔✔✔✔]   1 : test commit 3",
+		"[✔✔✔✔]   1 : test commit 2",
+		"[✔✔✔✔]   1 : test commit 1",
+	}, lines[:6])
 	output.Reset()
 
 	// 'git spr -m' :: MergePullRequest :: commits=[a1, a2, a3, a4]