cmd/gopherbot: handle gone issues in setMilestone too

The "label proposals" task adds the "Proposal" label and sets the
"Proposal" milestone, whichever is missing, to proposal issues.
It got stuck processing a no-longer-existing proposal issue 67913:

milestone-Proposal
	https://go.dev/issue/67913  proposal: exp/xerrors: add try and handle
2024/06/17 15:24:10 label proposals: PATCH https://github.com/api/repos/golang/go/issues/67913: 404 Not Found []
2024/06/17 15:24:10 gopherbot ran in 33.445731908s
2024/06/17 15:24:10 sleeping 30s after previous error.

Because it happens to set the milestone first, and setting the milestone
was missing the path to add to deletedIssues to work around for maintner
not reflecting deleted issues in its corpus (see go.dev/issue/30184).

Add that path to setMilestone. Also make the task add label first,
set milestone second, for consistency with the task's description.

While here, also mark a few issues that are known to be gone.
Tested in -dry-run mode.

For golang/go#28320.
Updates golang/go#30184.

Change-Id: I5878c944c8e8f52ab375cd433032953fb8a58d54
Reviewed-on: https://go-review.googlesource.com/c/build/+/593056
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: dmitshur

cmd/gopherbot/gopherbot.go

@@ -330,6 +330,7 @@ func main() {
 			{goRepo, 40593}: true,
 			{goRepo, 40600}: true,
 			{goRepo, 41211}: true,
+			{goRepo, 41268}: true, // transferred to https://github.com/golang/tour/issues/1042
 			{goRepo, 41336}: true,
 			{goRepo, 41649}: true,
 			{goRepo, 41650}: true,
@@ -365,6 +366,8 @@ func main() {
 			{goRepo, 45201}: true,
 			{goRepo, 45202}: true,
 			{goRepo, 47140}: true,
+			{goRepo, 62987}: true,
+			{goRepo, 67913}: true,
 
 			{vscode, 298}:  true,
 			{vscode, 524}:  true,
@@ -373,6 +376,9 @@ func main() {
 			{vscode, 773}:  true,
 			{vscode, 959}:  true,
 			{vscode, 1402}: true,
+			{vscode, 2260}: true, // transferred to https://go.dev/issue/53080
+			{vscode, 2548}: true,
+			{vscode, 2781}: true, // transferred to https://go.dev/issue/60435
 		},
 	}
 	for n := int32(55359); n <= 55828; n++ {
@@ -639,9 +645,15 @@ func (b *gopherbot) setMilestone(ctx context.Context, repoID maintner.GitHubRepo
 	if *dryRun {
 		return nil
 	}
-	_, _, err := b.ghc.Issues.Edit(ctx, repoID.Owner, repoID.Repo, int(gi.Number), &github.IssueRequest{
+	_, resp, err := b.ghc.Issues.Edit(ctx, repoID.Owner, repoID.Repo, int(gi.Number), &github.IssueRequest{
 		Milestone: github.Int(m.Number),
 	})
+	if err != nil && resp != nil && (resp.StatusCode == http.StatusNotFound || resp.StatusCode == http.StatusGone) {
+		// An issue can become gone on GitHub without maintner realizing it. See go.dev/issue/30184.
+		log.Printf("setMilestone: Issue %v#%v returned %s when trying to set milestone. Skipping. See go.dev/issue/30184.", repoID, gi.Number, resp.Status)
+		b.deletedIssues[githubIssue{repoID, gi.Number}] = true
+		return nil
+	}
 	return err
 }
 
@@ -954,18 +966,18 @@ func (b *gopherbot) labelProposals(ctx context.Context) error {
 		if !strings.HasPrefix(gi.Title, "proposal:") && !strings.HasPrefix(gi.Title, "Proposal:") {
 			return nil
 		}
-		// Add Milestone if missing:
-		if gi.Milestone.IsNone() && !gi.HasEvent("milestoned") && !gi.HasEvent("demilestoned") {
-			if err := b.setMilestone(ctx, b.gorepo.ID(), gi, proposal); err != nil {
-				return err
-			}
-		}
 		// Add Proposal label if missing:
 		if !gi.HasLabel("Proposal") && !gi.HasEvent("unlabeled") {
 			if err := b.addLabel(ctx, b.gorepo.ID(), gi, "Proposal"); err != nil {
 				return err
 			}
 		}
+		// Add Milestone if missing:
+		if gi.Milestone.IsNone() && !gi.HasEvent("milestoned") && !gi.HasEvent("demilestoned") {
+			if err := b.setMilestone(ctx, b.gorepo.ID(), gi, proposal); err != nil {
+				return err
+			}
+		}
 
 		// Remove NeedsDecision label if exists, but not for Go 2 issues:
 		if !isGo2Issue(gi) && gi.HasLabel("NeedsDecision") && !gopherbotRemovedLabel(gi, "NeedsDecision") {