devapp: account {,de}milestoned events in issue burndown chart

Previously the calculation of open issues over previous days only took
into account when the issue was created and closed, but not that issues
may have been added to or removed from the current development milestone
over their lifetime.

This was especially noticeable when relui moved 100 open issues from
Go1.21 to Go1.22, yet the burndown graph made it seem like there had
always been around 225 issues in it, pretty misleading. :( maintner
already tracks milestoned and demilestoned events along with event
creation time, so take them into account to de-gaslight the graph.

Change-Id: Iba3f3f68e4ddc140d5612317c02bdfefe4693d8d
Reviewed-on: https://go-review.googlesource.com/c/build/+/517716
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>

Author: dmitshur

devapp/release.go

@@ -7,6 +7,7 @@ package main
 import (
 	"bytes"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"html/template"
 	"log"
@@ -265,22 +266,12 @@ func (s *server) updateReleaseData() {
 	curMilestoneStart := time.Date(2021, time.August+monthsSinceGo117Release, 1, 0, 0, 0, 0, time.UTC)
 
 	dirToIssues := map[string][]*maintner.GitHubIssue{}
-	var curMilestoneIssues []*maintner.GitHubIssue
 	s.repo.ForeachIssue(func(issue *maintner.GitHubIssue) error {
-		// Only include issues in active milestones.
-		if issue.Milestone.IsUnknown() || issue.Milestone.Closed || issue.Milestone.IsNone() {
-			return nil
-		}
-
-		if issue.Milestone.Title == curMilestoneTitle {
-			curMilestoneIssues = append(curMilestoneIssues, issue)
-		}
-
-		// Only open issues are displayed on the page using dirToIssues.
-		if issue.Closed {
+		// Only open issues in active milestones are displayed on the page using dirToIssues.
+		if issue.Closed ||
+			issue.Milestone.IsUnknown() || issue.Milestone.Closed || issue.Milestone.IsNone() {
 			return nil
 		}
-
 		dirs := titleDirs(issue.Title)
 		if len(dirs) == 0 {
 			dirToIssues[""] = append(dirToIssues[""], issue)
@@ -292,13 +283,52 @@ func (s *server) updateReleaseData() {
 		return nil
 	})
 
+	// Find issues that have been in the current milestone.
+	var curMilestoneIssues []*maintner.GitHubIssue
+	s.repo.ForeachIssue(func(issue *maintner.GitHubIssue) error {
+		if issue.Closed && issue.ClosedAt.Before(curMilestoneStart) {
+			// Old issue, couldn't be relevant to current milestone.
+			return nil
+		}
+		if !issue.Milestone.IsUnknown() && issue.Milestone.Title == curMilestoneTitle {
+			// Easy case: the issue is still in current milestone.
+			curMilestoneIssues = append(curMilestoneIssues, issue)
+			return nil
+		}
+		// Check if the issue was ever in the current milestone.
+		issue.ForeachEvent(func(e *maintner.GitHubIssueEvent) error {
+			if e.Type == "milestoned" && e.Milestone == curMilestoneTitle {
+				curMilestoneIssues = append(curMilestoneIssues, issue)
+				return errStopIteration
+			}
+			return nil
+		})
+		return nil
+	})
+
 	bd := burndownData{Milestone: curMilestoneTitle}
 	for t, now := curMilestoneStart, time.Now(); t.Before(now); t = t.Add(24 * time.Hour) {
 		var e burndownEntry
 		for _, issue := range curMilestoneIssues {
 			if issue.Created.After(t) || (issue.Closed && issue.ClosedAt.Before(t)) {
 				continue
 			}
+			var inCurMilestoneAtT bool
+			issue.ForeachEvent(func(e *maintner.GitHubIssueEvent) error {
+				if e.Created.After(t) {
+					return errStopIteration
+				}
+				switch e.Type {
+				case "milestoned":
+					inCurMilestoneAtT = e.Milestone == curMilestoneTitle
+				case "demilestoned":
+					inCurMilestoneAtT = false
+				}
+				return nil
+			})
+			if !inCurMilestoneAtT {
+				continue
+			}
 			if issue.HasLabel("release-blocker") {
 				e.Blockers++
 			}
@@ -522,3 +552,7 @@ func (s *server) handleRelease(t *template.Template, w http.ResponseWriter, r *h
 		return
 	}
 }
+
+// errStopIteration is used to stop iteration over issues or comments.
+// It has no special meaning.
+var errStopIteration = errors.New("stop iteration")