feat(dist): auto-update changelog '## Unreleased' section

Teach the release script to update the heading of the '## Unreleased'
section in CHANGELOG.md and also add the 'Downloads' link.

Author: strager

dist/release.go

@@ -50,8 +50,13 @@ var Steps []Step = []Step{
 	Step{
 		Title: "Update release notes file",
 		Run: func() {
-			fmt.Printf("Update the release notes file: docs/CHANGELOG.md\n")
-			WaitForDone()
+			changelogPath := "docs/CHANGELOG.md"
+			if err := UpdateUnreleasedChangelogFile(changelogPath, VersionFileInfo{
+				VersionNumber: ReleaseVersion,
+				ReleaseDate:   ReleaseDate,
+			}); err != nil {
+				Stopf("failed to update changelog file %s: %v", changelogPath, err)
+			}
 		},
 	},
 
@@ -712,6 +717,40 @@ func UpdateDebianChangelog(changelogFilePath string, versionInfo VersionFileInfo
 	return nil
 }
 
+func UpdateUnreleasedChangelogFile(changelogFilePath string, versionInfo VersionFileInfo) error {
+	originalData, err := os.ReadFile(changelogFilePath)
+	if err != nil {
+		return err
+	}
+
+	newData, err := UpdateUnreleasedChangelog(originalData, versionInfo)
+	if err != nil {
+		return err
+	}
+
+	fileMode := fs.FileMode(0644) // Unused, because the file should already exist.
+	if err := os.WriteFile(changelogFilePath, newData, fileMode); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func UpdateUnreleasedChangelog(changelog []byte, versionInfo VersionFileInfo) ([]byte, error) {
+	unreleasedHeadingRegexp := regexp.MustCompile("(?m:^## Unreleased\n)")
+	newHeading := []byte(fmt.Sprintf(
+		"## %s (%s)\n\n[Downloads](https://c.quick-lint-js.com/releases/%s/)\n",
+		versionInfo.VersionNumber,
+		versionInfo.ReleaseDate.Format("2006-01-02"),
+		versionInfo.VersionNumber,
+	))
+	newChangelog := unreleasedHeadingRegexp.ReplaceAllLiteral(changelog, newHeading)
+	if bytes.Equal(newChangelog, changelog) {
+		return nil, fmt.Errorf("could not find '## Unreleased' heading in changelog")
+	}
+	return newChangelog, nil
+}
+
 func EnsureEmptyDirectory(path string) {
 Retry:
 	err := os.Mkdir(path, 0700)

dist/release_test.go

@@ -102,6 +102,45 @@ func TestUpdateDebianChangelog(t *testing.T) {
 	})
 }
 
+func TestUpdateUnreleasedChangelog(t *testing.T) {
+	t.Run("replaces unreleased heading and adds downlods link", func(t *testing.T) {
+		losAngeles, err := time.LoadLocation("America/Los_Angeles")
+		Check(t, err)
+
+		newChangelog, err := UpdateUnreleasedChangelog(
+			[]byte("## Unreleased\n\n## 1.0.0 (2022-01-01)\n"),
+			VersionFileInfo{
+				VersionNumber: "2.0.0",
+				ReleaseDate:   time.Date(2022, 2, 8, 16, 56, 37, 0, losAngeles),
+			},
+		)
+		Check(t, err)
+		AssertStringsEqual(t, string(newChangelog),
+			"## 2.0.0 (2022-02-08)\n"+
+				"\n"+
+				"[Downloads](https://c.quick-lint-js.com/releases/2.0.0/)\n"+
+				"\n"+
+				"## 1.0.0 (2022-01-01)\n")
+	})
+
+	t.Run("fails if no unreleased heading", func(t *testing.T) {
+		losAngeles, err := time.LoadLocation("America/Los_Angeles")
+		Check(t, err)
+
+		_, err = UpdateUnreleasedChangelog(
+			[]byte("## 1.0.0 (2022-01-01)\n\n"),
+			VersionFileInfo{
+				VersionNumber: "2.0.0",
+				ReleaseDate:   time.Date(2022, 2, 8, 16, 56, 37, 0, losAngeles),
+			},
+		)
+		if err == nil {
+			t.Fatalf("expected error")
+		}
+		AssertStringsEqual(t, err.Error(), "could not find '## Unreleased' heading in changelog")
+	})
+}
+
 func TestUpdateReleaseVersions(t *testing.T) {
 	t.Run("only version number", func(t *testing.T) {
 		updated, err := UpdateReleaseVersions(UpdateReleaseVersionsOptions{