Add linter to check truncate tag in blog posts (#4139)

Closes #4048.

tools/checkdocs/checkdocs.go

@@ -45,12 +45,13 @@ func checkFiles(files []string, logf, fatalf func(string, ...any)) {
 	var failed bool
 
 	for _, file := range files {
-		b, err := os.ReadFile(file)
+		fileInBytes, err := os.ReadFile(file)
 		if err != nil {
 			fatalf("Couldn't read file %s: %s", file, err)
 		}
 
-		if b, err = extractFrontMatter(b); err != nil {
+		b, err := extractFrontMatter(fileInBytes)
+		if err != nil {
 			fatalf("Couldn't extract front matter from %s: %s", file, err)
 		}
 
@@ -68,13 +69,27 @@ func checkFiles(files []string, logf, fatalf func(string, ...any)) {
 			logf("%q: %s", file, err)
 			failed = true
 		}
+
+		if err = verifyTruncateString(fileInBytes); err != nil {
+			logf("%q: %s", file, err)
+			failed = true
+		}
 	}
 
 	if failed {
 		fatalf("One or more blog posts are not correctly formatted")
 	}
 }
 
+// verifyTruncateString checks that the truncate string is present.
+func verifyTruncateString(b []byte) error {
+	if !bytes.Contains(b, []byte("<!--truncate-->")) {
+		return fmt.Errorf("<!--truncate--> must be included to have \"Read more\" link on the homepage")
+	}
+
+	return nil
+}
+
 // extractFrontMatter returns the front matter of a blog post.
 func extractFrontMatter(fm []byte) ([]byte, error) {
 	var in bool

tools/checkdocs/checkdocs_test.go

@@ -62,3 +62,8 @@ func TestVerifyTags(t *testing.T) {
 	err := verifyTags(fm)
 	assert.EqualError(t, err, `tag "documents databases" is not in the allowed list`)
 }
+
+func TestVerifyTruncateString(t *testing.T) {
+	err := verifyTruncateString(fm)
+	assert.EqualError(t, err, "<!--truncate--> must be included to have \"Read more\" link on the homepage")
+}

website/blog/2024-01-08-new-ferretdb-v118-support-oplog-functionality.md

@@ -13,6 +13,8 @@ tags: [release]
 We are starting the new year with a major feature addition!
 The latest version of FerretDB v1.18.0 has just been released with support for basic OpLog functionality, along with other exciting features.
 
+<!--truncate-->
+
 OpLog (operations log) functionality has been one of our most requested features, and it would make it possible for developers to build real-time applications using the Meteor framework.
 We made this a priority in the last quarter and are super excited that it's now available.
 We can't wait to see what you build with FerretDB!