Documentation: add link checker

Signed-off-by: Hank Donnay <hdonnay@redhat.com>

Documentation/SUMMARY.md

@@ -7,10 +7,13 @@
   - [API Definition](./howto/api.md)
   - [Testing ClairV4](./howto/testing.md)
 - [Concepts](./concepts.md)
+  - [Indexing](./concepts/indexing.md)
+  - [Matching](./concepts/matching.md)
   - [Internal API](./concepts/api_internal.md)
   - [Authentication](./concepts/authentication.md)
   - [Notifications](./concepts/notifications.md)
   - [Updaters and Airgap](./concepts/updatersandairgap.md)
+  - [Operation](./concepts/operation.md)
 - [Contribution](./contribution.md)
   - [Releases](./contribution/releases.md)
   - [Commit Style](./contribution/commit_style.md)

Documentation/listing_test.go

@@ -0,0 +1,72 @@
+package Documentation
+
+import (
+	"bufio"
+	"io/fs"
+	"os"
+	"path"
+	"regexp"
+	"sort"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+// TestListing fails if the SUMMARY.md falls out of sync with the markdown files
+// in this directory.
+func TestListing(t *testing.T) {
+	// Check that this is the docs test.
+	// These files are copied into the "book" directory, so when left around in
+	// a work tree, test will run there as well.
+	if _, err := os.Stat("index.html"); err == nil {
+		t.Skip("skip listing check in compiled docs")
+	}
+
+	linkline, err := regexp.Compile(`\s*- \[.+\]\((.+)\)`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	f, err := os.Open("SUMMARY.md")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	linked := []string{"SUMMARY.md"}
+	s := bufio.NewScanner(f)
+	for s.Scan() {
+		ms := linkline.FindSubmatch(s.Bytes())
+		switch {
+		case ms == nil, len(ms) == 1:
+			continue
+		case len(ms) == 2:
+			linked = append(linked, path.Clean(string(ms[1])))
+		}
+	}
+	if err := s.Err(); err != nil {
+		t.Error(err)
+	}
+	sort.Strings(linked)
+
+	var files []string
+	err = fs.WalkDir(os.DirFS("."), ".", func(p string, d fs.DirEntry, err error) error {
+		switch {
+		case err != nil:
+			return err
+		case d.IsDir():
+			return nil
+		case path.Ext(d.Name()) != ".md":
+			return nil
+		}
+		files = append(files, p)
+		return nil
+	})
+	if err != nil {
+		t.Error(err)
+	}
+	sort.Strings(files)
+
+	if !cmp.Equal(linked, files) {
+		t.Error(cmp.Diff(linked, files))
+	}
+}