Vellum/internal/arch/arch_test.go

// Package arch holds no code. It exists so that the import rules this project
// depends on are enforced by the test suite instead of by good intentions.
//
// A layering rule that is only written down in a comment is a rule that will be
// broken within a month, and the breakage is invisible: an upward import does
// not fail to compile, it just quietly welds two layers together until nothing
// can be moved. These tests fail loudly instead.
package arch

import (
	"go/parser"
	"go/token"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"testing"
)

const modulePath = "github.com/mithral/vellum/"

// Layers, lowest first. A package may import from a LOWER layer or its own
// layer, never from a higher one. Peers are allowed because they genuinely
// collaborate (metadata enriches items that media serves); upward imports are
// not, because they invert the dependency and make the lower layer unusable on
// its own.
var layers = map[string]int{
	// 0 — foundation. No knowledge of the application.
	"internal/core":    0,
	"internal/httpx":   0,
	"internal/natsort": 0,
	"internal/arch":    0,
	"migrations":       0,

	// 1 — primitives the application is built from.
	"internal/auth":     1,
	"internal/jobs":     1,
	"internal/stream":   1,
	"internal/progress": 1,
	"internal/probe":    1,
	"internal/engine":   1,
	"internal/apitoken": 1,

	// 2 — leaves that the application is built from: the data seam, and the
	// external catalogues. providers sits here so it stays a leaf — a
	// metadata client must be usable from a test with no database.
	"internal/catalog":   2,
	"internal/providers": 2,

	// 3 — the application.
	"internal/library":        3,
	"internal/scanner":        3,
	"internal/media":          3,
	"internal/home":           3,
	"internal/search":         3,
	"internal/metadata":       3,
	"internal/playback":       3,
	"internal/modules/comics": 3,
	"internal/modules/ebooks": 3,
	"internal/modules/games":  3,
	"internal/modules/photos": 3,

	// 4 — adapters: HTTP surfaces and foreign protocols.
	"internal/httpapi":      4,
	"internal/jellyfin":     4,
	"internal/jellyfin/dto": 3,
	"internal/subsonic":     4,
	"internal/subsonic/dto": 3,
	"internal/cli":          4,

	// 5 — the only place everything is wired together.
	"cmd/vellum": 5,
}

// repoRoot walks up until it finds go.mod, so the test works regardless of
// where `go test` is invoked from.
func repoRoot(t *testing.T) string {
	t.Helper()
	dir, err := os.Getwd()
	if err != nil {
		t.Fatal(err)
	}
	for {
		if _, err := os.Stat(filepath.Join(dir, "go.mod")); err == nil {
			return dir
		}
		parent := filepath.Dir(dir)
		if parent == dir {
			t.Fatal("could not find go.mod above the test directory")
		}
		dir = parent
	}
}

// importsOf parses a directory and returns the internal packages it imports.
func importsOf(t *testing.T, dir string) []string {
	t.Helper()
	entries, err := os.ReadDir(dir)
	if err != nil {
		t.Fatalf("read %s: %v", dir, err)
	}
	seen := map[string]bool{}
	for _, e := range entries {
		if e.IsDir() || !strings.HasSuffix(e.Name(), ".go") || strings.HasSuffix(e.Name(), "_test.go") {
			continue
		}
		fset := token.NewFileSet()
		f, err := parser.ParseFile(fset, filepath.Join(dir, e.Name()), nil, parser.ImportsOnly)
		if err != nil {
			t.Fatalf("parse %s: %v", e.Name(), err)
		}
		for _, imp := range f.Imports {
			p := strings.Trim(imp.Path.Value, `"`)
			if strings.HasPrefix(p, modulePath) {
				seen[strings.TrimPrefix(p, modulePath)] = true
			}
		}
	}
	out := make([]string, 0, len(seen))
	for p := range seen {
		out = append(out, p)
	}
	sort.Strings(out)
	return out
}

// TestNoUpwardImports is the rule that keeps the layers separable.
func TestNoUpwardImports(t *testing.T) {
	root := repoRoot(t)
	for pkg, layer := range layers {
		dir := filepath.Join(root, filepath.FromSlash(pkg))
		if _, err := os.Stat(dir); err != nil {
			continue // package not present (e.g. a module was removed)
		}
		for _, dep := range importsOf(t, dir) {
			depLayer, known := layers[dep]
			if !known {
				continue // a package outside the map; TestEveryPackageIsLayered catches it
			}
			if depLayer > layer {
				t.Errorf("%s (layer %d) imports %s (layer %d): upward dependency",
					pkg, layer, dep, depLayer)
			}
		}
	}
}

// TestCoreStaysPure is the module rule: core may not know about any feature, or
// it stops being the thing everything else can depend on.
func TestCoreStaysPure(t *testing.T) {
	root := repoRoot(t)
	for _, dep := range importsOf(t, filepath.Join(root, "internal", "core")) {
		if dep != "migrations" {
			t.Errorf("internal/core imports %s; core may depend only on migrations", dep)
		}
	}
}

// TestCatalogHasNoHTTP keeps the data seam a data seam. If it grows an
// http.ResponseWriter it can no longer be reused by a job, a CLI or a test
// without dragging a request in.
func TestCatalogHasNoHTTP(t *testing.T) {
	root := repoRoot(t)
	for _, dep := range importsOf(t, filepath.Join(root, "internal", "catalog")) {
		if dep == "internal/httpx" || dep == "internal/httpapi" {
			t.Errorf("internal/catalog imports %s; the data seam must not know about HTTP", dep)
		}
	}
}

// TestModulesDoNotImportEachOther keeps modules independently removable. A
// module that imports a sibling cannot be dropped without dropping both.
func TestModulesDoNotImportEachOther(t *testing.T) {
	root := repoRoot(t)
	modsDir := filepath.Join(root, "internal", "modules")
	mods, err := os.ReadDir(modsDir)
	if err != nil {
		t.Skip("no modules directory")
	}
	for _, m := range mods {
		if !m.IsDir() {
			continue
		}
		self := "internal/modules/" + m.Name()
		for _, dep := range importsOf(t, filepath.Join(modsDir, m.Name())) {
			if strings.HasPrefix(dep, "internal/modules/") && dep != self {
				t.Errorf("%s imports %s; modules must not depend on each other", self, dep)
			}
		}
	}
}

// TestEveryPackageIsLayered stops the map above from silently going stale. A
// new package that nobody placed in a layer is a package with no rules, which
// is how the layering erodes.
func TestEveryPackageIsLayered(t *testing.T) {
	root := repoRoot(t)
	var found []string

	var walk func(base string)
	walk = func(base string) {
		entries, err := os.ReadDir(base)
		if err != nil {
			return
		}
		for _, e := range entries {
			if !e.IsDir() {
				continue
			}
			full := filepath.Join(base, e.Name())
			rel, _ := filepath.Rel(root, full)
			rel = filepath.ToSlash(rel)
			// Only directories that actually contain Go files are packages.
			if hasGoFiles(full) {
				found = append(found, rel)
			}
			walk(full)
		}
	}
	walk(filepath.Join(root, "internal"))
	found = append(found, "cmd/vellum")

	for _, pkg := range found {
		if _, ok := layers[pkg]; !ok {
			t.Errorf("%s is not in the layers map; add it so the import rules apply to it", pkg)
		}
	}
}

func hasGoFiles(dir string) bool {
	entries, err := os.ReadDir(dir)
	if err != nil {
		return false
	}
	for _, e := range entries {
		if !e.IsDir() && strings.HasSuffix(e.Name(), ".go") {
			return true
		}
	}
	return false
}