Load every setting from one table in internal/config (closes #320)

Config.settings lists each setting once: its key, its environment
variable and the function that loads it. Most rows load through field,
whose arguments give the type, the default and the check. db_url and
cache_max_bytes keep their own load functions. Loading, the unknown-key
and unknown-variable checks and the names in error messages all read
the table. allowlist_hosts now loads through parseHostList, as
referer_blocklist does. isKnownConfigKey, envVarNames, strictLoader,
validate, validateAllowlistHostsValue and getStringSlice are gone, with
the getStringSlice tests. The key constants move to the tests, their
only users. A new test checks that README.md and
configs/config.example.yml name the keys of the table.

Model: opus-5-5

internal/config/documented_settings_internal_test.go

@@ -0,0 +1,108 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+// TestReadmeNamesEverySetting checks that the Configuration section of
+// README.md names every key of the settings table in backticks. It also
+// checks that every name in backticks there written like a key with an
+// underscore, such as state_dir, is a key of the table. The other keys,
+// such as port and metrics.username, are checked only the first way.
+func TestReadmeNamesEverySetting(t *testing.T) {
+	t.Parallel()
+
+	readme, err := os.ReadFile(filepath.Join("..", "..", "README.md"))
+	if err != nil {
+		t.Fatalf("failed to read README.md: %v", err)
+	}
+
+	_, section, found := strings.Cut(string(readme), "\n### Configuration\n")
+	if !found {
+		t.Fatal("README.md has no Configuration section")
+	}
+
+	// The section ends at the next heading of its level or above.
+	section, _, _ = strings.Cut(section, "\n## ")
+	section, _, _ = strings.Cut(section, "\n### ")
+
+	named := map[string]bool{}
+
+	for _, match := range regexp.MustCompile("`([^`]+)`").
+		FindAllStringSubmatch(section, -1) {
+		named[match[1]] = true
+	}
+
+	for _, s := range new(Config).settings() {
+		if !named[s.key] {
+			t.Errorf("README.md's Configuration section does not name %s", s.key)
+		}
+
+		delete(named, s.key)
+	}
+
+	keyWithUnderscore := regexp.MustCompile(`^[a-z]+(_[a-z]+)+$`)
+
+	for name := range named {
+		if keyWithUnderscore.MatchString(name) {
+			t.Errorf("README.md's Configuration section names %s, "+
+				"which is not a setting", name)
+		}
+	}
+}
+
+// TestExampleConfigNamesEverySetting checks that
+// configs/config.example.yml names exactly the keys of the settings
+// table, each one set or commented out with "# ". A key line holds the
+// key, a colon and at most a one-word value, which tells it from a
+// comment line that starts with a word and a colon. An indented key is
+// inside the map of the key above it, as username is inside metrics.
+func TestExampleConfigNamesEverySetting(t *testing.T) {
+	t.Parallel()
+
+	const examplePath = "configs/config.example.yml"
+
+	example, err := os.ReadFile(filepath.Join("..", "..", examplePath))
+	if err != nil {
+		t.Fatalf("failed to read %s: %v", examplePath, err)
+	}
+
+	keyLine := regexp.MustCompile(`(?m)^(?:# )?( *)([a-z_]+):(?: \S+)?$`)
+	named := map[string]bool{}
+
+	var mapKey string
+
+	for _, match := range keyLine.FindAllStringSubmatch(string(example), -1) {
+		indent, key := match[1], match[2]
+		if indent == "" {
+			mapKey = key
+			named[key] = true
+
+			continue
+		}
+
+		// The key holding the map is not itself a setting.
+		delete(named, mapKey)
+
+		named[mapKey+"."+key] = true
+	}
+
+	// The env section sets environment variables; it is not a setting.
+	delete(named, "env")
+
+	for _, s := range new(Config).settings() {
+		if !named[s.key] {
+			t.Errorf("%s does not name %s", examplePath, s.key)
+		}
+
+		delete(named, s.key)
+	}
+
+	for key := range named {
+		t.Errorf("%s names %s, which is not a setting", examplePath, key)
+	}
+}