Keep the bans, the clients and GeoJS's answers in state files (closes #17)

smallwebwaf now copies its state to bans.json, clients.json and
lookups.json in SWWAF_STATE_DIR, as "Persistent state" in SPEC.md
describes, and reads them back at start, so a restart lifts no ban and
gives no client a fresh allowance. Each client gains a history, and a
ban's notes count the netblock's requests. bans.json is written
SWWAF_STATE_WRITE_DELAY after a ban, every file every
SWWAF_STATE_COUNTER_INTERVAL and at the stop, each through a synced
temporary file renamed over it. A file that does not parse, an unknown
version or an unwritable directory stops the start. The image gets
/var/lib/smallwebwaf, which the run script gives to the smallwebwaf user.

Deviation: no AS number or name, and no ban cause, reason or lifting yet.

Model: opus-5-5

internal/config/config.go

@@ -12,6 +12,7 @@ import (
 	"net/http"
 	"net/netip"
 	"net/url"
+	"path/filepath"
 	"slices"
 	"strconv"
 	"strings"
@@ -94,6 +95,14 @@ type Config struct {
 	// BanScopeV4Prefix is the length of the netblock around an IPv4
 	// client that a ban covers (SWWAF_BAN_SCOPE_V4_PREFIX).
 	BanScopeV4Prefix int
+	// StateDir is the directory of the state files, an absolute path
+	// (SWWAF_STATE_DIR). bans.json is written StateWriteDelay after a ban
+	// is made (SWWAF_STATE_WRITE_DELAY), and every state file every
+	// StateCounterInterval (SWWAF_STATE_COUNTER_INTERVAL). Neither can be
+	// off.
+	StateDir             string
+	StateWriteDelay      time.Duration
+	StateCounterInterval time.Duration
 
 	// settings are the values read, as given or by default, for the
 	// log line at start.
@@ -139,6 +148,8 @@ var (
 	errNotBanResponse = errors.New("is not 403, 429 or close")
 	errNotV4Prefix    = errors.New(
 		"is not the length of an IPv4 netblock, from 0 to 32, such as 24")
+	errNotAbsolutePath = errors.New(
+		"is not an absolute path, such as /var/lib/smallwebwaf")
 )
 
 // FromEnvironment reads the settings with lookupEnv, normally
@@ -174,6 +185,9 @@ func FromEnvironment(lookupEnv func(string) (string, bool)) (*Config, error) {
 		MaxBanDuration:       env.durationNotOff("SWWAF_MAX_BAN_DURATION", "7d"),
 		MaxBans:              env.numberNotOff("SWWAF_MAX_BANS", "5000"),
 		BanScopeV4Prefix:     env.v4Prefix("SWWAF_BAN_SCOPE_V4_PREFIX", "32"),
+		StateDir:             env.absolutePath("SWWAF_STATE_DIR", "/var/lib/smallwebwaf"),
+		StateWriteDelay:      env.durationNotOff("SWWAF_STATE_WRITE_DELAY", "10s"),
+		StateCounterInterval: env.durationNotOff("SWWAF_STATE_COUNTER_INTERVAL", "15m"),
 	}
 
 	for _, country := range cfg.ExclusivelyAllowedCountries {
@@ -329,6 +343,16 @@ func (e *environment) v4Prefix(name, defaultValue string) int {
 	return length
 }
 
+// absolutePath reads a setting that is an absolute path.
+func (e *environment) absolutePath(name, defaultValue string) string {
+	path := e.value(name, defaultValue)
+	if !filepath.IsAbs(path) {
+		e.check(name, fmt.Errorf("%q %w", path, errNotAbsolutePath))
+	}
+
+	return path
+}
+
 // parseDuration reads a duration in Go's syntax, such as 90s or 15m, a
 // whole number of days such as 7d, or off.
 func parseDuration(value string) (time.Duration, error) {