Take in an admin's edits of the state files while running (closes #68)

smallwebwaf watches SWWAF_STATE_DIR with fsnotify and takes in a saved
edit of a state file in place of what it held. It knows its own writes
by the SHA-256 of what it last read or wrote; each write first takes in
an edit made since. An edit that does not parse is renamed to
<name>.bad at the next write. Each edit taken in or set aside is logged
and counted. Every ban on a netblock is checked, and the next ban is
worked out from the one that ended last. README.md says how to add and
lift a ban.

Judgement call: a broken edit is set aside at the next write, since an
editor's file can be read half written.

Model: opus-5-5

internal/bans/snapshot_test.go

@@ -130,6 +130,75 @@ func TestLoadedBanRefusesEveryClientInItsNetblock(t *testing.T) {
 	}
 }
 
+func TestPermanentBanStartedBeforeAnEndedOneRefuses(t *testing.T) {
+	t.Parallel()
+
+	// As when an admin adds a permanent ban to bans.json with a start
+	// before that of the netblock's ban that has ended.
+	netblock := netip.MustParsePrefix("203.0.113.0/24")
+	permanent := bans.Ban{Netblock: netblock, Start: midnight().Add(-time.Hour)}
+	ended := bans.Ban{
+		Netblock: netblock,
+		Start:    midnight(),
+		Expires:  midnight().Add(time.Hour),
+	}
+
+	ledger := bans.New(defaultRules())
+	ledger.Load([]bans.Ban{permanent, ended})
+
+	now := midnight().Add(2 * time.Hour)
+	client := netip.MustParseAddr("203.0.113.9")
+
+	ban, banned := ledger.Find(client, now)
+	if !banned || !ban.Permanent() {
+		t.Errorf("find gives %+v and %t, want the permanent ban", ban, banned)
+	}
+
+	ban, banned = ledger.Check(client, now)
+	if !banned || !ban.Permanent() {
+		t.Errorf("the client is refused: %t, under %+v, want under the permanent ban",
+			banned, ban)
+	}
+
+	// A limit broken now makes no shorter ban over the permanent one.
+	ban = ledger.BanForLimit(netblock, now, bans.Notes{})
+	if !ban.Permanent() || len(ledger.Bans(netblock)) != 2 {
+		t.Errorf("a broken limit returned %+v and left the netblock %d bans, "+
+			"want the permanent ban and 2", ban, len(ledger.Bans(netblock)))
+	}
+}
+
+func TestNextBanWorkedOutFromTheBanThatEndedLast(t *testing.T) {
+	t.Parallel()
+
+	// A 9-hour ban smallwebwaf made, the third in a row, and an admin's
+	// 1-hour ban added to bans.json over it, with no notes.
+	netblock := netip.MustParsePrefix("203.0.113.9/32")
+	nineHours := bans.Ban{
+		Netblock: netblock,
+		Start:    midnight(),
+		Expires:  midnight().Add(9 * time.Hour),
+		Notes:    bans.Notes{EarlierBans: 2},
+	}
+	admins := bans.Ban{
+		Netblock: netblock,
+		Start:    midnight().Add(time.Hour),
+		Expires:  midnight().Add(2 * time.Hour),
+	}
+
+	ledger := bans.New(defaultRules())
+	ledger.Load([]bans.Ban{nineHours, admins})
+
+	// Once both have ended, a limit broken within the repeat window bans
+	// for three times the 9 hours, and the notes count the two bans
+	// before the 9-hour one, it, and the admin's.
+	ban := ledger.BanForLimit(netblock, nineHours.Expires.Add(time.Hour), bans.Notes{})
+	if ban.Expires.Sub(ban.Start) != 27*time.Hour || ban.Notes.EarlierBans != 4 {
+		t.Errorf("the next ban lasts %s with %d earlier bans, want 27h and 4",
+			ban.Expires.Sub(ban.Start), ban.Notes.EarlierBans)
+	}
+}
+
 func TestLoadKeepsAtMostMaxBansDroppingTheEarliest(t *testing.T) {
 	t.Parallel()
 
@@ -151,6 +220,41 @@ func TestLoadKeepsAtMostMaxBansDroppingTheEarliest(t *testing.T) {
 	}
 }
 
+func TestLoadReplacesTheBansHeld(t *testing.T) {
+	t.Parallel()
+
+	// Room for three bans, so that the second load, were it added to the
+	// two bans held, would drop none of them to make room.
+	rules := defaultRules()
+	rules.MaxBans = 3
+	ledger := bans.New(rules)
+	kept := bans.Ban{Netblock: netip.MustParsePrefix("2001:db8::/64"), Start: midnight()}
+	ledger.Load([]bans.Ban{
+		{Netblock: netip.MustParsePrefix("203.0.113.0/24"), Start: midnight()},
+		kept,
+	})
+
+	// Loaded again without the first ban, as when an admin's edit of
+	// bans.json is taken in, that ban is lifted.
+	ledger.Load([]bans.Ban{kept})
+
+	_, banned := ledger.Check(netip.MustParseAddr("203.0.113.9"), midnight())
+	if banned {
+		t.Error("a ban left out of the second load still refuses")
+	}
+
+	// The ledger holds one ban, so it makes two more without dropping any.
+	first := ledger.BanForLimit(netip.MustParsePrefix("198.51.100.7/32"), midnight(),
+		bans.Notes{})
+	second := ledger.BanForLimit(netip.MustParsePrefix("198.51.100.8/32"), midnight(),
+		bans.Notes{})
+
+	want := []bans.Ban{first, second, kept}
+	if got := ledger.Snapshot(); !slices.Equal(got, want) {
+		t.Errorf("the ledger holds %+v, want %+v", got, want)
+	}
+}
+
 func TestLoadCutsTheTextsTo256Bytes(t *testing.T) {
 	t.Parallel()