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/bans.go

@@ -26,9 +26,9 @@ const maxTextBytes = 256
 type Rules struct {
 	// LimitBanDuration is how long a first ban lasts.
 	LimitBanDuration time.Duration
-	// LimitBanRepeatWindow is how soon after the netblock's last ban
-	// ended a broken limit counts as a repeat, which bans for
-	// repeatFactor times as long as that ban.
+	// LimitBanRepeatWindow is how soon after the end of the netblock's
+	// ban that ended last a broken limit counts as a repeat, which bans
+	// for repeatFactor times as long as that ban.
 	LimitBanRepeatWindow time.Duration
 	// MaxBanDuration is the longest ban; a ban that would be longer is
 	// permanent instead.
@@ -176,9 +176,23 @@ func (l *Ledger) Find(client netip.Addr, now time.Time) (Ban, bool) {
 	return *ban, true
 }
 
+// activeBan returns the ban in bans, a netblock's bans oldest first, that
+// is active at now, or nil when none is. If several are, it returns the
+// one that started last. Every ban is looked at, since a ban an admin adds
+// to bans.json can start before the netblock's others and outlast them.
+func activeBan(bans []Ban, now time.Time) *Ban {
+	for i := len(bans) - 1; i >= 0; i-- {
+		if bans[i].ActiveAt(now) {
+			return &bans[i]
+		}
+	}
+
+	return nil
+}
+
 // BanForLimit bans netblock at now for a broken limit, with notes, and
 // returns the ban. A first ban lasts LimitBanDuration. A ban made within
-// LimitBanRepeatWindow after the netblock's last ban ended lasts
+// LimitBanRepeatWindow after the netblock's ban that ended last lasts
 // repeatFactor times as long as that one. A ban that would be longer
 // than MaxBanDuration is permanent instead. If a ban on netblock is still
 // active, as when two of its requests break a limit at once, that ban is
@@ -192,12 +206,22 @@ func (l *Ledger) BanForLimit(netblock netip.Prefix, now time.Time, notes Notes)
 
 	bans, found := l.netblocks.Get(netblock)
 	if found {
-		last = &(*bans)[len(*bans)-1]
-		if last.ActiveAt(now) {
-			return *last
+		active := activeBan(*bans, now)
+		if active != nil {
+			return *active
 		}
 
-		notes.EarlierBans = last.Notes.EarlierBans + 1
+		// No ban is active, so each has an end. A ban an admin adds to
+		// bans.json can start after another and end before it, so the
+		// ban that ended last is looked for among them all.
+		ended := slices.MaxFunc(*bans, func(a, b Ban) int {
+			return a.Expires.Compare(b.Expires)
+		})
+		last = &ended
+
+		// The netblock's first ban held counts the bans it had before that
+		// one, since dropped to make room, and each ban held adds one.
+		notes.EarlierBans = (*bans)[0].Notes.EarlierBans + len(*bans)
 	}
 
 	notes.Request = notes.Request.cut()
@@ -282,21 +306,25 @@ func (l *Ledger) Snapshot() []Ban {
 	return held
 }
 
-// Load puts bans read from bans.json into a ledger that holds none yet,
-// in the order they started, so that a netblock whose last ban started
-// latest counts as the most recently seen. Each netblock is masked to its
-// length, so that 203.0.113.9/24 is 203.0.113.0/24, and each text in the
-// notes is cut to 256 bytes. Past MaxBans the earliest bans are dropped,
-// as when they are made.
+// Load puts bans read from bans.json into the ledger, in place of the
+// bans it holds, in the order they started, so that a netblock whose last
+// ban started latest counts as the most recently seen. Each netblock is
+// masked to its length, so that 203.0.113.9/24 is 203.0.113.0/24, and
+// each text in the notes is cut to 256 bytes. Past MaxBans the earliest
+// bans are dropped, as when they are made.
 func (l *Ledger) Load(bans []Ban) {
-	l.mu.Lock()
-	defer l.mu.Unlock()
-
 	bans = slices.Clone(bans)
 	slices.SortStableFunc(bans, func(a, b Ban) int {
 		return a.Start.Compare(b.Start)
 	})
 
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	l.netblocks.Purge()
+	l.held = 0
+	l.v4Lengths, l.v6Lengths = nil, nil
+
 	for _, ban := range bans {
 		ban.Netblock = ban.Netblock.Masked()
 		ban.Notes.Request = ban.Notes.Request.cut()
@@ -318,11 +346,9 @@ func (l *Ledger) active(client netip.Addr, now time.Time) *Ban {
 			continue
 		}
 
-		// A ban is made only once the one before has ended, so only the
-		// last can be active.
-		last := &(*bans)[len(*bans)-1]
-		if last.ActiveAt(now) {
-			return last
+		ban := activeBan(*bans, now)
+		if ban != nil {
+			return ban
 		}
 	}
 
@@ -358,8 +384,8 @@ func (l *Ledger) add(ban Ban) {
 }
 
 // expiry returns when a ban for a broken limit made at now ends, or zero
-// when it is permanent. last is the netblock's last ban, which has ended,
-// or nil when it has none.
+// when it is permanent. last is the netblock's ban that ended last, or nil
+// when it has none.
 func (l *Ledger) expiry(last *Ban, now time.Time) time.Time {
 	length := l.rules.LimitBanDuration