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

smallwebwaf watches SWWAF_STATE_DIR with fsnotify and takes in an edit of
a state file as soon as it is saved, in place of what it held. It tells
its own writes from an admin's by the SHA-256 of what it last read or
wrote, and each write takes in an edit made since first. An edit that
does not parse is renamed to <name>.bad at the file's next write. Every
ban on a netblock is checked, so an added ban that starts before the
others still refuses. 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

@@ -160,20 +160,32 @@ func (l *Ledger) Check(client netip.Addr, now time.Time) (Ban, bool) {
 			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) {
-			last.Notes.Requests++
-			last.Notes.Refused++
+		ban := activeBan(*bans, now)
+		if ban != nil {
+			ban.Notes.Requests++
+			ban.Notes.Refused++
 
-			return *last, true
+			return *ban, true
 		}
 	}
 
 	return Ban{}, false
 }
 
+// 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
@@ -190,11 +202,12 @@ 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
 		}
 
+		last = &(*bans)[len(*bans)-1]
 		notes.EarlierBans = last.Notes.EarlierBans + 1
 	}
 

internal/bans/snapshot_test.go

@@ -130,6 +130,38 @@ 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)
+
+	ban, banned := ledger.Check(netip.MustParseAddr("203.0.113.9"), 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 TestLoadKeepsAtMostMaxBansDroppingTheEarliest(t *testing.T) {
 	t.Parallel()
 
@@ -154,8 +186,10 @@ 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 = 2
+	rules.MaxBans = 3
 	ledger := bans.New(rules)
 	kept := bans.Ban{Netblock: netip.MustParsePrefix("2001:db8::/64"), Start: midnight()}
 	ledger.Load([]bans.Ban{
@@ -164,18 +198,22 @@ func TestLoadReplacesTheBansHeld(t *testing.T) {
 	})
 
 	// Loaded again without the first ban, as when an admin's edit of
-	// bans.json is taken in: that ban is lifted, and the ledger, holding
-	// one ban, makes another without dropping any.
+	// bans.json is taken in, that ban is lifted.
 	ledger.Load([]bans.Ban{kept})
-	made := ledger.BanForLimit(netip.MustParsePrefix("198.51.100.7/32"), midnight(),
-		bans.Notes{})
 
 	_, banned := ledger.Check(netip.MustParseAddr("203.0.113.9"), midnight())
 	if banned {
 		t.Error("a ban left out of the second load still refuses")
 	}
 
-	if got, want := ledger.Snapshot(), []bans.Ban{made, kept}; !slices.Equal(got, want) {
+	// 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)
 	}
 }

internal/state/state.go

@@ -378,9 +378,11 @@ func (f *Files) writeFile(name string) error {
 		return err
 	}
 
+	// The file holds data from here on, even if the directory sync fails,
+	// so that its next read does not take it for an admin's edit.
 	f.sums[name] = sha256.Sum256(data)
 
-	return nil
+	return syncDirectory(f.params.Dir)
 }
 
 // encode returns the state file name as smallwebwaf writes it, from a
@@ -620,7 +622,7 @@ func position(data []byte, err error) string {
 // write writes data to the file name in dir so that a crash at any
 // moment leaves either the old file or the new one, whole: data goes to a
 // temporary file in the same directory, which is synced and renamed over
-// name, and then the directory is synced, so that the rename lasts.
+// name. syncDirectory must follow, so that the rename lasts.
 func write(dir, name string, data []byte) error {
 	path := filepath.Join(dir, name)
 	temporary := path + ".tmp"
@@ -632,10 +634,13 @@ func write(dir, name string, data []byte) error {
 
 	if err != nil {
 		_ = os.Remove(temporary)
-
-		return err
 	}
 
+	return err
+}
+
+// syncDirectory syncs dir to the disk, so that a rename in it lasts.
+func syncDirectory(dir string) error {
 	directory, err := os.Open(dir) //nolint:gosec // SWWAF_STATE_DIR itself
 	if err != nil {
 		return err