A changed jail filter restarts fail2ban

fail2ban restarts when the composed jail file changes, and each filter is
a file of its own, so a module that changed only its failregex left the
running jail on the old pattern. The jail file now names each filter's
digest.

internal/catalogue/jails_into.go

@@ -1,6 +1,8 @@
 package catalogue
 
 import (
+	"crypto/sha256"
+	"encoding/hex"
 	"fmt"
 	"sort"
 	"strings"
@@ -43,8 +45,16 @@ func jailsInto(modules []Manifest, j *Jailing) []map[string]any {
 
 	out := make([]map[string]any, 0, len(jails)+1)
 	for _, d := range jails {
-		fmt.Fprintf(&composed, "\n# from %s\n[%s]\nenabled = true\nfilter = %s\n%s\n",
+		// **The filter's digest rides in the jail file.** fail2ban is restarted when this file
-			d.module, d.jail.Name, d.jail.Name, strings.TrimRight(d.jail.Jail, "\n"))
+		// changes, and the filter is a file of its own: a module that changed only what a failure
+		// looks like rewrote the filter on disk and left the running jail on the old pattern, with
+		// nothing said (novox/hq issue 191's rollout found it on gitea's sshd). Naming the filter's
+		// digest here makes a changed pattern a changed jail file, so the restart the service
+		// already takes on it covers the filter too.
+		sum := sha256.Sum256([]byte(d.jail.Failregex))
+		fmt.Fprintf(&composed, "\n# from %s, filter %s\n[%s]\nenabled = true\nfilter = %s\n%s\n",
+			d.module, hex.EncodeToString(sum[:])[:12], d.jail.Name, d.jail.Name,
+			strings.TrimRight(d.jail.Jail, "\n"))
 		// The filter is a file of its own, named as the jail's filter= references it.
 		out = append(out, map[string]any{
 			"id":   "filter-" + d.jail.Name,

internal/catalogue/jails_into_test.go

@@ -44,3 +44,29 @@ func TestTheComposedJailFileIsWrittenEvenWhenEmpty(t *testing.T) {
 		t.Fatalf("the empty composed jail file was not written alone: %v", files)
 	}
 }
+
+// A changed pattern restarts fail2ban (novox/hq issue 191's rollout): the service restarts when the
+// composed jail file changes, and the filter is a file of its own, so the jail file names the
+// filter's digest. Changing only the failregex must change the jail file; the same pattern must not.
+func TestAChangedFilterChangesTheJailFile(t *testing.T) {
+	jailFile := func(failregex string) string {
+		modules := []Manifest{
+			{Module: "fail2ban", Jailing: &Jailing{Into: "/etc/fail2ban/jail.d/mesh.conf", FilterInto: "/etc/fail2ban/filter.d"}},
+			{Module: "gitea", Jails: []Jail{{Name: "gitea", Failregex: failregex, Jail: "port = 222"}}},
+		}
+		for _, f := range jailsInto(modules, modules[0].Jailing) {
+			if f["id"] == ComposedJailsID() {
+				return f["content"].(string)
+			}
+		}
+		t.Fatal("no composed jail file")
+		return ""
+	}
+	before := jailFile("web login failed from <HOST>")
+	if again := jailFile("web login failed from <HOST>"); again != before {
+		t.Errorf("the same pattern composed a different jail file, which would restart fail2ban for nothing")
+	}
+	if after := jailFile("web login failed from <HOST>\n            Invalid user .* from <HOST>"); after == before {
+		t.Errorf("a changed pattern left the jail file as it was, so fail2ban keeps the old filter:\n%s", after)
+	}
+}