Diffs name their chain; history shows the ruleset's diff

The ruleset's diff names the chain each hunk is in on its @@ line, as
git names the function. A history entry shows what it did to the
ruleset (web page, hc history diff, GET /history/{id}/diff), rebuilt
from the current configuration by stepping back through the later
entries. A changed setting is listed by its own fields
(builtins.drop_invalid) instead of the whole object.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/diff_test.go

@@ -0,0 +1,72 @@
+package server
+
+import (
+	"strings"
+	"testing"
+
+	udiff "github.com/aymanbagabas/go-udiff"
+
+	"housecarl/internal/api"
+	"housecarl/internal/model"
+)
+
+// TestBlockHeaders: each hunk says which chain it is in, even when the
+// chain's own line is far above it.
+func TestBlockHeaders(t *testing.T) {
+	var b strings.Builder
+	b.WriteString("table inet housecarl {\n\tset lan_v4 { type ipv4_addr; }\n\tchain input {\n")
+	for i := 0; i < 20; i++ {
+		b.WriteString("\t\tcounter comment \"in\"\n")
+	}
+	b.WriteString("\t}\n\tchain forward {\n")
+	for i := 0; i < 20; i++ {
+		b.WriteString("\t\tcounter comment \"fw\"\n")
+	}
+	b.WriteString("\t}\n}\n")
+	now := b.String()
+	then := strings.Replace(now, "\t\tcounter comment \"in\"\n\t}", "\t\tcounter comment \"in\"\n\t\taccept comment \"new in\"\n\t}", 1)
+	then = strings.Replace(then, "\t\tcounter comment \"fw\"\n\t}", "\t\taccept comment \"new fw\"\n\t}", 1)
+
+	diff := blockHeaders(udiff.Unified("a", "b", now, then), now, then)
+	var heads []string
+	for _, l := range strings.Split(diff, "\n") {
+		if strings.HasPrefix(l, "@@") {
+			heads = append(heads, l)
+		}
+	}
+	if len(heads) != 2 || !strings.HasSuffix(heads[0], "@@ chain input") || !strings.HasSuffix(heads[1], "@@ chain forward") {
+		t.Fatalf("hunk headers %q, want chain input then chain forward\n%s", heads, diff)
+	}
+	if blockHeaders("", now, then) != "" {
+		t.Fatal("no diff should stay empty")
+	}
+}
+
+// TestHistoryDiff: an entry's diff is its own change, rebuilt from the
+// current configuration however many entries came after it.
+func TestHistoryDiff(t *testing.T) {
+	e := newEnv(t)
+	var mr api.MutationResponse
+	e.call(e.token, "POST", "/zones", model.Zone{Name: "lan", Interfaces: []string{"eth1"}}, nil)
+	e.call(e.token, "POST", "/rules", model.Rule{Chain: "input", Match: model.Match{Services: []string{"ssh"}}, Comment: "first"}, &mr)
+	first := mr.HistoryID
+	e.call(e.token, "POST", "/rules", model.Rule{Chain: "forward", Match: model.Match{Services: []string{"http"}}, Comment: "second"}, nil)
+	e.call(e.token, "PATCH", "/settings", map[string]any{"builtins": map[string]any{"drop_invalid": true}}, nil)
+
+	var out struct{ Diff string }
+	if code := e.call(e.token, "GET", "/history/"+itoa(first)+"/diff", nil, &out); code != 200 {
+		t.Fatalf("diff: %d", code)
+	}
+	var added []string
+	for _, l := range strings.Split(out.Diff, "\n") {
+		if strings.HasPrefix(l, "+") && !strings.HasPrefix(l, "+++") {
+			added = append(added, l)
+		}
+	}
+	if len(added) == 0 || !strings.Contains(strings.Join(added, "\n"), ` first"`) || strings.Contains(out.Diff, ` second"`) || !strings.Contains(out.Diff, "@@ chain input") {
+		t.Errorf("entry %d's diff:\n%s", first, out.Diff)
+	}
+	if code := e.call(e.token, "GET", "/history/9999/diff", nil, nil); code != 404 {
+		t.Errorf("unknown entry: %d", code)
+	}
+}

internal/server/h_config.go

@@ -101,7 +101,68 @@ func (s *Server) rulesetDiff(before, after *model.State) string {
 	if before == nil || after == nil {
 		return ""
 	}
-	return udiff.Unified("ruleset.nft (now)", "ruleset.nft (after)", s.fw.Render(before).Text, s.fw.Render(after).Text)
+	now, then := s.fw.Render(before).Text, s.fw.Render(after).Text
+	return blockHeaders(udiff.Unified("ruleset.nft (now)", "ruleset.nft (after)", now, then), now, then)
+}
+
+// blockHeaders names, after each hunk's @@ line, the chain (or other
+// block of the table) the hunk starts in, as git names the function:
+// "@@ -40,6 +40,8 @@ chain forward". Without it a change deep in a long
+// chain doesn't say which chain it is in.
+func blockHeaders(diff, now, then string) string {
+	if diff == "" {
+		return diff
+	}
+	oldBlocks, newBlocks := enclosingBlocks(now), enclosingBlocks(then)
+	lines := strings.SplitAfter(diff, "\n")
+	for i, l := range lines {
+		if !strings.HasPrefix(l, "@@ ") {
+			continue
+		}
+		// "@@ -a,b +c,d @@": the new side's start, or the old side's
+		// when nothing of the hunk is left.
+		f := strings.Fields(l)
+		if len(f) < 4 || f[3] != "@@" {
+			continue
+		}
+		block := blockAt(newBlocks, f[2])
+		if block == "" {
+			block = blockAt(oldBlocks, f[1])
+		}
+		if block != "" {
+			lines[i] = strings.TrimRight(l, "\n") + " " + block + "\n"
+		}
+	}
+	return strings.Join(lines, "")
+}
+
+// enclosingBlocks gives each line of a rendered ruleset the block of the
+// table it is in ("chain input"), "" outside one. A block opens with a
+// line of one tab ending in "{" and closes with "\t}".
+func enclosingBlocks(text string) []string {
+	var out []string
+	cur := ""
+	for _, l := range strings.Split(text, "\n") {
+		if strings.HasPrefix(l, "\t") && !strings.HasPrefix(l, "\t\t") && strings.HasSuffix(l, "{") {
+			cur = strings.TrimSpace(strings.TrimSuffix(l, "{"))
+		}
+		out = append(out, cur)
+		if l == "\t}" {
+			cur = ""
+		}
+	}
+	return out
+}
+
+// blockAt is the block at a hunk side's start ("+40,8" or "-40"); lines
+// count from 1, and an empty side (",0") sits after its line.
+func blockAt(blocks []string, side string) string {
+	start, _, _ := strings.Cut(side[1:], ",")
+	n, err := strconv.Atoi(start)
+	if err != nil || n < 1 || n > len(blocks) {
+		return ""
+	}
+	return blocks[n-1]
 }
 
 // kindOf maps the API's plural path names to kinds.
@@ -616,6 +677,38 @@ func (s *Server) getHistory(w http.ResponseWriter, r *http.Request, p *auth.Prin
 	return nil
 }
 
+// historyDiff is the ruleset's diff for one history entry: the
+// configurations before and after it, got back by stepping back from
+// the current one through every later entry, rendered by this version.
+func (s *Server) historyDiff(w http.ResponseWriter, r *http.Request, p *auth.Principal) error {
+	if err := require(p, auth.ResConfig, false); err != nil {
+		return err
+	}
+	id, err := pathID(r, "id")
+	if err != nil {
+		return err
+	}
+	hs, err := s.store.HistorySince(r.Context(), id)
+	if err != nil {
+		return err
+	}
+	if len(hs) == 0 || hs[len(hs)-1].ID != id {
+		return notFound("history entry %d not found", id)
+	}
+	st := s.eng.State()
+	var after *model.State
+	for _, h := range hs {
+		if h.ID == id {
+			after = st.Clone()
+		}
+		if err := engine.Rewind(st, h); err != nil {
+			return err
+		}
+	}
+	writeJSON(w, http.StatusOK, map[string]string{"diff": s.rulesetDiff(st, after)})
+	return nil
+}
+
 func (s *Server) revertHistory(w http.ResponseWriter, r *http.Request, p *auth.Principal) error {
 	if err := require(p, auth.ResConfig, true); err != nil {
 		return err

internal/server/routes.go

@@ -75,6 +75,7 @@ func (s *Server) routes() {
 	m.HandleFunc("GET "+p+"/stats/heatmap", s.authed(s.statsHeatmap))
 	m.HandleFunc("GET "+p+"/history", s.authed(s.listHistory))
 	m.HandleFunc("GET "+p+"/history/{id}", s.authed(s.getHistory))
+	m.HandleFunc("GET "+p+"/history/{id}/diff", s.authed(s.historyDiff))
 	m.HandleFunc("POST "+p+"/history/{id}/revert", s.authed(s.revertHistory))
 	m.HandleFunc("GET "+p+"/probation", s.authed(s.getProbation))
 	m.HandleFunc("POST "+p+"/confirm", s.authed(s.confirmChanges))