explain: shows a duplicate's skipped delete, leaves the cache alone, walks only for duplicate tests

2 files changed, 42 insertions, 3 deletions

diff --git a/internal/engine/exclude_test.go b/internal/engine/exclude_test.go
index ceb8442..cd1cc4b 100644
--- a/internal/engine/exclude_test.go
+++ b/internal/engine/exclude_test.go
@@ -426,3 +426,25 @@ func TestPartlyReadableDocument(t *testing.T) {
 		t.Errorf("no warning for the keyword the unreadable part might hold")
 	}
 }
+
+// TestExplainLeavesTheCacheAlone: explain never writes the keyword cache -
+// not even to remove one a directory without content tests no longer uses
+// (triage 28e: explain runs without the directory's lock).
+func TestExplainLeavesTheCacheAlone(t *testing.T) {
+	h, dl := excludeTree(t, map[string]string{"a.txt": "x"})
+	main := writeConfig(t, h, `(include "dl")`, map[string]string{"dl": "(path \"~/dl\")\n(rule \"all\" (move \"Out\"))\n"})
+	e, errs := Load(main)
+	if len(errs) > 0 {
+		t.Fatal(errs)
+	}
+	e.CacheDir = filepath.Join(h, "cache")
+	os.MkdirAll(e.CacheDir, 0o700)
+	cache := filepath.Join(e.CacheDir, "dl.cache")
+	os.WriteFile(cache, []byte("old"), 0o600)
+	if _, err := e.Explain(context.Background(), filepath.Join(dl, "a.txt")); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := os.Stat(cache); err != nil {
+		t.Errorf("explain removed the cache: %v", err)
+	}
+}
diff --git a/internal/engine/match.go b/internal/engine/match.go
index 162730b..0e8b3d5 100644
--- a/internal/engine/match.go
+++ b/internal/engine/match.go
@@ -227,6 +227,7 @@ type Explanation struct {
 	Excludes []ExcludeTrace
 	Excluded string // the first exclude that matches, which sets the file aside; "" when none does
 	Rules    []RuleTrace
+	NoDelete string // why a delete from the matching rules would be skipped (spec §5.5); "" when it would not
 }
 
 // Explain reports, for one file, whether krino's ordinary scan would ever
@@ -266,8 +267,18 @@ func (e *Engine) Explain(ctx context.Context, path string) (*Explanation, error)
 	excl := e.excludeDirs(d)
 	skip := explainSkip(d, sf, excl, now)
 
-	run := newMatchRun(e, d, ctx, now, e.filesForExplain(d, sf, excl, now))
-	e.openCache(run) // read only: Explain never writes the cache
+	// The directory is walked only for a duplicate test, the one thing that
+	// needs its other files (triage 21).
+	files := []scan.File{sf}
+	if len(d.DupScopes) > 0 {
+		files = e.filesForExplain(d, sf, excl, now)
+	}
+	run := newMatchRun(e, d, ctx, now, files)
+	if len(d.ContentKeywords) > 0 {
+		// Loaded only: openCache would remove the cache of a directory with
+		// no content tests, and Explain holds no lock (triage 28e).
+		e.openCache(run)
+	}
 	f := newFacts(run, sf)
 
 	var excludes []ExcludeTrace
@@ -282,6 +293,7 @@ func (e *Engine) Explain(ctx context.Context, path string) (*Explanation, error)
 	}
 
 	var rules []RuleTrace
+	var matched []RuleMatch
 	stoppedBy := ""
 	for _, r := range d.Rules {
 		if stoppedBy != "" {
@@ -292,14 +304,19 @@ func (e *Engine) Explain(ctx context.Context, path string) (*Explanation, error)
 		match := trace.Value
 		if match {
 			f.matched = true
+			matched = append(matched, RuleMatch{Rule: r})
 		}
 		rules = append(rules, RuleTrace{Rule: r, Match: match, Trace: trace})
 		if match && r.Conf.Stop {
 			stoppedBy = r.Name
 		}
 	}
+	noDel := ""
+	if len(d.DupScopes) > 0 && deletes(matched) {
+		noDel = noDelete(f, d.DupScopes)
+	}
 
-	return &Explanation{Dir: d, File: sf, Skip: skip, Excludes: excludes, Excluded: excluded, Rules: rules}, nil
+	return &Explanation{Dir: d, File: sf, Skip: skip, Excludes: excludes, Excluded: excluded, Rules: rules, NoDelete: noDel}, nil
 }
 
 // cacheFingerprint identifies what a cached keyword answer of d depends on