web: don't clobber books.toml when the settings books field is empty

2 files changed, 37 insertions, 2 deletions

diff --git a/internal/web/server.go b/internal/web/server.go
index 34371b7..d7f0ebc 100644
--- a/internal/web/server.go
+++ b/internal/web/server.go
@@ -651,8 +651,13 @@ func settingsPost(s *server) http.HandlerFunc {
 			renderSettingsError(w, cfg, booksText, "saving config: "+err.Error())
 			return
 		}
-		if p, err := config.BooksPath(); err == nil {
-			_ = os.WriteFile(p, []byte(booksText), 0o644)
+		// Only write books.toml when the editor actually carried content, so a
+		// submit with an empty/absent books field never clobbers an existing
+		// override (the form always pre-fills the textarea).
+		if strings.TrimSpace(booksText) != "" {
+			if p, err := config.BooksPath(); err == nil {
+				_ = os.WriteFile(p, []byte(booksText), 0o644)
+			}
 		}
 
 		s.apply(cfg, newTbl)
diff --git a/internal/web/server_test.go b/internal/web/server_test.go
index ee36318..61546b8 100644
--- a/internal/web/server_test.go
+++ b/internal/web/server_test.go
@@ -567,3 +567,33 @@ func TestSettingsPostInvalidBooks(t *testing.T) {
 }
 
 func bibleDefaultBooks() []byte { return bible.DefaultBooksTOML() }
+
+// TestSettingsPostEmptyBooksPreservesFile guards that a submit without a books
+// field never clobbers an existing books.toml override.
+func TestSettingsPostEmptyBooksPreservesFile(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("LECTIO_CONFIG", filepath.Join(dir, "config.toml"))
+	booksPath := filepath.Join(dir, "books.toml")
+	original := []byte("[en]\nJohn = [\"Jn\", \"John\"]\n")
+	if err := os.WriteFile(booksPath, original, 0o644); err != nil {
+		t.Fatal(err)
+	}
+	srv := NewServer(config.Default())
+	form := url.Values{}
+	form.Set("lectionary", "new")
+	form.Set("ui_language", "en")
+	form.Set("web_display", "vertical")
+	form.Set("web_theme", "transfiguration")
+	form.Set("default_version", "bt")
+	form["versions"] = []string{"bt"} // no "books" field
+	post := httptest.NewRequest("POST", "/settings", strings.NewReader(form.Encode()))
+	post.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	rec := httptest.NewRecorder()
+	srv.ServeHTTP(rec, post)
+	if rec.Code != http.StatusSeeOther {
+		t.Fatalf("status %d", rec.Code)
+	}
+	if got, _ := os.ReadFile(booksPath); string(got) != string(original) {
+		t.Errorf("empty books submit clobbered books.toml: got %q", got)
+	}
+}