fix: handle newlines in SSE events to prevent content truncation

alekseysotnikov · alekseysotnikov · commit d565e3517364 · 2026-03-13T10:24:51.000+07:00
When HTML content contains newline characters (\n), the SSE event formatting
was not escaping them properly. This caused SSE events to be prematurely
terminated, resulting in truncated content on the client side during
live-reload updates.

Root cause: format-datastar-fragment was outputting HTML directly in a
single 'data: elements' line without handling newlines. Any \n\n in
the HTML would prematurely end the SSE event according to the SSE spec.

Fix: Split HTML by \n and emit multiple 'data: elements' lines, which
Datastar concatenates. This follows Datastar's documented multi-line
SSE format and prevents \n\n from breaking the event stream.

Tests added:
- Unit tests for textarea and pre elements with newline content
- E2E test for content with newlines during route redefinition
diff --git a/src/hyper/render.clj b/src/hyper/render.clj
@@ -2,7 +2,8 @@
   "Rendering pipeline.
 
    Handles rendering hiccup to HTML and formatting Datastar SSE events."
-  (:require [dev.onionpancakes.chassis.core :as c]
+  (:require [clojure.string :as string]
+            [dev.onionpancakes.chassis.core :as c]
             [hyper.context :as context]
             [hyper.routes :as routes]
             [hyper.state :as state]
@@ -27,10 +28,16 @@
    event: datastar-patch-elements
    data: elements <html content>
 
-   (blank line to end event)"
+   For multi-line HTML content, emits multiple 'data: elements' lines
+   that Datastar will concatenate. This prevents \n in HTML from
+   prematurely terminating the SSE event."
   [html]
-  (str "event: datastar-patch-elements\n"
-       "data: elements " html "\n\n"))
+  (let [lines (string/split-lines html)]
+    (str "event: datastar-patch-elements\n"
+         (->> lines
+              (map (fn [line] (str "data: elements " line "\n")))
+              (apply str))
+         "\n")))
 
 (defn mark-head-elements
   "Add `{:data-hyper-head true}` to each top-level hiccup element in a
diff --git a/test/hyper/e2e_test.clj b/test/hyper/e2e_test.clj
@@ -578,8 +578,22 @@
 
           (is (= "Live Reloaded!" (w/text-content "h1")))
           (is (= "This content was hot-swapped"
-                 (w/text-content "#reloaded-marker")))))
+                 (w/text-content "#reloaded-marker"))))
 
+        ;; Test that content with newlines renders correctly
+        (testing "Content with newlines preserved in route handler"
+          (alter-var-root #'*test-routes*
+                          (constantly
+                            [["/" {:name  :home
+                                   :title "Newlines Test"
+                                   :get   (fn [_]
+                                            [:div
+                                             [:textarea#newline-content "line1\nline2"]
+                                             [:pre#pre-content "code\nwith\n\nnew\n\nlines\n\n"]])}]]))
+          (w/navigate (str base-url "/"))
+          (wait-for-sse)
+          (is (= "line1\nline2" (w/text-content "#newline-content")))
+          (is (= "code\nwith\n\nnew\n\nlines\n\n" (w/text-content "#pre-content")))))
       (finally
         (close-browser! browser-info)))))
 
diff --git a/test/hyper/render_test.clj b/test/hyper/render_test.clj
@@ -38,7 +38,24 @@
     (let [html     "<span>test</span>"
           fragment (render/format-datastar-fragment html)]
       (is (.contains fragment html))
-      (is (.startsWith fragment "event: datastar-patch-elements\n")))))
+      (is (.startsWith fragment "event: datastar-patch-elements\n"))))
+
+  (testing "HTML with 2 newlines emits multiple data lines"
+    (let [html     "<pre>code\nwith\nnewline</pre>"
+          fragment (render/format-datastar-fragment html)]
+      (is (= 3 (count (re-seq #"data: elements" fragment))))
+      (is (.contains fragment "<pre>code"))
+      (is (.contains fragment "with"))
+      (is (.contains fragment "newline</pre>"))
+      (is (.endsWith fragment "\n\n"))))
+
+  (testing "HTML with double newlines emits multiple data lines"
+    (let [html     "<textarea>line1\n\nline2</textarea>"
+          fragment (render/format-datastar-fragment html)]
+      (is (= 3 (count (re-seq #"data: elements" fragment))))
+      (is (.contains fragment "<textarea>line1"))
+      (is (.contains fragment "line2</textarea>"))
+      (is (.endsWith fragment "\n\n")))))
 
 (deftest test-render-tab
   (testing "render-tab returns nil when no render-fn is registered"
