fix(_inject_tooldrawer): fix "bad escape \u"

We were using re.sub() to inject the tooldrawer HTML.  Re.sub treats
backslashes in the replacement string specially.  When the injected
HTML includes a backslash (e.g. in JSON string data), this was
resulting in a "bad escape \u" exception.

lektor/admin/modules/serve.py

@@ -80,7 +80,10 @@ def _inject_tooldrawer(
             tooldrawer_config=dataclasses.asdict(tooldrawer_config),
             tooldrawer_js=url_for("static", filename="tooldrawer.js"),
         ).encode("utf-8")
-        html = re.sub(rb"(?i)(?=</\s*head\s*>|\Z)", tooldrawer_html, html, count=1)
+        match = re.search(rb"(?i)</\s*head\s*>|\Z", html)
+        assert match is not None
+        head_end = match.start()
+        html = html[:head_end] + tooldrawer_html + html[head_end:]
     return html
 
 

tests/admin/test_serve.py

@@ -75,6 +75,10 @@ def test_inject_tooldrawer(html_text, expect_at_tail):
 
 @pytest.mark.usefixtures("dummy_app_context")
 def test_inject_tooldrawer_adds_livereload(dummy_app, make_dummy_artifact):
+    # Exercises a bug: the injected HTML includes backslash escapes within JSON
+    # strings. re.sub(pat, repl, s) doesn't work in that case, as it treats backslashes
+    # in repl specially.  This results in an exception: "re.error: bad escape \u at
+    # position [...]"
     dummy_app.register_blueprint(livereload.bp)
     artifact = make_dummy_artifact(artifact_name="ARTIFACT_NAME")
     config = make_tooldrawer_config("EDIT_URL", artifact)
@@ -83,6 +87,12 @@ def test_inject_tooldrawer_adds_livereload(dummy_app, make_dummy_artifact):
     )
 
 
+@pytest.mark.usefixtures("dummy_app_context")
+def test_inject_tooldrawer_unicode_escapes():
+    config = make_tooldrawer_config("http://example.com/EDIT_URL?path=/foo&alt=de")
+    assert b"TOOLDRAWER_CONFIG" in serve._inject_tooldrawer(b"", config)
+
+
 @pytest.fixture
 def make_dummy_artifact(tmp_path):
     def make_dummy_artifact(