perf(utils): Read source context from linecache before the module loader

sentry_sdk/utils.py

@@ -482,21 +482,19 @@ def get_lines_from_file(
     if has_data_collection_enabled(client_options):
         context_lines = client_options["data_collection"]["frame_context_lines"]
 
-    source = None
-    if loader is not None and hasattr(loader, "get_source"):
+    try:
+        source = linecache.getlines(filename)
+    except (OSError, IOError):
+        source = []
+
+    if not source and loader is not None and hasattr(loader, "get_source"):
         try:
             source_str: "Optional[str]" = loader.get_source(module)
         except (ImportError, IOError):
             source_str = None
         if source_str is not None:
             source = source_str.splitlines()
 
-    if source is None:
-        try:
-            source = linecache.getlines(filename)
-        except (OSError, IOError):
-            return [], None, []
-
     if not source:
         return [], None, []
 

tests/test_utils.py

@@ -1330,6 +1330,47 @@ def fake_getlines(filename):
         assert result == expected_result
 
 
+def test_get_lines_from_file_prefers_linecache(tmp_path):
+    filename = tmp_path / "source.py"
+    filename.write_text("before\ntarget\nafter\n")
+    calls = []
+
+    class Loader:
+        def get_source(self, module):
+            calls.append(module)
+            return "loader source\n"
+
+    result = get_lines_from_file(
+        str(filename), 1, loader=Loader(), module="source_module"
+    )
+
+    assert result == (["before"], "target", ["after"])
+    assert calls == []
+
+
+def test_get_lines_from_file_falls_back_to_loader(tmp_path):
+    filename = tmp_path / "missing.py"
+    calls = []
+
+    class Loader:
+        def get_source(self, module):
+            calls.append(module)
+            return "before\none\ntarget\nafter\n"
+
+    result = get_lines_from_file(
+        str(filename), 2, loader=Loader(), module="fallback_module"
+    )
+
+    assert result == (["before", "one"], "target", ["after"])
+    assert calls == ["fallback_module"]
+
+
+def test_get_lines_from_file_without_source(tmp_path):
+    result = get_lines_from_file(str(tmp_path / "missing.py"), 0)
+
+    assert result == ([], None, [])
+
+
 @pytest.mark.parametrize(
     "options,expected_context_lines",
     [