gh-149430: Fix edge-cases in profiling.sampling outputs (GH-149431)

The line highlights on the heatmap are driven by the URL hash and the
`:target` selector. When clicking a caller/callee link for the line that
was already selected, the hash doesn't change, so the browser keeps the
existing target state and doesn't restart the animation. Due to this the
highlight only works the first time.

With this fix, line navigation goes through JavaScript. If the target
URL already points to the current location, the highlight is replayed by
clearing the animation, forcing style recalculation, and restoring it.

The `baseline_self` variable isn't initialized for structural elided
roots. This variable is accessed later unconditionally and leads to a
crash.

The child process ends up being invoked with `--diff_flamegraph` instead
of the correct argument.
(cherry picked from commit 9587726)

Co-authored-by: László Kiss Kollár <kiss.kollar.laszlo@gmail.com>

Author: lkollar

Lib/profiling/sampling/_heatmap_assets/heatmap.js

@@ -84,7 +84,7 @@ function showNavigationMenu(button, items, title) {
 
         item.appendChild(funcDiv);
         item.appendChild(createElement('div', 'callee-menu-file', linkData.file));
-        item.addEventListener('click', () => window.location.href = linkData.link);
+        item.addEventListener('click', () => navigateToLine(linkData.link));
         menu.appendChild(item);
     });
 
@@ -105,7 +105,7 @@ function handleNavigationClick(button, e) {
 
     const navData = button.getAttribute('data-nav');
     if (navData) {
-        window.location.href = JSON.parse(navData).link;
+        navigateToLine(JSON.parse(navData).link);
         return;
     }
 
@@ -117,11 +117,29 @@ function handleNavigationClick(button, e) {
     }
 }
 
+function restartLineHighlight(target) {
+    target.style.animation = 'none';
+    // Force style recalculation so restoring the animation restarts it.
+    void target.offsetWidth;
+    target.style.animation = '';
+}
+
+function navigateToLine(link) {
+    const url = new URL(link, window.location.href);
+
+    if (url.href === window.location.href) {
+        scrollToTargetLine();
+    } else {
+        window.location.href = link;
+    }
+}
+
 function scrollToTargetLine() {
     if (!window.location.hash) return;
     const target = document.querySelector(window.location.hash);
     if (target) {
         target.scrollIntoView({ behavior: 'smooth', block: 'start' });
+        restartLineHighlight(target);
     }
 }
 

Lib/profiling/sampling/cli.py

@@ -167,7 +167,9 @@ def _build_child_profiler_args(args):
         child_args.extend(["--mode", mode])
 
     # Format options (skip pstats as it's the default)
-    if args.format != "pstats":
+    if args.format == "diff_flamegraph":
+        child_args.extend(["--diff-flamegraph", args.diff_baseline])
+    elif args.format != "pstats":
         child_args.append(f"--{args.format}")
 
     return child_args

Lib/profiling/sampling/stack_collector.py

@@ -698,6 +698,8 @@ def _add_elided_metadata(self, node, baseline_stats, scale, path):
         func_key = self._extract_func_key(node, self._baseline_collector._string_table)
         current_path = path + (func_key,) if func_key else path
 
+        baseline_self = 0
+        baseline_total = 0
         if func_key and current_path in baseline_stats:
             baseline_data = baseline_stats[current_path]
             baseline_self = baseline_data["self"] * scale

Lib/test/test_profiling/test_sampling_profiler/test_children.py

@@ -109,6 +109,39 @@ def _wait_for_process_ready(proc, timeout):
     return proc.poll() is None
 
 
+@unittest.skipIf(
+    _build_child_profiler_args is None,
+    "profiling.sampling.cli unavailable",
+)
+class TestChildProfilerArgBuilder(unittest.TestCase):
+    """Tests for child profiler CLI argument construction."""
+
+    def test_build_child_profiler_args_diff_flamegraph(self):
+        """Test child args use the real --diff-flamegraph flag."""
+        args = argparse.Namespace(
+            sample_interval_usec=1000,
+            duration=None,
+            all_threads=False,
+            realtime_stats=False,
+            native=False,
+            gc=True,
+            opcodes=False,
+            async_aware=False,
+            mode="wall",
+            format="diff_flamegraph",
+            diff_baseline="baseline.bin",
+        )
+
+        child_args = _build_child_profiler_args(args)
+
+        self.assertIn("--diff-flamegraph", child_args)
+        self.assertNotIn("--diff_flamegraph", child_args)
+
+        flag_index = child_args.index("--diff-flamegraph")
+        self.assertGreater(len(child_args), flag_index + 1)
+        self.assertEqual(child_args[flag_index + 1], "baseline.bin")
+
+
 @requires_remote_subprocess_debugging()
 class TestGetChildPids(unittest.TestCase):
     """Tests for the get_child_pids function."""

Lib/test/test_profiling/test_sampling_profiler/test_collectors.py

@@ -18,6 +18,7 @@
     )
     from profiling.sampling.jsonl_collector import JsonlCollector
     from profiling.sampling.gecko_collector import GeckoCollector
+    from profiling.sampling.heatmap_collector import _TemplateLoader
     from profiling.sampling.collector import extract_lineno, normalize_location
     from profiling.sampling.opcode_utils import get_opcode_info, format_opcode
     from profiling.sampling.constants import (
@@ -82,6 +83,18 @@ def test_mock_frame_info_with_empty_and_unicode_values(self):
         self.assertEqual(frame.location.lineno, 999999)
         self.assertEqual(frame.funcname, long_funcname)
 
+    def test_heatmap_navigation_restarts_line_highlight(self):
+        """Test heatmap navigation can replay target line highlights."""
+        loader = _TemplateLoader()
+
+        self.assertIn(".code-line:target", loader.file_css)
+        self.assertIn("function restartLineHighlight(target)", loader.file_js)
+        self.assertIn("target.style.animation = 'none'", loader.file_js)
+        self.assertIn("void target.offsetWidth", loader.file_js)
+        self.assertIn("url.href === window.location.href", loader.file_js)
+        self.assertIn("navigateToLine(JSON.parse(navData).link)", loader.file_js)
+        self.assertIn("navigateToLine(linkData.link)", loader.file_js)
+
     def test_pstats_collector_with_extreme_intervals_and_empty_data(self):
         """Test PstatsCollector handles zero/large intervals, empty frames, None thread IDs, and duplicate frames."""
         # Test with zero interval
@@ -1403,6 +1416,39 @@ def test_diff_flamegraph_elided_stacks(self):
         self.assertGreater(child["baseline"], 0)
         self.assertAlmostEqual(child["diff"], -child["baseline"])
 
+    def test_diff_flamegraph_elided_top_level_root(self):
+        """Elided top-level roots do not crash metadata generation."""
+        baseline_frames_1 = [
+            MockInterpreterInfo(0, [
+                MockThreadInfo(1, [
+                    MockFrameInfo("file.py", 10, "kept_leaf"),
+                    MockFrameInfo("file.py", 20, "kept_root"),
+                ])
+            ])
+        ]
+        baseline_frames_2 = [
+            MockInterpreterInfo(0, [
+                MockThreadInfo(1, [
+                    MockFrameInfo("file.py", 30, "old_leaf"),
+                    MockFrameInfo("file.py", 40, "old_root"),
+                ])
+            ])
+        ]
+
+        diff = make_diff_collector_with_mock_baseline([
+            baseline_frames_1,
+            baseline_frames_2,
+        ])
+        diff.collect(baseline_frames_1)
+
+        data = diff._convert_to_flamegraph_format()
+        elided = data["stats"]["elided_flamegraph"]
+        elided_strings = elided.get("strings", [])
+        children = elided.get("children", [])
+
+        self.assertEqual(len(children), 1)
+        self.assertIn("old_root", resolve_name(children[0], elided_strings))
+
     def test_diff_flamegraph_function_matched_despite_line_change(self):
         """Functions match by (filename, funcname), ignoring lineno."""
         baseline_frames = [