gh-142927: Fix heatmap caller navigation for interior lines

The heatmap was only showing caller buttons on function definition
lines, not on interior lines within a function. This happened because
callers were recorded against the function definition line but looked
up by the current line number when building navigation buttons.

Added a line_to_function mapping to track which function each sampled
line belongs to. When building navigation buttons, callers are now
looked up via the function definition line so all lines in a function
show who calls that function. Callees remain line-specific since only
actual call sites should show what they call. Added tests covering
root, middle, and leaf frame behavior in call stacks.

Author: pablogsal

Lib/profiling/sampling/heatmap_collector.py

@@ -472,6 +472,10 @@ def __init__(self, *args, **kwargs):
         self.callers_graph = collections.defaultdict(set)
         self.function_definitions = {}
 
+        # Map each sampled line to its function for proper caller lookup
+        # (filename, lineno) -> funcname
+        self.line_to_function = {}
+
         # Edge counting for call path analysis
         self.edge_samples = collections.Counter()
 
@@ -596,6 +600,10 @@ def _record_line_sample(self, filename, lineno, funcname, is_leaf=False,
         if funcname and (filename, funcname) not in self.function_definitions:
             self.function_definitions[(filename, funcname)] = lineno
 
+        # Map this line to its function for caller/callee navigation
+        if funcname:
+            self.line_to_function[(filename, lineno)] = funcname
+
     def _record_bytecode_sample(self, filename, lineno, opcode,
                                 end_lineno=None, col_offset=None, end_col_offset=None,
                                 weight=1):
@@ -1150,13 +1158,37 @@ def _format_specialization_color(self, spec_pct: int) -> str:
         return f"rgba({r}, {g}, {b}, {alpha})"
 
     def _build_navigation_buttons(self, filename: str, line_num: int) -> str:
-        """Build navigation buttons for callers/callees."""
+        """Build navigation buttons for callers/callees.
+
+        - Callers: All lines in a function show who calls this function
+        - Callees: Only actual call site lines show what they call
+        """
         line_key = (filename, line_num)
-        caller_list = self._deduplicate_by_function(self.callers_graph.get(line_key, set()))
+
+        # Find which function this line belongs to
+        funcname = self.line_to_function.get(line_key)
+
+        # Get callers: look up by function definition line, not current line
+        # This ensures all lines in a function show who calls this function
+        if funcname:
+            func_def_line = self.function_definitions.get((filename, funcname), line_num)
+            func_def_key = (filename, func_def_line)
+            caller_list = self._deduplicate_by_function(self.callers_graph.get(func_def_key, set()))
+        else:
+            caller_list = self._deduplicate_by_function(self.callers_graph.get(line_key, set()))
+
+        # Get callees: only show for actual call site lines (not every line in function)
         callee_list = self._deduplicate_by_function(self.call_graph.get(line_key, set()))
 
         # Get edge counts for each caller/callee
-        callers_with_counts = self._get_edge_counts(line_key, caller_list, is_caller=True)
+        # For callers, use the function definition key for edge lookup
+        if funcname:
+            func_def_line = self.function_definitions.get((filename, funcname), line_num)
+            caller_edge_key = (filename, func_def_line)
+        else:
+            caller_edge_key = line_key
+        callers_with_counts = self._get_edge_counts(caller_edge_key, caller_list, is_caller=True)
+        # For callees, use the actual line key since that's where the call happens
         callees_with_counts = self._get_edge_counts(line_key, callee_list, is_caller=False)
 
         # Build navigation buttons with counts

Lib/test/test_profiling/test_heatmap.py

@@ -367,6 +367,96 @@ def test_process_frames_with_file_samples_dict(self):
         self.assertEqual(collector.file_samples['test.py'][10], 1)
 
 
+def frame(filename, line, func):
+    """Create a frame tuple: (filename, location, funcname, opcode)."""
+    return (filename, (line, line, -1, -1), func, None)
+
+
+class TestHeatmapCollectorNavigationButtons(unittest.TestCase):
+    """Test navigation button behavior for caller/callee relationships.
+
+    For every call stack:
+    - Root frames (entry points): only DOWN arrow (callees)
+    - Middle frames: both UP and DOWN arrows
+    - Leaf frames: only UP arrow (callers)
+    """
+
+    def collect(self, *stacks):
+        """Create collector and process frame stacks."""
+        collector = HeatmapCollector(sample_interval_usec=100)
+        for stack in stacks:
+            collector.process_frames(stack, thread_id=1)
+        return collector
+
+    def test_deep_call_stack_relationships(self):
+        """Test root/middle/leaf navigation in a 5-level call stack."""
+        # Stack: root -> A -> B -> C -> leaf
+        stack = [
+            frame('leaf.py', 5, 'leaf'),
+            frame('c.py', 10, 'func_c'),
+            frame('b.py', 15, 'func_b'),
+            frame('a.py', 20, 'func_a'),
+            frame('root.py', 25, 'root'),
+        ]
+        c = self.collect(stack)
+
+        # Root: only callees (no one calls it)
+        self.assertIn(('root.py', 25), c.call_graph)
+        self.assertNotIn(('root.py', 25), c.callers_graph)
+
+        # Middle frames: both callers and callees
+        for key in [('a.py', 20), ('b.py', 15), ('c.py', 10)]:
+            self.assertIn(key, c.call_graph)
+            self.assertIn(key, c.callers_graph)
+
+        # Leaf: only callers (doesn't call anyone)
+        self.assertNotIn(('leaf.py', 5), c.call_graph)
+        self.assertIn(('leaf.py', 5), c.callers_graph)
+
+    def test_all_lines_in_function_see_callers(self):
+        """Test that interior lines map to their function for caller lookup."""
+        # Same function sampled at different lines (12, 15, 10)
+        c = self.collect(
+            [frame('mod.py', 12, 'my_func'), frame('caller.py', 100, 'caller')],
+            [frame('mod.py', 15, 'my_func'), frame('caller.py', 100, 'caller')],
+            [frame('mod.py', 10, 'my_func'), frame('caller.py', 100, 'caller')],
+        )
+
+        # All lines should map to same function
+        for line in [10, 12, 15]:
+            self.assertEqual(c.line_to_function[('mod.py', line)], 'my_func')
+
+        # Function definition line should have callers
+        func_def = c.function_definitions[('mod.py', 'my_func')]
+        self.assertIn(('mod.py', func_def), c.callers_graph)
+
+    def test_multiple_callers_and_callees(self):
+        """Test multiple callers/callees are recorded correctly."""
+        # Two callers -> target, and caller -> two callees
+        c = self.collect(
+            [frame('target.py', 10, 'target'), frame('caller1.py', 20, 'c1')],
+            [frame('target.py', 10, 'target'), frame('caller2.py', 30, 'c2')],
+            [frame('callee1.py', 5, 'f1'), frame('dispatcher.py', 40, 'dispatch')],
+            [frame('callee2.py', 6, 'f2'), frame('dispatcher.py', 40, 'dispatch')],
+        )
+
+        # Target has 2 callers
+        callers = c.callers_graph[('target.py', 10)]
+        self.assertEqual({x[0] for x in callers}, {'caller1.py', 'caller2.py'})
+
+        # Dispatcher has 2 callees
+        callees = c.call_graph[('dispatcher.py', 40)]
+        self.assertEqual({x[0] for x in callees}, {'callee1.py', 'callee2.py'})
+
+    def test_edge_samples_counted(self):
+        """Test that repeated calls accumulate edge counts."""
+        stack = [frame('callee.py', 10, 'callee'), frame('caller.py', 20, 'caller')]
+        c = self.collect(stack, stack, stack)
+
+        edge_key = (('caller.py', 20), ('callee.py', 10))
+        self.assertEqual(c.edge_samples[edge_key], 3)
+
+
 class TestHeatmapCollectorExport(unittest.TestCase):
     """Test HeatmapCollector.export() method."""