gh-158121: Preserve instruction events when line monitoring is disabled

Lib/test/test_free_threading/test_monitoring.py

@@ -1,6 +1,7 @@
 """Tests monitoring, sys.settrace, and sys.setprofile in a multi-threaded
 environment to verify things are thread-safe in a free-threaded build"""
 
+import dis
 import sys
 import threading
 import time
@@ -251,6 +252,62 @@ def append(self, trace):
 
 @threading_helper.requires_working_threading()
 class MonitoringMisc(MonitoringTestMixin, TestCase):
+    def test_disable_line_keeps_instruction_events(self):
+        def func(x):
+            a = x + 1
+            b = a * 2
+            return b
+
+        code = func.__code__
+        expected = [instr.offset for instr in dis.get_instructions(func)
+                    if instr.opname != "RESUME"]
+        # Two threads, each with two lists of instruction offsets (one per call).
+        # Check that disabling LINE preserves INSTRUCTION events in every
+        # thread-local bytecode copy, including on subsequent calls.
+        records = [[[], []] for _ in range(2)]
+        local = threading.local()
+        ready = Barrier(len(records) + 1)
+        start = threading.Event()
+
+        def worker(calls):
+            # Create each thread's bytecode copy before enabling monitoring.
+            func(1)
+            ready.wait()
+            start.wait()
+            for instructions in calls:
+                local.instructions = instructions
+                func(1)
+
+        def instruction(code, offset):
+            local.instructions.append(offset)
+
+        def line(code, lineno):
+            return monitoring.DISABLE
+
+        threads = [Thread(target=worker, args=(calls,)) for calls in records]
+        try:
+            with threading_helper.start_threads(threads, unlock=start.set):
+                ready.wait()
+                monitoring.register_callback(
+                    self.tool_id, monitoring.events.INSTRUCTION, instruction)
+                monitoring.register_callback(
+                    self.tool_id, monitoring.events.LINE, line)
+                monitoring.set_local_events(
+                    self.tool_id, code,
+                    monitoring.events.INSTRUCTION | monitoring.events.LINE)
+                start.set()
+        finally:
+            monitoring.set_local_events(self.tool_id, code, 0)
+            monitoring.register_callback(
+                self.tool_id, monitoring.events.INSTRUCTION, None)
+            monitoring.register_callback(
+                self.tool_id, monitoring.events.LINE, None)
+            monitoring.restart_events()
+
+        for calls in records:
+            for instructions in calls:
+                self.assertEqual(instructions, expected)
+
     def register_callback(self, barrier):
         barrier.wait()
 

Lib/test/test_monitoring.py

@@ -1192,6 +1192,80 @@ def __call__(self, code, offset):
 class TestLineAndInstructionEvents(CheckEvents):
     maxDiff = None
 
+    def test_disable_line_keeps_instruction_events(self):
+        for local in (False, True):
+            # Use the same tool or different tools for LINE and INSTRUCTION.
+            for line_tool, instr_tool in (
+                (TEST_TOOL, TEST_TOOL),
+                (TEST_TOOL2, TEST_TOOL),
+            ):
+                with self.subTest(local=local, line_tool=line_tool,
+                                  instr_tool=instr_tool):
+                    self.check_disable_line_keeps_instruction_events(
+                        local, line_tool, instr_tool)
+
+    def check_disable_line_keeps_instruction_events(self, local, line_tool,
+                                                   instr_tool):
+        def func(x):
+            a = x + 1
+            # Split the assignment so line events also move backwards from
+            # the expression's line to the assignment's line.
+            b = (
+                a * 2
+            )
+            return b
+
+        code = func.__code__
+        instructions = []
+        lines = []
+        disable = False
+
+        def instruction(code_arg, offset):
+            if code_arg is code:
+                instructions.append(offset)
+
+        def line(code_arg, lineno):
+            if code_arg is code:
+                lines.append(lineno)
+                if disable:
+                    return sys.monitoring.DISABLE
+
+        sys.monitoring.register_callback(instr_tool, E.INSTRUCTION, instruction)
+        sys.monitoring.register_callback(line_tool, E.LINE, line)
+        events = {instr_tool: E.INSTRUCTION}
+        events[line_tool] = events.get(line_tool, 0) | E.LINE
+        try:
+            for tool, mask in events.items():
+                if local:
+                    sys.monitoring.set_local_events(tool, code, mask)
+                else:
+                    sys.monitoring.set_events(tool, mask)
+            func(1)
+            expected_instructions = instructions[:]
+            expected_lines = lines[:]
+            self.assertEqual(expected_instructions, [
+                inst.offset for inst in dis.get_instructions(func)
+                if inst.opname != "RESUME"
+            ])
+            self.assertTrue(expected_lines)
+            disable = True
+            for restart in (False, True):
+                if restart:
+                    sys.monitoring.restart_events()
+                for call in range(2):
+                    instructions.clear()
+                    lines.clear()
+                    func(1)
+                    self.assertEqual(instructions, expected_instructions)
+                    self.assertEqual(lines, expected_lines if call == 0 else [])
+        finally:
+            for tool in events:
+                sys.monitoring.set_local_events(tool, code, 0)
+                sys.monitoring.set_events(tool, 0)
+            sys.monitoring.register_callback(instr_tool, E.INSTRUCTION, None)
+            sys.monitoring.register_callback(line_tool, E.LINE, None)
+            sys.monitoring.restart_events()
+
     def test_simple(self):
 
         def func1():

Misc/NEWS.d/next/Core_and_Builtins/2026-09-29-14-35-00.gh-issue-158121.Kj4nQ8.rst

@@ -0,0 +1,2 @@
+Fix missing :mod:`sys.monitoring` ``INSTRUCTION`` events when a ``LINE``
+callback returns :data:`sys.monitoring.DISABLE`.

Python/instrumentation.c

@@ -715,9 +715,6 @@ de_instrument_line(PyCodeObject *code, _Py_CODEUNIT *bytecode, _PyCoMonitoringDa
     }
     _PyCoLineInstrumentationData *lines = monitoring->lines;
     int original_opcode = _PyCode_GetOriginalOpcode(lines, i);
-    if (original_opcode == INSTRUMENTED_INSTRUCTION) {
-        set_original_opcode(lines, i, monitoring->per_instruction_opcodes[i]);
-    }
     CHECK(original_opcode != 0);
     CHECK(original_opcode == _PyOpcode_Deopt[original_opcode]);
     FT_ATOMIC_STORE_UINT8(instr->op.code, original_opcode);
@@ -882,7 +879,15 @@ remove_line_tools(PyCodeObject * code, int offset, int tools)
         should_de_instrument = ((single_tool & tools) == single_tool);
     }
     if (should_de_instrument) {
+        /* Restore all thread-local bytecodes before updating the shared
+         * original opcode. */
         MODIFY_BYTECODE(code, de_instrument_line, monitoring, offset);
+        if (_PyCode_GetOriginalOpcode(monitoring->lines, offset) ==
+            INSTRUMENTED_INSTRUCTION)
+        {
+            set_original_opcode(monitoring->lines, offset,
+                                monitoring->per_instruction_opcodes[offset]);
+        }
     }
 }
 
@@ -1424,6 +1429,11 @@ _Py_call_instrumentation_line(PyThreadState *tstate, _PyInterpreterFrame* frame,
     uint8_t original_opcode;
 done:
     original_opcode = _PyCode_GetOriginalOpcode(line_data, i);
+    if (instr->op.code == INSTRUMENTED_INSTRUCTION) {
+        /* A LINE callback may have disabled the last LINE tool while
+         * leaving INSTRUCTION monitoring enabled. */
+        original_opcode = INSTRUMENTED_INSTRUCTION;
+    }
     assert(original_opcode != 0);
     assert(original_opcode != INSTRUMENTED_LINE);
     assert(_PyOpcode_Deopt[original_opcode] == original_opcode);