fix(ci_visibility): fix corner case in Python 3.9 code coverage instrumentation (#15697)

## Description

In Python 3.9, `dis.findlinestarts()` (used to determine where to insert
instrumentation calls) can return imprecise line number values in some
corner cases. This PR ensures that we do not explode in such cases (even
though the line number information may not be 100% precise).

## Testing

Unit tests.

## Risks

None.

## Additional Notes

Perhaps a better solution would be not to rely on `dis.findlinestarts()`
and implement our own routine for identifying line starts in Python 3.9.
But I don't think it's worth doing given that Python 3.9 is itself past
its end of life.

(cherry picked from commit fbc2b78)

Author: vitor-de-araujo

ddtrace/internal/coverage/instrumentation_py3_9.py

@@ -145,7 +145,7 @@ def update_location_data(
                 new_data.append(line_delta)
 
                 # Also add the current trap size to the accumulated offset
-                accumulated_new_offset = trap_map[current_orig_offset] << 1
+                accumulated_new_offset = trap_map.get(current_orig_offset, 0) << 1
                 current_new_offset += accumulated_new_offset
 
         return bytes(new_data)

pyproject.toml

@@ -145,6 +145,7 @@ exclude = [
     "tests/contrib/grpc/hello_pb2.py",
     "tests/contrib/django_celery/app/*",
     "tests/contrib/protobuf/schemas/**/*.py",
+    "tests/coverage/included_path/py39_line_numbers.py",
     "tests/appsec/iast/fixtures/ast/str/non_utf8_content.py",
     "tests/appsec/iast/fixtures/aspects/str/non_utf8_content.py",
     "tests/lib-injection/dd-lib-python-init-test-protobuf-old/addressbook_pb2.py"

releasenotes/notes/ci_visibility-fix-coverage-py39-line-numbers-f93fe457098505cb.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    CI Visibility: This fix resolves an issue where code coverage instrumentation in Python 3.9 would raise an exception
+    while handling line numbers in some corner cases.

tests/coverage/included_path/py39_line_numbers.py

@@ -0,0 +1,19 @@
+x = {}
+
+def g(*args, **kwargs):
+    pass
+
+
+def precise_line_numbers():
+    g(None,
+      **x)
+
+
+def imprecise_line_numbers():
+    g(
+        None, **x)
+
+
+def call_all_functions():
+    precise_line_numbers()
+    imprecise_line_numbers()

tests/coverage/test_coverage_py39_line_numebers.py

@@ -0,0 +1,52 @@
+import pytest
+
+
+@pytest.mark.subprocess
+def test_coverage_py39_line_numbers():
+    """
+    In Python 3.9, `dis.findlinestarts()` (used to determine where to insert instrumentation calls) can return
+    imprecise line number values in some corner cases. This test ensures that we do not explode in such cases (even
+    though the line number information may not be 100% precise).
+    """
+    import os
+    from pathlib import Path
+
+    from ddtrace.internal.coverage.code import ModuleCodeCollector
+    from ddtrace.internal.coverage.installer import install
+    from tests.coverage.utils import _get_relpath_dict
+
+    cwd_path = os.getcwd()
+    include_path = Path(cwd_path + "/tests/coverage/included_path/")
+
+    install(include_paths=[include_path], collect_import_time_coverage=True)
+
+    from tests.coverage.included_path.py39_line_numbers import call_all_functions
+
+    ModuleCodeCollector.start_coverage()
+    call_all_functions()
+    ModuleCodeCollector.stop_coverage()
+
+    executable = _get_relpath_dict(cwd_path, ModuleCodeCollector._instance.lines)
+    covered = _get_relpath_dict(cwd_path, ModuleCodeCollector._instance._get_covered_lines(include_imported=False))
+    covered_with_imports = _get_relpath_dict(
+        cwd_path, ModuleCodeCollector._instance._get_covered_lines(include_imported=True)
+    )
+
+    expected_executable = {
+        "tests/coverage/included_path/py39_line_numbers.py": {1, 3, 4, 7, 8, 9, 12, 13, 14, 17, 18, 19},
+    }
+    expected_covered = {
+        "tests/coverage/included_path/py39_line_numbers.py": {4, 8, 9, 13, 14, 18, 19},
+    }
+    expected_covered_with_imports = {
+        "tests/coverage/included_path/py39_line_numbers.py": {1, 3, 4, 7, 8, 9, 12, 13, 14, 17, 18, 19},
+    }
+
+    assert executable == expected_executable, (
+        f"Executable lines mismatch: expected={expected_executable} vs actual={executable}"
+    )
+    assert covered == expected_covered, f"Covered lines mismatch: expected={expected_covered} vs actual={covered}"
+    assert covered_with_imports == expected_covered_with_imports, (
+        f"Covered lines with imports mismatch: expected={expected_covered_with_imports} "
+        f"vs actual={covered_with_imports}"
+    )