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gh-151022: Fix remote debugging linetable reads #151036

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gh-151022: Fix remote debugging linetable reads #151036

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35 changes: 35 additions & 0 deletions Lib/test/test_external_inspection.py
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Original file line number Diff line number Diff line change
Expand Up @@ -438,6 +438,41 @@ def _extract_coroutine_stacks_lineno_only(self, stack_trace):
# ============================================================================


class TestSelfStackTrace(RemoteInspectionTestBase):

@maurycy maurycy Jun 9, 2026

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@goutamadwant I think there's a missing @requires_remote_subprocess_debugging() decorator :-)

@skip_if_not_supported
@unittest.skipIf(
sys.platform == "linux" and not PROCESS_VM_READV_SUPPORTED,
"Test only runs on Linux with process_vm_readv support",
)
def test_self_trace_with_large_linetable(self):
script = textwrap.dedent("""\
import os
import _remote_debugging

assignments = "\\n".join(
f"value_{i} = {i}" for i in range(1000)
)
source = (
f"{assignments}\\n"
"_remote_debugging.RemoteUnwinder(os.getpid()).get_stack_trace()\\n"
)
code = compile(source, "large_linetable.py", "exec")
assert len(code.co_linetable) > 4096, len(code.co_linetable)
exec(code)
""")

result = subprocess.run(
[sys.executable, "-c", script],
capture_output=True,
text=True,
timeout=SHORT_TIMEOUT,
)
self.assertEqual(
result.returncode, 0,
f"stdout: {result.stdout}\nstderr: {result.stderr}"
Comment on lines +470 to +472

@maurycy maurycy Jun 9, 2026

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@goutamadwant It would be interesting to ensure that the returned linetable is also correct, not -1 :-)

)


@requires_remote_subprocess_debugging()
class TestGetStackTrace(RemoteInspectionTestBase):
@skip_if_not_supported
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2026-06-06-15-04-37.gh-issue-151022.1Aw8Tk.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix ``_remote_debugging`` stack traces for code objects with large line
tables.
7 changes: 5 additions & 2 deletions Modules/_remote_debugging/code_objects.c
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,9 @@

#include "_remote_debugging.h"

#define MAX_LINETABLE_SIZE (1024 * 1024)

@maurycy maurycy Jun 9, 2026

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@goutamadwant Why exactly 1024 * 1024? :-)

I know that's an arbitrary number but my hunch would be to try to check what sizes are we seeing in the wild, or at least in the stdlib. There's linetable_dist.py script in the original issue that should be helpful!

#define MAX_LINETABLE_ENTRIES 65536

@maurycy maurycy Jun 9, 2026 •
edited
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@goutamadwant

I did not check it but my guess would be that there's much more head room in the MAX_LINETABLE_SIZE now, than in MAX_LINETABLE_ENTRIES. If I'm reading the code right, it fallbacks to -1:

cpython/Modules/_remote_debugging/code_objects.c

Lines 495 to 500 in 29a920e

if (!ok) {
info.lineno = -1;
info.end_lineno = -1;
info.column = -1;
info.end_column = -1;
}

@pablogsal

To be honest, I'd get rid of MAX_LINETABLE_ENTRIES and entry_count. There will be always some inherit whacka-a-mole between the cap and the entries.

Anotehr solution would be along the lines of ob_size (bytes_object.ob_size?) and code_units (in CachedCodeMetadata?) but I think it's beyond the scope.


/* ============================================================================
* TLBC CACHING FUNCTIONS (Py_GIL_DISABLED only)
* ============================================================================ */
Expand Down Expand Up @@ -186,7 +189,6 @@ parse_linetable(const uintptr_t addrq, const char* linetable, Py_ssize_t linetab
int computed_line = firstlineno; // Running accumulator, separate from output
int val; // Temporary for varint results
uint8_t byte; // Temporary for byte reads
const size_t MAX_LINETABLE_ENTRIES = 65536;
size_t entry_count = 0;

while (ptr < end && *ptr != '\0' && entry_count < MAX_LINETABLE_ENTRIES) {
Expand Down Expand Up @@ -387,7 +389,8 @@ parse_code_object(RemoteUnwinderObject *unwinder,
}

linetable = read_py_bytes(unwinder,
GET_MEMBER(uintptr_t, code_object, unwinder->debug_offsets.code_object.linetable), 4096);
GET_MEMBER(uintptr_t, code_object, unwinder->debug_offsets.code_object.linetable),
MAX_LINETABLE_SIZE);
if (!linetable) {
set_exception_cause(unwinder, PyExc_RuntimeError, "Failed to read linetable from code object");
goto error;
Expand Down
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