SEGFAULT when running a given coroutine #72968
Comments
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Hi, I stumbled upon a SEGFAULT while trying Python 3.6.0 on a project using The bug appears between 3.6.0a4 (most recent version tested not affected) and I also produced a traceback using gdb (see bellow). The segfault happens around the "await" in the body of Cursor._read_data(), Cheers, Case (also attached as test.py) : loop = asyncio.get_event_loop()
class Connection:
def read_until(self, *args, **kwargs):
return self
async def all(self):
return b"\n"
class Cursor:
def __init__(self):
self._connection = Connection()
self._max_bytes = 100
self._data = bytearray()
async def _read_data(self):
# XXX segfault there, if I change anything in the code, it works...
while True:
data = await self._connection.read_until(
b'\n', max_bytes=self._max_bytes).all()
self._max_bytes -= len(data)
if data == b'\n':
break
self._data.extend(data)
async def main():
await Cursor()._read_data()
loop.run_until_complete(main())Traceback extract (with Python3.6.0b4, --with-pydebug on Linux): Program received signal SIGSEGV, Segmentation fault. |
Martiusweb
added
3.7
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I'm marking this as a release blocker since it is a (new) segfault. |
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FWIW, hg bisect finds: changeset 103444:a77756e480c2: bad Also, FWIW, the successful runs prior to this revision report: base_events.py:481: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=False> |
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Wow, Martin, this is a very interesting one. Thanks so much for tracking this down and reducing the test case. So far this doesn't seem to be related to async/await: the interpreter stack seems to enter some strange state under some conditions, and async/await just happens to be the thing that explodes. To those trying to debug this: you don't need asyncio, just replace |
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Oh wow, the bug is tricky. _PyGen_yf() checks if the next instruction is YIELD_FROM using code[f_lasti+2]. The problem is that WORDCODE kept f_lasti == -1 for a frame not executed yet: f_lasti+2 is 1 in this case. Problem: code[1] is not an operation code, but the argment. Python 3.6 bytecode now uses the "wordcode" format: 16-bit units of (operation, argument). The obvious and simple fix is to add a special case for f_lasti==-1 in _PyGen_yf(): see attached patch. pygen_yf.patch fixes the crash. -- Another much larger change would be to change f_lasti to -2... In the rewiew of the huge WORDCODE patch, if I recall correctly, I asked Demur to use -1 for backward compatibility. Or maybe I asked to kept the backward compatibility at the Python level using a getter converting -2 to -1... I don't recall correctly. See http://bugs.python.org/issue26647 for wordcode. |
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It would be nice if Demur Rumed and/or Serhiy Storchaka can review pygen_yf.patch since Demur wrote the wordcode patch and Serhiy reviewed the wordcode patch. |
Attached lasti.patch implements this idea. I consider that it makes the C code simpler because getting the next instruction (f_lasti + 2) doesn't require a special case anymore. My patch keeps f_lasti == -1 at the Python level for backward compatibility. lasti.patch is only a backward incompatible change at the C level. -- Between pygen_yf.patch and lasti.patch, I prefer lasti.patch even if 3.6 is at its last beta version before the final version. I prefer to fix the C API. Later it will be much harder to fix it. -- I read again the wordcode issue bpo-26647: I wrote on the review of wpy7.patch: "The overall change LGTM, but I'm no more 100% sure that starting f_lasti=-1 is safe." I wrote: "IMHO it's ok to break the C API, but I would prefer to keep the backward compatibility for the Python API (replace any negative number with -1 for the Python API)." Serhiy: "I think we should make yet few related changes: (...) * Change f_lasti, tb_lasti etc to count code units instead of bytes." |
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I had considered this, for some reason I didn't realize code[1] could be equal to YIELD_FROM, felt it'd always be false f_lasti being -2 has always been my preference, lasti.patch lgtm |
How about we commit pygen_yf.patch to 3.6 and whatever else in 3.7? lasti.patch is too invasive for 3.6 at this point, -1 for it in 3.6. |
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I prefer pygen_yf.patch (with addressing Yury's suggestions). It is much simpler, and in any case I want to change f_lasti to count words rather than bytes (bpo-27129). It would be again -1 at the start. |
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Updated patch. I added a NEWS entry and the two assertions from lasti.patch. These assertions are basic sanity checks, not directly related to this issue, they shouldn't harm. A started frame must never go back to the not-started state (f_lasti < 0). Yury: "How about we commit pygen_yf.patch to 3.6 and whatever else in 3.7?" Wordcode is already a massive change in our bytecode, I would prefer to not touch f_lasti just as a late update of WORDCODE in 3.7. I'm ok to keep f_lasti == -1 for frames not started yet. Serhiy: "I prefer pygen_yf.patch (with addressing Yury's suggestions)." Ok, fine. I'm also more confident in smaller changes when we are very close to the final release! So let's forget lasti.patch ;-) (Sorry Demur!) -- FYI attached test.py reproduces the bug because Cursor._read_data() starts with the instruction "SETUP_LOOP 72" and 72 is the code of the operation YIELD_FROM :-) To reproduce the bug, you must have a code object where the second byte is 72. It's not easy to control the generated bytecode from the Python code, so I decided to not write an unit test for this bug. |
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pygen_yf-2.patch LGTM. Agreed about tests. |
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LGTM |
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New changeset 303cedfb9e7a by Victor Stinner in branch '3.6': |
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Thanks Martin Richard to continue to find (and report!) crazy corner cases ;-) |
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Thank you all for fixing this so quickly, it's been done amazingly fast! |
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Martin Richard: "Thank you all for fixing this so quickly, it's been done amazingly fast!" Even if I didn't write the commit a77756e480c2 which introduced the regression, I pushed it and so I felt responsible. It's a good motivation to fix it quickly. So nobody notice :-D |
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(Victor also merged this into default branch for 3.7.0 in 6453ff3328b8) |
Misc/NEWSso that it is managed by towncrier #552Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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