Jiri Daněk created DISPATCH-1962:
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Summary: Make LSan suppressions more targetted and specific
(within reason)
Key: DISPATCH-1962
URL: https://issues.apache.org/jira/browse/DISPATCH-1962
Project: Qpid Dispatch
Issue Type: Test
Affects Versions: 1.15.0
Reporter: Jiri Daněk
Attachments: dispatch_leaking_agent.png
There are some unfortunate suppressions in {{tests/lsan.supp}} at this moment:
{code}
leak:*libwebsockets*
{code}
This is way too broad. It suppresses leaks in Dispatch files, since it matches
e.g. {{src/http-libwebsockets.c}}.
The stars at the beginning and end are actually assumed implicitly. If you do
not want substring match, you have to do ^foo$. LSan suppression format is
simplistic, very unlike Valgrind's.
{code}
leak:run_unit_tests.c
{code}
Same thing, any leaks revealed by running unit_tests get suppressed. This
suppression suppresses all leak traces that include run_unit_tests.c anywhere
in the stack. What't the point of running such tests under leak detector, then?
{code}
leak:run_unit_tests.c
leak:^libqpid-proton.so$
{code}
Same thing. The patterns suppress all leaks that include Python (or Proton)
anywhere in the stacktrace. That means there are huge blind spots where
dispatch leaks can hide. This is a weakness of the lsan.supp syntax (Valgrind
suppressions can be much more targeted and discerning).
h3. Python leaks
Leaks are known and there is ongoing effort to fight them:
https://bugs.python.org/issue1635741 (https://bugs.python.org/issue25302) and
https://www.python.org/dev/peps/pep-3121
Here's valgrind suppression file from somebody who actually investigated the
Python leaks and identified the harmless ones:
https://github.com/libgit2/pygit2/blob/master/misc/valgrind-python.supp
One example of a hidden leak in dispatch, which is revealed by making Python
suppressions more targetted:
{code|
9: Direct leak of 56 byte(s) in 1 object(s) allocated from:
9: #0 0x7f78a3606e8f in __interceptor_malloc
(/nix/store/g40sl3zh3nv52vj0mrl4iki5iphh5ika-gcc-10.2.0-lib/lib/libasan.so.6+0xace8f)
9: #1 0x7f78a2d64afb in qd_malloc ../include/qpid/dispatch/ctools.h:229
9: #2 0x7f78a2d657da in qdr_core_subscribe
../src/router_core/route_tables.c:149
9: #3 0x7f78a2c83072 in IoAdapter_init ../src/python_embedded.c:711
9: #4 0x7f78a2353a6c in type_call
(/nix/store/r85nxfnwiv45nbmf5yb60jj8ajim4m7w-python3-3.8.5/lib/libpython3.8.so.1.0+0x165a6c)
{code}
The problem is in
{code}
class Agent:
...
def activate(self, address):
...
self.io = IoAdapter(self.receive, address, 'L', '0',
TREATMENT_ANYCAST_CLOSEST)
{code}
IoAdapter refers to Agent (through the bound method reference self.receive) and
Agent refers to IoAdapter (through property self.io). Since IoAdapter is
implemented in C and does not implement support for Python's cyclic GC, there
is no way to break the cycle.
Heap dump in attachment. The bound method is at the top of the picture. (Ignore
the Mock objects, I was trying to simplify the picture while not getting
crashes due to too much meddling).
.h3 Random observations
It is possible to build special Debug build of Python, which has tools to
detect leaks, asserts to prevent negative refcounts, etc.
https://pythonextensionpatterns.readthedocs.io/en/latest/debugging/debug_python.html#debug-version-of-python-memory-alloc-label
Use the following to detect python leaks (instead of valgrind)
https://docs.python.org/3/library/tracemalloc.html
Use https://pypi.org/project/objgraph (with graphviz) to view heap object trees.
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