bigfile/py: Dump pybuf referrers if pybuf->ob_refcnt != 1 before dying in loadblk epilogue
Instead of only printing "BUG" let's print information about objects which still refer to pybuf - to help debugging. For example with the following artificial pybuf leak ``` diff --git a/bigfile/tests/test_basic.py b/bigfile/tests/test_basic.py index c737621..f5e057a 100644 --- a/bigfile/tests/test_basic.py +++ b/bigfile/tests/test_basic.py @@ -126,6 +126,7 @@ def test_basic(): # test that python exception state is preserved across pagefaulting def test_pagefault_savestate(): + zzz =  class BadFile(BigFile): def loadblk(self, blk, buf): # simulate some errors in-between to overwrite thread exception @@ -154,6 +155,7 @@ def loadblk(self, blk, buf): # which result in holding additional ref to buf, but loadblk caller # will detect and handle this situation via garbage-collecting # above cycle. + zzz.append(buf) self.loadblk_run = 1 ``` it dies this way: bigfile/_bigfile.c:567 pybigfile_loadblk WARN: pybuf->ob_refcnt != 1 even after GC: pybuf (ob_refcnt=2): <read-write buffer ptr 0x7f08d3e88000, size 2097152 at 0x7f08d48b7070> pybuf referrers: [[<read-write buffer ptr 0x7f08d3e88000, size 2097152 at 0x7f08d48b7070>]] bigfile/_bigfile.c:573 pybigfile_loadblk BUG!
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