When a subtransaction is aborted in plpython because of an SPIexception, it tries to find a matching python exception in a hash`PLy_spi_exceptions` and to make python vm raise it.That hash is generated during module initialization, but the exceptionobjects are not marked to prevent the garbage collector from collectingthem, which can lead to a segmentation fault when processing any SPIexception.PoC to reproduce the issue:```sqlCREATE OR REPLACE FUNCTION server_crashes()RETURNS VOIDAS $$    import gc    gc.collect()    plan = plpy.prepare('SELECT raises_an_spi_exception();', [])    plpy.execute(plan)$$ LANGUAGE plpythonu;CREATE OR REPLACE FUNCTION raises_an_spi_exception()RETURNS VOIDAS $$DECLARE  sql TEXT;BEGIN  sql = format('%I', NULL); -- NullValueNotAllowedEND$$ LANGUAGE plpgsql;SELECT server_crashes(); -- segfault here```Stacktrace of the problem (using PostgreSQL `REL9_5_STABLE` and python`2.7.3-0ubuntu3.8` on a Ubuntu 12.04):``` Program received signal SIGSEGV, Segmentation fault. 0x00007f3155c7670b in PyObject_Call (func=0x7f31b7db2a30, arg=0x7f31b87d17d0, kw=0x0) at ../Objects/abstract.c:2525 2525    ../Objects/abstract.c: No such file or directory. (gdb) bt #0  0x00007f3155c7670b in PyObject_Call (func=0x7f31b7db2a30, arg=0x7f31b87d17d0, kw=0x0) at ../Objects/abstract.c:2525#1  0x00007f3155d81ab1 in PyEval_CallObjectWithKeywords (func=0x7f31b7db2a30, arg=0x7f31b87d17d0, kw=0x0) at ../Python/ceval.c:3890#2  0x00007f3155c766ed in PyObject_CallObject (o=0x7f31b7db2a30, a=0x7f31b87d17d0) at ../Objects/abstract.c:2517#3  0x00007f31561e112b in PLy_spi_exception_set (edata=0x7f31b8743d78, excclass=0x7f31b7db2a30) at plpy_spi.c:547#4  PLy_spi_subtransaction_abort (oldcontext=<optimized out>, oldowner=<optimized out>) at plpy_spi.c:527#5  0x00007f31561e2185 in PLy_spi_execute_plan (ob=0x7f31b87d0cd8, list=0x7f31b7c530d8, limit=0) at plpy_spi.c:307#6  0x00007f31561e22d4 in PLy_spi_execute (self=<optimized out>, args=0x7f31b87a6d08) at plpy_spi.c:180#7  0x00007f3155cda4d6 in PyCFunction_Call (func=0x7f31b7d29600, arg=0x7f31b87a6d08, kw=0x0) at ../Objects/methodobject.c:81#8  0x00007f3155d82383 in call_function (pp_stack=0x7fff9207e710, oparg=2) at ../Python/ceval.c:4021#9  0x00007f3155d7cda4 in PyEval_EvalFrameEx (f=0x7f31b8805be0, throwflag=0) at ../Python/ceval.c:2666#10 0x00007f3155d82898 in fast_function (func=0x7f31b88b5ed0, pp_stack=0x7fff9207ea70, n=0, na=0, nk=0) at ../Python/ceval.c:4107#11 0x00007f3155d82584 in call_function (pp_stack=0x7fff9207ea70, oparg=0) at ../Python/ceval.c:4042#12 0x00007f3155d7cda4 in PyEval_EvalFrameEx (f=0x7f31b8805a00, throwflag=0) at ../Python/ceval.c:2666#13 0x00007f3155d7f8a9 in PyEval_EvalCodeEx (co=0x7f31b88aa460, globals=0x7f31b8727ea0, locals=0x7f31b8727ea0, args=0x0, argcount=0, kws=0x0, kwcount=0, defs=0x0, defcount=0, closure=0x0) at ../Python/ceval.c:3253#14 0x00007f3155d74ff4 in PyEval_EvalCode (co=0x7f31b88aa460, globals=0x7f31b8727ea0, locals=0x7f31b8727ea0) at ../Python/ceval.c:667#15 0x00007f31561dc476 in PLy_procedure_call (kargs=kargs@entry=0x7f31561e5690 "args", vargs=<optimized out>, proc=0x7f31b873b2d0, proc=0x7f31b873b2d0) at plpy_exec.c:801#16 0x00007f31561dd9c6 in PLy_exec_function (fcinfo=fcinfo@entry=0x7f31b7c1f870, proc=0x7f31b873b2d0) at plpy_exec.c:61#17 0x00007f31561de9f9 in plpython_call_handler (fcinfo=0x7f31b7c1f870) at plpy_main.c:291```

`pg_catalog.pg_label` shows `labname` and `labkind`.`labname` is the name of the label.`labkind` presents `v` for vertex and `e` for edge.Closepostgres#7

fixespostgres#7.