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With a clean build of release27-maint (r84317), test_dbm.py fails on ppc64 with this error:
 File "test_dbm.py", line 24, in test_keys
 self.assert_(k in self.d)
AssertionError
I'm building gainst gdbm-1.8.0 (specifically, on a prerelease of RHEL6, with gdbm-devel-1.8.0-36.el6.ppc64)
All of the headers define datum as:
 typedef struct {
 char *dptr;
 int dsize;
 } datum;
Note the use of "int" for dsize.
This fragment of code in python's Modules/dbmmodule.c:dbm_contains:
 if (PyString_AsStringAndSize(v, (char **)&key.dptr,
 (Py_ssize_t *)&key.dsize)) {
 return -1;
 }
 appears to assume that
 sizeof(datum.dsize) == sizeof(Py_ssize_t)
which is not correct on these architectures:
(gdb) p sizeof(key.dsize)
25ドル = 4
(gdb) p sizeof(Py_ssize_t)
26ドル = 8
On ppc64, when PyString_AsStringAndSize writes the 0x00000000000000001 value for the ob_size of "a" to &key.dsize, I believe the 0x00000000 part is written to &key.size, and the 0x00000001 part is written to the 4 bytes following it, due to the incorrect cast from (int*) to (Py_ssize_t*)
Thankfully
(gdb) p sizeof(key)
28ドル = 16
so it writes this value to padding within the "datum key", rather than corrupting the stack.
The dbm_fetch() invocation is thus passed a 0 dsize, and doesn't find the key, hence the test fails.
The various other uses with that source file appear correct:
(i) there are various PyArg_Parse* calls using s#, with int, which is correct, given the absence of the PY_SSIZE_T_CLEAN macro.
(ii) there are various calls of PyString_FromStringAndSize(, datum.dsize), which I believe is correct: I believe the compiler will coerce this int to the wider Py_ssize_t type.
I'm attaching a patch which (I hope) correctly coerces the size of the key from Py_ssize_t to "int" within gdb_contains.

Note to self: I'm tracking this one in RH's downstream tracker as:
 https://bugzilla.redhat.com/show_bug.cgi?id=626756 

Should be fixed in issue17926. AFAICT the issue doesn't exist on 3.x.