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Fixes SIGSEGV and improve thread safety of journal.Reader class #144

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Fixes SIGSEGV and improve thread safety of journal.Reader class #144

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245784c
improve thread safety of journal.Reader class by simple protection wi…
sebres Apr 1, 2025
7fa91c8
amend: close and decrement count for initial reference only once (avo…
sebres Apr 1, 2025
7c99a39
repair GHA-CI (update versions, pip install using --break-system-pack…
sebres Apr 2, 2025
646b3b3
remove noop, unneeded by dealloc
sebres Jul 1, 2025
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108 changes: 69 additions & 39 deletions systemd/_reader.c
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Original file line number Diff line number Diff line change
Expand Up @@ -64,6 +64,8 @@
typedef struct {
PyObject_HEAD
sd_journal *j;
unsigned closed;
unsigned ref_count;
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In the future, please explain the reasoning behind a patch in the commit message. For something like thread safety, this is particularly important. Without that, the reader is left to guess.

I don't understand how this is supposed to help. This is a python object, and PyObject_HEAD above already embeds a reference counter. A second reference counter in the same struct is unexpected.

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Oh, you did provide an explanation, but in the pull request ticket. I was looking at the patches and missed that. Sorry. (The explanation still belongs in the commit message, not on a pull request.)

I'll reply further in #143, because you make some good points there.

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The explanation still belongs in the commit message, not on a pull request.

Well, matter of taste either (many repo-owners like to hold the commit messages as short as possible)...
But it can be added to the merge commit, can not it?
Or do you rather want that the PR branch gets rebased instead (with details in the commit)?

} Reader;
static PyTypeObject ReaderType;

Expand All @@ -89,6 +91,31 @@ static PyStructSequence_Desc Monotonic_desc = {
2,
};

static PyObject *Reader_new(PyTypeObject *type, PyObject *args, PyObject *kwds) {
Reader *self = (Reader *)PyType_GenericNew(type, args, kwds);
self->j = NULL;
self->closed = 0;
self->ref_count = 1; /* initial reference */
return (PyObject *)self;
}

static inline void decr_ref_count(Reader *self) {
if (!self->ref_count) return;
if (!--self->ref_count && self->j) {
sd_journal_close(self->j);
self->j = NULL;
}
}

#define INCR_REF_BEGIN_ALLOW_THREADS(self) \
self->ref_count++; \
Py_BEGIN_ALLOW_THREADS
#define DECR_REF_END_ALLOW_THREADS(self) \
Py_END_ALLOW_THREADS \
decr_ref_count(self);



/**
* Convert a str or bytes object into a C-string path.
* Returns NULL on error.
Expand Down Expand Up @@ -220,7 +247,10 @@ static int intlist_converter(PyObject* obj, int **_result, size_t *_len) {
}

static void Reader_dealloc(Reader* self) {
sd_journal_close(self->j);
if (self->j) {
sd_journal_close(self->j);
self->j = NULL;
}
Py_TYPE(self)->tp_free((PyObject*)self);
}

Expand Down Expand Up @@ -271,9 +301,9 @@ static int Reader_init(Reader *self, PyObject *args, PyObject *keywds) {
return -1;

#if HAVE_JOURNAL_OPEN_DIRECTORY_FD
Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_open_directory_fd(&self->j, (int) fd, flags);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)
#else
r = -ENOSYS;
#endif
Expand All @@ -285,9 +315,9 @@ static int Reader_init(Reader *self, PyObject *args, PyObject *keywds) {
if (!path)
return -1;

Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_open_directory(&self->j, path, flags);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)
}
} else if (_files) {
_cleanup_Py_DECREF_ PyObject *item0 = NULL;
Expand All @@ -300,9 +330,9 @@ static int Reader_init(Reader *self, PyObject *args, PyObject *keywds) {
return -1;

#if HAVE_JOURNAL_OPEN_FILES
Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_open_files(&self->j, (const char**) files, flags);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)
#else
r = -ENOSYS;
#endif
Expand All @@ -314,9 +344,9 @@ static int Reader_init(Reader *self, PyObject *args, PyObject *keywds) {
return -1;

#if HAVE_JOURNAL_OPEN_DIRECTORY_FD
Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_open_files_fd(&self->j, fds, n_fds, flags);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)
#else
r = -ENOSYS;
#endif
Expand All @@ -329,16 +359,16 @@ static int Reader_init(Reader *self, PyObject *args, PyObject *keywds) {
if (!namespace)
return -1;

Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_open_namespace(&self->j, namespace, flags);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)
#else
r = -ENOSYS;
#endif
} else {
Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_open(&self->j, flags);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)
}

return set_error(r, NULL, "Opening the journal failed");
Expand Down Expand Up @@ -438,8 +468,8 @@ static PyObject* Reader_close(Reader *self, PyObject *args) {
assert(self);
assert(!args);

sd_journal_close(self->j);
self->j = NULL;
self->closed = 1;
decr_ref_count(self); /* decrement initial reference (without incr) */
Py_RETURN_NONE;
}

Expand Down Expand Up @@ -501,7 +531,7 @@ static PyObject* Reader_next(Reader *self, PyObject *args) {
return NULL;
}

Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
if (skip == 1LL)
r = sd_journal_next(self->j);
else if (skip == -1LL)
Expand All @@ -512,7 +542,7 @@ static PyObject* Reader_next(Reader *self, PyObject *args) {
r = sd_journal_previous_skip(self->j, -skip);
else
assert(!"should be here");
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)

if (set_error(r, NULL, NULL) < 0)
return NULL;
Expand Down Expand Up @@ -782,9 +812,9 @@ PyDoc_STRVAR(Reader_seek_head__doc__,
"See :manpage:`sd_journal_seek_head(3)`.");
static PyObject* Reader_seek_head(Reader *self, PyObject *args) {
int r;
Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_seek_head(self->j);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)

if (set_error(r, NULL, NULL) < 0)
return NULL;
Expand All @@ -800,9 +830,9 @@ PyDoc_STRVAR(Reader_seek_tail__doc__,
static PyObject* Reader_seek_tail(Reader *self, PyObject *args) {
int r;

Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_seek_tail(self->j);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)

if (set_error(r, NULL, NULL) < 0)
return NULL;
Expand All @@ -820,9 +850,9 @@ static PyObject* Reader_seek_realtime(Reader *self, PyObject *args) {
if (!PyArg_ParseTuple(args, "K:seek_realtime", &timestamp))
return NULL;

Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_seek_realtime_usec(self->j, timestamp);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)

if (set_error(r, NULL, NULL) < 0)
return NULL;
Expand Down Expand Up @@ -850,17 +880,17 @@ static PyObject* Reader_seek_monotonic(Reader *self, PyObject *args) {
if (set_error(r, NULL, "Invalid bootid") < 0)
return NULL;
} else {
Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_id128_get_boot(&id);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)

if (set_error(r, NULL, NULL) < 0)
return NULL;
}

Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_seek_monotonic_usec(self->j, id, timestamp);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)

if (set_error(r, NULL, NULL) < 0)
return NULL;
Expand Down Expand Up @@ -924,9 +954,9 @@ static PyObject* Reader_process(Reader *self, PyObject *args) {

assert(!args);

Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_process(self->j);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)
if (set_error(r, NULL, NULL) < 0)
return NULL;

Expand All @@ -950,9 +980,9 @@ static PyObject* Reader_wait(Reader *self, PyObject *args) {
if (!PyArg_ParseTuple(args, "|L:wait", &timeout))
return NULL;

Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_wait(self->j, timeout);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)

if (set_error(r, NULL, NULL) < 0)
return NULL;
Expand All @@ -970,9 +1000,9 @@ static PyObject* Reader_seek_cursor(Reader *self, PyObject *args) {
if (!PyArg_ParseTuple(args, "s:seek_cursor", &cursor))
return NULL;

Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_seek_cursor(self->j, cursor);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)

if (set_error(r, NULL, "Invalid cursor") < 0)
return NULL;
Expand Down Expand Up @@ -1035,9 +1065,9 @@ static PyObject* Reader_query_unique(Reader *self, PyObject *args) {
if (!PyArg_ParseTuple(args, "s:query_unique", &query))
return NULL;

Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_query_unique(self->j, query);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)

if (set_error(r, NULL, "Invalid field name") < 0)
return NULL;
Expand Down Expand Up @@ -1172,9 +1202,9 @@ static PyObject* Reader_get_catalog(Reader *self, PyObject *args) {
assert(self);
assert(!args);

Py_BEGIN_ALLOW_THREADS
INCR_REF_BEGIN_ALLOW_THREADS(self)
r = sd_journal_get_catalog(self->j, &msg);
Py_END_ALLOW_THREADS
DECR_REF_END_ALLOW_THREADS(self)

if (r == -ENOENT) {
const void* mid;
Expand Down Expand Up @@ -1264,7 +1294,7 @@ static int Reader_set_data_threshold(Reader *self, PyObject *value, void *closur
PyDoc_STRVAR(closed__doc__,
"True iff journal is closed");
static PyObject* Reader_get_closed(Reader *self, void *closure) {
return PyBool_FromLong(!self->j);
return PyBool_FromLong(self->closed || !self->j);
}

static PyGetSetDef Reader_getsetters[] = {
Expand Down Expand Up @@ -1330,7 +1360,7 @@ static PyTypeObject ReaderType = {
.tp_methods = Reader_methods,
.tp_getset = Reader_getsetters,
.tp_init = (initproc) Reader_init,
.tp_new = PyType_GenericNew,
.tp_new = Reader_new,
};

static PyMethodDef methods[] = {
Expand Down
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