Acquire/release the Python GIL where needed to avoid threading issues.

With these additions, frontends can now call libsigrokdecode API
functions from different threads without running into threading issues.

The backend releases the GIL when it is performing tasks that might take
a while and it doesn't need to run Python/C API calls during that time.

This allows frontends to run multiple PD stacks (in multiple frontend
threads) "at the same time" in a time-sharing, "interlocked" manner.
Whenever one of the decoders is inside e.g. self.wait() it releases the
GIL and thus allows other decoders to do some work in the mean time.

The user-visible effect is that for use-cases such as running 3 different
decoder stacks at the same time for an acquisition, the user will not
have to wait for PD 1 to finish decoding, then wait for PD 2 to finish
decoding, and only *then* being able to see annotations from PD 3.

Instead, all three PDs will decode some chunks of data from time to
time, thus the user is able to inspect annotations from all 3 PDs while
the acquisition and decoding is still going on.

1 files changed, 79 insertions, 7 deletions

diff --git a/decoder.c b/decoder.c
index d7395a6..2fece88 100644
--- a/decoder.c
+++ b/decoder.c
@@ -152,11 +152,15 @@ static void decoder_option_free(void *data)
 
 static void decoder_free(struct srd_decoder *dec)
 {
+	PyGILState_STATE gstate;
+
 	if (!dec)
 		return;
 
+	gstate = PyGILState_Ensure();
 	Py_XDECREF(dec->py_dec);
 	Py_XDECREF(dec->py_mod);
+	PyGILState_Release(gstate);
 
 	g_slist_free_full(dec->options, &decoder_option_free);
 	g_slist_free_full(dec->binary, (GDestroyNotify)&g_strfreev);
@@ -183,10 +187,15 @@ static int get_channels(const struct srd_decoder *d, const char *attr,
 	struct srd_channel *pdch;
 	GSList *pdchl;
 	ssize_t i;
+	PyGILState_STATE gstate;
+
+	gstate = PyGILState_Ensure();
 
-	if (!PyObject_HasAttrString(d->py_dec, attr))
+	if (!PyObject_HasAttrString(d->py_dec, attr)) {
 		/* No channels of this type specified. */
+		PyGILState_Release(gstate);
 		return SRD_OK;
+	}
 
 	pdchl = NULL;
 
@@ -227,6 +236,8 @@ static int get_channels(const struct srd_decoder *d, const char *attr,
 	Py_DECREF(py_channellist);
 	*out_pdchl = pdchl;
 
+	PyGILState_Release(gstate);
+
 	return SRD_OK;
 
 except_out:
@@ -235,6 +246,7 @@ except_out:
 err_out:
 	g_slist_free_full(pdchl, &channel_free);
 	Py_XDECREF(py_channellist);
+	PyGILState_Release(gstate);
 
 	return SRD_ERR_PYTHON;
 }
@@ -246,10 +258,15 @@ static int get_options(struct srd_decoder *d)
 	struct srd_decoder_option *o;
 	GVariant *gvar;
 	ssize_t opt, i;
+	PyGILState_STATE gstate;
 
-	if (!PyObject_HasAttrString(d->py_dec, "options"))
+	gstate = PyGILState_Ensure();
+
+	if (!PyObject_HasAttrString(d->py_dec, "options")) {
 		/* No options, that's fine. */
+		PyGILState_Release(gstate);
 		return SRD_OK;
+	}
 
 	options = NULL;
 
@@ -342,6 +359,7 @@ static int get_options(struct srd_decoder *d)
 	}
 	d->options = options;
 	Py_DECREF(py_opts);
+	PyGILState_Release(gstate);
 
 	return SRD_OK;
 
@@ -350,6 +368,7 @@ except_out:
 err_out:
 	g_slist_free_full(options, &decoder_option_free);
 	Py_XDECREF(py_opts);
+	PyGILState_Release(gstate);
 
 	return SRD_ERR_PYTHON;
 }
@@ -362,9 +381,14 @@ static int get_annotations(struct srd_decoder *dec)
 	GSList *annotations;
 	char **annpair;
 	ssize_t i;
+	PyGILState_STATE gstate;
+
+	gstate = PyGILState_Ensure();
 
-	if (!PyObject_HasAttrString(dec->py_dec, "annotations"))
+	if (!PyObject_HasAttrString(dec->py_dec, "annotations")) {
+		PyGILState_Release(gstate);
 		return SRD_OK;
+	}
 
 	annotations = NULL;
 
@@ -396,6 +420,7 @@ static int get_annotations(struct srd_decoder *dec)
 	}
 	dec->annotations = annotations;
 	Py_DECREF(py_annlist);
+	PyGILState_Release(gstate);
 
 	return SRD_OK;
 
@@ -404,6 +429,7 @@ except_out:
 err_out:
 	g_slist_free_full(annotations, (GDestroyNotify)&g_strfreev);
 	Py_XDECREF(py_annlist);
+	PyGILState_Release(gstate);
 
 	return SRD_ERR_PYTHON;
 }
@@ -417,9 +443,14 @@ static int get_annotation_rows(struct srd_decoder *dec)
 	struct srd_decoder_annotation_row *ann_row;
 	ssize_t i, k;
 	size_t class_idx;
+	PyGILState_STATE gstate;
+
+	gstate = PyGILState_Ensure();
 
-	if (!PyObject_HasAttrString(dec->py_dec, "annotation_rows"))
+	if (!PyObject_HasAttrString(dec->py_dec, "annotation_rows")) {
+		PyGILState_Release(gstate);
 		return SRD_OK;
+	}
 
 	annotation_rows = NULL;
 
@@ -492,6 +523,7 @@ static int get_annotation_rows(struct srd_decoder *dec)
 	}
 	dec->annotation_rows = annotation_rows;
 	Py_DECREF(py_ann_rows);
+	PyGILState_Release(gstate);
 
 	return SRD_OK;
 
@@ -501,6 +533,7 @@ except_out:
 err_out:
 	g_slist_free_full(annotation_rows, &annotation_row_free);
 	Py_XDECREF(py_ann_rows);
+	PyGILState_Release(gstate);
 
 	return SRD_ERR_PYTHON;
 }
@@ -513,9 +546,14 @@ static int get_binary_classes(struct srd_decoder *dec)
 	GSList *bin_classes;
 	char **bin;
 	ssize_t i;
+	PyGILState_STATE gstate;
 
-	if (!PyObject_HasAttrString(dec->py_dec, "binary"))
+	gstate = PyGILState_Ensure();
+
+	if (!PyObject_HasAttrString(dec->py_dec, "binary")) {
+		PyGILState_Release(gstate);
 		return SRD_OK;
+	}
 
 	bin_classes = NULL;
 
@@ -548,6 +586,7 @@ static int get_binary_classes(struct srd_decoder *dec)
 	}
 	dec->binary = bin_classes;
 	Py_DECREF(py_bin_classes);
+	PyGILState_Release(gstate);
 
 	return SRD_OK;
 
@@ -557,6 +596,7 @@ except_out:
 err_out:
 	g_slist_free_full(bin_classes, (GDestroyNotify)&g_strfreev);
 	Py_XDECREF(py_bin_classes);
+	PyGILState_Release(gstate);
 
 	return SRD_ERR_PYTHON;
 }
@@ -568,17 +608,23 @@ static int check_method(PyObject *py_dec, const char *mod_name,
 {
 	PyObject *py_method;
 	int is_callable;
+	PyGILState_STATE gstate;
+
+	gstate = PyGILState_Ensure();
 
 	py_method = PyObject_GetAttrString(py_dec, method_name);
 	if (!py_method) {
 		srd_exception_catch("Protocol decoder %s Decoder class "
 				"has no %s() method", mod_name, method_name);
+		PyGILState_Release(gstate);
 		return SRD_ERR_PYTHON;
 	}
 
 	is_callable = PyCallable_Check(py_method);
 	Py_DECREF(py_method);
 
+	PyGILState_Release(gstate);
+
 	if (!is_callable) {
 		srd_err("Protocol decoder %s Decoder class attribute '%s' "
 			"is not a method.", mod_name, method_name);
@@ -601,15 +647,20 @@ SRD_PRIV long srd_decoder_apiver(const struct srd_decoder *d)
 {
 	PyObject *py_apiver;
 	long apiver;
+	PyGILState_STATE gstate;
 
 	if (!d)
 		return 0;
 
+	gstate = PyGILState_Ensure();
+
 	py_apiver = PyObject_GetAttrString(d->py_dec, "api_version");
 	apiver = (py_apiver && PyLong_Check(py_apiver))
 			? PyLong_AsLong(py_apiver) : 0;
 	Py_XDECREF(py_apiver);
 
+	PyGILState_Release(gstate);
+
 	return apiver;
 }
 
@@ -629,6 +680,7 @@ SRD_API int srd_decoder_load(const char *module_name)
 	long apiver;
 	int is_subclass;
 	const char *fail_txt;
+	PyGILState_STATE gstate;
 
 	if (!srd_check_init())
 		return SRD_ERR;
@@ -636,8 +688,11 @@ SRD_API int srd_decoder_load(const char *module_name)
 	if (!module_name)
 		return SRD_ERR_ARG;
 
+	gstate = PyGILState_Ensure();
+
 	if (PyDict_GetItemString(PyImport_GetModuleDict(), module_name)) {
 		/* Module was already imported. */
+		PyGILState_Release(gstate);
 		return SRD_OK;
 	}
 
@@ -775,6 +830,8 @@ SRD_API int srd_decoder_load(const char *module_name)
 		goto err_out;
 	}
 
+	PyGILState_Release(gstate);
+
 	/* Append it to the list of loaded decoders. */
 	pd_list = g_slist_append(pd_list, d);
 
@@ -791,6 +848,7 @@ err_out:
 	if (fail_txt)
 		srd_err("Failed to load decoder %s: %s", module_name, fail_txt);
 	decoder_free(d);
+	PyGILState_Release(gstate);
 
 	return SRD_ERR_PYTHON;
 }
@@ -809,6 +867,7 @@ SRD_API char *srd_decoder_doc_get(const struct srd_decoder *dec)
 {
 	PyObject *py_str;
 	char *doc;
+	PyGILState_STATE gstate;
 
 	if (!srd_check_init())
 		return NULL;
@@ -816,12 +875,14 @@ SRD_API char *srd_decoder_doc_get(const struct srd_decoder *dec)
 	if (!dec)
 		return NULL;
 
+	gstate = PyGILState_Ensure();
+
 	if (!PyObject_HasAttrString(dec->py_mod, "__doc__"))
-		return NULL;
+		goto err;
 
 	if (!(py_str = PyObject_GetAttrString(dec->py_mod, "__doc__"))) {
 		srd_exception_catch("Failed to get docstring");
-		return NULL;
+		goto err;
 	}
 
 	doc = NULL;
@@ -829,7 +890,14 @@ SRD_API char *srd_decoder_doc_get(const struct srd_decoder *dec)
 		py_str_as_str(py_str, &doc);
 	Py_DECREF(py_str);
 
+	PyGILState_Release(gstate);
+
 	return doc;
+
+err:
+	PyGILState_Release(gstate);
+
+	return NULL;
 }
 
 /**
@@ -880,9 +948,12 @@ static void srd_decoder_load_all_zip_path(char *path)
 	Py_ssize_t pos = 0;
 	char *prefix;
 	size_t prefix_len;
+	PyGILState_STATE gstate;
 
 	set = files = prefix_obj = zipimporter = zipimporter_class = NULL;
 
+	gstate = PyGILState_Ensure();
+
 	zipimport_mod = py_import_by_name("zipimport");
 	if (zipimport_mod == NULL)
 		goto err_out;
@@ -951,6 +1022,7 @@ err_out:
 	Py_XDECREF(zipimporter_class);
 	Py_XDECREF(zipimport_mod);
 	PyErr_Clear();
+	PyGILState_Release(gstate);
 }
 
 static void srd_decoder_load_all_path(char *path)