Release GIL while compiling Yara rules (#183)

* Release GIL when compiling rules.

The compilation is a blocking operation and can be potentially CPU
intensive. There is no reason to hold GIL.

* Lock the GIL in callback to prevent segmentation fault.

To be uniform with other callbacks and to prevent the SIGSEGV we have to
lock the GIL while we want to interact with python objects. The callback
is called from a C code releases GIL. Other callback functions use the same
locking strategy.

yara-python.c

@@ -1891,6 +1891,8 @@ void raise_exception_on_error(
     const char* message,
     void* user_data)
 {
+  PyGILState_STATE gil_state = PyGILState_Ensure();
+
   if (error_level == YARA_ERROR_LEVEL_ERROR)
   {
     if (file_name != NULL)
@@ -1925,6 +1927,8 @@ void raise_exception_on_error(
     PyList_Append(warnings, warning_msg);
     Py_DECREF(warning_msg);
   }
+
+  PyGILState_Release(gil_state);
 }
 
 
@@ -2228,8 +2232,10 @@ static PyObject* yara_compile(
 
       if (fh != NULL)
       {
+        Py_BEGIN_ALLOW_THREADS
         error = yr_compiler_add_file(compiler, fh, NULL, filepath);
         fclose(fh);
+        Py_END_ALLOW_THREADS
       }
       else
       {
@@ -2238,7 +2244,9 @@ static PyObject* yara_compile(
     }
     else if (source != NULL)
     {
+      Py_BEGIN_ALLOW_THREADS
       error = yr_compiler_add_string(compiler, source, NULL);
+      Py_END_ALLOW_THREADS
     }
     else if (file != NULL)
     {
@@ -2246,9 +2254,11 @@ static PyObject* yara_compile(
 
       if (fd != -1)
       {
+        Py_BEGIN_ALLOW_THREADS
         fh = fdopen(fd, "r");
         error = yr_compiler_add_file(compiler, fh, NULL, NULL);
         fclose(fh);
+        Py_END_ALLOW_THREADS
       }
       else
       {
@@ -2268,7 +2278,9 @@ static PyObject* yara_compile(
 
           if (source != NULL && ns != NULL)
           {
+            Py_BEGIN_ALLOW_THREADS
             error = yr_compiler_add_string(compiler, source, ns);
+            Py_END_ALLOW_THREADS
 
             if (error > 0)
               break;
@@ -2305,8 +2317,10 @@ static PyObject* yara_compile(
 
             if (fh != NULL)
             {
+              Py_BEGIN_ALLOW_THREADS
               error = yr_compiler_add_file(compiler, fh, ns, filepath);
               fclose(fh);
+              Py_END_ALLOW_THREADS
 
               if (error > 0)
                 break;