Merge pull request #292 from benfred/gil_deadlock_fix

Fix deadlock in child threads launched from python

README.md

@@ -5,7 +5,7 @@
 
 Non-Metric Space Library (NMSLIB) 
 =================
-The latest **pre**-release is [1.7](https://github.com/searchivarius/nmslib/releases/tag/v1.7). Note that the manual is not updated to reflect some of the changes. In particular, we changed the build procedure for Windows. Also note that the manual targets primiarily developers who will extend the library. For most other folks, [Python binding docs should be sufficient](python_bindings).
+The latest **pre**-release is [1.7.1](https://github.com/searchivarius/nmslib/releases/tag/v1.7.1). Note that the manual is not updated to reflect some of the changes. In particular, we changed the build procedure for Windows. Also note that the manual targets primiarily developers who will extend the library. For most other folks, [Python binding docs should be sufficient](python_bindings).
 -----------------
 Non-Metric Space Library (NMSLIB) is an **efficient** cross-platform similarity search library and a toolkit for evaluation of similarity search methods. The core-library does **not** have any third-party dependencies.
 

python_bindings/nmslib.cc

@@ -74,12 +74,12 @@ struct IndexWrapper {
                                                                    loadParams(space_params))) {
     auto vectSpacePtr = dynamic_cast<VectorSpace<dist_t>*>(space.get());
     if (data_type == DATATYPE_DENSE_VECTOR && vectSpacePtr == nullptr) {
-      throw std::invalid_argument("The space type " + space_type + 
+      throw std::invalid_argument("The space type " + space_type +
                                   " is not compatible with the type DENSE_VECTOR, only dense vector spaces are allowed!");
     }
     auto vectSiftPtr = dynamic_cast<SpaceL2SqrSift*>(space.get());
     if (data_type == DATATYPE_DENSE_UINT8_VECTOR && vectSiftPtr == nullptr) {
-      throw std::invalid_argument("The space type " + space_type + 
+      throw std::invalid_argument("The space type " + space_type +
                                   " is not compatible with the type DENSE_UINT8_VECTOR!");
     }
   }
@@ -361,6 +361,20 @@ struct IndexWrapper {
   ObjectVector data;
 };
 
+// pybind11::gil_scoped_acquire can deadlock when acquiring the GIL on threads
+// created from python (https://github.com/searchivarius/nmslib/issues/291)
+// This might be fixed in a future version of pybind11 (https://github.com/pybind/pybind11/pull/1211)
+// but until then, lets fall back to the python c-api to fix.
+struct AcquireGIL {
+  PyGILState_STATE state;
+  AcquireGIL()
+    : state(PyGILState_Ensure()) {
+  }
+  ~AcquireGIL() {
+    PyGILState_Release(state);
+  }
+};
+
 class PythonLogger
   : public Logger {
  public:
@@ -374,7 +388,7 @@ class PythonLogger
            int line,
            const char * function,
            const std::string & message) {
-    py::gil_scoped_acquire l;
+    AcquireGIL l;
     switch(severity) {
       case LIB_DEBUG:
         inner.attr("debug")(message);