I built your example with 3.6:

git clone https://github.com/pybind/pybind11
wget https://bugs.python.org/file48923/decimal_crash.zip
unzip decimal_crash.zip

git checkout v3.6.7
./configure --with-pydebug
make

g++ -std=c++11 -pthread -Wno-unused-result -Wsign-compare -g -Og -Wall -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -I. -I./Include -I./pybind11/include -c main.cpp

g++ -pthread -Xlinker -export-dynamic -o main main.o libpython3.6dm.a -lpthread -ldl -lutil -lm


cp python python3
PATH=.:$PATH
./main



And I literally get this error (not always, it may take 10 runs or so):

$ ./main
Fatal Python error: Python memory allocator called without holding the GIL

Thread 0x00007f1e73fff700 (most recent call first):

Thread 0x00007f1e7b836700 (most recent call first):

Thread 0x00007f1e7a834700 (most recent call first):

Thread 0x00007f1e7b035700 (most recent call first):

Thread 0x00007f1e7d039700 (most recent call first):

Thread 0x00007f1e7c838700 (most recent call first):

Current thread 0x00007f1e7c037700 (most recent call first):

Thread 0x00007f1e7e84f740 (most recent call first):
Aborted (core dumped)



So no, I don't think the GIL handling is correct.