Hello

When openning a netcdf file in read mode more than one time in a ThreadPool results in a segfault. When using a process pool there is no issue and the concurrent read works fine. It can be an issue in a web environment where the server is using threads to handle its clients and two or more clients make a request that need to read the same netcdf file.

I have a simple python script and a dataset that reproduces the segfault:
netcdfSegfault.zip

The python script provided in the zip:

import netCDF4
from multiprocessing.pool import ThreadPool

def read_netcdf(path):
    ieast = 1750
    iwest = 1760
    inorth = 1380
    isouth = 1370

    with netCDF4.Dataset(path, 'r') as ncf:
        return ncf['/Depth/ndepths'][:][iwest:ieast, isouth:inorth]


if __name__ == "__main__":
    print("Netcdf4 version: %s" % netCDF4.__version__)
    path = "./max_depth_ndepth_quonops.nc"
    with ThreadPool(2) as p:
        print(p.map(read_netcdf, [path for i in range(2)]))

Will output the following when executed.

Netcdf4 version: 1.4.0
Segmentation fault (core dumped)