Write array file fails when running a parallel pool of FreeFEM processes

General Discussion
1

Dear community,

I noticed a weird bug when trying to export arrays to files when running several FreeFem++ processes with a Python parallel for loop. The bug seems to be related to the interpretation of macros imported from external files.

I am using the following Python code calling pyfreefem for exporting an array in multiple output folders:

# This code does not work !
from pyfreefem import FreeFemRunner
from multiprocessing.pool import ThreadPool as Pool
    
code = """  
IMPORT "io.edp"
    
real[int] x = [1,2,3,4,5];  
exportArray(x); 
"""

def process_data(index):    
    FreeFemRunner(code, config={"FILE": str(index)},    
                  run_dir="test/output_"+str(index), debug=1).execute()
        
indices=range(100)
with Pool(processes=8) as pool: 
    pool.map(process_data, indices)

The code is supposed to save hundred times the file test/output_{i}/ffexport/array_x where i is an integer between 0 and 99.
For running this code, pyfreefem needs to be installed with pip install pyfreefem.

The macro exportArray is defined here: it does

func int saveArray(string fileName, real[int] &value){
	ofstream file(fileName);
	file.precision(16);
	file << value;
	dispDebug("Saved "+fileName,2,"blue");
}

macro exportArray(a) saveArray("$FFEXPORTDIR/array_"+Stringification(a),a)//

When executed, the line IMPORT "io.edp" is substituted into a regular import of a local copy of the file io.edp and $FFEXPORTDIR is replaced with test/output_{i}/ffexport.

Running this code with Python fails for some of the processes (not all) with the message

    4 : real[int] x = [1,2,3,4,5];  
    5 : exportArray(x)  saveArray("test/output_68/ffexport/array_"+Stringification((xx)),x);  sizestack + 1024 =4568  ( 3544 )

 Error opening file test/output_68/ffexport/array_x
  current line = 85
 call saveArray  at  line 5
Exec error : Error opening file
   -- number :1

The problem seems to be related to the interpretation of the macro exportArray which saves the file. Curiously, if we replace exportArray(x) with exportArray2(x) where exportArray2 is a copy of the macro exportArray that we insert directly in the running file , then it works:

# THIS CODE WORKS !
from pyfreefem import FreeFemRunner
from multiprocessing.pool import ThreadPool as Pool
    
code = """  
IMPORT "io.edp"
    
real[int] x = [1,2,3,4,5];  

macro exportArray2(a) saveArray("$FFEXPORTDIR/array_"+Stringification(a),a)//
exportArray2(x); 
"""

def process_data(index):    
    FreeFemRunner(code, config={"FILE": str(index)},    
                  run_dir="test/output_"+str(index), debug=1).execute()
        
indices=range(100)
with Pool(processes=8) as pool: 
    pool.map(process_data, indices)

Would you have an idea about why the original version fails ?

PS : Sorry for not providing a more minimal code which would not rely on pyfreefem, but I hope my message and the instructions are sufficiently clear.