Enable multiprocessing with spawn context for cloud safety

src/mdio/segy/parsers.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import multiprocessing as mp
 import os
 from concurrent.futures import ProcessPoolExecutor
 from itertools import repeat
@@ -46,15 +47,18 @@ def parse_index_headers(
     trace_ranges = []
     for idx in range(n_blocks):
         start, stop = idx * block_size, (idx + 1) * block_size
-        if stop > trace_count:
-            stop = trace_count
+        stop = min(stop, trace_count)
 
         trace_ranges.append((start, stop))
 
+    # For Unix async reads with s3fs/fsspec & multiprocessing,
+    # use 'spawn' instead of default 'fork' to avoid deadlocks
+    # on cloud stores. Slower but necessary. Default on Windows.
     num_workers = min(n_blocks, NUM_CPUS)
+    context = mp.get_context("spawn")
 
     tqdm_kw = dict(unit="block", dynamic_ncols=True)
-    with ProcessPoolExecutor(num_workers) as executor:
+    with ProcessPoolExecutor(num_workers, mp_context=context) as executor:
         # pool.imap is lazy
         lazy_work = executor.map(
             header_scan_worker,  # fn