reddit.rexport: experiment with using optional cpu pool (used by all of HPI)

Enabled by the env variable, specifying how many cores to dedicate, e.g.

HPI_CPU_POOL=4 hpi query ...

my/reddit/rexport.py

@@ -8,6 +8,7 @@ REQUIRES = [
 ]
 
 from dataclasses import dataclass
+import inspect
 from pathlib import Path
 from typing import TYPE_CHECKING, Iterator, Sequence
 
@@ -45,14 +46,16 @@ def migration(attrs: Attrs) -> Attrs:
         ex: uconfig.rexport = attrs['rexport']
         attrs['export_path'] = ex.export_path
     else:
-        warnings.high("""DEPRECATED! Please modify your reddit config to look like:
+        warnings.high(
+            """DEPRECATED! Please modify your reddit config to look like:
 
 class reddit:
     class rexport:
         export_path: Paths = '/path/to/rexport/data'
-            """)
+            """
+        )
         export_dir = 'export_dir'
-        if export_dir in attrs: # legacy name
+        if export_dir in attrs:  # legacy name
             attrs['export_path'] = attrs[export_dir]
             warnings.high(f'"{export_dir}" is deprecated! Please use "export_path" instead."')
     return attrs
@@ -93,8 +96,19 @@ Upvote      = dal.Upvote
 
 
 def _dal() -> dal.DAL:
-    inp = list(inputs())
-    return dal.DAL(inp)
+    sources = list(inputs())
+
+    ## backwards compatibility (old rexport DAL didn't have cpu_pool argument)
+    cpu_pool_arg = 'cpu_pool'
+    pass_cpu_pool = cpu_pool_arg in inspect.signature(dal.DAL).parameters
+    if pass_cpu_pool:
+        from my.core._cpu_pool import get_cpu_pool
+
+        kwargs = {cpu_pool_arg: get_cpu_pool()}
+    else:
+        kwargs = {}
+    ##
+    return dal.DAL(sources, **kwargs)
 
 
 cache = mcachew(depends_on=inputs)