don't run more than 61 workers on Windows (#838)

README.md

@@ -986,6 +986,8 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 
 ### 19.5b0
 
+* don't crash when run on a Windows machine with more than 61 cores (#838)
+
 * remove unnecessary parentheses around `yield` expressions (#834)
 
 * add parentheses around long tuples in unpacking assignments (#832)

black.py

@@ -440,22 +440,10 @@ def main(
             report=report,
         )
     else:
-        loop = asyncio.get_event_loop()
-        executor = ProcessPoolExecutor(max_workers=os.cpu_count())
-        try:
-            loop.run_until_complete(
-                schedule_formatting(
-                    sources=sources,
-                    fast=fast,
-                    write_back=write_back,
-                    mode=mode,
-                    report=report,
-                    loop=loop,
-                    executor=executor,
-                )
-            )
-        finally:
-            shutdown(loop)
+        reformat_many(
+            sources=sources, fast=fast, write_back=write_back, mode=mode, report=report
+        )
+
     if verbose or not quiet:
         bang = "💥 💔 💥" if report.return_code else "✨ 🍰 ✨"
         out(f"All done! {bang}")
@@ -497,6 +485,36 @@ def reformat_one(
         report.failed(src, str(exc))
 
 
+def reformat_many(
+    sources: Set[Path],
+    fast: bool,
+    write_back: WriteBack,
+    mode: FileMode,
+    report: "Report",
+) -> None:
+    """Reformat multiple files using a ProcessPoolExecutor."""
+    loop = asyncio.get_event_loop()
+    worker_count = os.cpu_count()
+    if sys.platform == "win32":
+        # Work around https://bugs.python.org/issue26903
+        worker_count = min(worker_count, 61)
+    executor = ProcessPoolExecutor(max_workers=worker_count)
+    try:
+        loop.run_until_complete(
+            schedule_formatting(
+                sources=sources,
+                fast=fast,
+                write_back=write_back,
+                mode=mode,
+                report=report,
+                loop=loop,
+                executor=executor,
+            )
+        )
+    finally:
+        shutdown(loop)
+
+
 async def schedule_formatting(
     sources: Set[Path],
     fast: bool,