Avoid hanging by using non-blocking pipe

CHANGELOG.md

@@ -10,21 +10,29 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ### Added
 
 * RELAP-5 Plugin
+* MCNP Plugin
+* Serpent Plugin
 
 ### Changes
 
 * The `Plugin.__call__` method now allows arbitrary keyword arguments to be
   passed on to the `Plugin.run` method
 * The `Database` class now acts like a sequence
 * Database directory names use random strings to avoid clashes when multiple
-  instances of WATTS are running simulataneously
+  instances of WATTS are running simultaneously
 * File template-based plugins now accept an `extra_template_inputs` argument
   indicating extra template files that should be rendered
 * The `PluginOpenMC` class now takes an optional `function` argument that
   specifies an arbitrary execution sequence
 * All plugins consistently use an attribute `executable` for specifying the path
   to an executable
 
+### Fixed
+
+* Use non-blocking pipe when capturing output to avoid some plugins stalling.
+* Avoid use of Unix-specific features in the Python standard library when
+  running on Windows
+
 ## [0.2.0]
 
 ### Added

src/watts/fileutils.py

@@ -2,6 +2,7 @@
 # SPDX-License-Identifier: MIT
 
 from contextlib import contextmanager
+import errno
 import os
 import platform
 import select
@@ -10,6 +11,9 @@
 import tempfile
 from typing import Union
 
+if sys.platform != 'win32':
+    import fcntl
+
 # Type for arguments that accept file paths
 PathLike = Union[str, bytes, os.PathLike]
 
@@ -91,18 +95,39 @@ def run(args):
     Based on https://stackoverflow.com/a/12272262 and
     https://stackoverflow.com/a/7730201
     """
-    p = subprocess.Popen(args, stdout=subprocess.PIPE, stderr=subprocess.PIPE,
-                         universal_newlines=True)
+    # Windows doesn't support select.select and fcntl module so just default to
+    # using subprocess.run. In this case, show_stdout/show_stderr won't work.
+    if sys.platform == 'win32':
+        subprocess.run(args)
+        return
+
+    # Helper function to add the O_NONBLOCK flag to a file descriptor
+    def make_async(fd):
+        fcntl.fcntl(fd, fcntl.F_SETFL, fcntl.fcntl(fd, fcntl.F_GETFL) | os.O_NONBLOCK)
+
+    # Helper function to read some data from a file descriptor, ignoring EAGAIN errors
+    def read_async(fd):
+        try:
+            return fd.read()
+        except IOError as e:
+            if e.errno != errno.EAGAIN:
+                raise e
+            else:
+                return ''
+
+    p = subprocess.Popen(args, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+    make_async(p.stdout)
+    make_async(p.stderr)
 
     while True:
-        select.select([p.stdout, p.stderr], [], [])
+        select.select([p.stdout, p.stderr], [], [], 0)
 
-        stdout_data = p.stdout.read()
-        stderr_data = p.stderr.read()
+        stdout_data = read_async(p.stdout)
+        stderr_data = read_async(p.stderr)
         if stdout_data:
-            sys.stdout.write(stdout_data)
+            sys.stdout.write(stdout_data.decode())
         if stderr_data:
-            sys.stderr.write(stderr_data)
+            sys.stderr.write(stderr_data.decode())
 
         if p.poll() is not None:
             break