Make waiter cross-platform

Waiter depended on a couple of posix tricks. The first was a clever
use of os.waitpid(-1, 0) to wait for any spawned processes to return.
On Windows, this raises an exception - Windows has no concept of a
process group.

The second was the use of NamedTempFile(). On anything but Windows,
a file can be open for writing and for reading at the same time.
This trick actually isn't necessary the way NamedTempFile is used
here. It exposes the actual file object on an attribute named file,
and it's already opened in binary mode. Typeshed does not know about
this yet, I'll submit a PR to fix the typeshed stub for tempfile.

Author: James Tatum

mypy/waiter.py

@@ -31,38 +31,23 @@ def __init__(self, name: str, args: List[str], *, cwd: str = None,
         self.end_time = None  # type: float
 
     def start(self) -> None:
-        self.outfile = tempfile.NamedTemporaryFile()
+        self.outfile = tempfile.TemporaryFile()
         self.start_time = time.time()
         self.process = Popen(self.args, cwd=self.cwd, env=self.env,
                              stdout=self.outfile, stderr=STDOUT)
         self.pid = self.process.pid
 
-    def handle_exit_status(self, status: int) -> None:
-        """Update process exit status received via an external os.waitpid() call."""
-        # Inlined subprocess._handle_exitstatus, it's not a public API.
-        # TODO(jukka): I'm not quite sure why this is implemented like this.
-        self.end_time = time.time()
-        process = self.process
-        assert process.returncode is None
-        if os.WIFSIGNALED(status):
-            process.returncode = -os.WTERMSIG(status)
-        elif os.WIFEXITED(status):
-            process.returncode = os.WEXITSTATUS(status)
-        else:
-            # Should never happen
-            raise RuntimeError("Unknown child exit status!")
-        assert process.returncode is not None
-
     def wait(self) -> int:
         return self.process.wait()
 
     def status(self) -> Optional[int]:
         return self.process.returncode
 
     def read_output(self) -> str:
-        with open(self.outfile.name, 'rb') as file:
-            # Assume it's ascii to avoid unicode headaches (and portability issues).
-            return file.read().decode('ascii')
+        file = self.outfile
+        file.seek(0)
+        # Assume it's ascii to avoid unicode headaches (and portability issues).
+        return file.read().decode('ascii')
 
     def cleanup(self) -> None:
         self.outfile.close()
@@ -178,17 +163,25 @@ def _record_time(self, name: str, elapsed_time: float) -> None:
         name2 = re.sub('( .*?) .*', r'\1', name)  # First two words.
         self.times2[name2] = elapsed_time + self.times2.get(name2, 0)
 
+    def _poll_current(self) -> Tuple[int, int]:
+        while True:
+            time.sleep(.25)
+            for pid in self.current:
+                cmd = self.current[pid][1]
+                code = cmd.process.poll()
+                if code is not None:
+                    cmd.end_time = time.time()
+                    return pid, code
+
     def _wait_next(self) -> Tuple[List[str], int, int]:
         """Wait for a single task to finish.
 
         Return tuple (list of failed tasks, number test cases, number of failed tests).
         """
-        pid, status = os.waitpid(-1, 0)
+        pid, status = self._poll_current()
         num, cmd = self.current.pop(pid)
         name = cmd.name
 
-        cmd.handle_exit_status(status)
-
         self._record_time(cmd.name, cmd.elapsed_time)
 
         rc = cmd.wait()