Initial Cygwin support module with just a bit more than bare-minimum

functionality.

Many tests are set to skip on Cygwin for now, but can be re-enabled as
more functionality is added back in.

The C extension module is just a stub for now and might end up being
removed altogether, especially if we can add
[PyCygwin](https://pycygwin.readthedocs.io/en/latest/) as a dependency.

psutil/__init__.py

@@ -16,6 +16,7 @@
  - NetBSD
  - Sun Solaris
  - AIX
+ - Cygwin (experimental)
 
 Works with Python versions from 2.6 to 3.4+.
 """
@@ -77,6 +78,7 @@
 
 from ._common import AIX
 from ._common import BSD
+from ._common import CYGWIN
 from ._common import FREEBSD  # NOQA
 from ._common import LINUX
 from ._common import MACOS
@@ -173,6 +175,10 @@
     # via sys.modules.
     PROCFS_PATH = "/proc"
 
+elif CYGWIN:
+    PROCFS_PATH = "/proc"
+    from . import _pscygwin as _psplatform
+
 else:  # pragma: no cover
     raise NotImplementedError('platform %s is not supported' % sys.platform)
 
@@ -201,7 +207,7 @@
     "POWER_TIME_UNKNOWN", "POWER_TIME_UNLIMITED",
 
     "BSD", "FREEBSD", "LINUX", "NETBSD", "OPENBSD", "MACOS", "OSX", "POSIX",
-    "SUNOS", "WINDOWS", "AIX",
+    "SUNOS", "WINDOWS", "AIX", "CYGWIN",
 
     # classes
     "Process", "Popen",
@@ -357,7 +363,7 @@ def __init__(self, seconds, pid=None, name=None):
 if POSIX:
     from . import _psposix
     _psposix.TimeoutExpired = TimeoutExpired
-if LINUX:
+if LINUX or CYGWIN:
     from . import _pslinux_common
     _pslinux_common.NoSuchProcess = NoSuchProcess
     _pslinux_common.ZombieProcess = ZombieProcess
@@ -662,11 +668,16 @@ def parent(self):
         if self.pid == lowest_pid:
             return None
         ppid = self.ppid()
+        if CYGWIN:
+            # See note for Process.create_time in _pscygwin.py
+            rounding_error = 1
+        else:
+            rounding_error = 0
         if ppid is not None:
             ctime = self.create_time()
             try:
                 parent = Process(ppid)
-                if parent.create_time() <= ctime:
+                if parent.create_time() <= (ctime + rounding_error):
                     return parent
                 # ...else ppid has been reused by another process
             except NoSuchProcess:
@@ -1495,7 +1506,15 @@ def pids():
     """Return a list of current running PIDs."""
     global _LOWEST_PID
     ret = sorted(_psplatform.pids())
-    _LOWEST_PID = ret[0]
+
+    if CYGWIN:
+        # Note: The _LOWEST_PID concept is not very meaningful since every
+        # "top-level" process has a non-1 PID; they do have PPID of 1 but that
+        # does not correspend to an actual process that shows up in the pids()
+        # list
+        _LOWEST_PID = 1
+    else:
+        _LOWEST_PID = ret[0]
     return ret
 
 
@@ -2467,7 +2486,7 @@ def test():  # pragma: no cover
         else:
             cputime = ''
 
-        user = p.info['username']
+        user = p.info['username'] or ''
         if not user and POSIX:
             try:
                 user = p.uids()[0]

psutil/_common.py

@@ -44,7 +44,7 @@
 __all__ = [
     # constants
     'FREEBSD', 'BSD', 'LINUX', 'NETBSD', 'OPENBSD', 'MACOS', 'OSX', 'POSIX',
-    'SUNOS', 'WINDOWS',
+    'SUNOS', 'WINDOWS', 'CYGWIN',
     'ENCODING', 'ENCODING_ERRS', 'AF_INET6',
     # connection constants
     'CONN_CLOSE', 'CONN_CLOSE_WAIT', 'CONN_CLOSING', 'CONN_ESTABLISHED',
@@ -86,6 +86,7 @@
 BSD = FREEBSD or OPENBSD or NETBSD
 SUNOS = sys.platform.startswith(("sunos", "solaris"))
 AIX = sys.platform.startswith("aix")
+CYGWIN = sys.platform.startswith("cygwin")
 
 
 # ===================================================================