cli.py: no fcntl on Windowsaevri/no_win32_fcntl

Work around the fact that we can't import 'fcntl' on Windows, and
confine the workaround to as small a scope as we can.

This enables us to run at least these commands on Windows:

	bst help
	bst init

We can't run any commands that require a Platform object though, which
is most commands.

1 files changed, 21 insertions, 8 deletions

diff --git a/src/buildstream/_frontend/cli.py b/src/buildstream/_frontend/cli.py
index 9abab6473..fa31d756f 100644
--- a/src/buildstream/_frontend/cli.py
+++ b/src/buildstream/_frontend/cli.py
@@ -2,7 +2,6 @@ import multiprocessing
 import os
 import sys
 from functools import partial
-import fcntl
 
 import shutil
 import click
@@ -187,6 +186,26 @@ def override_completions(orig_args, cmd, cmd_param, args, incomplete):
     raise CompleteUnhandled()
 
 
+def validate_output_streams():
+    try:
+        import fcntl
+    except ImportError:
+        # When we move onto Python 3.6+, we'll probably want to check for
+        # ModuleNotFoundError instead, as that's more specifically the problem
+        # we're working around.
+        if sys.platform != 'win32':
+            raise
+        return
+
+    for stream in (sys.stdout, sys.stderr):
+        fileno = stream.fileno()
+        flags = fcntl.fcntl(fileno, fcntl.F_GETFL)
+        if flags & os.O_NONBLOCK:
+            click.echo("{} is currently set to O_NONBLOCK, try opening a new shell"
+                       .format(stream.name), err=True)
+            sys.exit(-1)
+
+
 def override_main(self, args=None, prog_name=None, complete_var=None,
                   standalone_mode=True, **extra):
 
@@ -211,13 +230,7 @@ def override_main(self, args=None, prog_name=None, complete_var=None,
     # Check output file descriptor at earliest opportunity, to
     # provide a reasonable error message instead of a stack trace
     # in the case that it is blocking
-    for stream in (sys.stdout, sys.stderr):
-        fileno = stream.fileno()
-        flags = fcntl.fcntl(fileno, fcntl.F_GETFL)
-        if flags & os.O_NONBLOCK:
-            click.echo("{} is currently set to O_NONBLOCK, try opening a new shell"
-                       .format(stream.name), err=True)
-            sys.exit(-1)
+    validate_output_streams()
 
     # We can only set the global multiprocessing start method once; for that
     # reason we're advised to do it inside the entrypoint, where it is easy to