_frontend/cli.py: Exit with error if output streams are set to nonblocking

This is better than raising a stack trace later on when logging gets
intense with a BlockingIOError.

This fixes #929

1 files changed, 12 insertions, 0 deletions

diff --git a/src/buildstream/_frontend/cli.py b/src/buildstream/_frontend/cli.py
index 9a753f77a..e9d67ca87 100644
--- a/src/buildstream/_frontend/cli.py
+++ b/src/buildstream/_frontend/cli.py
@@ -3,6 +3,7 @@ import sys
 from contextlib import ExitStack
 from functools import partial
 from tempfile import TemporaryDirectory
+import fcntl
 
 import click
 from .. import _yaml
@@ -191,6 +192,17 @@ def override_main(self, args=None, prog_name=None, complete_var=None,
         # Regular client return for test cases
         return
 
+    # 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)
+
     original_main(self, args=args, prog_name=prog_name, complete_var=None,
                   standalone_mode=standalone_mode, **extra)